JP2010145782A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus which is capable of suppressing the degradation in image quality due to transfer dust and transfer non-uniformity which are caused when transferring a toner image to a transfer material. <P>SOLUTION: The image forming apparatus includes a photoreceptor 1 carrying a toner image, a developing device 4 for forming the toner image on a surface of the photoreceptor, a corona transfer device 20 which faces the photoreceptor and charges a surface of a web 7 with a polarity opposite to that of electric charges of the toner image to transfer the toner image, and a transporting means 11 for transporting the web 7. In this image forming apparatus, an electric charge application means 21 is provided which applies electric charges whose polarity is opposite to the charging polarity for transfer, to a surface opposite to a surface brought into contact with the photoreceptor 1, of the web 7 without contacting with the web 7 in a region before and after a position wherein the photoreceptor 1 and the web 7 are brought into contact with each other above the corona transfer device 20. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an image forming apparatus using an electrophotographic process, and more particularly to an image forming apparatus for transferring toner from an image carrier such as a photosensitive member and an intermediate transfer member to a recording medium. The present invention relates to an image forming apparatus capable of effectively suppressing deterioration in image quality such as transfer dust and transfer non-uniformity when the transfer member is used.

  As an image forming apparatus represented by a printer or the like, various systems have been put into practical use. After the latent image is formed on the photoreceptor, a direct transfer method in which the latent image is visualized with toner and the toner image formed is transferred onto a recording paper, or a plurality of color toner images are overlaid on the intermediate transfer member. Later, there is a method (intermediate transfer method) in which the intermediate transfer member is collectively transferred onto a transfer material. Transfer dust is a typical image deterioration when transferred from a photosensitive member or an intermediate transfer member to a transfer material. Transfer dust is a phenomenon in which when a toner image is transferred, it is not transferred to a position where it should originally be transferred, but is diffused and transferred around the toner image, resulting in a blurred image. This will impair the sharpness of the image.

  The cause of transfer dust is that the toner on the photosensitive member and the intermediate transfer member is electrostatically attracted by the action of an electric field at the entrance of the transfer nip, and is formed on the recording paper in a region where there is a gap between the photosensitive member and the recording paper. Generated by pre-transfer. In addition, for example, when a discharge occurs at the entrance of the transfer nip, the charge amount (Q / M) of the toner affected by the discharge is reduced and the electrostatic adhesion is reduced. As a result, it becomes impossible to overcome the coulomb repulsive force and further increase the generation of transfer dust. In order to suppress this transfer dust, many inventions have been made so far.

JP 2000-221806 A JP 2003-248380 A

  In Japanese Patent Application Laid-Open No. 9-236992, when transferring to paper, the toner to be transferred is influenced by the discharge at the entrance of the transfer nip by bringing the paper into close contact (pre-contact) with the photoconductor upstream of the transfer region. However, it is very difficult to pre-contact the paper in a stable state at all times. Paper strain or the like varies depending on environmental conditions and paper types, and conversely, it may be considered that the image is rubbed when paper enters.

  Further, in a continuous paper printer that uses continuous forms, since the recording medium is continuous, it is not possible to bring the recording medium into close contact (pre-contact) with the photoreceptor upstream from the transfer area as described above. The above technology is applied to a continuous paper printer as it is because of the problem of ensuring the print quality of the first page and the last line of the first page and the last page when printing is started / stopped. It is difficult to do.

  In order to prevent transfer dust generated at the entrance of the transfer nip, there is a means to prevent corona irradiation to the pre-nip part, but there is a charge opposite to the toner via the recording paper. Since it flows into the pre-nip portion, a sufficient effect was not obtained.

  Accordingly, the present invention solves the problems of the conventional devices as described above, and image quality due to transfer dust and transfer non-uniformity generated when transferring a toner image from an image carrier to various transfer materials such as paper. An object of the present invention is to provide an image forming apparatus capable of suppressing deterioration of the image.

