JP4590716B2 - Image forming apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a copying machine, a printer, or a facsimile using an electrophotographic method, and more particularly to an image forming apparatus capable of extending the life of an image carrier including a photosensitive drum.
[0002]
[Prior art]
In recent years, documents processed in offices and the like have been rapidly colored, and image forming apparatuses such as copying machines, printers, and facsimiles that handle these documents have also been rapidly colored. Currently, these color devices have been further improved in image quality and speed as the quality of office processing in offices and the like has become higher and faster. As a color device that can meet such a demand, for example, image forming portions of each color of yellow (Y), magenta (M), cyan (C), and black (K) are arranged on a line and formed by each image forming portion. Various types of so-called tandem type color image forming apparatuses have been proposed in which multiple images having different colors are transferred onto an intermediate transfer belt or a recording medium transported by a paper transport belt to form a color image. Actually commercialized.
[0003]
Conventionally, as this type of tandem type color image forming apparatus, for example, there is the following. As shown in FIG. 12, the tandem type color image forming apparatus includes a yellow color image forming unit 100Y that forms a yellow (Y) image and a magenta color image forming unit that forms a magenta (M) color image. The image forming unit 100 includes four image forming units 100M, a cyan image forming unit 100C that forms a cyan (C) color image, and a black image forming unit 100K that forms a black (K) image.
[0004]
These four image forming units 100Y, 100M, 100C, and 100K are horizontally arranged with a certain distance from each other. Further, below the black (K), yellow (Y), magenta (M), and cyan (C) image forming units 100Y, 100M, 100C, and 100K, the image forming units 100Y, 100M, 100C, An intermediate transfer belt 101 that transfers toner images sequentially formed at 100K in a state where they are superimposed on each other is disposed so as to be rotatable along the direction of the arrow.
[0005]
The black (K), yellow (Y), magenta (M), and cyan (C) image forming units 100Y, 100M, 100C, and 100K are all configured in the same manner, and these four image forming units 100Y, 100Y, In 100M, 100C, and 100K, as described above, yellow, magenta, cyan, and black toner images are sequentially formed. The image forming units 100Y, 100M, 100C, and 100K of the respective colors include a photoconductive drum 102. The surface of the photoconductive drum 102 is uniformly charged by a charging roll 103 for primary charging, and then an image is formed. The writing device 104 scans and exposes image forming laser light in accordance with image information to form an electrostatic latent image. The electrostatic latent image formed on the surface of the photosensitive drum 102 is developed with a yellow, magenta, cyan, and black toner image by the developing unit 105 of each image forming unit to form a visible toner image. These visible toner images are transferred onto the intermediate transfer belt 101 while being superimposed on each other by the transfer charger 106. The yellow, magenta, cyan, and black toner images transferred onto the intermediate transfer belt 101 in a multiple manner are collectively transferred onto the recording medium 107 and then subjected to a fixing process by a fixing device (not shown). Then, a color image is formed.
[0006]
In FIG. 12, reference numeral 108 denotes a cleaning device for cleaning the surface of the photosensitive drum 102.
[0007]
By the way, the tandem type color image forming apparatus configured in this way is a system in which toner images sequentially formed by a plurality of image forming units 100Y, 100M, 100C, and 100K are transferred onto the intermediate transfer belt 101 in a multiple manner. Therefore, the image forming units 100Y, 100M, 100C, and 100K for all colors are operated even when an image that does not use all four colors, particularly a monochrome image, is recorded. Therefore, there has been a problem that the life of the photosensitive drum 102 of the color that has not been used is undesirably shortened.
[0008]
More specifically, in the tandem type color image forming apparatus, as shown in FIG. 13, in order to uniformly charge the surface of the photosensitive drum 102 to a predetermined potential, a direct current in which an alternating voltage is superimposed on the charging roll 103 is used. It is comprised so that a voltage may be applied. However, when a DC voltage on which an AC voltage is superimposed is applied to the charging roll 103, a photosensitive vibration as shown in FIG. 11 is caused by fine vibration generated between the charging roll 103 and the photosensitive drum 102 due to the AC voltage being superimposed. It is known that the wear on the surface of the body drum 102 is promoted. Therefore, the tandem color image forming apparatus operates the image forming units 100Y, 100M, 100C, and 100K for all colors even when an image that does not use all four colors, particularly a black and white image, is recorded. There is a problem in that the life of the photosensitive drum 102 of the color that has not been used is undesirably shortened.
[0009]
Accordingly, as a technique for improving the life of the photosensitive drum by switching the image formation between a single color and multiple colors in a tandem color image forming apparatus, for example, (1) JP-A-10-186774 (2) JP-A-3-288273, JP-A-6-258914, JP-A-10-142932, JP-A-9-292753, JP-A-10-31964, etc. Has already been proposed.
[0010]
The color image forming apparatus according to the above-mentioned Japanese Patent Application Laid-Open No. 10-186774 is provided with an endless conveyance belt that conveys a recording medium as it travels, and opposed to the conveyance belt. And a plurality of image forming units for forming a toner image on the photosensitive drum when placed at the image forming position, and transfer means disposed to face each of the photosensitive drums with the conveyance belt interposed therebetween. And image forming unit moving means for selectively placing the image forming units at the retreat position and the image forming position.
[0011]
In addition, the image forming apparatus according to Japanese Patent Laid-Open No. 3-288273 includes an image forming station that includes a plurality of image forming stations that form images of different colors and a transfer material transport unit that transports a transfer material to these image forming stations. In the apparatus, an image forming station that first or lastly transfers an image formed among the plurality of image forming stations to the transfer material is designated as an image forming station of a designated color, and the transfer material is conveyed when a monochrome image of the designated color is formed. The means is configured to be separated from the image carrier of the remaining image forming stations not involved in image formation.
[0012]
Further, the image forming apparatus according to the above-mentioned Japanese Patent Application Laid-Open No. 6-258914 is an image forming apparatus capable of color image formation and single color image formation, and is provided corresponding to each of a plurality of sequentially arranged image carriers. A plurality of image forming means for forming images of different colors on the respective image carriers, and each image carrier is stretched so that a midway portion is opposed to the image carriers, and the image carriers are sequentially arranged. Conveying means having a transfer material conveying belt for conveying a transfer material, and provided on each image carrier for each transfer material provided corresponding to each of the image carriers, and formed on each image carrier. A plurality of transfer means for respectively transferring the image to be transferred and an image carrier corresponding to the image forming means for forming an image of a color corresponding to the color image at the time of color image formation; Means for displacing the transfer material conveying belt without changing the shape of the belt so as to come into contact only with an image carrier corresponding to an image forming means that performs only image formation of a color corresponding to the monochrome image. It is comprised so that it may do.
