JP5315680B2 - Color image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a color image forming apparatus capable of suppressing an increase in drive torque of an image carrier with simple constitution. <P>SOLUTION: The color image forming apparatus 1 includes a plurality of image forming units 13 each of which includes an image carrier 15, an electrifying means 16, an exposure means 14, a developing means 17 having a developing roll 17a and developing an electrostatic latent image to be a toner image by application of predetermined developing bias, a transfer means 26 transferring the toner image to a recording medium 30, a cleaning means 18 removing remaining toner on the image carrier 15 after transfer by a plate-like cleaning blade 19, and a destaticizing means 50 uniformizing residual potential by uniformly exposing the surface of the image holding body 15 after transfer. In the image forming units 13Y, 13M and 13C which are not used when performing a series of image forming operation, rotation of the corresponding developing rolls 17Ya, 17Ma and 17Ca and exposure by the corresponding destaticizing means 50Y, 50M and 50C are stopped. <P>COPYRIGHT: (C)2009,JPO&amp;INPIT

Description

  The present invention relates to an improvement in a color image forming apparatus that reduces the driving torque of an image carrier accompanying the adhesion of discharge products.

JP 2005-025008 A JP 2006-171358 A

  Conventionally, as a color image forming apparatus that forms a full-color or black-and-white image using an electrophotographic method, a latent image corresponding to image data is formed on the surface of an image carrier charged to a predetermined potential, A device having a plurality of image forming units for developing the latent image into a toner image is known.

  In such an image forming apparatus, the discharge product during charging adheres to the image carrier, and the frictional force with the cleaning blade that cleans the surface of the image carrier increases, resulting in the image carrier. There has been a problem that the driving torque increases with time.

  In particular, in recent years, a photosensitive drum that is an image bearing member has been used to improve wear resistance by using a harder resin (for example, a resin having a cross-linked structure) as the outermost layer to extend the life. However, when such a photosensitive drum is used, the friction between the cleaning blade and the photosensitive drum becomes larger because it becomes difficult to remove the discharge products adhering to the surface of the photosensitive drum during charging. The driving torque of the photosensitive drum tends to increase further. In order to solve such a problem, various color image forming apparatuses that reduce the driving torque of the photosensitive drum have been proposed (see, for example, Patent Documents 1 and 2).

  Here, Patent Document 1 discloses a color image forming apparatus in which an AC component in a charging unit is reduced by a predetermined amount for an image forming unit that does not require image formation.

  Patent Document 2 discloses a color image forming apparatus configured to be able to supply a predetermined developer as a lubricant via a transfer medium from another image forming unit to a cleaning unit of the predetermined image forming unit. .

  An object of the present invention is to provide a color image forming apparatus capable of suppressing an increase in driving torque of an image carrier with a simple configuration.

In order to achieve the above object, the color image forming apparatus according to claim 1, an image holding body on which an electrostatic latent image is held, and a surface of the image holding body by applying a predetermined charging bias. A charging means for charging the surface of the image holding member to a predetermined potential, an exposure means for exposing the surface of the image holding member to form a desired electrostatic latent image, and a developing roll that is disposed so as to face the image holding member and is rotatable. A developing means for applying a predetermined developing bias to form a toner image from the electrostatic latent image formed on the image carrier, and transferring the toner image formed on the image carrier to a recording medium. Transfer means, cleaning means for cleaning residual toner on the image carrier after transfer with a plate-like cleaning blade, and static elimination for uniformly exposing the surface of the image carrier after transfer to uniform residual potential A plurality of image forming units having A color image forming apparatus, the image forming unit is not used during the series of image forming operation, while performing the charging of the image carrier by the corresponding said charging means, according to the rotation and the charge removing means of the developing roll The exposure is stopped.

According to a second aspect of the present invention, in the color image forming apparatus according to the first aspect, the image forming unit that is not used in the series of image forming operations forms a black and white image in a mixed color / monochrome image forming operation. It is a color image forming unit when performing the above.

  According to a third aspect of the present invention, in the configuration of the first or second aspect, the outermost surface layer of the image carrier is formed of a crosslinkable resin.

  According to a fourth aspect of the present invention, in the color image forming apparatus according to the first aspect, a charging bias in which an alternating current component is superimposed on a direct current component is applied to the charging unit. It is characterized by.