  In order to achieve the above object, the first means of the present invention includes an image carrier that carries a toner image, a developing unit that forms a toner image on the surface of the image carrier, and a conveyance unit that conveys a recording medium. And an image transfer means for transferring the toner image onto the recording medium by charging the surface of the recording medium facing the image carrier in a non-contact manner with a polarity opposite to the charge of the toner image. In the forming apparatus, in the region before and after the position where the image carrier upstream of the transfer means and the recording medium contact, the opposite surface of the recording medium that contacts the toner image carrier has a polarity opposite to the charging polarity of the transfer. The present invention is characterized in that a charge imparting means for imparting charges without contact with the recording medium is provided.

  According to a second means of the present invention, in the first means, the position of the charge applying means is a region within a range of 50 mm before and after the contact start position of the transfer region where the recording medium and the photosensitive member abut. The recording medium is characterized in that the charging polarity of the surface potential of the recording medium is the same polarity as the polarity of the toner image.

  According to a third means of the present invention, in the first or second means, the charging means is a corona discharger comprising a wire and a shield.

  According to a fourth means of the present invention, in the first or second means, the charging means is a sawtooth discharge needle.

  According to a fifth means of the present invention, in the first or second means, a shielding plate is provided between the transfer means and the charging means.

  The sixth means of the present invention is characterized in that, in the first or second means, the timing at which the voltage is applied by the charging means is controlled with a time difference from the timing at which the voltage of the transfer means is applied. It is what.

  The present invention has the above-described configuration, solves the problems of the conventional ones, and generates transfer dust and transfer defects that occur when transferring a toner image from an image carrier to a variety of transfer materials such as paper. An image forming apparatus capable of suppressing deterioration in image quality due to uniformity can be provided.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 8 is a schematic configuration diagram of the entire image forming apparatus according to the embodiment.

(1) Description of Entire Image Forming Apparatus As shown in FIG. 8, a charger 2, an exposure device 3, a developing device 4, a transfer unit 5, a cleaning brush 6, and the like are arranged along the rotation direction of the photosensitive member 1. . After the charger 2 uniformly charges the surface of the photosensitive member 1, the exposure device 3 irradiates the surface of the photosensitive member 1 with laser light according to image data. As a result, an electrostatic latent image is formed on the surface of the photoconductor 1 and developed by the developing device 4, and a toner image is formed on the surface of the photoconductor 1.

  As the photoconductor 1, a positively charged photoconductor such as a selenium photoconductor, an organic photoconductor (OPC), or amorphous silicon (a-Si) was used. The developing method of the photoreceptor 1 is a reversal developing method, and the charging polarity of the toner is a positive polarity.

The web 7 made of continuous paper is conveyed to the transfer unit 5 by the web conveyance devices 8 to 12, and the toner image is transferred onto the web 7 by the transfer unit 5. The toner image on the web 7 is heated to the vicinity of the transition temperature of the toner resin when passing through the pre-heater 13, and then the toner image is fused and fixed to the web 7 by a fixing device 16 including a heating roller 14 and a backup roller 15 incorporating a heater. Is done. In the case of duplex printing, two image forming apparatuses are used as shown in FIG. The present invention is characterized by the configuration of the transfer unit 5 in the image forming apparatus. Next, the configuration of the transfer unit 5 will be described.
(2) Configuration of Transfer Unit 5 FIG. 1 is a schematic configuration diagram in the vicinity of the transfer unit, showing a printing state, and FIG. 2 is a schematic configuration diagram showing when printing is stopped (retracted). As shown in FIG. 1, tractor-type web conveyance devices 11 and 12 are installed on the upstream and downstream sides of the transfer unit 5 in the web conveyance direction, and the web conveyance devices 11 and 12 rotating in the feed holes of the web 7 are installed. The pins are sequentially fitted to convey the web 7 in a predetermined direction.

