JP2007171633A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of preventing image failure such as transfer memory ghost with simple configuration without disposing additional composing members, for example, around an image carrier. <P>SOLUTION: The image forming apparatus includes a control means (50) for changing the charging biases of charging means (2 and 12) so as to increase the charging potential of the image carrier (1) during image formation. For example, the control means (50) changes the charging biases so that, during the formation of a monochrome image, the charging potential of the image carrier corresponding to the area where a toner image composing the monochrome image is formed successively increases. And, the control means (50) changes the charging biases so that, during the formation of a multicolor image, the charging potential of the image carrier corresponding to the area where each of toner images composing the multicolor image is formed successively increases. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an image forming apparatus such as a printer, a copying machine, a facsimile machine, or a multi-function machine that can form an image made of toner as a dry developer, and in particular, to prevent an image defect such as a transfer memory ghost from occurring. The present invention relates to an image forming apparatus capable of

  In an image forming apparatus such as a printer to which an electrophotographic method or an electrostatic recording method is applied, basic image formation is generally performed through the following process. First, an image carrier such as a rotating photoconductor is uniformly charged by a charging unit to which a charging bias is applied, and then the charged image carrier is exposed based on image information from the image exposure unit to electrostatically charge the image carrier. A latent image is formed. Subsequently, the electrostatic latent image on the image carrier is developed with toner supplied from the developing means to form a toner image, and the toner image is directly applied to a recording medium such as recording paper by the transfer means to which a transfer bias is applied. Alternatively, the toner image is transferred via an intermediate transfer member, and finally, the toner image is fixed on the recording medium by a fixing unit. Thus, an image made of toner is formed.

  However, in such an image forming apparatus, an image defect called a transfer memory ghost described below may occur.

  Regarding the generation of the transfer memory ghost, for example, the portion (exposure portion) where the potential is lowered by exposure after charging the image carrier to a negative polarity charging potential is used as an electrostatic latent image, and the exposure becomes the electrostatic latent image. An example is an image forming apparatus in which a negatively charged toner is attached to a portion by reversal development to form a toner image, and then the toner image is transferred to a recording sheet or an intermediate transfer member with a positive polarity transfer bias applied. This is as follows.

  That is, in the transfer process at the time of image formation by the image forming apparatus, a positive polarity charge is injected into the image carrier due to the application of the transfer bias. At that time, the surface potential of the image carrier is generally positive. In particular, there is a difference between the exposed portion where the toner is present and the non-exposed portion where the toner is not present. In this state, a potential difference is generated.

  In the subsequent exposure process during image formation, when the portion of the image carrier where the potential difference has occurred becomes an exposure portion (latent image portion), the potential difference state corresponding to the potential difference has a positive polarity in the electrostatic latent image. It remains as a charge history. Next, at the stage where the latent image portion where such charge history exists is developed, a density difference occurs at the step portion due to the potential difference of the charge history, and the density difference portion is an image (for example, a uniform halftone image). ) May be visually recognized as if it were part of the image drawn inside. This is a so-called transfer memory ghost.

  On the other hand, conventionally, there has been known an image forming apparatus in which a countermeasure for preventing such a transfer memory ghost is taken.

  For example, there is known an image forming apparatus in which an exposure unit that exposes the entire surface of the image carrier and removes electricity between the transfer unit and the cleaning unit along the rotation direction of the image carrier is known (Patent Document 1). . According to this image forming apparatus, after the transfer of the toner image and before the cleaning of the image carrier, the entire surface of the image carrier is exposed by the exposure means (for example, a neutralization lamp) and the charge is eliminated. In the potential difference portion, the potential difference almost disappears and does not remain as a history of potential, so that a memory ghost is not generated during subsequent image formation.

  In addition, an image forming apparatus is known in which a bias applying unit that applies a bias voltage having the same polarity as that of charging to the image carrier is provided between a charging unit and a transfer unit arranged around the image carrier (see FIG. Patent Document 2). According to this image forming apparatus, the image carrier is charged with the same polarity as the charging polarity by the bias applying means (for example, a cleaning blade) after the toner image is transferred and before the next charging is performed. When the charged parts of the opposite polarity (transfer material non-passing part and non-development part on the image carrier) are restored to the original charged polarity and then charged by the charging means, the charged state on the surface of the image carrier is It becomes uniform.

Japanese Patent Laid-Open No. 11-133824 JP 2002-132022 A

  However, this conventional image forming apparatus has the following problems.

  That is, in any image forming apparatus, it is necessary to add and install new components such as an exposure unit or a bias application unit around the image carrier, and thus the installation space must be secured separately. As a result, layout changes and restrictions occur, and this is disadvantageous in reducing the size of the image forming apparatus.

  The present invention has been made in view of such problems, and prevents the occurrence of image defects such as transfer memory ghosts with a simple configuration without installing new components around the image carrier. An image forming apparatus capable of performing the above is provided.

  The image forming apparatus of the present invention includes an image carrier, a charging unit that charges the image carrier by applying a charging bias, and an image that forms an electrostatic latent image on the charged image carrier by exposure based on image information. In an image forming apparatus comprising: an exposure unit; a developing unit that develops the electrostatic latent image with toner to form a toner image; and a transfer unit that applies a transfer bias to transfer the toner image to a transfer member. It has a control means for changing the charging bias of the charging means so that the charging potential of the image carrier increases at the time of formation.

