JP3996566B2 - Image forming apparatus and image forming method - Google Patents

Description

  The present invention relates to an image forming apparatus such as a copying machine or a laser printer using electrophotography.

    In an image forming apparatus using electrophotography, the surface of a photoconductor as an image carrier is uniformly charged, and the surface is image-exposed to form an electrostatic latent image corresponding to the exposed image. The electrostatic latent image is developed with, for example, a toner that is a colorant for making the image visible. The toner image on the surface of the image carrier formed by this development is transferred to a transfer material (sheet) such as paper that is appropriately conveyed, the sheet is separated from the photoconductor, and passed through a fixing device as a hard copy. I try to finish it.

  On the other hand, after the toner image is transferred to the sheet, a part of the toner image toner remains on the photoconductor. In order to repeatedly use this photoconductor, the residual toner is cleaned and the residual charge is removed. To eliminate static electricity. This makes it possible to repeat image formation on the photosensitive member with the photosensitive member in a clean state before the start of each image forming process.

  The surface potential of the photoreceptor has a great influence on image formation. In other words, in order to always obtain the same image quality state, it is important to stabilize the surface potential of the entire photosensitive member.

  However, in a general inexpensive photoconductor, when the photoconductor is new, there is no particular problem if the charging means is stable. However, as the image formation is repeated repeatedly, the charging potential at the first rotation at the start of use cannot be increased to a predetermined potential. As a result, in the image formed by the first image, a surface potential difference occurs between the first rotation area and the second and subsequent rotation areas of the photoconductor, resulting in non-uniform image density and background fogging. However, there is a problem that causes a reduction in image quality.

  Due to the recent development of high image quality technology, the toner tends to have a smaller particle size, and a toner having an average particle size of 8 μm or less and a sharper particle size distribution has been developed. Along with this, the carrier also tends to be reduced in particle size. In addition, in order to reduce the toner consumption per copy, studies have been made to increase the concentration (content) of the pigment in the toner, which was conventionally 5 to 6%, to about 8 to 20%. ing. Thus, by reducing the particle size of the toner and the carrier, the specific surface area per unit weight of the developer is increased, and further, the pigment concentration is increased, thereby changing the behavior of the developer with respect to the change in surface potential. Also grows.

  These problems remarkably appear as a halftone density step and a fogging step in a white background as a background in image formation using reversal development. In addition, as a photoconductor, a phenomenon that remarkably occurs in a general photoconductor used for digital use using a phthalocyanine-based compound has also become a phenomenon.

  Laminated photoreceptors using a phthalocyanine compound as a charge generation layer have been widely put into practical use, but such a phenomenon was observed in all but somewhat different degrees. Further, since it was not observed in the multilayer photoreceptor using the azo pigment in the charge generation layer, it was found that this was a problem specific to the multilayer photoreceptor using the phthalocyanine compound. The reason why such a phenomenon occurs is not clear, but when various phenomena were examined, the mechanism was estimated as follows.

  In a photostatic discharge process in a normal electrophotographic process, excess carriers are generated in the charge generation layer of the multilayer photoconductor, and the residual potential is neutralized and discharged. Here, if electron wrap exists in the charge generation layer, the carriers generated earlier are temporarily captured, and if they remain until the next charging step, a part of them is released, which causes a decrease in charging potential. In the second and subsequent rotations, the electron wrap is already buried, so that the carrier emission in the next charging process is suppressed and the charging potential is reduced less. If left unattended, the carriers trapped in the electron trap are thermally relaxed, and the electron trap is returned to the first free state and the potential is lowered. The reason why the reduction width increases when the photoreceptor is fatigued is thought to be that the amount of electron traps in the charge generation layer gradually increases due to fatigue.

  As one means for solving the above-mentioned problems, in an image forming apparatus that uniformly charges a photosensitive member using contact charging means (roller charging), charging conditions, exposure conditions, and phenomenon conditions change between the first rotation and the second and subsequent rotations. There is an image forming apparatus. According to this image forming apparatus, image creation can be started from the first time of the photosensitive member, and a hard copy with stable image quality can be output from the first sheet. In other words, the charging potential for the first rotation and the charging potential for the second and subsequent rotations can be made substantially equal, and the exposure and phenomenon conditions are controlled so that the same image quality can be obtained. (For example, refer to Patent Documents 1 and 2).

