JP2005115003A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus in which image fogging caused by developer deterioration during running for printing is effectively prevented, the image forming apparatus being designed to form a toner image by the development of an electrostatic latent image formed on an image carrier. <P>SOLUTION: The image forming apparatus includes a control means which performs a first development control in which in order to form an image, the image carrier is electrified and then exposed to obtain an electrostatic latent image and a development bias voltage is applied to develop the image, and second development control in which the image carrier is electrified during image non-formation and then development is carried out. The second development control is exerted such that a potential difference ¾VO2-Vb2¾ between the surface potential V02 of the image carrier and the surface potential Vb2 of the development bias voltage is made larger than a potential difference ¾V01-Vb1¾ between the surface potential V01 of the image carrier and the development bias voltage Vb1 in the first development control. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an image forming apparatus such as a copying machine, a printer, or a fax machine, and more specifically, forms a latent image on a latent image carrier and carries the developer on a developer carrier to which a developing bias voltage is applied. The present invention relates to an image forming apparatus that develops a latent image on a latent image carrier by transporting it to a developing region facing the carrier.

  Conventionally, among image forming apparatuses such as copying machines, printers, and fax machines, for example, in an electrophotographic image recording apparatus employed in a laser printer or the like, after the surface of the latent image carrier is uniformly charged, A latent image is formed on the latent image carrier by selectively irradiating the surface of the latent image carrier with light based on the image information. This latent image is developed by a two-component developing device using a two-component developer containing toner and magnetic particles, or a one-component developing device using toner as a one-component developer, and then transferred to a transfer material by a transfer device. The In this case, it is general to use a reversal development method in which development is performed using toner having the same polarity as the charged polarity of the latent image carrier.

  That is, when the latent image carrier is charged by the charging member and a latent image is formed by the exposure optical system, a developing bias having the same polarity as that of the latent image carrier is applied to the toner carrier, and the toner in the developer chamber The toner supplied to the carrier is charged with the same polarity as that of the latent image carrier, and further, a thin layer is formed by a thin layer forming member adjacent to the toner carrier and is frictionally charged. When the toner contacts the latent image carrier, the toner moves from the toner thin layer toward the exposed portion of the surface of the latent image carrier. At this time, the portion where the latent image is formed on the surface of the latent image carrier has the potential polarity reduced by exposure, but the unexposed portion where the latent image is not formed remains high in potential. The toner particles charged to the same polarity repel the unexposed part and move to the exposed part. As a result, the latent image on the electrostatic latent image carrier is developed.

  However, as the development process is performed for a long period of time, a so-called fog phenomenon occurs in which a large number of small black spots appear on a white background portion on a recorded recording sheet. The cause of this is deterioration of the developer due to printing running, etc., and defective charged toner such as uncharged toner or low-charged toner is generated in the developing device and gradually increases. It is considered that image fogging occurs on the image due to the toner.

In order to solve this problem, for example, as shown in the timing chart of FIG. 8, a method of turning off the developing bias at the time of non-image formation or applying a bias having a polarity opposite to that at the time of image formation has been proposed. According to this proposed method, after the toner charged to the opposite polarity is attached to the surface of the electrostatic latent image carrier during non-image formation, the cleaning means removes the toner remaining on the surface of the electrostatic latent image carrier. Therefore, image fogging by reverse polarity toner is suppressed.
JP-A-9-127773

  However, even with the above proposal, the image fog is not completely eliminated. This is because the toner charged to the opposite polarity in this method is discharged from the developing device, so that the image fogging by the opposite polarity toner is suppressed, but the charge amount is uncharged or more than the normal charge amount. Since low low-charged toner is not discharged from the inside of the developing device, it is estimated that this uncharged / low-charged toner causes image fogging.

  Accordingly, in view of the problems of the prior art, the image forming apparatus according to the present invention develops an electrostatic latent image formed on an image carrier to form a toner image. An object of the present invention is to provide an image forming apparatus capable of effectively preventing the above-described problem.

  Accordingly, in order to solve such a problem, the present invention provides an image forming apparatus that develops an electrostatic latent image formed on an image carrier to form a toner image, and then charges the image carrier and exposes the image carrier. In the first development control in which an electrostatic latent image is obtained and development is performed by applying a development bias voltage, and in the first development control without exposure after charging the image carrier at a predetermined timing during non-image formation. Control for performing second development control in which development is performed with the potential difference between the surface potential of the image carrier and the development bias voltage being equal or larger than the potential difference between the surface potential of the image carrier and the development bias voltage. An image forming apparatus characterized by having means is proposed.