  Here, the image carrier is a member on which a toner image is formed through charging, image exposure (formation of an electrostatic latent image), and development (process), such as a photoconductor. The transfer member is capable of carrying a toner image formed on the image carrier by transfer, and is, for example, a recording medium such as recording paper, cardboard, or OHP sheet, an intermediate transfer member, or the like.

  The charging potential of the image carrier increases when a toner image constituting an image finally formed (transferred and fixed) on one side of a recording medium is continuously formed on the same image carrier. That is, the charged potential (absolute value) of the image carrier corresponding to at least the toner image formation region increases in a predetermined pattern and increases. When the charging bias is changed, the initial value of the charging potential of the image carrier that increases (the voltage value that charges the charging tip of the toner image forming region) is equal to or higher than the minimum charging potential necessary for charging. Must be a value. Further, the degree of increasing the charging potential is preferably set on the basis of obtaining a potential equal to or higher than the charging potential necessary for canceling the difference in surface potential when a transfer memory ghost occurs.

  The object of the charging bias to be changed by the control means is, for example, the value of the direct current (DC) charging method in which a direct current voltage is applied as the charging bias, and the direct current voltage is changed to the direct current voltage as the charging bias. When the alternating current (AC) superposition charging method for applying the superimposition of is applied, the direct current voltage value is obtained.

  Further, the image forming apparatus has a function of forming a single color toner image on the image carrier and forming a single color image composed of the single color toner image, and the control unit includes: When the monochrome image is formed by the function, the charging bias can be changed so that the charging potential of the image carrier corresponding to the toner image forming area constituting the monochrome image continuously increases. .

  The above-mentioned continuous increase usually means that the charging potential increases in a state that is directly proportional to the elapsed time (always has a constant slope), but the charging potential increases with respect to the elapsed time as necessary. It does not matter if it increases at a predetermined rate of increase (a state in which the slope increases in a predetermined pattern). The same applies to “continuously increasing” thereafter. In addition, the formation of a single color image includes a case where a single color toner image is formed in an image forming apparatus that sequentially forms a plurality of color toner images on an image carrier to be described later to form a multicolor image. In the image forming apparatus provided with a plurality of image forming apparatuses corresponding to each of a plurality of color toners, which will be described later, only a single color image is obtained by using only one of the image forming apparatuses. The monochromatic image is usually a black one-color image (monochrome image), but may be a monochromatic image of other colors.

  In the case of such a configuration, for example, when different monochrome images are continuously formed, or when the same monochrome image is continuously formed at different (shifted) positions on the image carrier, the toner In the charging process when images are continuously formed, charging in which the charging potential continuously increases is performed. This eliminates or reduces the charge history on the image carrier generated during the formation of the preceding monochrome image (particularly during the transfer process) in the charging process in which the charging potential is increased when the subsequent monochrome image is formed ( The potential difference of the history can be reduced). In this case, since the charging bias is changed so that the charging potential continuously increases, the charging bias is reduced as compared with the case where the charging bias is changed so as to increase the charging potential stepwise. A stepped portion does not occur in the reflected surface potential of the image carrier, and a density difference due to the stepped portion does not occur particularly in a halftone image, so that a better image quality can be obtained.

  Further, the image forming apparatus has a function of sequentially forming a plurality of color toner images on the image carrier and forming a multicolor image composed of the plurality of color toner images, and the control means. However, when the multicolor image is formed by the function, the charging bias is changed so that each charging potential of the image carrier corresponding to each toner image forming area constituting the multicolor image continuously increases. It can also be configured as follows.

  In the case of an image forming apparatus that sequentially forms a plurality of color toner images on such an image carrier, the developing means is usually provided with the same number of developing means as the number of colors of toner images constituting a multicolor image. In addition, it is necessary to employ, for example, both the recording medium and the intermediate transfer body as the member to be transferred, or a recording medium transport body that holds and transports the recording medium and the recording medium so as to pass through the transfer position. is there. Further, it is necessary to make the same conditions for image forming apparatuses having similar functions thereafter.

  In such a configuration, for example, when one multicolor image is formed, a plurality of color toner images constituting the multicolor image are sequentially formed on the same image carrier. In the charging process when the toner image is formed, charging is performed such that the charging potential continuously increases. As a result, the charge history on the image carrier generated during the formation of the toner image of the color to be formed in advance (particularly during the transfer process), the toner image of the remaining color is formed thereafter. It can be eliminated or reduced by a charging process in which the charging potential is increased.

  Further, the image forming apparatus has a function of sequentially forming a plurality of color toner images on the image carrier and forming a multicolor image composed of the plurality of color toner images, and the control means. However, at the time of multicolor image formation by the function, each charging potential of the image carrier corresponding to each toner image forming area constituting the multicolor image is increased stepwise between the forming areas. The charging bias can be changed.

  The rate at which the charging voltage is increased in each step when increasing stepwise may be the same rate in each step, or may be different depending on the step if necessary.

  In this case, for example, when one multicolor image is formed, a plurality of color toner images constituting the multicolor image are sequentially formed on the same image carrier. In each charging process when forming a toner image, charging is performed such that the charging potential increases stepwise. As a result, the charge history on the image carrier generated during the formation of the toner image of the color to be formed in advance (particularly during the transfer process) is obtained, and the toner image of the remaining color is formed thereafter. It can be eliminated or reduced by a charging process in which the charging potential is increased.