  The transfer section 5 has a lower guide 17a for guiding the web 7 and a transfer guide housing 18 provided with an upper guide 17b at the opening end, and the surface of the web 7 accommodated therein has a negative polarity charge opposite to the charge polarity of the toner. The corona transfer device 20 to be supplied, the corona charger 21 for supplying a positive polarity charge opposite to the transfer polarity to the surface of the web 7 on the upstream side of the corona transfer device 20, and between the corona transfer device 20 and the corona charger 21. An insulating plate 22 is provided to shield each corona charge. Further, the corona transfer device 20 and the corona charger 21 are provided with high-voltage power supplies 23a and 23b for applying reverse charging polarities, respectively.

  The phenomenon of transfer dust will be described with reference to FIG. FIG. 3 is an enlarged schematic view of the transfer region upstream side.

  The negative corona generated from the corona transfer device 20 is irradiated onto the surface of the web 7 and electrostatically attracts toner having a positive polarity on the photoreceptor in the transfer region (transfer nip) to transfer the toner onto the web 7. In the pre-nip region upstream of the transfer region (the gap portion upstream of the nip start point), there is a gap between the photoconductor 1 and the web 7, so it is necessary to prevent irradiation of the pre-nip portion outside the transfer region. In order to prevent this, there is a means for preventing the irradiation to the pre-nip portion by providing a shielding plate 24. However, the negative charge is reversed in the direction of the arrow in the figure through the wraparound and the web 7. Therefore, it is difficult to suppress transfer dust. Therefore, it was necessary to block the electric charge flowing through the web 7.

  Here, the effect of the configuration in which the corona charger 21 for applying a voltage having a polarity opposite to that of the transfer according to the present invention is provided on the upstream side in the conveyance direction of the web 7 will be described with reference to FIGS. 4 and 5. FIG. 4 is an enlarged schematic view of the pre-nip portion when a corona charger 21 for applying a voltage having a polarity opposite to that of transfer is added. The minus radiated from the corona transfer device 20 is charged on the web 7 and electrostatically attracts the toner image, and tries to go upstream via the web 7, but the plus provided on the upstream side. Are blocked by positive charges emitted from the corona charger 21 (point A in the figure). As a result, the paper potential in the pre-nip region having a gap existing upstream from the nip start point in the figure can be made the same polarity as the charging polarity of the toner, and as a result, unnecessary toner dust is suppressed. It becomes possible.

  FIG. 5 shows the distribution of the sheet surface potential at the transfer section. The horizontal axis indicates each position of the web passing through the transfer portion, and the vertical axis indicates the surface potential of the web. In the figure, the broken line shows the distribution when only the corona transfer device 20 is applied. Due to the negative corona transfer, the transfer region becomes about −200 V, but negative charges are transmitted to the upstream side via the web, which is negative. The paper potential at the pre-nip where the transfer dust is generated is also negative, which is opposite to the toner charging polarity (in this embodiment, positive), and is attracted electrostatically. It is a state that has occurred. However, when a positive charging polarity is applied to the upstream side by the corona charger 21, the potential of the paper can be made positive in the pre-nip region. From these, it can be seen that the addition of the corona charger 21 is effective in suppressing transfer dust and improving transfer uniformity.

  FIG. 6 is a graph showing the state of the surface potential of the paper at a certain point using the voltage applied to the corona charger as a parameter. It can be seen that if the voltage of the positive corona charger is increased, the negative charging polarity from the corona transfer can be blocked in front of the pre-transfer portion.

Further, when two corona dischargers having opposite polarities are used close to each other, the corona due to mutual interference cancels each other, and efficiency reduction is expected. In order to prevent this phenomenon, a shielding plate 22 is provided between the two corona dischargers. The material of this shielding board should just be an insulating member.
(3) Oscillating mechanism of the transfer unit housing As shown in FIG. 2, the transfer unit housing 18 is supported so as to be swingable about a shaft core part 25 provided at the upper part. One end of a link 26 is connected to the rear wall of the transfer unit housing 18 opposite to the photoreceptor 1, and the other end of the link 26 is eccentrically attached to the output shaft of the housing drive motor 27. Accordingly, the transfer device housing 18 can swing around the swing shaft core portion 25 by the rotation of the housing drive motor 27.