The predetermined timing at the time of non-image formation is, for example, when the power is turned on, specifically, when the power switch is turned on and the image forming apparatus main body is energized to start image formation, and the stabilization operation is known per se. Immediately after the stabilization operation is completed. In particular, during the stabilization operation, depending on the image forming apparatus, this means the time until the fixing temperature is stabilized after various simulation operations are performed and abnormality determination of various processes of the image forming apparatus is performed.
Also, when returning from the preheat state (power saving state) for energy saving or when the image forming apparatus is restarted after the jam processing is completed, the toner cartridge is replaced in the case of the developing device of the toner cartridge replacement type. It means when. In addition, the counter unit that stores the number of printed sheets of the image forming apparatus can be used, for example, after the end of a series of image forming operations after 10,000 times of image forming.
As described above, in addition to the first development control during normal image formation, the surface potential of the image carrier in the first development control after charging the image carrier at a predetermined timing during non-image formation. By performing the second development control for performing development by making the potential difference between the image carrier and the development bias voltage equal or larger than the potential difference between the image development bias voltage and the development bias voltage, the second development control is performed. Uncharged / low-charged toner moves to the surface of the image carrier and is removed by the image carrier cleaning means to be discharged out of the developing device. This reduces the amount of toner adhering to the unexposed portion of the image carrier during image formation, and effectively prevents image fogging, even when using a developer that has deteriorated due to long-term development processing. Is prevented.

  In the image forming apparatus according to the present invention, the control unit sets the surface potential of the image carrier in the second development control to be the same as the surface potential of the image carrier in the first development control. The development bias voltage in the second development control may be changed, or the surface potential of the image carrier in the second development control may be changed to change the development bias voltage in the second development control. The development bias voltage may be the same as that in the first development control.

  As described above, the second development control can be easily realized only by changing either the surface potential of the image carrier or the development bias voltage.

In the image forming apparatus according to the present invention, the potential difference in the second development control is a potential difference that allows the uncharged toner and the low-charged toner having a lower charge amount than the normal charge amount to move to the image carrier. It is characterized by that.
Thus, during the second development control, the uncharged / low-charged toner is positively moved to the image carrier and discharged to the outside of the developing device, so that image fog can be effectively prevented.

Further, in the image forming apparatus according to the present invention, the control unit applies a developing bias having a polarity opposite to the developing bias applied in the first developing control before or after executing the second developing control. It is characterized by doing so.
As described above, since the developing bias having the opposite polarity to the developing bias applied in the first developing control is applied before or after the second developing control is performed, the uncharged / low-charged toner is removed. Before or after the toner is removed from the developing device, the toner charged to the opposite polarity can be discharged out of the developing device, and image fogging can be prevented more effectively.

  As described above, the present invention relates to an image forming apparatus that develops an electrostatic latent image formed on an image carrier into a toner image, and applies uncharged / low-charged toner to the image carrier surface during non-image formation. Therefore, there is an advantage that image fogging caused by deterioration of the developer during printing running can be effectively prevented.

  Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the drawings. However, the dimensions, materials, shapes, relative arrangements, and the like of the components described in this embodiment are not intended to limit the scope of the present invention unless otherwise specified, but are merely illustrative examples. Not too much.

  An embodiment in which the present invention is applied to a color printer which is an image forming apparatus of a tandem type intermediate transfer system will be described. FIG. 1 is a schematic configuration diagram of an image forming apparatus to which the present invention is applicable. In the figure, reference numeral 100 denotes an image forming apparatus main body, 200 denotes a paper feed table on which the image forming apparatus main body 100 is placed, 300 denotes a scanner attached to the image forming apparatus main body 100, and 400 denotes an automatic document feeder (ADF) attached thereon. is there. The image forming apparatus main body 100 is provided with an intermediate transfer belt 10 as a transfer belt in the center.

  Here, when a start switch (not shown) is pressed and, for example, single-sided copying is started, sheets are fed out from one of the sheet feeding cassettes 44 of the sheet feeding table 200, and the sheets are separated one by one by the separation roller 45. The paper enters the paper feed path 46, is transported, is abutted against a registration roller 49 in the image forming apparatus main body 100, and temporarily stops. Then, the registration roller 49 is rotated in synchronization with the composite color image on the intermediate transfer belt 10, the sheet is fed between the intermediate transfer belt 10 and the secondary transfer device 22, and is transferred by the secondary transfer device 22. A color image is recorded on the sheet. The sheet after the image transfer is conveyed by the secondary transfer device 22, sent to the fixing device 25 and fixed, and then discharged from the discharge roller 56 and stacked on the discharge tray 57.