  In addition to the above, the image forming apparatus includes a plurality of image forming apparatuses each having the image carrier, a charging unit, an image exposure unit, a developing unit and a transfer unit. Each of the toner images of different colors is formed, and has a function of forming a multi-color image composed of the toner images. It is also possible to change the charging bias of the charging means of each image forming device so that the charging potential of each image carrier on which each toner image forming the color image is continuously increased.

  In the case of an image forming apparatus in which a plurality of color toner images are distributed and formed separately for each color by such a plurality of image forming apparatuses, the plurality of toner images constituting the multicolor image are usually used as the developing device. The developing means for storing the developer corresponding to the color is distributed to each image forming device and used, and for example, both the recording medium and the intermediate transfer body are adopted as the transfer member, or the recording medium and the recording medium It is necessary to adopt a carrier.

  In the case of the image forming apparatus configured as described above, for example, when forming one multicolor image, each of the image carriers of the image forming apparatus to which the toner images of a plurality of colors constituting the multicolor image correspond are provided. Although it is formed individually, in the charging process of each image forming device when forming each toner image, the charging potential increases continuously in the toner image forming area of each image carrier. Done. As a result, the charge history on each image carrier that is generated when the toner image previously formed on the image carrier of each image forming device is transferred to the member to be transferred can be recorded thereafter. It becomes possible to disappear or reduce in the charging process in which the charging potential is increased when forming the subsequent toner image formed by the body.

  Further, the control means in the image forming apparatus including the plurality of image forming devices may be configured such that, when the multicolor image is formed by the function, the charging potential of each image carrier on which each toner image constituting the multicolor image is formed. However, the charging bias of the charging means of each image forming device can be changed so as to increase in a stepwise manner in accordance with the transfer order of the toner images.

  When configured in this way, in addition to the charging potential continuously increasing as described above in the charging process of each image forming device when forming each color toner image constituting the multicolor image, During the charging process of each image forming device, charging is performed such that each charging potential increases in a stepwise manner along the order of transfer of the toner images. As a result, the charge history generated when the preceding toner image is formed on the image carrier of each image forming device can be eliminated or reduced by the charging process during the subsequent toner image formation in which the above-described charging potential continuously increases. It becomes possible. In addition to this, as the image carrier of the image forming apparatus in which the transfer order of each toner image is later, the charge history becomes larger due to the execution of the transfer process in the presence of the toner image transferred in advance. The phenomenon can be eliminated or reduced in the charging process at the time of subsequent toner image formation in which the charging potential increases in accordance with the order of transfer.

  According to the image forming apparatus of the present invention, it is possible to perform charging so that the charging potential of the image carrier increases at the time of image formation by the control means. Therefore, it occurs in the transfer process at the time of forming the preceding toner image. It is possible to eliminate or reduce the charge history on the image carrier by a charging process in which a charging potential is increased when a subsequent toner image is formed. As a result, it is possible to prevent an image defect such as a transfer memory ghost from occurring with a simple configuration without installing new components around the image carrier.

[Embodiment 1]
FIG. 1 shows an outline of a full-color printer according to Embodiment 1 of the present invention.

  The full-color printer includes an image forming unit 10, an intermediate transfer unit 20, a fixing device 40, and the like. Reference numeral 50 in the figure denotes a system control device.

  First, the image forming unit 10 is provided with a photosensitive drum 1 as an image carrier that rotates in the direction of an arrow. Around the photosensitive drum 1, a charging roll 2 as a charging unit and a ROS (image exposure unit) are provided. Raster Output Scanner) 3, and rotation switching (4), four color developing devices 4Y, 4M, 4C, 4K of yellow (Y), magenta (M), cyan (C), and black (black, K) as developing means. A rotary type developing device 4 and a primary transfer roll 5 as a (primary) transfer means are disposed. Reference numeral 6 in the figure denotes a drum cleaner for the photosensitive drum 1, which is mainly composed of a cleaning blade 6a installed in contact with the photosensitive drum 1 and a waste toner box 16b for storing unnecessary toner scraped off by the blade 6a. It is configured.

  Among these, the photosensitive drum 1 has a photosensitive layer made of an organic photosensitive material formed on the peripheral surface of a grounded cylindrical conductive substrate, and is installed to rotate at a predetermined speed by a driving device (not shown). .

  As shown in FIG. 2, the charging roll 2 is formed by forming an elastic layer 2b or the like on a roll base material 2a such as metal. The charging roll 2 is supported so as to be driven and rotated in contact with the peripheral surface of the photosensitive drum 1 with a predetermined pressure, and a charging bias is applied to the roll base 2a from the charging power source 12 during the charging process. Is done. In this example, an AC superposition charging method in which a DC bias with an AC voltage superimposed thereon is applied as the charging bias is applied. In addition, the charging roll 2 is cleaned with a cleaning member such as a foam roll or a brush, in order to remove deposits that adhere to the surface of the charging roll 2 and impair the chargeability. A mechanism is installed. In the case of a foam roll, it is configured to be driven and rotated while in contact with the charging roll 2.

  The ROS 3 is mainly composed of a semiconductor laser emission source and a scanning optical system device such as a polygon mirror, a reflection mirror, and a lens. At the time of the image exposure process, a laser beam LB corresponding to an image signal processed and generated by an image processing apparatus (not shown) is emitted from a semiconductor laser based on image information input to the printer, and is exposed to light by a scanning optical system apparatus. Scanning exposure is performed toward the drum 1.