  As another method, it has been proposed to use a photoconductor having the same surface potential of the photoconductor after the first and second rotations of the photoconductor due to the start of image formation. In this photoreceptor, the charge generation layer constituting the photoreceptor contains a phthalocyanine compound as a charge generation material, and further contains an azo compound, thereby suppressing a decrease in charge potential at the first rotation as a photoreceptor characteristic. . For this reason, image formation can be performed from the first rotation of the photosensitive member to cope with a higher speed (for example, see Patent Document 3).

In addition, there has been proposed an apparatus that enables image formation from the first rotation by controlling the charge output at the first rotation and the second rotation according to the cumulative number of specifications and the frequency of use (see, for example, Patent Document 4). ).
JP-A-4-161963 Japanese Patent Laid-Open No. 11-15238 Japanese Patent Laid-Open No. 9-127711 JP 2000-56543 A

  The above-described conventional techniques are methods in which image formation is performed after the surface potential of the photoconductor is stabilized by continuously applying the same charge output to the charging means until the photoconductor is stabilized. With this method, although the first copy time is delayed, stable image formation is performed. However, for example, when the photosensitive member is set to −650 V, the phenomenon bias is controlled to about −500 V so that the image does not fog and the phenomenon agent carrier does not adhere.

  When the control for reducing the difference between the first rotation and the second rotation is not performed, for example, when the surface potential of the photoconductor is desired to be −650 V, it is lower than −650 V for the first rotation. It may be about -400V. As described above, when the surface potential of the first rotation of the photosensitive member becomes −450 V or less, the toner adheres to the non-image portion of the photosensitive member, and the image is fogged. Therefore, until the surface potential of the photoconductor is stabilized, toner that is not necessary for image formation is taken away from the developing unit on the surface of the photoconductor. Such a phenomenon is not preferable because the cost per unit copy increases. In addition, since a lot of unnecessary toner accumulates in the cleaning unit, the maintainability also deteriorates. Further, it is impossible to satisfy both the fog and the image density by a method in which the surface potential is not controlled and only the developing conditions such as the exposure condition and the developing bias are changed.

  Further, the surface potential of the first rotation of the photosensitive member is greatly different after storage for a long time and immediately after copying. As described above, it is known that the surface potential of the first rotation of the photoconductor greatly varies depending on the usage number of the image forming apparatus, the accumulated number of use of the photoconductor, temperature, humidity, and the like.

  Accordingly, it can be understood that in any environment or situation as described above, it is necessary to perform very complicated control to make the surface potential of the first rotation the same as that of the second rotation.

Further, such control is excessively performed, and the surface potential of the first rotation of the photosensitive member may be higher than −700V. If the surface potential of the photoconductor becomes too high, toner adhesion to the surface of the photoconductor is suppressed, but conversely, the carrier adheres to the photoconductor, and as a result, the carrier is sandwiched between the cleaning blade and the photoconductor, and the photoconductor There is a possibility that the photosensitive layer may be scratched in the circumferential direction. Such scratches appear as vertical stripes in the image, greatly reducing the image quality. The image quality can be restored to the original only by exchanging the photoconductor.

  The object of the present invention is to cope with uncertainties in which the characteristics and surface potential characteristics of the photoreceptor cannot be stabilized by repeating the image forming process and the uncertainties due to environmental changes such as humidity and temperature. The object is to propose a method for controlling the surface potential on the photosensitive member that can perform more stable image formation in the second and subsequent times.

The present invention uses a laminated electrophotographic photoreceptor having a charge generation layer and a charge transport layer containing a phthalocyanine compound on a conductive support,
Charging means for charging the photoreceptor;
Exposure means for forming an electrostatic latent image on the photoreceptor by irradiating the charged photoreceptor with light;
Developing means for developing an electrostatic latent image formed on the photoreceptor;
In an image forming apparatus having
The charging unit and the developing unit are driven at 1/6 to 2/3 of the rated output at the first rotation of the photoconductor after the image forming process starts, and
In the second and subsequent rotations of the photosensitive member, the charging unit and the developing unit are each driven at a rated output to form an image.