  As shown in FIG. 1, the intermediate transfer belt 10 is wound around three support rollers 14, 15, and 16 so as to be able to rotate and convey clockwise in the drawing. In this illustrated example, an intermediate transfer body cleaning device 17 for removing residual toner remaining on the intermediate transfer belt 10 after image transfer is provided on the left of the second support roller 15 among the three. Further, among the three, four images of yellow, cyan, magenta, and black are arranged on the intermediate transfer belt 10 stretched between the first support roller 14 and the second support roller 15 along the conveyance direction. The tandem image forming unit 20 is configured by arranging the forming units 18Y, 18C, 18M, and 18B side by side. However, these four color orders are merely examples, and the present invention is not limited to these.

  In FIG. 2, one of the four image forming units 18Y, 18C, 18M, and 18B is enlarged and shown, and this configuration will be described. An image forming unit 18Y, which is a toner image forming unit, has a charging device 60, a developing device 61, a primary transfer device 62, a photoconductor cleaning device 63, and a charge eliminating device 64 around a photoconductive drum 40 as a drum-shaped image carrier. And so on. The charging device 60 is a roller charging roller 60 in the illustrated example, and charges the photosensitive drum 40 by applying a voltage by contacting the photosensitive drum 40. Of course, charging can also be performed with a non-contact scorotron charger.

  The developing device 61 uses a two-component developer composed of a magnetic carrier and a non-magnetic toner, and is provided with a developing sleeve 65 as a developer carrying member facing the photosensitive drum 40 through the opening of the developing case. A magnet 72 is fixedly provided in the developing sleeve 65. Further, a panning roller 73 is provided with the surface approaching the developing sleeve 65. Further, two agitation paddles 68 are provided inside the developing device, and the two-component developer is conveyed and circulated while being agitated by the two paddles 68 and supplied to the developing sleeve 65. The developer supplied to the developing sleeve 65 is pumped and held by the magnet 72 to form a magnetic brush on the developing sleeve 65. The magnetic brush is trimmed to an appropriate amount by the trimming roller 73 as the developing sleeve 65 rotates. The developer that has been cut off is returned to the stirring unit 66.

  FIG. 4 is a timing chart showing the timing of charging of the surface of the photosensitive drum 40, exposure, application of a developing bias, and transfer of a toner image in the first development control operation performed during normal image formation. Referring also to a), when rotation of the photosensitive drum 40 is started by a motor (not shown), the photosensitive drum 40 is charged by the charging device 60, and its surface potential becomes V01. Thereafter, writing light L from the exposure device 21 by a laser, LED, or the like is applied to the photosensitive drum 40 to form an electrostatic latent image on the photosensitive drum 40. At this time, the exposed portion has a small charge polarity and its surface potential is VL. Thereafter, the toner on the developing sleeve 65 of the developing device 61 is charged with the same polarity as the charging polarity of the unexposed portion on the surface of the photosensitive drum 40 by friction. The bias voltage at this time is Vb1. As shown in FIG. 3, since the unexposed portion on the surface of the photosensitive drum 40 and the toner are charged with the same polarity, the toner repels from the unexposed portion on the surface of the photosensitive drum 40, and the effective potential difference (FIG. 3 (indicated by a broken-line arrow in FIG. 3) is transferred to a latent image portion having a small charge polarity as | Vb1-VL |, and the electrostatic latent image on the photosensitive drum 40 is visualized as a toner image. The toner image is transferred onto the sheet. Thereafter, when the exposure operation is completed, the application of the developing bias is also turned off, the charging operation is also turned off, and the transferred paper is conveyed from the image forming unit 18Y to the fixing unit.