  The rotation switching type developing device 4 includes four color developing devices 4Y, 4M, 4C, and 4K arranged at an angle of 90 ° in the circumferential direction of the rotation support frame 8. Further, in the developing device 4, the developing support 4Y, 4M, and 4D corresponding to the color to be developed is obtained by rotating the rotation support frame 8 by a predetermined amount around the rotation shaft 8a at a predetermined timing during the developing process. 4C and 4K are moved to development positions facing the photosensitive drum 1, respectively.

  Each of the developing devices 4Y, 4M, 4C, and 4K has a housing 4a in which a housing portion that houses a required developer (for example, a two-component developer) is formed. The housing 4a includes a developing roll 4b and a developing roller. An agent stirring member and the like are provided. A developing bias is applied to the developing roll 4a from a developing power source (not shown). Reference numerals 9Y, 9M, 9C, and 9K in the figure denote developer cartridges that individually store developer of a predetermined color to be replenished in the housing 4a and are detachably attached to the rotation support frame 8. In this example, a negatively chargeable toner is used as the toner for the two-component developer.

  The primary transfer roll 5 is obtained by forming an elastic layer or the like on a roll base material such as metal. The primary transfer roll 5 is supported so as to rotate while being in contact with the peripheral surface portion serving as the primary transfer position of the photosensitive drum 1, and a primary transfer power source (not shown) is provided on the roll base material at the time of the primary transfer process. To the primary transfer bias. As the primary transfer bias, a DC voltage having a polarity opposite to the charging polarity of the toner (in this example, a positive polarity) is applied.

  Next, the intermediate transfer unit 20 is installed in the vicinity of the transfer position of the photosensitive drum 1 in the image forming unit 10, and includes an intermediate transfer belt 21 as an intermediate transfer member, a secondary transfer roll 25 as a secondary transfer unit, and the like. Its main part is composed. In the figure, reference numeral 26 denotes a belt cleaner that cleans the surface of the intermediate transfer belt 21, and 27 denotes a reference mark (determining the start reference timing of the printing operation) formed at a predetermined position on the outer peripheral surface of the intermediate transfer belt 21. It is a mark detection sensor for detection.

  The intermediate transfer belt 21 in this example is supported by being wound around a plurality of rolls 22 (a, b, c, d, e,...) And is wound around an outer peripheral surface serving as a transfer position of the photosensitive drum 1. In such a state (wrapped state), it is installed so as to be driven and rotated in synchronization with the photosensitive drum 1. The primary transfer roll 5 is disposed in contact with the inner peripheral surface portion of the intermediate transfer belt 21 that is in contact with the photosensitive drum 1. The roll 22a is a backup roll constituting a part of the secondary transfer portion, the roll 22d is a wrap-in roll, and the roll 22e is a wrap-out roll.

  The secondary transfer roll 25 is formed by forming an elastic layer or the like on a roll base material such as metal, and has an outer periphery that becomes a secondary transfer position of the intermediate transfer belt 21 (a belt portion supported by the backup roll 22a). It is arranged so that it can contact the surface. Further, the secondary transfer roll 25 is supported so as to be able to come into contact with and separate from the intermediate transfer belt 21 and contacts the intermediate transfer belt 21 only when the secondary transfer process is executed. Further, when the secondary transfer roll 25 is in the secondary transfer process, a secondary transfer voltage is applied to the roll base material from a secondary transfer power source (not shown). As the secondary transfer bias, a voltage having a polarity opposite to the charging polarity of the toner is applied.

  Further, the fixing device 40 includes a box-shaped housing (cover) 41 in which an inlet and an outlet of the paper 13 are opened, a heating roll 42 installed so as to be driven to rotate inside the housing 41, and the heating roll 43. And a pressure roll (or belt) 43 that is driven to rotate so as to form a fixing nip, and a discharge roll pair 44 for discharging the sheet 13 after fixing.

  In such a printer, image formation (printing) is performed as follows.

  First, when a detection signal from the mark detection sensor 27 is received after receiving a print start command, the surface of the photosensitive drum 1 in the image forming unit 10 has a predetermined polarity (in this example, a negative polarity) by the charging roll 2 at a predetermined timing. ), The laser beam LB corresponding to the image signal is scanned and exposed from the ROS 3 to form an electrostatic latent image. Thereafter, the electrostatic latent image is developed by a developer (in this example, toner of a two-component developer) supplied from a predetermined developing device 4 (Y, M, C, K) in a rotation switching type developing device. Thus, a toner image of a predetermined color is formed. In this development, the developer is conveyed to the development area by the developing roll 4a of each developing device, and the toner component is applied only to the latent image portion of the photosensitive drum 1 in a state where the development bias is applied in the development area. It is done by moving toward and adhering electrostatically.

  In particular, in the image forming unit 10, the above-described charging, exposure, and development processes are repeated a predetermined number of times according to the color of the image to be formed. For example, in a print mode (full color mode) that forms a full color image composed of toner images of a total of four colors of yellow, magenta, cyan, and black, the charging, exposure, and development processes correspond to the four colors. The toner images corresponding to the four colors constituting the full color image are sequentially formed on the surface of the photosensitive drum 1. It is also possible to select and execute a print mode (monochrome mode) for forming only a black and white image consisting of a black toner image. When this mode is selected, a black toner image is formed. Each process of charging, exposure, and development is executed only once.