[0013]
Furthermore, in the electrophotographic apparatus according to the above-mentioned Japanese Patent Application Laid-Open No. 10-142932, a plurality of image forming units having different colors including a photoconductor are provided in series, and a toner image is sequentially formed on a transfer sheet conveyed by a conveying belt. Is an electrophotographic apparatus that forms a full-color image by transferring a toner image mark that forms a toner image mark for color misregistration detection on the conveyor belt in order to detect and correct color misregistration during full-color image formation. In a photographic device, when forming a full-color image, the toner is transferred by adsorbing a conveyor belt that covers the entire plurality of image forming units, and when a monochrome image is formed, the conveyor belt is retracted and conveyed. The image is transferred by a transfer device different from the belt. When a black monochrome image is formed, only the black image forming unit operates and the other color image forming units are stopped. Also it includes a configuration that.
[0014]
In addition, the image forming apparatus according to the above-mentioned Japanese Patent Application Laid-Open No. 9-292753 has an image forming apparatus in which a plurality of color image forming stations are arranged in parallel and a conveying belt is provided that moves through each image forming station. , A roller is disposed so as to be substantially in contact with the transfer position of the image forming station of a specific color located at either end, and the conveyor belt is wound around the roller and the conveyor belt is displaced with the axis of the roller as a fulcrum. Thus, a means for separating the conveying belt from the transfer position of the other image forming station is provided.
[0015]
Further, the image forming apparatus according to the above Japanese Patent Application Laid-Open No. 10-319664 is an image forming apparatus in which a color mode for forming a color image and a single color mode for forming a single color image can be selected as the image forming mode. A plurality of image forming stations provided with a body, a recording medium transporting means for transporting the recording medium to the image forming station, and the recording medium transporting means in contact with the image carriers of all the image forming stations when forming a color image. A contact / separation means for separating the recording medium conveying means from an image carrier of an image forming station not involved in image formation at the time of monochromatic image formation, a recording medium sending means positioned upstream of the recording medium conveying means, and the recording medium Arranged between the feeding means and the recording medium means, depending on the selected image forming mode When the switching of the position of said recording medium conveying means by moving means has been performed, which is constituted to include the recording medium guiding means for switching the position in conjunction with the switching of the position.
[0016]
[Problems to be solved by the invention]
However, the above prior art has the following problems. That is, in the case of the color image forming apparatus according to Japanese Patent Laid-Open No. 10-186774 of (1) above, image formation in which each image forming unit including the photosensitive drum is selectively placed at the retracted position and the image forming position. Part moving means, and (2) JP-A-3-288273, JP-A-6-258914, JP-A-10-142932, JP-A-9-292773, and In the case of the image forming apparatus according to Japanese Laid-Open Patent Application No. 10-31964, the transfer material conveying unit and the like are configured so as to be capable of being pressed and retracted with respect to the image carrier of the remaining image forming station not involved in image formation. .
[0017]
Therefore, in the case of the image forming apparatus according to the above proposal, in order to move the image forming unit or the transfer material conveying unit, it is necessary to secure a place for retracting the image forming unit or the transfer material conveying unit. In addition, there is a problem in that driving means including a gear, a cam, and the like are required, resulting in an increase in size and cost of the apparatus. In the case of the image forming apparatus according to the above proposal, since the image forming unit and the transfer material conveying unit repeatedly press and retract, the contact between the image forming unit and the transfer material conveying unit is a contact part of the transfer unit. There is also a problem that the state becomes unstable and it becomes difficult to set transfer conditions. Further, in the case of the image forming apparatus according to the above proposal, since the image forming unit and the transfer material conveying unit repeatedly press and retract, the toner images formed by the plurality of image forming units are transferred by the transfer material conveying unit or the like. When transferring onto a transported transfer material, the toner images of the respective colors are likely to be misaligned, making it difficult to form a high-quality image.
[0018]
Therefore, the photosensitive member is solved without solving such problems and without increasing the size and cost of the apparatus, and preventing the setting of transfer conditions from being difficult and the positional deviation of the toner images of the respective colors. Techniques that can improve the life of the drum include, for example, (3) those disclosed in JP-A-8-190252 and JP-A-8-137204, and (4) JP-A-10-186809. Has already been proposed.
[0019]
An image forming apparatus according to the above-mentioned Japanese Patent Application Laid-Open No. 8-190252 includes an image carrier on which an image forming region and a non-image forming region can be formed on a surface, and a charging unit capable of applying a charging voltage to the surface of the image carrier. An exposure unit that can freely expose the surface of the image carrier, and the exposure unit controls image exposure when the exposure unit corresponds to the image forming area, and the charge unit is neutralized after the image exposure control. An exposure control unit that controls exposure, and a toner image that forms an electrostatic latent image that is exposed and formed in an image forming region of the image carrier as an electrostatic toner image, and that can transfer the electrostatic toner image to a predetermined transfer material When the forming transfer means and the charging means correspond to the image forming area, a charging voltage in which a direct current and an alternating current are superimposed is supplied to the charging means, and the charging means corresponds to the non-image forming area. When at least alternating current And voltage supply means for stopping the supply of by charging current, which is constituted to include.
[0020]
In addition, an image forming apparatus according to the above-mentioned Japanese Patent Application Laid-Open No. 8-137204 includes a photoconductor, a rotary contact charging device that applies a voltage from a power supply device to charge the surface of the photoconductor, and photoconductor charging by the charging device. A developing device that develops an electrostatic latent image formed by image exposure in a region to form a visible toner image, and the rotary contact charging device is configured such that after the visible toner image is transferred to a transfer material, The toner remaining on the surface of the photoconductor is disturbed to form a non-pattern and the surface of the photoconductor is charged. After the transfer, the developing device collects the toner remaining on the photoconductor and simultaneously collects the electrostatic latent image. In the image forming apparatus that develops the image, the power supply device that applies a voltage to the charging device applies a vibration voltage obtained by adding a vibration component to a direct current component during a period in which the charging device corresponds at least to the image forming region of the photoconductor. death The period corresponding to the non-image forming region of the photoreceptor, which is constituted of which at least a predetermined period of time, so as to apply a DC voltage of only a DC component.
[0021]
Further, the image forming apparatus according to the above-mentioned Japanese Patent Application Laid-Open No. 10-186809 has an image bearing member having a movable surface to be charged, and a predetermined voltage applied thereto in contact with the image bearing member. A charging member that charges the image carrier, an exposure unit that exposes the charged image carrier to form a latent image, a developing member that visualizes the latent image with a developer, and a contact member that contacts the image carrier. An image forming apparatus comprising: a transfer unit that transfers a visualized image to a transfer body; a DC power source that applies a DC voltage to the charging member; and an AC superimposed power source that superimposes an AC voltage on the DC voltage; The DC power supply or the AC superimposed power supply is configured to have switching means that can be manually switched selectively.
[0022]
However, in the case of the technique disclosed in Japanese Patent Laid-Open No. 8-190252 and Japanese Patent Laid-Open No. 8-137204 in (3) above, the supply of the charging current by the alternating current is simply turned off in the non-image portion. In the case of the technique disclosed in Japanese Patent Application Laid-Open No. 10-186809, etc., the DC power supply or the AC superimposed power supply is selectively switched manually.
[0023]
By the way, when charging the surface of the photosensitive drum, even if the same charging roll is used, as shown in FIG. 14, the alternating current necessary for obtaining the same surface potential (−700 V) of the photosensitive drum is used. The applied voltage of DC voltage (-730V) when applying a DC voltage on which the voltage is superimposed is usually different from the applied voltage (approx. -1400V) when only the DC voltage is applied.