Next, the second development control performed when the power is turned on, which is an example of a predetermined timing at the time of non-image formation, will be described with reference to FIGS. 2, 3, and 4 (b). In FIG. 4B, the charging timing of the surface of the photosensitive drum 40, exposure, application of a developing bias, and transfer of the toner image are indicated by solid lines.
First, when the main power switch of the image forming apparatus is turned on, various stabilization operations such as cleaning of the charger and potential adjustment are performed as the fixing device is heated. Subsequent to this stabilization operation, the second development control of the present invention is executed. That is, when the rotation of the photosensitive drum 40 is started by a motor (not shown), the photosensitive drum 40 is charged by the charging device 60 and its surface potential becomes V02. Thereafter, the exposure operation is not performed, and the surface potential of the photoconductive drum 40 is uniformly maintained at V02. Next, the toner of the developer on the developing sleeve 65 of the developing device 61 is charged to the same polarity as the charging polarity of the unexposed portion on the surface of the photosensitive drum 40 by friction. The developing bias voltage at this time is Vb2, and is indicated by a solid line in FIG. V02 and Vb2 are set so that | V01−Vb1 (indicated by solid line arrows in FIG. 3) | ≦ | V02−Vb2 (indicated by thick line arrows in FIG. 3) |. At this time, uncharged / low-charged toner moves to the photosensitive drum 40. The uncharged / low-charged toner that has moved to the photosensitive drum 40 may be removed by the cleaning device 63. Thereafter, after a predetermined time has elapsed, the development bias is turned off and the charging unit is turned off. Even in the case of collection by the cleaning device 63, after development by the second development control, it may be transferred to unnecessary paper and discarded.

  FIG. 5 shows timings for performing the first development control and the second development control. If the control unit determines that the image forming apparatus is powered on or not performing image formation (step S1), the driving of the photosensitive drum 40 is turned on and the photosensitive drum is turned on. 40 rotates (step S2), and second development control is performed (step S3). When this is completed (step S4), if it is determined by the control unit that an instruction to perform image formation as it is thereafter (step S5), an image forming operation is performed in the first development control (step S6). Otherwise, the image forming apparatus enters a standby state (step S7).

FIG. 6 shows the result of an experiment conducted to determine whether or not image fogging occurs by performing control so as to perform the first development control and the second development control.
As the experimental conditions, an α-Si photosensitive member (amorphous silicon photosensitive member) having a diameter of 60 mm is used as the photosensitive drum 40, and a two-component developer composed of a magnetic carrier and a nonmagnetic toner is used as the developer. In the same manner as above, a developing device comprising a developer agitation paddle, a spike roller, and a developing roller (a magnet and a developing sleeve existing on the outer peripheral surface thereof) was used. Also, the stirring paddle is operated during both non-image formation and image formation, but during image formation, the panning roller is set to a position where developer is supplied to the developing roller, and during non-image formation, the panning roller is placed on the developing roller with developer. It was in the position not to supply. Under this condition, the second development control was performed for 40 seconds following the stabilization operation after the power was turned on. The image fog degree was measured by measuring the image-formed paper with a Macbeth reflection densitometer. The fog value (FD value) is a difference between the measured density value of the sheet before image formation and the measured density value of the non-image area (white background portion) of the sheet after image formation. This indicates that there is fog on the white background after image formation. When there is no fog at all, the FD value is zero.

  First, when the photoreceptor surface potential V0 is set to 420V, the development bias potential Vb is set to 240V, and only the normal image formation is performed without performing the second development control, the initial fog value (FD value) is 0. Although it was 0.005, it became 0.015 after printing 100,000 sheets, and further increased to 0.020 when operating in a high temperature environment.

Next, when the first development control and the second development control are controlled so that the photoreceptor surface potential V0 and the development bias potential Vb are 420 V and 240 V, respectively, as described above (X), the photoreceptor surface potential Experiments were conducted for the case where only V0 was increased by 20V to 440V (Y), and the case where only the photoreceptor surface potential V0 was increased by 40V to 460V (Z). As a result, (X) was 0.005 after printing 100,000 sheets, and even when operated in a high temperature environment, it was almost the same as 0.005. In the case of (Y) and (Z), it is 0.004 after printing 100,000 sheets, and further 0.004 when operated in a high temperature environment, and image fog is further reduced. Was confirmed. That is, by performing the second development control without exposure, it is confirmed that the image fog is reduced. Further, this effect is obtained by calculating the difference between the photosensitive member surface potential V0 and the development bias potential Vb during the normal image formation (the first image formation). It became even more noticeable when it was made larger than during development control. In this embodiment, the photosensitive member surface potential V0 is increased as a method of widening the difference between the photosensitive member surface potential V0 and the developing bias potential Vb. However, the photosensitive member surface potential V0 is also decreased by decreasing the developing bias potential Vb. And the developing bias potential Vb increased, and the same effect was obtained.
The printing of 100,000 sheets was performed on the image forming apparatus at a rate of 30 sheets per minute continuously for 8 hours per day and finished on the seventh day. For this reason, the second development control was performed seven times when printing 100,000 sheets.