  According to a fifth aspect of the present invention, in the color image forming apparatus according to the first to fourth aspects, the charging unit generates a discharge between the image holding member and the surface of the image holding member. The method is characterized in that it is charged to a predetermined potential.

A color image forming apparatus according to a sixth aspect of the present invention is the color image forming apparatus according to any one of the first to fifth aspects, wherein the exposure of the image carrier is stopped when the exposure by the charge eliminating unit corresponding to the unused image forming unit is stopped. The DC component in the developing bias applied to the developing roll is controlled so as to reduce the potential difference between the charging potential charged by the charging device and the developing bias.

A color image forming apparatus according to a seventh aspect of the present invention is the color image forming apparatus according to any one of the first to fifth aspects, wherein the exposure of the image carrier is stopped when the exposure by the charge eliminating unit corresponding to the unused image forming unit is stopped. The direct current component in the charging bias applied to the charging means is controlled so that the charging potential does not change.

  According to the first aspect of the present invention, by increasing the contact pressure between the cleaning blade and the image carrier, the image carrier driving torque can be increased with a simple configuration by stopping only a predetermined operation of a predetermined component. Can be suppressed.

  According to the second aspect of the present invention, it is possible to effectively suppress an increase in the driving torque of the image carrier, which is a problem when forming a black and white image in a color / monochrome mixed original.

  According to the third aspect of the present invention, it is more preferable to increase driving torque in an image forming apparatus having an image carrier whose surface is hardened with a crosslinkable resin, in which adhesion of discharge products becomes a significant problem. Can be suppressed.

  According to the fourth aspect of the present invention, an increase in driving torque in an image forming apparatus in which a charging bias in which an alternating current component is superimposed on a direct current component, in which the influence of a discharge product due to charging becomes more prominent, is applied. It can suppress more suitably.

  According to the fifth aspect of the present invention, it is possible to more suitably suppress an increase in driving torque in an image forming apparatus having a contact-type (or proximity-type) charging unit in which the influence of a discharge product due to charging becomes more serious. can do.

  According to the sixth aspect of the present invention, it is possible to easily compensate for fluctuations in the potential difference between the charged potential of the image carrier and the developing bias accompanying the stop of the static eliminating means, while preventing toner and carrier from scattering. can do.

  According to the seventh aspect of the present invention, it is possible to easily compensate for fluctuations in the charged potential of the image carrier accompanying the stop of the charge eliminating means, while preventing the toner and carrier from scattering.

  An embodiment of the present invention will be described below with reference to the drawings.

  First, a schematic configuration of an embodiment of an image forming apparatus to which the present invention can be applied will be described with reference to FIG. Here, FIG. 1 is a schematic configuration diagram of a tandem color image forming apparatus to which the present invention is applicable.

  The image forming apparatus according to the present embodiment receives, for example, color image information of a color document read by a document reading device, color image information sent from a personal computer or an image data input device (not shown), and the like. Image processing is performed on the input image information.

  In FIG. 1, reference numeral 1 denotes a main body of a tandem type digital color image forming apparatus, and an automatic document feeder (ADF) that automatically feeds documents 2 one by one on the top of the main body 1. 3 and a document reading device 4 for reading an image of the document 2 conveyed by the automatic document conveying device 3. The document reader 4 illuminates a document 2 placed on a platen glass 5 with a light source 6, and reflects a reflected light image from the document 2 from a full-rate mirror 7, half-rate mirrors 8 and 9, and an imaging lens 10. The image reading element 11 composed of a CCD or the like is scanned and exposed through a reduction optical system, and the color material reflected light image of the document 2 is formed at a predetermined dot density (for example, 16 dots / mm) by the image reading element 11. It is supposed to read.

  The image data that has been subjected to the predetermined image processing by the IPS 12 as described above is also four colors of yellow (Y), magenta (M), cyan (C), and black (K) (each 8 bits) by the IPS 12. As described below, the image forming units 13Y, 13M, 13C, and 13K for each color of yellow (Y), magenta (M), cyan (C), and black (K) are converted. Is sent to a ROS (Raser Output Scanner) 14, and the exposure means ROS 14 performs image exposure with laser light LB in accordance with original reproduction color material gradation data of a predetermined color.