[0024]
Therefore, simply turning off the AC voltage when applying a DC voltage on which the AC voltage is superimposed in the non-image area will result in an optimum DC voltage value for charging the surface of the photosensitive drum to a predetermined surface potential. As shown in FIG. 14, the surface potential of the photosensitive drum is significantly lower than the predetermined surface potential. Therefore, as shown in FIG. 15, the cleaning potential V determined by the difference between the surface potential of the photosensitive drum and the developing bias potential. _CLN However, it is out of the predetermined range for preventing background fogging and carrier adhesion called BCO, and as shown in FIG. 16, a new problem that image quality defects such as background fogging and carrier adhesion called BCO occur. Had a point. In the case of the above exemplified bias, bias development is performed (see FIG. 14).
[0025]
In order to prevent the occurrence of image quality defects such as background fogging and carrier adhesion called BCO, the adoption of a one-component development method that does not use a carrier and the development bias that is normally used is set to about 500 V lower than usual. It is necessary to adopt other development techniques that can be performed, and another problem arises that a development system including a development method and a development bias must be newly redesigned.
[0026]
Therefore, the present applicant has solved the above-described problems of the conventional art and has an image configured to reduce the charge amount of the photosensitive drum in the non-image area before and after image exposure for the purpose of improving the life of the photosensitive drum. A forming apparatus has already been proposed (Japanese Patent Application No. 2000-66188).
[0027]
However, in the image forming apparatus according to Japanese Patent Application No. 2000-66188, the toner image formed on the photosensitive drum is transferred to a transfer medium such as a sheet or an intermediate transfer member such as an intermediate transfer belt. No consideration is given to the transfer voltage to be applied.
[0028]
Therefore, before the image area of the photosensitive drum, as shown in FIG. 19, when the transfer voltage is not applied to the transfer unit and the transfer voltage is applied only at the time of image formation, the polarity opposite to the charged polarity of the photosensitive drum is obtained. Due to the influence of the transfer current, the charge amount of the first approximately one rotation portion of the photosensitive drum in the image area that is not affected by the transfer current is high, and the charge amount of the subsequent part that is affected by the transfer current is high. It has been found that there is a problem that the image becomes darker and darker (ghost) before and after the charge amount of the photosensitive drum is changed relatively.
[0029]
Further, when the transfer voltage is applied to the transfer unit from before the image area, as shown in FIG. 20, if the same transfer voltage as that at the time of image formation is applied to the transfer unit, During the first rotation of the photosensitive drum, the charge amount of the photosensitive drum becomes lower than a desired value due to fluctuations in the charging characteristic of the photosensitive drum, so-called “cycle 1-2”. It has been found that there is a problem in that ghosting occurs.
[0030]
Accordingly, the present invention has been made to solve the above-described problems of the prior art, and the object of the present invention is to make it difficult to set transfer conditions without increasing the size and cost of the apparatus. Image quality defects such as background fogging and adherence of carrier called BCO, as well as improving the life of the photosensitive drum, while preventing the misalignment of the toner images of each color. An object of the present invention is to provide an image forming apparatus capable of reliably preventing this.
[0031]
Another object of the present invention is that in an image forming apparatus capable of improving the life of the photosensitive drum, the density of the image (ghost) caused by applying the transfer voltage to the transfer means is generated. An object of the present invention is to provide an image forming apparatus capable of preventing the above-described problem.
[0032]
[Means for Solving the Problems]
In order to solve the above-described problems, an invention described in claim 1 is directed to an image carrier and a method for charging the image carrier. Consists of a charging roll that contacts the surface of the image carrier A charging means; an exposure means for performing image exposure on the surface of the image carrier charged by the charging means to form an electrostatic latent image; and a developing means for visualizing the electrostatic latent image; In the image forming apparatus provided with the transfer unit for transferring the visualized image, the charging bias voltage applied to the charging unit is a period from the start of charging to the start of image formation, and the image bearing is performed more than at the time of image formation. DC component to reduce body charge Apply an AC voltage with all or part of the AC component removed to The first charging bias voltage set in such a manner and the image formation start to the image formation end The AC component is superimposed on the DC component up to The image forming bias voltage is switched to and applied to the transfer unit. Consists only of DC components The transfer bias voltage is synchronized with the application of the first charging bias voltage to the charging unit, the transfer bias voltage having a DC component lower than the transfer voltage at the time of image formation, and the image from the start of image formation to the end of image formation. It is configured to be switched to the transfer bias voltage at the time of formation.
[0046]
Claim 2 In the invention described in, the transfer voltage applied to the transfer means is From the start of charging to the start of image formation Charge amount of image carrier During image formation The image carrier is set so as to be lower than the transfer voltage at the time of image formation in accordance with the rate of decrease with respect to the charge amount of the image carrier. Claim 1 The image forming apparatus described in the above.
[0064]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings.
[0065]
Embodiment 1
FIG. 2 shows a tandem digital color printer as an image forming apparatus according to Embodiment 1 of the present invention. FIG. 3 shows a tandem type digital color copying machine as an image forming apparatus according to Embodiment 1 of the present invention.
[0066]
2 and 3, reference numeral 1 denotes a main body of a tandem type digital color printer and copying machine. In the case of a digital color copying machine, as shown in FIG. An automatic document feeder (ADF) 3 that automatically conveys the documents one by one and a document reading device 4 that reads an image of the document 2 conveyed by the automatic document feeder 3 are provided. 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.
[0067]
The color material reflected light image of the document 2 read by the document reading device 4 is, for example, IPS (red (R), green (G), blue (B) (8 bits each) as document reflectance data of three colors. Image processing system) 12, and in this IPS 12, predetermined reflectivity data such as shading correction, positional deviation correction, brightness / color space conversion, gamma correction, frame deletion, color / movement editing, etc. are applied to the reflectance data of the document 2. Image processing is performed. The IPS 12 also performs predetermined image processing on image data sent from a personal computer or the like.
[0068]
The image data that has been subjected to the predetermined image processing by the IPS 12 as described above is also converted into four colors of yellow (Y), magenta (M), cyan (C), and black (K) (each 8 bits) by the IPS 12. As described below, the image reproduction units 13Y, 13M, 13C, and 13K for the respective colors of yellow (Y), magenta (M), cyan (C), and black (K) are converted into original reproduction color material gradation data. The image is sent to a ROS (Raster Output Scanner) 14, and the ROS 14 as the image exposure apparatus performs image exposure with laser light in accordance with document reproduction color material gradation data of a predetermined color.
[0069]
By the way, inside the tandem type digital color printer and the copying machine main body 1, as shown in FIGS. 2 and 3, four colors of yellow (Y), magenta (M), cyan (C), and black (K) are provided. The two image forming units 13Y, 13M, 13C, and 13K are arranged in parallel at regular intervals in the horizontal direction.