  As described above, in the case of the continuous paper printer, since the web 7 is continuous, the web 7 needs to be brought into contact with and separated from the photoreceptor 1 in a short time when printing is stopped. The reason for performing this in a short time is basically to prevent blank pages from being created, and this separation / connection operation must be performed within a limited print prohibition area between pages. Therefore, the winding length (nip region) of the web 7 around the photoreceptor 1 is not so large. As the winding length increases, the operation time for moving away from the photosensitive member increases, leading to image quality deterioration due to unnecessary contact. In the present embodiment, the corona transfer device 20, the corona charger 21, and the insulating plate 22 that is a shielding plate are not interlocked with the operation of the transfer device housing and do not perform the separation operation, and interfere with the web 7 during standby. It is placed in a position that does not.

  In order to effectively suppress transfer dust at the pre-nip portion, it is desired to make the corona transfer device 20 and the corona charger 21 compact so that a positive charge can be applied within the winding length. Since the distance between the wire and the shield is determined by the required applied voltage, it is difficult to reduce the size of.

  In summary, the position where the charge having the same polarity as the toner is applied is ideally downstream of the transfer nip start point in the figure. However, the phenomenon that the electric charge flows through the sheet can be said to be the same even when the positive charging polarity is used. Therefore, it is not always necessary to be upstream of the transfer nip start point.

  In order to prevent the image quality deterioration prevention at the time of starting / stopping printing, which is peculiar to continuous paper, the problem at the time of web separation / contact operation is solved in a limited transfer area (nip width), and the effect of suppressing transfer dust is achieved. Therefore, the charge application position by the corona charger 21 may be set within a range of 50 mm before and after the nip start point.

Next, the operation of the transfer unit 5 will be described.
(4) Operation of Transfer Unit 5 FIGS. 1 and 2 are diagrams showing states of the transfer unit 2 during printing and printing standby, and FIG. 7 is a timing chart showing respective operations.

  At the time of printing standby, the housing drive motor 27 rotates and the transfer device housing 18 stops in the state shown in FIG. 2, and the transfer device housing 18 and the web 7 are separated from the photoreceptor 1. A web conveyance motor (not shown) rotates based on a web conveyance start signal from a control unit (not shown), and accordingly, the web conveyance devices 11 and 12 are driven to start conveyance of the web 7. As shown in FIG. 7, the conveyance speed of the web 7 rises to a prescribed speed before the leading end of the printed page of the web 7 reaches the transfer point, and completes the preparation for the transfer operation.

  As shown in FIG. 7, this is performed at the timing when the corona charger 21 is turned on almost simultaneously with the start of the conveyance operation of the web 7, and the corona transfer device 20 is turned on after a predetermined delay time t. The predetermined delay time t is not necessarily set, but is set in the present invention to simplify the control.

  Thereby, the surface potential of the web 7 can be set to a positive potential having the same polarity as that of the toner in advance, and the transfer dust can be stably suppressed from the start of printing. It should be noted that when printing is stopped, the procedure is the reverse of the startup operation, and the next printing operation is prepared.

  FIG. 9 shows another embodiment using the transfer apparatus of the present invention. The configuration is the same as that of the first embodiment except that a discharge needle 28 is provided instead of the corona charger 21 on the upstream side of the corona transfer device 20.