  In the image forming apparatus according to the present embodiment, a plurality of image forming units 13Y, 13M, 13C, and 13K that form toner images of different colors are arranged in parallel, and the plurality of image forming units 13Y, 13Y, A belt-like intermediate transfer body 25 to which the toner images of the respective colors formed by the plurality of image forming units 13Y, 13M, 13C, and 13K are transferred is arranged over the upper portions of 13M, 13C, and 13K, and a plurality of image formation Below the units 13Y, 13M, 13C, 13K, image writing means 14Y, 14M, 14C for writing images on the image carriers 15Y, 15M, 15C, 15K of the image forming units 13Y, 13M, 13C, 13K. 14K is arranged.

  Further, in the present embodiment, an image formation that forms a black toner image on the most downstream side in the moving direction of the belt-shaped intermediate transfer body 25 among the plurality of image forming units 13Y, 13M, 13C, and 13K. The unit 13K is arranged.

  Next, constituent devices of the image forming units 13Y, 13M, 13C, and 13K will be described with reference to FIGS. Here, FIG. 2 is a schematic enlarged view for explaining the configuration of each image forming unit, and FIG. 3 is a schematic diagram for explaining the configuration of the surface layer of the photosensitive drum.

  As shown in an enlarged view in FIG. 2, the four image forming units 13Y, 13M, 13C, and 13K corresponding to the respective colors of yellow (Y), magenta (M), cyan (C), and black (K) The endless intermediate transfer belt 25 stretched by a stretch roll is arranged in parallel along the traveling direction at a certain interval in the horizontal direction, and each of yellow, magenta, cyan and black The toner images are sequentially formed at a predetermined timing. Since these image forming units 13Y, 13M, 13C, and 13K are all configured in the same manner, hereinafter, the reference numerals of the respective constituent devices are generically expressed (for example, the photosensitive drum 15).

  The image forming units 13Y, 13M, 13C, and 13K are roughly classified into a photosensitive drum 15 as an image carrier that is rotationally driven in a direction indicated by an arrow at a predetermined speed (for example, 200 mm / sec), and a surface of the photosensitive drum 15. A charging roll 16 as a charging means for uniformly charging the surface, a ROS 14 as an exposure means for forming an electrostatic latent image by exposing an image corresponding to a predetermined color on the surface of the photosensitive drum 15, and the photosensitive drum 15. A developing unit 17 as a developing unit that develops the electrostatic latent image formed thereon with toner of a predetermined color, a drum cleaning device 18 as a cleaning unit that cleans the surface of the photosensitive drum 15, and an exposure unit. In order to eliminate the influence of the latent image, it comprises an erase lamp 50 or the like as a charge eliminating means for exposing the entire surface of the photosensitive drum 15 before charging. In the present embodiment, the photosensitive drum 15 and the components disposed around the photosensitive drum 15 are integrally formed as a unit and can be individually replaced from the main body 1.

  In the present embodiment, the ROS 14 is configured in common to the four image forming units 13Y, 13M, 13C, and 13K, and modulates four semiconductor lasers (not shown) according to the original reproduction color material gradation data of each color. Thus, the laser beams LB-Y, LB-M, LB-C, and LB-K are emitted from these semiconductor lasers according to the gradation data. The ROS 14 may be individually configured for each of a plurality of image forming units. The laser beams LB-Y, LB-M, LB-C, and LB-K emitted from the semiconductor laser are irradiated to the polygon mirror 19 through an f-θ lens (not shown), and are deflected and scanned by the polygon mirror 19. The The laser beams LB-Y, LB-M, LB-C, and LB-K deflected and scanned by the polygon mirror 19 pass through an imaging lens (not shown) and a plurality of mirrors to an exposure point on the photosensitive drum 15. Then, scanning exposure is performed obliquely from below.

  In the present embodiment, since the ROS 14 scans and exposes an image on the photosensitive drum 15 from below, the ROS 14 develops four image forming units 13Y, 13M, 13C, and 13K located above. There is a risk that toner or the like may fall from the container 17 and be contaminated. Therefore, the periphery of the ROS 14 is hermetically sealed by a rectangular parallelepiped frame 20, and four laser beams LB-Y, LB-M, LB-C, and LB-K are placed on the upper portion of the frame 20. Transparent glass windows 21Y, 21M, 21C, and 21K as shield members are provided for exposure on the photosensitive drums 15 of the image forming units 13Y, 13M, 13C, and 13K.