[0070]
These four image forming units 13Y, 13M, 13C, and 13K are all configured in the same manner, and roughly divided, a photosensitive drum 15 as an image carrier that is rotationally driven at a predetermined speed, and the photosensitive drum. A charging roll 16 for primary charging as a charging means that uniformly charges the surface of 15 and 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. A developing device 17 as a developing means for developing the electrostatic latent image formed on the photosensitive drum 15 with toner of a predetermined color, and a toner image formed on the photosensitive drum 15. A transfer roll 26 as transfer means and a cleaning device 18 for cleaning the surface of the photosensitive drum 15 are configured.
[0071]
As shown in FIGS. 2 and 3, the ROS 14 is configured in common to the four image forming units 13Y, 13M, 13C, and 13K, and uses four semiconductor lasers (not shown) to reproduce the color reproduction gradation material for each color. The laser beams LB-Y, LB-M, LB-C, and LB-K are emitted from these semiconductor lasers according to the gradation data. Of course, the ROS 14 may be individually configured for each of the plurality of image forming units 13Y, 13M, 13C, and 13K. The laser beams LB-Y, LB-M, LB-C, and LB-K emitted from the semiconductor laser are irradiated to the rotary polygon mirror 19 through an f-θ lens (not shown), and deflected by the rotary polygon mirror 19. Scanned. The laser beams LB-Y, LB-M, LB-C, and LB-K deflected and scanned by the rotary polygon mirror 19 are scanned obliquely from below on the photosensitive drum 15 through a plurality of reflection mirrors (not shown). Exposed.
[0072]
From the IPS 12, each color is supplied 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). Are sequentially output, and the laser beams LB-Y, LB-M, LB-C, and LB-K emitted from the ROS 14 according to the image data are scanned and exposed on the surface of the corresponding photosensitive drum 15. An electrostatic latent image is formed. The electrostatic latent image formed on the photosensitive drum 15 is developed as a toner image of each color of yellow (Y), magenta (M), cyan (C), and black (K) by the developing unit 17. .
[0073]
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 25 arranged over the image forming units 13Y, 13M, 13C, and 13K, the images are transferred in multiple by the primary transfer roll 26. The intermediate transfer belt 25 is wound around the drive roll 27, the backup roll 28, and the tension roll 24 with a constant tension, and is driven to rotate by a dedicated drive motor (not shown) having excellent constant speed. The drive roll 27 is circulated at a predetermined speed in the direction of the arrow. As the intermediate transfer belt 25, for example, a synthetic resin film such as polyimide having flexibility is formed in a band shape, and both ends of the synthetic resin film formed in the band shape are connected by means such as welding, thereby being endless. A belt-shaped one is used.
[0074]
The yellow (Y), magenta (M), cyan (C), and black (K) toner images transferred onto the intermediate transfer belt 25 in a multiple manner are transferred to the backup roll 28 by a secondary transfer roll 29. The recording paper 30 that has been secondarily transferred onto the recording paper 30 as a recording medium by the pressure contact force and the electrostatic force, and onto which the toner images of these colors are transferred, is conveyed to a fixing device 31 positioned above. The secondary transfer roll 29 is located on the side of the backup roll 28, and is configured to secondary-transfer toner images of each color onto a recording paper 30 that is conveyed from below to above. 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
[0075]
As shown in FIGS. 2 and 3, the recording paper 30 is of a predetermined size from the paper feed cassette 34, and is fed via a paper transport path 37 by a paper feed roll 35 and a roll pair 36 for paper separation and transport. Once conveyed to the registration roll 38, it is stopped. The recording paper 30 supplied from the paper feed cassette 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.
[0076]
In the above digital color printer and copying machine, when making a full-color double-sided copy, the recording paper 30 having an image fixed on one side is not directly discharged onto the discharge tray 33 by the discharge roll 32, but is shown in the figure. The conveyance direction is switched by the switching gate not to be conveyed, and the sheet is conveyed to the duplex conveyance unit 40 via the sheet conveyance roll 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 conveyance roll pair (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.
[0077]
2 and 3, 44Y, 44M, 44C, and 44K supply toners of predetermined colors to the developing devices 17 of yellow (Y), magenta (M), cyan (C), and black (K), respectively. Each toner cartridge is shown.
[0078]
FIG. 4 shows the image forming units of the digital color printer and the copying machine.
[0079]
The four image forming units 13Y, 13M, 13C, and 13K of yellow, magenta, cyan, and black are all configured in the same manner as shown in FIG. 4, and these four image forming units 13Y, 13Y, In 13M, 13C, and 13K, as described above, yellow, magenta, cyan, and black toner images are sequentially formed at a predetermined timing. As described above, the image forming units 13Y, 13M, 13C, and 13K for the respective colors are provided with the photosensitive drums 15, and the surfaces of these photosensitive drums 15 are uniformly provided by the charging rolls 16 for primary charging. Charged. Thereafter, the surface of the photosensitive drum 15 is scanned and exposed to an image forming laser beam LB emitted from the ROS 14 according to the image data, and an electrostatic latent image corresponding to each color is formed. The laser beam LB scanned and exposed on the photosensitive drum 15 is set to be exposed at a predetermined inclination angle from an obliquely lower side slightly to the right of the photosensitive drum 15. The electrostatic latent image formed on the photosensitive drum 15 is converted into yellow, magenta, cyan, and black colors by the developing roll 17a of the developing unit 17 of each image forming unit 13Y, 13M, 13C, and 13K. The visible toner images are developed with toner, and these visible toner images are sequentially transferred in multiple onto the intermediate transfer belt 25 by the charging of the primary transfer roll 26.
[0080]
Residual toner, paper dust, and the like are removed from the surface of the photosensitive drum 15 after the toner image transfer process by the cleaning device 18 to prepare for the next image forming process. The cleaning device 18 includes a cleaning blade 42, and the cleaning blade 42 removes residual toner, paper dust, and the like on the photosensitive drum 18. Further, the surface of the intermediate transfer belt 25 after the toner image transfer step is finished, as shown in FIGS. 2 and 3, the residual toner and paper dust are removed by the cleaning device 43, and the next image forming process. Prepare for. The cleaning device 43 includes a cleaning brush 43a and a cleaning blade 43b, and residual toner and paper dust on the intermediate transfer belt 25 are removed by the cleaning brush 43a and the cleaning blade 43b.
[0081]
By the way, in this embodiment, in the image forming unit that does not perform image formation, the amount of charge by the charging unit of the image carrier is reduced as compared with the image forming unit that performs image formation, and the developing bias applied to the developing unit. Is configured to change.
[0082]
In addition, as the charging unit, for example, a charging unit to which a DC voltage on which an AC voltage is superimposed is applied is used, and a charging amount is reduced by applying a voltage from which all or a part of the AC voltage is removed. Composed.
[0083]
Further, in this embodiment, the charging bias voltage applied to the charging means is a first bias voltage from the start of charging to the start of image formation, and a second bias voltage from the start of image formation to the end of image formation. The third bias voltage from the end of image formation to the end of charging is switched. However, the first bias voltage and the third bias voltage are set equal to each other as necessary.
[0084]
In this embodiment, the charging unit is switched between the charging bias voltage applied to the charging unit while the developing unit is in operation and non-operating.