  In this configuration, the surface potential of the photosensitive member is reduced from 1/6 to 2/3 of the normal by driving the charging unit and the phenomenon unit in the first rotation at an output of 1/6 to 2/3 of the rated output. . However, due to this, the variation in the surface potential of the photoconductor caused by the usage frequency, the cumulative number of use, the temperature, the humidity, etc. of the photoconductor is also reduced from 1/6 to 2/3. On the other hand, the occurrence of fog due to phenomenon bias and the presence or absence of carrier adhesion are determined by the voltage ratio. Therefore, by making the phenomenon bias that drives the phenomenon means follow the magnitude of the charge output, the voltage ratio between the charge output and the phenomenon bias can be kept constant at all times. Only the potential variation can be reduced.

  It has been experimentally confirmed that more preferable results can be obtained if the output of the charging means and the developing means in the first rotation is set to 1/4 to 1/2 of the rated output.

Further , by preventing toner adhesion and carrier adhesion during the first rotation and performing image formation after the second rotation, more stable image formation becomes possible.

  According to the present invention, the surface potential of the photosensitive member is reduced from the normal 1/6 to 2/3 by driving the charging means and the phenomenon means at the output of 1/6 to 2/3 of the rated value in the first rotation. Become. However, in addition to the usage frequency of the photoconductor, the variation in the surface potential of the photoconductor caused by the cumulative number of use, temperature, humidity, and the like is reduced from 1/6 to 2/3 of the normal. On the other hand, since the presence or absence of fogging or carrier adhesion due to phenomenon bias is determined by the voltage ratio, the charging output is fixed and controlled at the rating by causing the driving of the phenomenon means to follow the driving of the charging means at the voltage ratio. Compared with the conventional one, it is possible to reduce only the variation in the surface potential of the photoreceptor due to charging.

  Hereinafter, an image forming method according to an embodiment of the present invention will be described in detail with reference to the drawings. FIG. 1 is an explanatory diagram showing the main configuration of an electrophotographic image forming apparatus. The photosensitive drum 21, the cleaner 9, the charging charger 2, the exposure scanning unit 3, the phenomenon unit 4, the toner supply container 8, and the transfer unit 5 are illustrated. A fixing device 7 and a paper feed unit 6.

  A photoconductor is provided on the surface of the photoconductor drum 21. The charging charge 2 uniformly charges the surface of the photoreceptor. When the exposure scanning unit 3 is exposed in such a uniformly charged state, an electrostatic latent image is formed on the surface of the photosensitive drum 21.

The charging charger 2 is provided with a wire or saw-tooth corona discharge electrode in an electrically insulated state in a case where the inner surface of the photosensitive member 21 is open. A grid electrode that is electrically insulated and held with respect to the case is attached to the open surface of the case to constitute a scorotron-type corona charger 2. The grid electrode controls the surface potential of the photoconductor 21, and the charging potential is determined by the voltage supplied to the grid electrode.

  The developing device includes a developing device 4 having a developing roller and a stirring device, and a toner cartridge 8 that is set to be detachable substantially horizontally with respect to the developing device and replenishes toner appropriately in the developing device 4. . The developing device 4 develops the electrostatic latent image formed on the photosensitive drum 21 into a visible image by a dry two-component magnetic brush developing method using a developer in which toner and a carrier are mixed.

  In synchronization with the rotation of the photosensitive drum 21, the transfer paper is conveyed from the paper supply unit 6 to a transfer position 5 in contact with the photosensitive drum 21 by a pair of paper supply roller and timing roller. Since the toner image transferred to the transfer paper is in an unstable state where it is peeled off immediately, it is conveyed to the fixing device 7. The fixing device 7 pressurizes the transfer paper at a high temperature to fix the toner on the transfer paper. Thereafter, the transfer paper is discharged onto the paper discharge tray.