  From the IPS 12, the image of each color is provided to the ROS 14 provided in common to the image forming units 13Y, 13M, 13C, and 13K of each color of yellow (Y), magenta (M), cyan (C), and black (K). Data is sequentially output, and the laser beams LB-Y, LB-M, LB-C, and LB-K emitted from the ROS 14 in accordance with the image data are scanned and exposed on the surface of the corresponding photosensitive drums 15 and electrostatically discharged. A latent image is formed. The electrostatic latent image formed on the photosensitive drum 15 is developed (toner image) by the developing unit 17 as toner images of yellow (Y), magenta (M), cyan (C), and black (K), respectively. ).

  In the photosensitive drum 15, a photosensitive layer made of an organic photosensitive material, an amorphous selenium photosensitive material, an amorphous silicon photosensitive material, or the like is formed on the surface of a metal drum that rotates in the direction of the arrow.

  More specifically, the photosensitive drum 15 according to the present exemplary embodiment includes a conductive support 155, a charge generation layer 150a, a first charge transport layer 150b, and a second charge as shown schematically in FIG. The charge transport layer 150c has a configuration in which these layers are stacked in this order. The charge generation layer 150a, the first charge transport layer 150b, and the second charge transport layer 150c constitute the photosensitive layer 150. In the present embodiment, the second charge transport layer 150c suppresses wear due to rubbing with the cleaning blade 19 to be described later, thereby extending the life of the photosensitive drum 15, and therefore the first charge transport layer 150b. For example, a crosslinkable resin such as phenol, melamine, and benzoguanamine is used.

  The yellow (Y), magenta (M), cyan (C), and black (K) toner images sequentially formed on the photosensitive drums 15 of the image forming units 13Y, 13M, 13C, and 13K are as follows. On the intermediate transfer belt (belt-shaped intermediate transfer member) 25 of the transfer unit 22 disposed over the image forming units 13Y, 13M, 13C, and 13K, the images are sequentially multiplexed by the four primary transfer rolls 26Y, 26M, 26C, and 26K. Is transcribed. These primary transfer rolls 26Y, 26M, 26C, and 26K are disposed on the back side of the intermediate transfer belt 25 corresponding to the photosensitive drums 15 of the image forming units 13Y, 13M, 13C, and 13K. Here, each primary transfer roll 26Y, 26M, 26C, and 26K is adjusted to a desired volume resistance value, and a transfer bias power source (not shown) is connected to these primary transfer rolls 26Y, 26M, 26C, and 26K. Thus, a transfer bias having a polarity opposite to the predetermined toner polarity (positive polarity in the present embodiment) is applied at a predetermined timing.

  The intermediate transfer belt 25 is wound around the drive roll 27, the tension roll 24, and the backup roll 28 with a constant tension, and is rotated by a dedicated drive motor with excellent constant speed (not shown). The drive roll 27 is driven to circulate at a predetermined speed in the direction of the arrow. As the intermediate transfer belt 25, for example, a belt whose resistance is adjusted with a belt material (rubber or resin) that does not cause charge-up is used.

  The yellow (Y), magenta (M), cyan (C), and black (K) toner images transferred onto the intermediate transfer belt 25 in multiple layers are backed up on the side of the intermediate transfer belt 25. The recording sheet 30 that has been secondarily transferred onto a recording sheet 30 as a recording medium by a pressing force and electrostatic attraction force by a secondary transfer roll 29 that is in pressure contact with the roll 28, and onto which the toner images of these colors are transferred, is directed upward. It is conveyed to the fixing device 31 located. The secondary transfer roll 29 is in pressure contact with the side of the backup roll 28, and secondary-transfers each color toner image onto a recording sheet 30 that is conveyed upward from below in the vertical direction. . The recording paper 30 onto which the toner images of the respective colors are transferred is subjected to a fixing process by heat and pressure by a fixing device 31 and then discharged onto a discharge tray 33 provided on the upper portion of the main body 1 by a discharge roll 32. The

  The recording paper 30 having a predetermined size is separated one by one by a nudger roll 35 and a paper separating / feeding feed roll 36 from a paper feeding device 34 disposed inside the apparatus main body 1. In this state, it is once transported to a registration roll 38 provided in the paper transport path 37 and stopped. The recording paper 30 supplied from the paper feeding device 34 is sent to the secondary transfer position of the intermediate transfer belt 25 by a registration roll 38 that rotates at a predetermined timing.