[0085]
Further, in this embodiment, switching of the charging bias voltage applied to the charging means of each image forming unit is sequentially performed for each image forming unit, and the timing of switching the charging bias voltage is determined by other image forming units. An image is being formed.
[0086]
FIG. 1 shows a power supply circuit of a digital color printer and a copying machine according to this embodiment.
[0087]
In FIG. 1, 50 is a DC high-voltage power source that applies a variable DC bias voltage to the charging roll 16, 51 is an AC high-voltage power supply that applies an AC bias voltage superimposed on the DC bias voltage, and 52 is a charging roll 16. A second DC high-voltage power supply that applies a variable DC bias voltage to 53; and 53, a switching means for switching the bias voltage applied to the charging roll 16 between a DC bias voltage and a DC bias voltage superimposed with an AC bias voltage, 54 is a DC high-voltage power supply that applies a variable DC bias voltage to the developing roll 17a, 55 is an AC high-voltage power supply that applies an AC bias voltage superimposed on the DC bias voltage, and 56 is variable to the developing roll 17a. A second DC high-voltage power supply for applying a DC bias voltage, 57 is a bias voltage applied to the developing roll 17a, Switching means for switching a DC bias voltage superimposed flow bias voltage and the direct current bias voltage, showing respectively.
[0088]
With the above configuration, in the image forming apparatus according to the present embodiment, the transfer condition is difficult to set without increasing the size and cost of the apparatus, and the position of each color toner image is as follows. While preventing the occurrence of misalignment, it is possible to improve the life of the photosensitive drum as well as to reliably prevent image quality defects such as background fogging and carrier adhesion called BCO. Is possible.
[0089]
That is, in the case of forming a full-color image in the digital color printer and the copying machine according to this embodiment, as shown in FIG. 5 and FIG. As shown in FIG. 1, the rotation drive is started and before the rotation speed of the photosensitive drum 15 reaches a predetermined speed, the charging roll 16 of each of the image forming units 13Y, 13M, 13C, and 13K A first charging bias voltage consisting of only a DC voltage is applied by the second DC high-voltage power supply 52 from the start of charging to the start of image formation. The first charging bias voltage is set to, for example, Vdc1 = −900V, and the surface of each photosensitive drum 15 is charged to Vh = −250V.
[0090]
Next, when the rotational speed of the photosensitive drum 15 reaches a predetermined speed, the developing roll 17a of each of the image forming units 13Y, 13M, 13C, and 13K has a DC high-voltage power supply 54 as shown in FIG. A first developing bias voltage composed of a DC voltage (for example, Vdev1 = −100 V) on which an AC voltage is superimposed is applied by the AC high-voltage power supply 55 until the development starts.
[0091]
Thereafter, as shown in FIG. 1, the charging unit 16 of the yellow image forming unit 13Y, which is the first color image forming unit, has a switching unit 53 connected to the DC high-voltage power supply 50 and the start of image formation. Switching to the AC high-voltage power supply 51 side, as shown in FIGS. 5 and 6, a second charging bias voltage composed of a DC voltage on which an AC voltage is superimposed is applied until the image forming operation is completed. For example, the second charging bias voltage is set such that the DC voltage is Vdc2 = −730 V, the AC voltage is Vac = 2 kV, and the frequency = 819 Hz, and the surface of each photosensitive drum 15 is charged to Vh = −700 V. .
[0092]
At that time, the output voltage of the DC high-voltage power supply 54 is switched to the developing roll 17a of the yellow developing device 17 as shown in FIG. The second developing bias voltage Vdev2 = −560 V is applied until the image forming operation is completed.
[0093]
Further, the surface of the photosensitive drum 15 of the yellow image forming unit 13Y is subjected to yellow image exposure LB-Y by the ROS 14, and an electrostatic latent image corresponding to the yellow color is formed. As shown in FIGS. 5 and 6, the yellow electrostatic latent image formed on the surface of the photosensitive drum 15 is developed by a developing roller 17a of a yellow developing device 17 to which a predetermined developing bias voltage is applied. The developed toner image becomes a yellow toner image.
[0094]
Next, when the image forming process in the yellow image forming unit 13Y is completed as described above, the switching means 53 is switched to the DC high voltage power supply 52 side as shown in FIG. The charging roll 16 is applied with a third charging bias voltage composed only of a DC voltage with the AC voltage superimposed off. For example, the third charging bias voltage is set to Vdc3 = −900 V, which is equal to the first charging bias voltage, and the surface of each photosensitive drum 15 is charged to Vh = −250 V.
[0095]
At that time, as shown in FIG. 1, the output voltage of the DC high-voltage power supply 54 is applied to the developing roll 17a of the yellow developing device 17 after a predetermined time delay from the application of the third bias voltage to the charging roll 16. For example, a voltage equal to the initial developing bias voltage is applied.
[0096]
Subsequently, as shown in FIG. 1, the charging unit 16 of the magenta image forming unit 13 </ b> M, which is the second color image forming unit, has a switching unit 53 connected to the DC high-voltage power supply 50 at the start of image formation. As shown in FIGS. 5 and 6, a second charging bias voltage composed of a DC voltage on which an AC voltage is superimposed is applied until the image forming operation is completed. For example, the second charging bias voltage is set such that the DC voltage is Vdc2 = −730 V, the AC voltage is Vac = 2 kV, and the frequency = 819 Hz, and the surface of each photosensitive drum 15 is charged to Vh = −700 V. .
[0097]
At that time, as shown in FIG. 1, the output voltage of the DC high-voltage power supply 54 is switched to the developing roll 17a of the magenta developing device 17 after a predetermined time delay from the application of the bias voltage to the charging roll 16. The second developing bias voltage Vdev = −560 V is applied until the image forming operation is completed.
[0098]
Further, the surface of the photosensitive drum 15 of the magenta image forming unit 13M is subjected to magenta image exposure LB-M by the ROS 14 to form an electrostatic latent image corresponding to the magenta color. As shown in FIGS. 5 and 6, the magenta electrostatic latent image formed on the surface of the photosensitive drum 15 is developed by a developing roll 17a of a magenta developing device 17 to which a predetermined developing bias voltage is applied. The developed toner image becomes a magenta toner image.
[0099]
Next, when the image forming process in the magenta image forming unit 13M is completed as described above, the switching means 53 is switched to the DC high voltage power source 52 side as shown in FIG. 1, and the magenta image forming unit 13M The charging roll 16 is applied with a third charging bias voltage composed only of a DC voltage with the AC voltage superimposed off. For example, the third charging bias voltage is set to Vdc3 = −900 V, which is equal to the first charging bias voltage, and the surface of each photosensitive drum 15 is charged to Vh = −250 V.
[0100]
At that time, as shown in FIG. 1, the output voltage of the DC high-voltage power supply 54 is applied to the developing roll 17a of the magenta developer 17 after a predetermined time delay from the application of the third bias voltage to the charging roll 16. For example, a voltage equal to the initial developing bias voltage is applied.