Although the above experiment was performed only for B (black) among a plurality of developing devices (C, M, Y, B), it can be similarly applied to other colors.
The present invention can be applied not only to a tandem type color image forming apparatus but also to a one-drum type color image forming apparatus as shown in FIG. This image forming apparatus includes four developing devices 4 to 4 including a sheet feeding cassette (not shown), a photosensitive drum 1, an optical scanning unit 2, a charger 3, and developer containers for each color (C, M, Y, B). 7, an intermediate transfer body 12, a secondary transfer roller 13, a fixing device 14, and a conveying device 9 using an endless belt for conveying the recording medium 8 onto which an image has been transferred from the secondary transfer roller 13 to the fixing device 14. At the time of printing, the photosensitive drum 1 is charged by the charger 3 for each color, and the image developed by the developing devices 4 to 7 is transferred from the optical scanning unit 2 to the intermediate transfer member 12 and fed (not shown). The image is transferred to the recording medium 8 conveyed from the cassette by the secondary transfer roller 13, fixed by heating and pressurizing by the fixing device 14, and discharged. In this case, the developing devices 4, 5, 6, and 7 for each color facing the photosensitive drum 1 may be mixed, for example, from the photosensitive drum 1 so as not to mix colors while performing the second development control for one developing device. The other developing devices are separated from each other, or the panning roller of the other developing device is moved to a position where the supply of the developer to the developing sleeve is stopped so that the developing pan is not brought into contact.

Further, as shown by a thick line in FIG. 4B, when the second development control is performed, a bias of reverse polarity may be applied before the photosensitive drum 40 is charged. As a result, reverse polarity toner can also be discharged from the developing device, and image fog can be reduced more effectively.
In the above-described embodiment, the power-on is described as the predetermined timing at the time of non-development. However, the image forming apparatus is restarted when returning from the preheat state (power saving state) for energy saving or when the jam processing is completed. In the case of a developing device of a toner cartridge replacement system, it means when the toner cartridge is replaced. In addition, the counter unit that stores the number of printed sheets of the image forming apparatus can be used, for example, after the end of a series of image forming operations after 10,000 times of image forming.

  By eliminating image fog, the quality of image formation is improved and paper is less wasted, leading to a reduction in paper costs.

1 is a schematic configuration diagram of an image forming apparatus to which the present invention is applicable. FIG. 2 is an enlarged view around an image forming unit of the image forming apparatus in FIG. 1. It is a conceptual diagram which shows the relationship between an image carrier and developing bias. FIG. 4A is a timing chart showing timings of charging of the photosensitive drum surface, exposure, development bias application, and toner image transfer. FIG. 4A is a timing chart of the first development control, and FIG. 6 is a timing chart of the development control. It is a flowchart which shows the timing which performs 1st image development control and 2nd image development control. It is a table | surface figure which shows the experimental result which measured the degree of image fogging in the image forming apparatus concerning this invention. It is a schematic block diagram of the other image forming apparatus which can apply this invention. 6 is a timing chart illustrating a method for controlling an image carrier and a developing bias in a conventional image forming apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 40 Photosensitive drum 4, 5, 6, 7, 61 Image development apparatus 18Y, 18C, 18M, 18B Image formation means 60 Charging apparatus 63 Photoconductor cleaning apparatus 64 Static elimination apparatus 100 Image forming apparatus main body

Claims (6)

In an image forming apparatus that develops an electrostatic latent image formed on an image carrier into a toner image,
The image carrier is charged and then exposed to obtain the electrostatic latent image, and a development bias voltage is applied to perform development, and the image carrier is charged at a predetermined timing during non-image formation. After the exposure, the potential difference between the surface potential of the image carrier and the development bias voltage is made equal or larger than the potential difference between the surface potential of the image carrier and the development bias voltage in the first development control without exposure. An image forming apparatus comprising: a control unit that performs second development control for performing development.   The control means changes the development bias voltage in the second development control by making the surface potential of the image carrier in the second development control the same as the surface potential of the image carrier in the first development control. The image forming apparatus according to claim 1, wherein:   The control means changes the surface potential of the image carrier in the second development control, and makes the development bias voltage in the second development control the same as the development bias voltage in the first development control. The image forming apparatus according to claim 1, wherein:   2. The image according to claim 1, wherein the potential difference in the second development control is a potential difference at which an uncharged toner and a low-charged toner having a lower charge amount than a normal charge amount can move to the image carrier. Forming equipment.   2. The control unit according to claim 1, wherein a developing bias having a polarity opposite to the developing bias applied in the first developing control is applied before or after executing the second developing control. The image forming apparatus described.   The image forming apparatus according to claim 1, wherein the predetermined timing at the time of non-image formation is when power is turned on.

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