  In the digital color image forming apparatus according to the present embodiment, when full-color double-sided copying is performed, the recording paper 30 having an image fixed on one side is directly discharged onto the discharge tray 33 by the discharge roll 32. Instead, the conveyance direction is switched by a switching gate (not shown), and the sheet is conveyed to the duplex conveyance unit 40 via the sheet conveyance roller pair 39. In the double-sided conveyance unit 40, the recording paper 30 is conveyed again to the registration roll 38 with the front and back sides of the recording paper 30 reversed by a pair of conveyance rollers (not shown) provided along the conveyance path 41. Is discharged onto the discharge tray 33 after the image is transferred and fixed on the back surface of the recording paper 30. In the drawing, 44Y, 44M, 44C, and 44K are toners that supply toner of a predetermined color to the developing device 17 of each color of yellow (Y), magenta (M), cyan (C), and black (K). Each cartridge is shown.

  Next, the image forming operation in each of the image forming units 13Y, 13M, 13C, and 13K described above will be further described.

  First, along with the start of the image forming operation, the photosensitive drum 15 of each of the image forming units 13Y, 13M, 13C, and 13K starts to rotate and the rotational speed of the photosensitive drum 15 reaches a predetermined speed. A charging bias is applied to the charging roll 16, and the surface of the photosensitive drum 15 (photosensitive layer 150) is uniformly charged by the charging roll 16. In the present embodiment, the charging roll 16 is a contact (or proximity) charging unit that generates a discharge with the photosensitive drum 15 and charges the surface of the photosensitive drum 15 to a predetermined potential. A charging bias in which an AC component is superimposed on a DC component is applied at a predetermined timing by a charging bias power source (not shown).

  The surface of the photosensitive drum 15 charged to a predetermined potential by the charging roll 16 is scanned and exposed to an image forming laser beam LB emitted from the ROS 14 according to the image data, and a desired electrostatic force corresponding to each color is obtained. A latent image is formed. In the present embodiment, the laser beam LB that is scanned and exposed on the photosensitive drum 15 is set to be exposed from an obliquely lower side slightly to the right of the photosensitive drum 15.

  The electrostatic latent images formed on the photosensitive drum 15 are respectively yellow, magenta, cyan, and black toners by the developing rolls 17a of the developing units 17 of the image forming units 13Y, 13M, 13C, and 13K. Are developed into visible toner images, and these visible toner images are sequentially transferred onto the intermediate transfer belt 25 in multiple layers by charging of the primary transfer roll 26. Here, the developing roller 17a of each developing unit 17 is disposed to face the photosensitive drum 15 with a predetermined separation, is formed to be rotatable, and is connected to a developing bias power source (not shown), and has a direct current component. A predetermined developing bias with an alternating current component superimposed thereon is applied at a predetermined timing.

  Further, the surface of the photosensitive drum 15 of each of the image forming units 13Y, 13M, 13C, and 13K after the toner image transfer process is finished is subjected to full exposure by an erase lamp 50 that is ON / OFF controlled at a predetermined timing. Residual charges are removed (uniformized).

  Further, residual toner and the like are removed from the surface of the photosensitive drum 15 after the toner image transfer process is completed by the drum cleaning device 18 to prepare for the next image forming process. The drum cleaning device 18 includes a plate-like cleaning blade 19. The cleaning blade 19 is brought into contact with the photosensitive drum 15 to scrape and remove residual toner on the photosensitive drum 15. . Further, after the toner image transfer process is completed, the surface of the intermediate transfer belt 25 is freed of residual toner, paper dust, and the like by a belt cleaning device 43 including a cleaning brush 43a and a cleaning blade 43b, and the next image formation. Prepare for the process.