[0101]
Similarly, in the cyan and black image forming units 13C and 13K, the surface of the photosensitive drum 15 is charged by the charging roll 16, the image is exposed by the ROS 14, and the electrostatic is generated by the developing roll 17a of the developing unit 17 of a predetermined color. The latent image development process is sequentially performed.
[0102]
In each of the image forming units 13Y, 13M, 13C, and 13K for yellow, magenta, cyan, and black, an AC high voltage power source is connected to the developing roll 17a of each developing unit 17 until at least the image forming process for all colors is completed. The AC voltage is applied by 55.
[0103]
In the illustrated embodiment, the charging bias voltage applied to the charging roll 16 of each of the image forming units 13Y, 13M, and 13C for yellow, magenta, and cyan after the image forming process is completed is applied to the photosensitive drum 15. The voltage remains applied until just before the rotation stops, but after a predetermined time has elapsed from the timing of the rear end of the image exposure by the ROS 14 of the black image forming unit 13K, the charging bias voltage is changed to another voltage. You may switch.
[0104]
On the other hand, in the case of forming a black and white image in the digital color printer and copying machine according to this embodiment, as shown in FIGS. As shown in FIG. 1, the rotation drive is started and before the rotation speed of the photosensitive drum 15 reaches a predetermined speed, the charging roll 16 of each of the image forming units 13Y, 13M, 13C, and 13K A first charging bias voltage consisting of only a DC voltage is applied by the second DC high-voltage power supply 52 from the start of charging to the start of image formation. The first charging bias voltage is set to, for example, Vdc1 = −900V, and the surface of each photosensitive drum 15 is charged to Vh = −250V.
[0105]
Next, when the rotational speed of the photosensitive drum 15 reaches a predetermined speed, the developing roll 17a of each of the image forming units 13Y, 13M, 13C, and 13K has a DC high-voltage power supply 54 as shown in FIG. The second high-voltage power supply 55 applies a second developing bias voltage composed of a DC voltage on which an AC voltage is superimposed.
[0106]
Thereafter, in order to form a black-and-white image, the charging means 16 of the black image forming unit 13k has a DC high voltage power supply 50 and an AC high voltage power supply 50 at the start of image formation as shown in FIG. As shown in FIGS. 7 and 8, the second charging bias voltage composed of the DC voltage on which the AC voltage is superimposed is applied until the image forming operation is completed. For example, the second charging bias voltage is set such that the DC voltage is Vdc2 = −730 V, the AC voltage is Vac = 2 kV, and the frequency = 819 Hz, and the surface of each photosensitive drum 15 is charged to Vh = −700 V. .
[0107]
Further, as shown in FIGS. 7 and 8, the image forming units 13Y, 13M, and 13C of the yellow, magenta, and cyan image forming units 13Y, 13M, and 13C form images. Therefore, the second DC high voltage power source 52 applies a first charging bias voltage consisting of only a DC voltage from the start of charging to the end of image formation, and an AC voltage is applied to the charging roll 16 for image formation. It is not applied until the end.
[0108]
At that time, the output voltage of the DC high-voltage power supply 54 is switched to the developing roll 17a of the black developing unit 17 as shown in FIG. The development bias voltage Vdev = −560 V is applied until the image forming operation is completed.
[0109]
Further, the surface of the photosensitive drum 15 of the black image forming unit 13Y is subjected to black image exposure LB-B by the ROS 14 to form an electrostatic latent image corresponding to black. The black electrostatic latent image formed on the surface of the photosensitive drum 15 is developed by a developing roll 17a of a black developing unit 17 to which a predetermined developing bias voltage is applied, as shown in FIGS. A black toner image is obtained.
[0110]
Next, when the image forming process in the black image forming unit 13K is completed as described above, as shown in FIG. 1, the switching means 53 is switched to the DC high-voltage power supply 52 side, and the charging roll of the black image forming unit 13K. The third charging bias voltage consisting only of the DC voltage is applied to 16 with AC voltage superposition turned off. For example, the third charging bias voltage is set to Vdc3 = −900 V, which is equal to the first charging bias voltage, and the surface of each photosensitive drum 15 is charged to Vh = −250 V.
[0111]
At that time, the output voltage of the DC high-voltage power supply 54 is again applied to the developing roll 17a of the black developing device 17 again after a predetermined time from the application of the third bias voltage to the charging roll 16 as shown in FIG. For example, a voltage equal to the initial development bias voltage is applied.
[0112]
In each of the image forming units 13Y, 13M, 13C, and 13K for yellow, magenta, cyan, and black, an AC high voltage power source is connected to the developing roll 17a of each developing unit 17 until at least the image forming process for all colors is completed. The AC voltage is applied by 55.
[0113]
In the illustrated embodiment, the charging bias voltage applied to the charging roll 16 of each of the image forming units 13Y, 13M, and 13C for yellow, magenta, and cyan after the image forming process is completed is applied to the photosensitive drum 15. The voltage remains applied until just before the rotation stops, but after a predetermined time has elapsed from the timing of the rear end of the image exposure by the ROS 14 of the black image forming unit 13K, the charging bias voltage is changed to another voltage. You may switch.
[0114]
As described above, in the digital color printer and the copying machine according to the above-described embodiment, the AC voltage is superimposed only on the image area on the 13Y, 13M, and 13C charging rolls 16 of each image forming unit during full color image formation. Since the DC voltage is applied and only the DC voltage is applied to the non-image area, the AC voltage can be applied even in the non-image area as compared with the case where the AC voltage is always applied to the charging roll 16. The accompanying deterioration of the surface of the photosensitive drum 15 can be suppressed, and wear due to the deterioration of the surface of the photosensitive drum 15 can be suppressed.
[0115]
In addition, when the present inventors apply only a DC voltage to the charging roll 16 to charge the surface of the photosensitive drum 15, the relationship between the DC applied voltage and the surface potential of the photosensitive drum is changed. Or, the experiment was conducted by changing the environment.
[0116]
9 and 10 show the results of the above experiment. 9A shows a high temperature and high humidity environment, FIG. 10B shows a normal environment, FIG. 10A shows a low temperature and low humidity environment, and FIG. 9B shows a normal temperature and low humidity environment. .
[0117]
As can be seen from FIGS. 9 and 10 above, fogging and BCO can be more reliably prevented by changing the DC voltage applied to the charging roll according to the environmental conditions and the film thickness of the photoreceptor. Even in a state in which the film reduction of the photoconductor progresses, a constant charging voltage (for example, Vh = −250 V) can be reliably obtained by changing the set voltage.
[0118]
According to the experiments by the present inventors, the change (decrease) in the film thickness of the photosensitive drum 15 with the increase in the number of image forming steps (number of cycles) according to the ratio of the AC voltage and the DC voltage applied to the charging roll 16. It is known that the amount of wear of the photosensitive drum 15 changes as shown in FIG.
[0119]
As can be seen from FIG. 11, by changing the ratio of the AC voltage applied to the charging roll 16, the change (decrease) in the film thickness of the photosensitive drum 15 and the wear amount of the photosensitive drum 15 are greatly reduced. Therefore, the life of the photosensitive drum 15 can be greatly extended. For example, if the ratio of the AC voltage applied to the charging roll 16 is reduced by half to 50% AC, the change (decrease) in the film thickness of the photosensitive drum 15 and the wear amount of the photosensitive drum 15 are reduced to about ½. The life of the photosensitive drum 15 is approximately doubled.