  Subsequently, the toner image formed on the photosensitive drum 1 of the image forming unit 10 is primarily transferred to the intermediate transfer belt 21 in the intermediate transfer unit 20 when passing through the primary transfer position. The primary transfer at this time is transferred to the intermediate transfer belt 21 side by receiving an electrostatic action (electrostatic force) due to the primary transfer bias in which the toner on the photosensitive drum 1 is mainly applied to the primary transfer roll 5. It is done by being held. When the full color mode is selected, a plurality of color toner images sequentially formed on the photosensitive drum 1 are multiple-transferred in a state of being sequentially superimposed on the intermediate transfer belt 21. After completion of the primary transfer process, the photosensitive drum 1 is cleaned by removing residual toner and the like adhering to the surface thereof by the drum cleaner 6.

  When the toner image primarily transferred to the intermediate transfer belt 21 passes through the secondary transfer position, the toner image passes from the paper supply unit (not shown) between the intermediate transfer belt 21 and the secondary transfer roll 25 (secondary transfer position). Then, the toner image is secondarily transferred to the paper 13 that is supplied and fed by the registration roll 11 at a predetermined timing. In the secondary transfer at this time, the toner image receives an electrostatic action (electrostatic force) due to the secondary transfer bias applied to the secondary transfer roll 25 and moves to the sheet 13 side to be electrostatically held. Is done. After the secondary transfer process is completed, the secondary transfer roll 25 moves in a direction away from the intermediate transfer belt 21 and the intermediate transfer belt 21 is cleaned by the belt cleaner 26.

  Finally, the sheet 13 on which the toner image is secondarily transferred passes through the secondary transfer position and is conveyed toward the fixing device 40. In the fixing device 40, the sheet 13 after the secondary transfer is heated and pressed by being introduced into and passing through the fixing nip between the rotating heating roll 42 and the pressing roll 43. As a result, the toner image on the paper 13 is melt-pressed and thermally fixed to the paper 13. In the single-sided print mode, the sheet 13 after fixing is discharged as it is to a discharge tray section (not shown) by the discharge roll pair 44. A dashed-dotted line in FIG. 1 indicates a basic conveyance path of the paper 13.

  FIG. 2 shows a part of the control system in this printer.

  Reference numeral 50 in the figure is a system control device for controlling various operations (mainly printing operations) of the entire printer. The control device 50 includes an arithmetic processing device, a storage device, and the like, and follows a predetermined control program stored in the recording device triggered by an input instruction signal from the outside or a detection signal from a sensor. Executes the control operation related to the controlled object.

  As shown in FIG. 2 in particular, the control device 50 has a function capable of performing control to change the value of the charging bias of the charging power source 12 that applies the charging bias to the charging roll 2. Yes. This control basically changes the value of the charging bias in the charging power source 12 so that the charging potential of the photosensitive drum 1 increases during printing. In practice, the applied voltage value in the charging power source 12 is set in advance. It is configured to execute according to a prepared charging bias change control pattern (control program).

  Further, the control device 50 includes a detection signal (reference signal) from the mark detection sensor 27 and selection mode information from a print mode management means 51 such as an image processing device for grasping information on a print mode to be printed. Is input.

  For example, when a full-color print mode for forming a full-color image is selected, the control device 50 includes the four colors (Y, M, C, and K) constituting the full-color image as shown in FIG. The setting is such that control is performed to change the charging bias (actually the value of the DC voltage) so that the respective charging potentials of the photosensitive drum 1 on which the toner images are to be formed continuously increase.

The rate of continuous increase at this time is continuous with the charging bias start voltage (V _S ) (voltage applied to the tip of the region where the first color toner image is formed) as shown in FIG. The voltage difference (ΔV ₁ = V _E1 −V _S ) with respect to the final reached voltage (V _E1 ) (voltage applied to the terminal portion of the region where the final color toner image is formed) when the voltage is increased is 10 Assuming that it is about ˜20 V, the charged potential is in a directly proportional state (ΔV ₁ / V) within the required time (Ta) required to form a four-color toner image constituting a full-color image to be formed on one side of the paper 13. It is set so as to increase continuously with the slope of Ta). Further, the start voltage is set to a known voltage value used as a normal charging voltage.

When the monochrome print mode in which a plurality of monochrome images are continuously formed is selected, as shown in FIG. 4, the charged potential of the photosensitive drum 1 portion that forms the black toner image constituting the monochrome image is displayed. Is set to execute a control to change the charging bias so as to continuously increase. The rate of continuous increase at this time is based on the premise that the respective charged potentials of the black toner images that are continuously formed have the potential difference (ΔV ₂ = V _E2 −V _S ) corresponding to the order of formation. The charging potential is in a directly proportional state (ΔV ₂ / V) within the time (Tb) required for forming all of the plurality of black toner images for each black and white image to be continuously formed on one side of the plurality of sheets 13. (Slope of Tb) is set to be continuous.

  Note that the charging bias application start timing is, for example, when a predetermined elapsed time t1 from when the detection signal (reference signal) when the mark detection sensor 27 detects the reference mark on the intermediate transfer belt 21 is input is reached. Set to The start timing for increasing the charging bias reaches, for example, a predetermined elapsed time t2 set in time for the start of writing the electrostatic latent image of the first color toner image from the input of the reference signal. It is set at the time.

  Next, a printing operation when the control related to the change of the charging bias is performed will be described.

  When the full color print mode is selected, the control device 50 performs a control operation for applying the charging bias as shown in FIG. 3 according to the “charging control for changing the charging bias for the full color mode”.