  The photoreceptor 21 is configured to include, for example, an organic photosensitive layer. The structure will be described with reference to FIG. The photoconductor 21 is formed on a 1 mm-thick aluminum conductive substrate 21 a formed in a drum shape, and a charge (carrier) generation layer 21 b formed to a thickness of about 0.5 μm and a thickness of about 25 μm. In addition, this is a function-separated organic photoreceptor in which the charge (carrier) transfer layer 21c is laminated in this order. For the carrier generation layer 21b, a phthalocyanine pigment, which is a carrier generation agent, is mixed at a predetermined ratio with respect to the binder resin. The diameter of the drum of the photosensitive member 21 is, for example, 65 mm.

  Examples of the charge transport material include poly-N-vinylcarbazole and derivatives thereof, poly-g-carbazolylethyl glutamate and derivatives thereof, polyvinylpyrene, polyvinylphenanthrene, oxazole derivatives, oxadiazole derivatives, imidazole derivatives, 9 -(p-diethylaminostyryl) anthracene, 1,1-bis (4-dibenzylaminophenyl) propane, styrylanthracene, styrylpyrazoline, pyrazoline derivatives, phenylhydrazones, hydrazone derivatives, triphenylamine compounds, tetraphenyldiamine And electron donating substances such as azine compounds, stilbene compounds, and azine compounds having a 3-methyl-2-benzothiazoline ring.

The binder resin may be any resin that is compatible with the charge transport material, such as polycarbonate and copolymer polycarbonate, polyarylate, polyvinyl butyral, polyamide, polyester, epoxy resin, polyurethane, polyketone, polyvinyl ketone, polystyrene. , Polyacrylamide, phenol resin, phenoxy resin and polysulfone resin, and copolymer resins thereof. You may use these resin individually or in mixture of 2 or more types. Among the above-mentioned binder resins, resins such as polystyrene, polycarbonate, copolymer polycarbonate, polyarylate, and polyester have a volume low efficiency of 10 ¹³ Ω or more, and are excellent in film formability and potential characteristics.

  In order to realize reversal development by the developing device 4, the surface potential of the photosensitive member 21, the developing bias potential (developing device drive output) supplied to the developing roller constituting the developing device 4, and the relationship shown in FIG. The electrostatic latent image potential formed by lowering the surface potential of the photoreceptor 21 by laser exposure is set. That is, the surface of the photoconductor 21 is uniformly charged to the charging potential Vo (for example, −650 V) by the charger 2. When exposed by laser light irradiation, the charge on the surface of the photoconductor 21 is canceled according to the amount of light, and an electrostatic latent image is formed with a latent image potential VL (for example, −100 V). A developing bias potential DVB (for example, −500 V) is supplied between the photosensitive member 21 and the developing roller of the developing device 4. For example, negatively charged toner is supplied from the developing roller to the surface of the photosensitive member 21 using the difference between the developing bias potential DVB and the latent image potential VL as a developing potential. Such potential setting enables reversal development in which toner adheres to the exposed portion of the surface of the photoreceptor 21.

FIG. 4 shows the relationship between the grid voltage ratio for controlling the charge output of the first rotation and the surface potential on the photoreceptor. The grid voltage ratio is the charge output for the first rotation / charge output for the second and subsequent rotations. When the charge output of the first rotation is equal to the charge output of the second rotation and the grid voltage ratio is 1, if the rated surface potential of the photoreceptor is controlled to about −650 V, the photoreceptor of the first rotation May vary from VB2 (−700 V) or more to V B1 (−400 V). Since this state exceeds the phenomenon bias safe area ΔDV, it is understood that some kind of treatment is necessary. On the other hand, when the grid voltage of the first rotation is lowered, it can be seen that the variation becomes smaller as the surface potential of the photoconductor of the first rotation is lowered. When the grid voltage ratio falls below 2/3, the surface potential variation ΔV of the photoreceptor becomes smaller than ΔDV. At this time, it was found that by controlling the developing bias at 2/3 of the rated output, it is possible to maintain the surface potential state without fogging or carrier adhesion at the first rotation of the photoreceptor. Furthermore, it can be seen that by making the surface potential of the photoreceptor less than or equal to 1/2, it is possible to further suppress variations, which is more effective.