[0120]
Further, in the digital color printer and the copying machine according to the above-described embodiment, the charging operation of the surface of the photosensitive drum 15 by the charging roll until the start of the image formation and the image forming operation is finished. Thereafter, Vdc3 = −900 V, which is the first charging bias voltage and the third charging bias voltage (equal to the first charging bias voltage in this embodiment), is applied to the charging roll, The surface of the photosensitive drum 15 is charged to Vh = −250V. At that time, a DC voltage Vdev = −100 V on which an AC voltage is superimposed is applied to the developing rolls 17a of 13Y, 13M, and 13C of each image forming unit.
[0121]
Therefore, the surface of each of the photosensitive drums 15 has a cleaning potential of Vh−Vdev = −250V − (− 100V) = − 150V, and as is apparent from FIG. 16, generation of fog toner in the background, BCO and It is possible to surely prevent the carrier from being called. In addition, since the occurrence of fog toner in the background can be surely prevented, the consumption of unauthorized toner can be eliminated accordingly, and therefore the running cost of the image forming unit including development can be reduced. In particular, when the two-component development method is adopted, it is possible to almost completely prevent BCO during AC-OFF and to achieve both fog and BCO.
[0122]
In the above-described embodiment, since a mechanism for retracting the image forming unit, the transfer belt, and the like is not used, space can be saved and transfer and registration are not affected. Also, the printing speed is not impaired when continuously creating jobs in which single color / multicolor images are mixed.
[0123]
Furthermore, in the above-described embodiment, in the case of the tandem method, even when one image creation unit creates an image region, not all image formation units are necessarily in the image region. Therefore, due to the effects described above, each image forming unit may increase the charge amount only after image region image formation, regardless of whether other image forming units are performing image region image formation. If it becomes, it becomes possible to reduce the charge amount immediately. Therefore, a significant improvement in life can be achieved in a tandem system that tends to have many non-imaging cycles that are not used for actual imaging.
[0124]
In the above-described embodiment, the AC image bias is not applied to the color image forming unit in black and white, and the total cost (TCO) of the print system is reduced.
[0125]
In the above-described embodiment, the image forming apparatus including a plurality of image forming units has been described. However, the same applies to an image forming apparatus including a single image forming unit including one image carrier. Of course you can.
[0126]
Also, by applying the above embodiment, it is possible to reduce the charge amount and reduce the photoreceptor damage while preventing image defects such as fogging and BCO.
[0127]
Furthermore, at this time, the cleaning potential can be set to Vcln = −150 V by setting the DC charging appropriate for the charging roll (for example, by applying −800 V and the surface potential of the photosensitive drum is Vh = −150 V). The developing roll of the developing unit facing the photosensitive drum may be stopped and at the same time the developing bias may be turned off (0 V). In this case, it is useful for reducing stress on the developing system. .
[0128]
In addition, when the developing roll is stopped, the toner that adheres to the photosensitive drum as fog is only required near the nip portion between the developing roll and the photosensitive drum, and the developing roll is stopped. At the same time, the development bias is turned off (0 V), and the DC application bias is also turned off (0 V application, Vh = 0 V, Vcln = 0 V), so that the toner adheres to the photosensitive drum more than when DC is applied. Damage can be reduced.
[0129]
Embodiment 2
17 and 18 show a second embodiment of the present invention. The same parts as those in the first embodiment will be described with the same reference numerals. A charging means for charging the image carrier, an exposure means for forming an electrostatic latent image by exposing the surface of the image carrier, and for visualizing the electrostatic latent image In the image forming apparatus comprising the developing unit and the transfer unit for transferring the visualized image, a transfer voltage is applied to the transfer unit when the image carrier is charged by the charging unit in a non-image area. It is comprised so that it may do.
[0130]
In the second embodiment, the transfer voltage applied to the transfer unit is configured to be lower than the transfer voltage at the time of image formation.
[0131]
Further, in this second embodiment, the transfer voltage applied to the transfer means is determined according to the ratio of the charge amount of the image carrier in the non-image area to the charge amount of the image carrier in the image area. It is set to be lower than the transfer voltage at the time of formation.
[0132]
Furthermore, in the second embodiment, the non-image area means that the charging bias voltage applied to the charging means is at least one level from the start of charging to the start of image formation, and from the start of image formation. Application of bias voltage excluding the image formation bias voltage from the start of image formation to the end of image formation when switching between the image formation bias voltage until the end of image formation and the bias voltage of at least one level from the end of image formation to the end of charging It is structured to be time.
[0133]
That is, in the case of forming a full-color image in the digital color printer and the copying machine according to the second embodiment, as shown in FIG. 17, the photosensitive drum 15 is driven to rotate as the image forming operation starts. Is started, and after the rotational speed of the photosensitive drum 15 reaches a predetermined speed, the charging roll 16 of each of the image forming units 13Y, 13M, 13C, and 13K is moved to the second as shown in FIG. A first charging bias voltage composed of a DC voltage is applied by the DC high-voltage power supply 52 from the start of charging to the start of image formation. The first charging bias voltage is set to, for example, Vdc1 = −900V, and the surface of each photosensitive drum 15 is charged to Vh = −250V.
[0134]
Thereafter, as shown in FIG. 1, the charging unit 16 of the yellow image forming unit 13Y, which is the first color image forming unit, has a switching unit 53 connected to the DC high-voltage power supply 50 and the start of image formation. As shown in FIG. 17, a second charging bias voltage composed of a DC voltage on which an AC voltage is superimposed is applied until the image forming operation is completed. For example, the second charging bias voltage is set such that the DC voltage is Vdc2 = −730 V, the AC voltage is Vac = 2 kV, and the frequency = 819 Hz, and the surface of each photosensitive drum 15 is charged to Vh = −700 V. .
[0135]
Next, when the image forming process in the yellow image forming unit 13Y is completed, as shown in FIG. 1, the switching means 53 is switched to the DC high voltage power supply 52 side, and the charging roll 16 of the yellow image forming unit 13Y is switched. The third charging bias voltage consisting only of the DC voltage is applied with the superposition of the AC voltage turned off. For example, the third charging bias voltage is set to Vdc3 = −900 V, which is equal to the first charging bias voltage, and the surface of each photosensitive drum 15 is charged to Vh = −250 V.
[0136]
By the way, in the second embodiment, as shown in FIG. 18, in synchronization with the application of the first charging bias voltage to the charging roll 16, the transfer rolls 26Y, 26M, 26C, and 26K as transfer means are The transfer voltages TRY, TRM, TRC, and TRK are sequentially applied.