That is, when a time t1 elapses after the reference start of the reference signal in response to the print start command, the charging bias of the start voltage V _S starts to be applied from the charging power source 12 to the charging roll 2, and then the reference signal When the time t2 elapses from the input time point, a charging bias is applied to the charging roll 2 such that the charging potential continuously increases in the above-described direct proportional state. The charging bias that has started to be applied is applied until the voltage is finally increased to the final voltage (V _E1 ) within the required time Ta. After reaching the final voltage (V _E1 ), the application of the charging bias is finished at a predetermined elapsed time t3.

  As a result, the photosensitive drum 1 is charged so that the charging potential continuously increases in accordance with the application of the continuously increasing charging bias, and then charged so that the charging potential continuously increases. The yellow (Y), magenta (M), cyan (C), and black (K) toner images are transferred to the formed photosensitive drum 1 through the process of transferring the toner image of the color formed in advance. As described above, the layers are sequentially formed.

  In particular, at this time, the charged potential of the photosensitive drum 1 portion where the toner image (including the electrostatic latent image) of the subsequent color (for example, M, C, K) formed after the second is formed precedes that. Thus, the relationship is always higher than the charged potential of the photosensitive drum 1 portion where the toner image of the color (Y, M or C) formed in this way is formed. Then, when a plurality of color toner images are continuously formed on such a single photosensitive drum 1, charging is performed such that the charging potential increases, and this occurs in the transfer process when the preceding color toner image is formed. The charge history on the photosensitive drum 1 can be eliminated or reduced by a charging process in which the charging potential is increased when a toner image of the subsequent color is formed. As a result, in the finally obtained full-color image, for example, an image defect such as a transfer memory ghost in which a toner image of a color formed in advance is visually recognized does not occur.

  When the monochrome print mode is selected, the control device 50 executes a control operation for applying the charging bias as shown in FIG. 4 according to the “charging control for changing the charging bias for the monochrome mode”. . Here, as a monochrome image to be continuously formed, for example, a similar monochrome image formed on two preceding sheets is a solid print (no background color: paper background color) image such as a character image or a grid, and the subsequent Assume that the black-and-white image formed on one sheet of paper is a halftone image.

That is, when a time t1 elapses from the input of the reference signal in response to the print start command, the charging bias of the start voltage V _S starts to be applied from the charging power source 12 to the charging roll 2, and then the input of the reference signal is performed. When the time t2 elapses from the time point, a charging bias is applied to the charging roll 2 so that the charging potential continuously increases in the above-described direct proportional state. The charging bias that has started to be applied is applied until the voltage is finally increased to the final voltage (V _E2 ) within the required time Tb. After reaching the final voltage (V _E2 ), the application of the charging bias is terminated at a predetermined elapsed time t3.

  As a result, the photosensitive drum 1 is charged so that the charging potential continuously increases in accordance with the application of the continuously increasing charging bias, and thereafter, the charging potential is continuously increased. Two black toner images as black-and-white images such as letters and black toner images as halftone images are transferred to the photosensitive drum 1 that has been previously formed through the transfer process of the black toner image formed in advance. Are sequentially formed.

  In particular, at this time, the charged potential of the photosensitive drum 1 portion where the black toner image (including the electrostatic latent image) for the subsequent halftone image to be formed third is formed in advance. The relationship is always higher than the charging potential of the photosensitive drum 1 portion where the black toner images for two character images are formed. Then, when a monochromatic toner image is continuously formed on such a single photosensitive drum 1, charging is performed so that the charging potential is increased, so that the toner image of the preceding monochrome image (character image or the like) is formed. The charge history on the photosensitive drum 1 generated in the transfer process can be eliminated or reduced by a charging process in which a charging potential is increased when a subsequent black and white (particularly halftone image) toner image is formed. As a result, in the halftone black-and-white image obtained at the end, an image defect such as a transfer memory ghost in which a toner image such as a character image formed in advance is visually recognized does not occur.

  A mechanism by which the generation of the transfer memory ghost is suppressed when the printing operation with the above control is performed will be described with reference to FIG.

FIG. 5 schematically shows the transition of the surface potential state of the photosensitive drum 1 (image carrier) during the printing operation. In this figure, similarly to the case described in Patent Document 1, the exposed portion on the surface of the photosensitive drum charged with negative polarity is developed by the reverse development method with the negatively charged toner, and transferred with positive polarity. This represents a case where transfer is performed by bias. Symbols a _{1 to} a ₄ in the figure indicate certain same portions of the surface of the photosensitive drum. Similarly, symbols b _{1 to} b ₄ and c _{1 to} c ₄ are different from the symbol a of the photosensitive drum. The same part is shown.

In FIG. 5, symbols a ₁ and c ₁ are non-exposed portions (non-image portions) after the exposure process of the photosensitive drum, and the surface potential is the same as the surface potential after charging of the photosensitive drum. Here, first, description will be made assuming a case of constant charging in which charging is always performed at the same charging potential. Reference numeral b ₁ denotes an exposure portion (image portion) after the exposure process of the photosensitive drum, and its surface potential moves to the positive polarity side by exposure. Then, in the developing process, a potential difference (V ₁ ) with the developing bias potential Vd of the sleeve of the developing device 4 is generated in the developing portion b ₁ , and the toner is electrostatically attached to develop the exposed portion b ₁ .

  Thereafter, in the transfer process, transfer is performed in a state where there is no toner in the non-exposed area on the photosensitive drum and there is toner in the exposed area. During this transfer, a positive transfer bias is applied to transfer the negatively charged toner on the photosensitive drum.