  On the other hand, if the charging output of the first rotation is reduced too much, the electron wrap is not filled up even after the second rotation. Therefore, the carrier is discharged even in the next charging process, and the surface potential of the second rotation is lowered. Therefore, a potential difference is generated between the second and third rotations. FIG. 5 shows the potential difference (V) in the surface potential between the second and third rotations of the photoreceptor when the charging output at the first rotation is controlled to each value. It can be seen that the potential difference decreases as the grid ratio increases. When the grid ratio is larger than 1/6, the difference in surface potential is smaller than -50 V, which is an allowable range that does not cause a difference in development density, so that there is no practical problem. It can be seen that if the grid ratio is greater than 1/4, this difference is further reduced.

  FIG. 6 shows a time chart for performing such control. At the first rotation of the photosensitive member, the grid voltage is controlled at Vo1 which is lower than the rated charging output Vo. Thereafter, the charging output after the second rotation is controlled at the rated Vo. When the portion where the surface potential of the photoreceptor is controlled by Vo1 reaches the developing portion with a delay of t time, the surface potential of the developing portion is controlled by DV1. When the portion to which the surface potential is applied after the second rotation reaches the developing portion, the surface potential of the developing portion is controlled by DVB. By performing the above control, image formation can be performed from the first rotation. Note that image formation may be performed from the first rotation, but toner adhesion and carrier adhesion may be prevented at the first rotation, and may be performed after entering the second rotation. By controlling in this way, more stable image formation becomes possible.

  By performing the control described above, it is possible to avoid fogging and carrier adhesion caused by variations in the surface potential of the photoreceptor at the first rotation. In addition, the variation in surface potential after the second rotation can be suppressed to a level where there is no practical problem.

Main configuration diagram of electrophotographic image forming apparatus Structure diagram of photoconductor 21 A diagram showing the relationship between the surface potential Vo of the photosensitive member 21, the developing bias potential DVB supplied to the phenomenon roller, and the electrostatic latent image potential. The figure which shows the relationship between the grid voltage ratio which controls the charge output of 1st rotation, and the surface potential on a photoconductor The figure which shows the electrical potential difference of the surface potential of the 2nd rotation of a photoreceptor, and the 3rd rotation when the charging output of the 1st rotation is controlled to each value. Time chart showing charging output and development bias control timing

Claims (4)

Using a laminated electrophotographic photosensitive member having a charge generation layer and a charge transport layer containing a phthalocyanine compound on a conductive support,
Charging means for charging the photoreceptor;
Exposure means for forming an electrostatic latent image on the photoreceptor by irradiating the charged photoreceptor with light;
Developing means for developing an electrostatic latent image formed on the photoreceptor;
In an image forming apparatus having
The charging unit and the developing unit for the first rotation of the photoconductor after the start of the image forming process are driven at 1/6 to 2/3 of the rated output, respectively.
An image forming apparatus, wherein after the second rotation of the photosensitive member, the charging unit and the developing unit are driven at a rated output to form an image.   Determined based on the rotation speed of the photosensitive member and the distance between the charging unit and the developing unit from the start of driving of the charging unit at the first and second rotations of the photosensitive member. The image forming apparatus according to claim 1, wherein driving of the developing unit is started at a timing delayed by a predetermined time. Using a laminated electrophotographic photosensitive member having a charge generation layer and a charge transport layer containing a phthalocyanine compound on a conductive support,
Charging the photoreceptor with charging means;
An electrostatic latent image is formed on the photoreceptor by irradiating the charged photoreceptor with light by an exposure unit,
The electrostatic latent image formed on the photoconductor is developed by a developing means,
The charging unit and the developing unit of the first rotation of the photosensitive member after the start of the image forming process are each driven by 1/6 to 2/3 of a predetermined output, the second and subsequent rotations are driven by a rated output, and An image forming method, wherein image formation is performed after the second rotation of the photoreceptor.   Determined based on the rotation speed of the photosensitive member and the distance between the charging unit and the developing unit from the start of driving of the charging unit at the first and second rotations of the photosensitive member. 4. The image forming method according to claim 3, wherein the driving of the developing unit is started at a timing delayed by a predetermined time.

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