  By the way, in the image forming apparatus configured to scrape and remove the residual toner and the like adhering to the photosensitive drum 15 with the plate-like cleaning blade 42 as described above, the contact between the cleaning blade 42 and the photosensitive drum 15 over time. The pressure increases, and as a result, there arises a problem that the driving torque of the photosensitive drum 15 increases. In particular, such a phenomenon is caused by contact type or proximity type charging means (more specifically, a discharge is generated between the photosensitive drum 15 and the photosensitive drum 15 in which adhesion of the discharge product due to charging to the photosensitive drum 15 becomes a problem. In this example, the surface of the photosensitive drum 15 is charged to a predetermined potential. In this example, a charging roll 16) is used, a charging bias in which an AC component is superimposed on a DC component, or a long time. For example, when the outermost surface layer of the photosensitive drum 15 is cured and formed of, for example, a crosslinkable resin in order to extend the service life, the driving torque of the photosensitive drum 15 increases excessively, and the worst case is that the apparatus stops. It becomes a problem.

  Therefore, in the color image forming apparatus according to the present invention, the above-described problem is prevented with a simple configuration as follows.

Specifically, in the image forming apparatus according to the present embodiment, in the image forming unit 13 that is not used in a series of image forming operations, the rotation of the corresponding developing roll 17a is stopped and the corresponding erase lamp 50 is stopped. It is configured to do. Here, as an example of the image forming unit 13 that is not used in the series of image forming operations, for example, the color image forming units 13Y, 13M, and 13C when forming a black and white image in a color / monochrome mixed image forming operation. Is mentioned. A specific operation in such a case will be described below.

  In the color image forming apparatus 1 according to the present embodiment, when a color / monochrome original is output, the developing devices 17Y, 17M, and 17C corresponding to the color image forming units 13Y, 13M, and 13C that do not consume developer during monochrome printing. When the developing rolls 17Ya, 17Ma, and 17Ca continue to rotate, the color developer deteriorates. Therefore, the developing rolls 17Ya, 17Ma, and 17Ca are stopped, and erase lamps 50Y, 50M, and 50C corresponding to the operation are stopped. Turn off the light. Here, the reason why only the developing rolls 17Ya, 17Ma, and 17Ca and the erase lamps 50Y, 50M, and 50C are stopped is that the number of prints per hour (productivity) accompanying the start / stop of the color image forming units 13Y, 13M, and 13C. This is to suppress the decrease in the minimum.

The erase lamps 50Y, 50M, and 50C are turned off when the rotation of the developing rolls 17Ya, 17Ma, and 17Ca is stopped and the supply of fresh toner and external additives to the cleaning blade 19 is stopped. This is to suppress an increase in driving torque of the photosensitive drum 15. In the present embodiment, as described above, in order to suppress the decrease in productivity, in the color image forming units 13Y, 13M, and 13C during monochrome printing, the photosensitive drum 15 is driven, the charging bias is applied, and the development is performed. The bias application is not stopped.

4 to the developing roller 17Ya, 17Ma, upon stopping the 17Ca, erase lamp 50Y, 50M, off and when to 50C lighting (ON) (OFF) is a photosensitive drum 15Y when allowed, 15M, 15C driven The result of having measured torque (static torque) is shown. In FIG. 4, the horizontal axis indicates the number of rotations of the photosensitive drum 15 (the number of black and white images formed), and the vertical axis indicates the driving torque of the color photosensitive drums 15Y, 15M, and 15C. ing.

  As understood from FIG. 4, when the erase lamps 50Y, 50M, and 50C are turned on, when about 4,000 black and white images are formed, the static torque rises to about 6.5 kgf · cm and then saturates. . On the other hand, when the erase lamps 50Y, 50M, and 50C are turned off, the static torque when about 4,000 black-and-white images are formed is about 4.8 kgf · cm, and the drive torque increases compared to when the lights are on. It turns out that it is suppressed.

  The reason why the drive torque is reduced when the erase lamp is extinguished in this way will be further described with reference to FIGS.

FIG. 5 shows the AC voltage waveform V ₀ applied to the charging roll 16 and the current waveform I ₁ flowing through the charging roll 16 as a result of voltage application. Here, since the charging roll 16 discharges and charges the photosensitive drum 15 when the applied voltage exceeds a predetermined voltage, the current waveform I ₁ deviates from the sine curve I ₀ during discharging. An overlay of the current waveform I ₁ a sine curve I ₀ is shown in FIG. 6, the discharge current Ic difference between the current waveform I ₁ and sine curve I ₀ (area) were used to discharge.