As a result, a positive polarity charge is applied to the photosensitive drum, and both the exposed portion and the non-exposed portion are shifted to the positive polarity side as a whole. A difference is generated between the non-exposed portion where no toner is present, and therefore, a potential difference (D1) is generated between the potential of the non-exposed portions a ₂ and c ₂ after the transfer and the exposed portion b ₂ . This remains as a positive charge history.

After this if you: charging and exposure processes, in the case where non-exposed portions a ₂ to c ₂ ahead becomes non-exposed portion a _{3'~c 3} 'indicated by the two-dot chain line, the non-exposed The potential difference in the parts a ₃ ′ to c ₃ ′ is D3, and there is almost no potential difference. On the other hand, the non-exposed portion a ₂ to c ₂ of previously exposed portion a _{4'~c 4} indicated by the two-dot chain line 'in case you, the exposed portion a _{4'~c 4'} potential difference is the non-exposed portion a _{3'~c 3} 'portion serving potential difference larger than the potential difference D3 of D2 (> D3) becomes, which is a positive polarity charge history in the latent image.

When development is performed in such a state, a difference in development density occurs in a portion corresponding to the stepped portion that becomes the potential difference D2 of the surface potential, and this appears as a transfer memory ghost in the subsequent image. Become. The difference in development density at this time is that the potential difference between the surface potential of the photosensitive drum after exposure after the transfer process and the development bias potential Vd is “V ₂ ′”, and this potential difference is higher than the previous potential difference (V ₁ ). Since the toner becomes larger, the toner adheres as much as the difference, and as a result, it becomes easier to visually recognize the density difference. In general, when the density is increased, there is a characteristic that the transfer memory ghost is easily visually sensed even with the same potential difference.

On the other hand, in the printing operation accompanied by the control by the printer, charging is performed such that the charging potential continuously increases. Therefore, the surface potential of the photosensitive drum after the exposure after the transfer process is the same as the previous non-printing potential. When the exposed portions a _{2 to} c ₂ become non-exposed portions a _{3 to} c ₃ indicated by solid lines, the potential in the non-exposed portions a _{3 to} c ₃ is increased as a whole (shifted to the negative polarity side). In addition, the potential difference almost disappears.

On the other hand, when the previous non-exposed portions a _{2 to} c ₂ become the exposed portions a _{4 to} c ₄ indicated by solid lines, the potential difference in the exposed portions a _{4 to} c ₄ becomes “D4”, which is a constant charge. In this case, the potential difference D2 between a ₄ ′ and c ₄ ′ is reduced. In addition, the potential difference from the developing bias potential Vd of the exposed portions a _{4 to} c ₄ is also “V ₂ ”, which is substantially the same level as the potential difference (V ₁ ) from the previous developing bias potential. That is, the positive charge history generated in the preceding transfer process is lost or reduced by increasing charging. Thus, the result density difference even when the exposure unit a ₄ to c ₄ and developed thereafter hardly occurs, transfer memory ghost will be prevented as a result.

  For example, when the full-color print mode is selected, the control device 50 is composed of the four colors (Y, M, C, K) constituting the full-color image as shown in FIG. It can also be set to execute control of changing the charging bias so that each charging potential of the photosensitive drum 1 portion where the toner image is to be formed increases stepwise.

In this case, the rate of increase in a stepwise manner may be increased in a stepwise manner with a predetermined potential difference (α) for each toner image forming region formed after the second. For example, as shown in the figure, the charging bias start voltage (V _S ) to be applied and the final voltage (V _E3 ) when increased stepwise (the end of the region where the final color toner image is formed) Assuming that the voltage difference (ΔV ₃ = V _E3 −V _S ) is approximately 10 to ₃₀ V, the voltage difference must be increased in steps (the number of toner images to be formed). A value evenly allocated by a value obtained by dividing 1 from the total) can be applied as the potential difference (α).

  Even when a plurality of color toner images are continuously formed on one photosensitive drum 1 as described above, even when charging is performed such that the charging potential increases stepwise, the preceding color toner image is formed. The charge history on the photosensitive drum 1 generated in the transfer process can be eliminated or reduced by a charging process in which the charging potential when forming a subsequent color toner image is increased stepwise. As a result, in the finally obtained full-color image, there is no occurrence of a transfer memory ghost in which a toner image of a color formed in advance is visually recognized.

  The color printer according to Embodiment 1 is a so-called four-cycle color printer that repeats a series of steps of charging, exposure, development, and transfer four times when forming a full-color image. In the above color printer, when the full color print mode is selected, charging in which the charging potential increases continuously or stepwise as described above is effective in preventing the transfer memory ghost.

  In other words, in this four-cycle color printer, when forming a full-color image, toner images of different colors are sequentially transferred onto the intermediate transfer belt (intermediate transfer member) 21, so the second and subsequent toner images are formed. As the toner image is transferred, there is a portion where the toner is transferred to the intermediate transfer belt in a bulky manner. Therefore, there is a difference in how positive charge is applied to the photosensitive drum by the transfer bias depending on the presence or absence of the toner. As a result, the charge history tends to occur. On the other hand, when increasing charging as described above, since the charging potential with respect to the toner image formed at least after the second is increased as the back is increased, the charge history generated during the preceding transfer is followed. This charging process can be effectively eliminated or reduced. This is effective in preventing a transfer memory ghost.

[Embodiment 2]
FIG. 7 shows an outline of a full-color printer according to Embodiment 2 of the present invention.