  Tractor-type web conveyance devices 11 and 12 are installed on the upstream and downstream sides of the transfer unit 5 in the web conveyance direction, and the pins of the web conveyance devices 11 and 12 rotating in the feed holes of the web 7 are sequentially fitted. The web 7 is conveyed in a predetermined direction. The transfer section 5 has a lower guide 17a for guiding the web 7 and a transfer housing 18 provided with an upper guide 17b at the open end, and the surface of the web 7 accommodated therein has a negative polarity charge opposite to the charged polarity of the toner. The corona transfer device 20 to be supplied, the corona charger 21 for supplying a positive polarity charge opposite to the transfer polarity to the surface of the web 7 on the upstream side of the corona transfer device 20, and between the corona transfer device 20 and the corona charger 21. An insulating plate 22 is provided to shield each corona charge.

  Further, the corona transfer device 20 and the corona charger 21 are provided with high-voltage power supplies 23a and 23b for applying reverse charging polarities, respectively. As described above, by using a sawtooth discharge needle for the charge applying means, it is possible to save the mounting space compared to the charger composed of the shield and the wire. Further, by using the discharge needle, a positive charge can be applied at a low voltage, which is effective in reducing the capacity of the high-voltage power supply, and the overall cost can be reduced.

  The present invention can be applied to an image recording apparatus such as an electrophotographic printer or a copying machine.

It is a schematic block diagram of the transfer part vicinity which concerns on the Example of this invention. It is a schematic block diagram at the time of the standby of the transfer part vicinity. It is an enlarged view of the pre-nip area | region of a transfer part. It is a schematic diagram which shows the state of the corona charge of the prenip part of a transfer part. FIG. 6 is a characteristic diagram illustrating a distribution of sheet potential in a transfer unit. FIG. 6 is a characteristic diagram illustrating a relationship between a voltage of a corona charger and a sheet potential. It is a timing chart which shows operation | movement of a transcription | transfer part. 1 is a schematic configuration diagram of an entire image forming apparatus according to an embodiment of the present invention. It is a schematic block diagram of the transfer part vicinity which concerns on the other Example of this invention.

Explanation of symbols

  1 is a photosensitive member, 2 is a charger, 3 is an exposure device, 4 is a developing device, 5 is a transfer unit, 6 is a cleaning brush, 7 is a web, 11 and 12 are web conveying devices, 13 is a preheater, and 14 is a heating roller. , 15 is a backup roller, 16 is a fixing device, 17 is a guide, 18 is a transfer device housing, 20 is a corona transfer device, 21 is a corona charger, 22 is a shielding member, 23a and 23b are high voltage power supplies, 24 is a shielding plate, Reference numeral 25 denotes an axial core portion, 26 denotes a link, 27 denotes a housing drive motor, and 28 denotes a discharge needle.

Claims (6)

An image carrier that carries a toner image, a developing unit that forms a toner image on the surface of the image carrier, a conveyance unit that conveys a recording medium, and a surface of the recording medium that faces the image carrier In an image forming apparatus comprising transfer means for charging the toner image onto the recording medium by charging in a non-contact manner with a polarity opposite to the charge of the toner image,
In the region before and after the position where the image carrier on the upstream side of the transfer means and the recording medium are in contact with each other, a charge having a polarity opposite to the charging polarity of the transfer is applied to the opposite surface of the recording medium in contact with the toner image carrier. An image forming apparatus, comprising: a charge applying unit that applies a charge in a non-contact manner.   2. The image forming apparatus according to claim 1, wherein the position of the charge applying unit is an area within a range of 50 mm before and after the contact start position of a transfer area where the recording medium and the photosensitive member contact each other. An image forming apparatus characterized in that the charging polarity of the potential is a position where the polarity is the same as the polarity of the toner image.   3. The image forming apparatus according to claim 1, wherein the charging unit is a corona discharger composed of a wire and a shield.   3. The image forming apparatus according to claim 1, wherein the charging unit is a sawtooth discharge needle.   3. The image forming apparatus according to claim 1, wherein a shielding plate is provided between the transfer unit and the charging unit.   3. The image forming apparatus according to claim 1, wherein the timing at which the voltage is applied by the charging unit is controlled with a time difference from the timing at which the voltage of the transfer unit is applied. .

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