FIG. 7 shows an alternating voltage (Vp−) applied to the charging roll 16 by the change on the + side and − side of the accumulated value (for 10 cycles) of the difference (discharge amount) between the current waveform I ₁ and the sine curve I ₀ of FIG. It was measured according to the change of p). Measurement conditions were as follows: process speed: 220 mm / sec; DC voltage component: 700 V; AC voltage frequency: 1500 Hz.

  As is apparent from FIG. 7, it is understood that when the erase lamp 50 is turned off, the discharge amount on the negative side (because the charged potential of the photosensitive drum 15 is negative) decreases. Therefore, when the erase lamp 50 is turned off, the stress due to charging (discharge) is reduced as compared with when the erasing lamp 50 is turned on, and the discharge stress (attachment of discharge products, etc.) received by the photosensitive drum 15 due to charging is suppressed. The friction is reduced, and the driving torque of the photosensitive drum 15 is reduced as shown in FIG.

  By the way, as described above, the photosensitive drum 15 is neutralized by the erase lamp 50 (the surface potential is lowered and uniformized) and then charged by the charging roll 16. Here, when the charging capability of the charging roll 16 is sufficiently high, the surface potential of the photosensitive drum 15 after charging becomes the same potential as the DC voltage (Vdc) applied to the charging roll 16, but the charging roll 16 has the same potential. When the charging capability is insufficient, such as when the process speed is higher than the charging capability, the charged potential of the photosensitive drum 15 tends to be lower than Vdc. If the erase lamp 50 is turned off in such a state, the surface potential of the photosensitive drum 15 entering the charging roll 16 becomes high, and the charged potential after charging may become higher than when the erase lamp 50 is turned on.

  Therefore, the image forming apparatus according to the present invention solves the above-described problem as follows.

  FIG. 8 shows a measurement of a difference ΔVH between the target value of the charging potential VH of the photosensitive drum 15 when the erase lamp 50 is turned on and when the erase lamp 50 is turned off. It can be seen that the potential increases. Here, in the reverse development type electrophotography, the image printing area on the surface of the photosensitive member charged to the potential VH is exposed to lower the potential to VL, and the potential difference Vcln with the developing bias Vbias applied to the developing unit 17 is used. The toner is placed on the image area and developed (see FIG. 9). For this reason, Vcln has a potential difference in the opposite direction to the image area so that toner does not fly to the non-image area. However, when this potential difference becomes large, a problem arises in that carriers are scattered and attached to the background area. Therefore, when the erase lamp 50 is turned off and VH changes, it is desirable to perform control to keep the potential difference Vcln between the charging potential VH of the photosensitive drum 15 and the developing bias Vbias constant.

  Therefore, in the image forming apparatus according to the present embodiment, the charging bias 16 applied to the charging roll 16 is increased so that the developing bias Vbias is increased according to the increase in VH due to the erase-off or the increase in VH due to the erase-off is offset and the VH does not change. The configuration is such that Vdc is lowered. When compensating for the increase in VH due to erase-off as described above, it is preferable to control the DC component in the developing bias or charging bias from the viewpoint of preventing adhesion due to scattering of toner or carrier.

A predetermined operation of a predetermined component of the image forming unit that is not used in this way (for example, rotation and erasing of the developing rolls 17Ya, 17Ma, and 17Ca corresponding to the color image forming units 13Y, 13M, and 13C when forming a monochrome image) Only the exposure of the photosensitive drums 15Y, 15M, and 15C by the lamps 50Y, 50M, and 50C is stopped, and the operations of the other component devices are continued in the same manner as in normal image formation without sacrificing productivity. In this case, the deterioration of the developer is suppressed, the discharge stress on the photosensitive drum 15 is reduced, and the toner and the external additive are supplied from another unit without using a lubrication means or a complicated control means. The increase in friction between the blade 19 and the photosensitive drum 15 is suppressed, and an increase in the driving torque of the photosensitive drum 15 is suppressed with a simple configuration. It becomes possible.

  Further, by controlling the DC component in the developing bias or the charging bias so that Vcln is kept constant when the erase lamp 50 is turned off, the erase lamp 50 is turned off while preventing toner and carriers from being scattered. As a result, it is possible to compensate for fluctuations in the potential difference Vcln between the charging potential VH of the photosensitive drum 15 and the developing bias Vbias.