[0137]
As shown in FIG. 18, the transfer voltage TR applied to each of the transfer rolls 26 is such that a transfer current smaller than that during image formation flows in synchronization with the application of the first charging bias voltage to the charging roll 16. Constant current control. For example, when the transfer current Itr energized to each transfer roll 26 is set to 10 μA (transfer voltage: 600 V) at the time of image formation, the charging voltage of the photosensitive drum 15 in the non-image area and the image It is set so that Itr = 10 × 250/700 = 3.5 μA according to the ratio with the charging voltage of the photosensitive drum 15 in the region.
[0138]
Next, a transfer current Itr suitable for image transfer is supplied to the transfer roll 26 of the yellow image forming unit 13Y, which is the first color image forming unit, with the start of image formation. Thus, for example, a transfer voltage of 600 V is applied.
[0139]
Thereafter, when the image forming operation is completed, as shown in FIG. 18, the same transfer voltage as that at the time of image formation may be applied to the transfer roll 26, or the same transfer voltage as that before the image formation may be applied. The transfer voltage may be turned off.
[0140]
In the above description, the transfer roll 26 of the yellow image forming unit 13Y has been mainly described. However, the timing of the transfer roll 26 of the magenta, cyan, and black image forming units 13M, 13C, and 13K is also described. However, the same transfer voltage as that of the transfer roll 26 of the yellow image forming unit 13Y is applied.
[0141]
As described above, in the second embodiment, as shown in FIG. 18, basically, when the photosensitive drum 15 is charged by the charging roll 16 in the non-image area, the transfer voltage is applied to the transfer roll 26. Thus, as in the case where the transfer voltage is applied to the transfer roll 26 only at the time of image formation, the photosensitive drum 15 is rotated during the first rotation of the photosensitive drum 15 in the image area due to the influence of the transfer current. Since the charge amount does not become larger than the subsequent charge amount and the charge amount of the photosensitive drum 15 can be kept constant, it is possible to prevent the occurrence of image shading (ghost). It becomes.
[0142]
In the second embodiment, as shown in FIG. 18, the transfer voltage applied to the transfer roll 26 is configured to be lower than the transfer voltage at the time of image formation. As in the case where the same transfer voltage as that at the time of image formation is applied, the charge amount of the photosensitive drum 15 is prevented from becoming lower than a desired value during the first rotation of the photosensitive drum 15 in the image area. As a result, it is possible to prevent the occurrence of shading (ghost) in the image.
[0143]
Further, in the second embodiment, the transfer voltage applied to the transfer roll 26 is in accordance with the rate at which the charge amount of the photosensitive drum 15 in the non-image area is reduced with respect to the charge amount of the photosensitive drum 15 in the image area. Since the transfer voltage is set to be lower than that at the time of image formation, when the photosensitive drum 15 is charged by the charging roll 16 in the non-image area, the transfer voltage is applied to the transfer roll 26. It is possible to prevent the transfer roller 26 from unintentionally affecting the chargeability of the photoconductive drum 15 and to reliably prevent the occurrence of image shading (ghost).
[0144]
Since other configurations and operations are the same as those of the first embodiment, description thereof is omitted.
[0145]
【The invention's effect】
As described above, according to the present invention, without increasing the size and cost of the apparatus, it is possible to prevent the setting of transfer conditions from being difficult and the occurrence of misalignment of toner images of the respective colors. It is possible to provide an image forming apparatus capable of reliably preventing the occurrence of image quality defects such as background fogging and adhesion of a carrier called BCO, as well as improving the life of the photosensitive drum.
[0146]
Further, according to the present invention, in an image forming apparatus capable of improving the life of the photosensitive drum, an image capable of preventing occurrence of image shading (ghost) caused by applying a transfer voltage to the transfer unit. A forming apparatus can be provided.
[Brief description of the drawings]
FIG. 1 is a circuit diagram showing a power supply circuit of a digital color printer and a copying machine as image forming apparatuses according to Embodiment 1 of the present invention.
FIG. 2 is a block diagram showing a digital color printer as an image forming apparatus according to Embodiment 1 of the present invention.
FIG. 3 is a block diagram showing a digital color copying machine as an image forming apparatus according to Embodiment 1 of the present invention.
FIG. 4 is a block diagram showing an image forming unit of a digital color printer as an image forming apparatus according to Embodiment 1 of the present invention.
FIG. 5 is a timing chart showing an image forming operation in the first embodiment.
FIG. 6 is a timing chart showing an image forming operation in the first embodiment.
FIG. 7 is a timing chart showing an image forming operation in the first embodiment.
FIG. 8 is a timing chart showing an image forming operation in the first embodiment.
FIGS. 9A and 9B are graphs showing the relationship between the voltage applied to the charging roll and the surface potential of the photosensitive drum during DC charging.
FIGS. 10A and 10B are graphs showing the relationship between the voltage applied to the charging roll and the surface potential of the photosensitive drum during DC charging.
FIG. 11 is a graph showing the relationship between the number of image forming cycles and the film thickness of the photosensitive drum.
FIG. 12 is a configuration diagram illustrating a tandem type image forming apparatus.
FIG. 13 is a graph showing the charging potential of the photosensitive drum.
FIG. 14 is a graph showing the charging potential of the photosensitive drum.
FIG. 15 is a graph showing the charging potential of the photosensitive drum.
FIG. 16 is a graph showing the relationship between cleaning potential, fogging, and BCO.
FIG. 17 is a timing chart showing an image forming operation in the second embodiment.
FIG. 18 is a timing chart showing a main part of an image forming operation in the second embodiment.
FIG. 19 is a timing chart showing a main part of a conventional image forming operation.
FIG. 20 is a timing chart showing a main part of a conventional image forming operation.
[Explanation of symbols]
13: Image forming unit, 14: ROS (exposure unit), 15: Photoconductor drum (image carrier), 16: Charging roll (charging unit), 17: Developer (developing unit), 26: Transfer roll (transfer unit) ), 50: DC high voltage power supply, 51: AC high voltage power supply, 52: DC high voltage power supply, 53: Switching means.

Claims (2)

An image carrier;
Charging means comprising a charging roll in contact with the surface of the image carrier for charging the image carrier;
Exposure means for performing image exposure on the surface of the image carrier charged by the charging means to form an electrostatic latent image;
Developing means for visualizing the electrostatic latent image;
In an image forming apparatus provided with a transfer means for transferring the visualized image,
Charging bias voltage applied to the charging unit for a period of up to image formation start from the charging start, all or part of the AC component to the DC component so than during image formation to lower the charge amount of the image bearing member The first charging bias voltage set to apply the removed AC voltage in a superimposed manner, and the image forming bias voltage in which the AC component is superimposed on the DC component from the start of image formation to the end of image formation.
The transfer bias voltage consisting of only the DC component applied to the transfer unit is a transfer bias voltage having a DC component lower than the transfer voltage at the time of image formation in synchronization with the application of the first charging bias voltage to the charging unit. And an image forming apparatus configured to be switched between a transfer bias voltage at the time of image formation from the start of image formation to the end of image formation. The transfer voltage applied to the transfer means is transferred during image formation according to the ratio of the charge amount of the image carrier from the start of charging to the start of image formation being reduced with respect to the charge amount of the image carrier during image formation The image forming apparatus according to claim 1 , wherein the image forming apparatus is set to be lower than a voltage.

Images