1 is a schematic configuration diagram showing an embodiment of an image forming apparatus according to the present invention. FIG. 3 is a schematic enlarged view for explaining a configuration of an image forming unit according to the present invention. It is a schematic diagram for demonstrating the structure of the surface layer of the photosensitive drum which concerns on this invention. FIG. 7 is a diagram showing the result of measuring the driving torque (static torque) of the photosensitive drum when the developing roll is stopped and the erase lamp is turned on (ON) and turned off (OFF). It is the figure which showed the alternating voltage waveform and electric current waveform which are applied to the charging roll. It is a figure which shows the electric current waveform of the charging roll at the time of discharge in contrast with a sine curve in order to demonstrate a discharge current. It is a figure which shows the result of having measured the change of + side and-side of the cumulative value (for 10 cycles) of discharge amount according to the change of the alternating voltage applied to a charging roll. It is a figure which shows the result of having measured the difference (DELTA) VH with the target value of the charging potential of the photosensitive drum when an erase lamp is turned on and when it put it out. FIG. 6 is a schematic diagram illustrating a relationship between a potential on the surface of a photoreceptor and a developing bias in electrophotographic reversal development.

Explanation of symbols

  1: color image forming apparatus main body, 3: automatic document feeder, 4: document reading device, 13Y, 13M, 13C, 13K: image forming unit, 15: photosensitive drum, 16: charging roll, 17: developing device, 17a: Developing roll, 18: Drum cleaning device, 19: Cleaning blade, 25: Intermediate transfer belt, 26: Primary transfer roll, 27: Drive roll, 28: Backup roll, 29: Secondary transfer roll, 30: Recording paper, 31: Fixing device, 34: paper feeding device, 38: registration roll, 40: duplex transport unit, 43: belt cleaning device, 50: erase lamp, 150: photosensitive layer, 150a: charge generation layer, 150b: first charge transport Layer, 150c: second charge transport layer, 155: conductive support, Ic: discharge current, Vbias: development bias, V ln: potential difference, Vdc: charging bias, VH: charging potential

Claims (7)

An image carrier that holds an electrostatic latent image on the surface, a charging unit that applies a predetermined charging bias to charge the surface of the image carrier to a predetermined potential, and exposes the surface of the image carrier. And an exposure unit that forms a desired electrostatic latent image, and a developing roll that is rotatably disposed and is opposed to the image carrier, and is formed on the image carrier by applying a predetermined development bias. Development means for converting the electrostatic latent image into a toner image, transfer means for transferring the toner image formed on the image carrier to a recording medium, and residual toner on the image carrier after transfer to a plate-shaped cleaning blade A color image forming apparatus comprising a plurality of image forming units each having a cleaning means for cleaning in step (a) and a charge eliminating means for uniformly exposing the surface of the image carrier after transfer to uniform the residual potential,
An image forming unit that is not used in a series of image forming operations is characterized in that the image carrier is charged by the corresponding charging unit, while the rotation of the developing roll and the exposure by the charge eliminating unit are stopped. A color image forming apparatus. 2. The color according to claim 1, wherein the image forming unit that is not used in the series of image forming operations is a color image forming unit for forming a monochrome image in a color / monochrome mixed image forming operation. Image forming apparatus.   The color image forming apparatus according to claim 1, wherein the outermost surface layer of the image carrier is formed of a crosslinkable resin.   4. The color image forming apparatus according to claim 1, wherein a charging bias in which an AC component is superimposed on a DC component is applied to the charging unit.   5. The color according to claim 1, wherein the charging unit is a system that generates a discharge with the image carrier and charges the surface of the image carrier to a predetermined potential. Image forming apparatus. Applied to the developing roll so as to reduce the potential difference between the charging potential charged by the charging means of the image carrier and the developing bias when the exposure by the charge eliminating means corresponding to the unused image forming unit is stopped. 6. The color image forming apparatus according to claim 1, wherein a direct current component in the developing bias is controlled. The direct current component in the charging bias applied to the charging unit is controlled so that the charging potential of the image holding member does not change when exposure by the neutralizing unit corresponding to the unused image forming unit is stopped. The color image forming apparatus according to claim 1.

Images