  In this full-color printer, the image forming unit 10 in Embodiment 1 is changed to the image forming units 10Y, 10M, 10C, and 10K that exclusively form the four color (Y, M, C, and K) toner images, The printer has the same configuration as that of the printer (FIG. 1) according to the first embodiment, except that the sheet conveying belt unit 30 is applied instead of the intermediate transfer unit 20 according to the first embodiment. For this reason, in the drawings, the same components as those in the printer according to the first embodiment are denoted by the same reference numerals, and the description thereof will be omitted in principle below.

  The image forming units 10Y, 10M, 10C, and 10K all correspond to the charging roll 2 and ROS 3 (which are shown in common but may be individually provided) with respect to the periphery of the photosensitive drum 1. The developing devices 4Y, 4M, 4C, and 4K that store the color developer, the transfer roll 5, and the cleaning device 6 are arranged, and the image forming units 10 are arranged in series in the vertical direction. . This type of image forming apparatus is generally called a tandem image forming apparatus.

  The paper transport belt unit 30 includes an endless paper transport belt 31 that can electrostatically attract and transport the paper 13, and a plurality of rolls 32 and 33 that rotatably stretch the paper transport belt 31. The main part is composed of a charging suction roll 34 for electrostatically adhering the paper 13 to the paper transport belt 31. The paper transport belt 31 transports the paper 13 so as to pass through the transfer position (nip portion with the transfer roll 5) of each photosensitive drum 1 of the image forming unit 10 (Y, M, C, K). is set up.

  The printing operation by this color printer is basically performed as follows.

  When printing a full-color image, first, a toner image is formed in each image forming unit 10 (Y, M, C, K) in substantially the same manner as in the case of the image forming unit 10 in the first embodiment. In addition, the toner images for each of the four colors are individually formed. Next, the sheet 13 conveyed from the sheet feeding unit in accordance with the toner image formation timing is adsorbed to the sheet conveying belt 31 by the charging adsorption roll 34, and the transfer positions of the respective image forming units 10 (Y, M, C, K). The toner images in the respective image forming units are sequentially electrostatically passed on the paper 13 on the paper transport belt 31 (in the order of Y, M, C, K). Transcribed. After the transfer, the sheet 13 is peeled off from the sheet conveying belt 31 and then conveyed to the fixing device 40 for fixing processing. As a result, a full-color image is formed on one side of the paper 13. In addition, when printing a black and white image, only the image forming unit 10K creates a black toner image and transfers the toner image onto the paper 13 conveyed by the paper conveying belt 31.

  FIG. 8 shows a part of the control system in this printer.

  Reference numeral 50 in the figure is a control device having basically the same configuration as the control device 50 in the first embodiment. Further, in this control device 50, as shown in the figure, each charging bias of the charging power source 12 for applying a charging bias to each charging roll 2 (Y, M, C, K) in each image forming unit 10 is provided. The function which can perform control which changes the value of is provided. This control basically changes the value of each charging bias in the charging power source 12 so that the charging potential of the photosensitive drum 1 in each image forming unit 10 increases during printing.

  For example, when the full-color print mode for forming a full-color image is selected, the control device 50, as shown in FIG. 9, the four colors (Y, M, C, K) constituting the full-color image. ) Is configured to execute a control to change the charging bias so that the charging potential of each photosensitive drum 1 unit for individually forming the toner image increases continuously.

The rate of continuous increase at this time is continuous with the charging bias start voltage (V _S ) (voltage applied to the tip of the region where the toner image is formed on each photosensitive drum) as shown in FIG. Assuming that the voltage difference (ΔV ₄ = V _E4 −V _S ) with respect to the final ultimate voltage (V _E4 ) when increased to about 10 to 30 V, one color to be formed on one side of the paper 13 The charging potential is set to continuously increase in a directly proportional state (inclination of ΔV ₄ / Tc) within a time required for forming a toner image (Tc: common to all colors). Further, the start voltage is set to a known voltage value used as a normal charging voltage.

  When the monochrome print mode for continuously forming a plurality of monochrome images is selected, as shown in FIG. 4, the black toner constituting the monochrome image is controlled in the same manner as the control contents of the first embodiment. The control is performed to change the charging bias so that the charging potential of the photosensitive drum 1 portion of the image forming unit 10K for forming an image continuously increases.

  Next, a printing operation when the control related to the change of the charging bias is performed will be described.

  When the full color print mode is selected, the control device 50 performs a control operation for applying the charging bias as shown in FIG. 9 according to the “charging control for changing the charging bias for the full color mode”.

That is, when the time t4 has elapsed from the input time point of the reference signal when the detection sensor 27 detects the reference mark of the paper transport belt 31 in response to the print start command, the charging bias of the starting voltage V _S is 4 from the charging power source 12. In the same manner, each charging roll 2 (Y, M, C, K) of the two image forming units 10 starts to be applied. Subsequently, when time t5 elapses from the input time of the reference signal, the charging potential continuously increases in the above-described direct proportional state with respect to the charging roll 2Y of the image forming unit 10Y on the most upstream side in the sheet conveyance direction. Such a charging bias is applied. Then, the charging bias that has started to be applied is applied until it finally increases to the final voltage (V _E4 ) within the required time Tc, and after reaching the final voltage (V _E4 ), the printing is completed. After a predetermined time has elapsed from the time, the application of the charging bias is terminated (this is the same for the application of the charging bias to the other charging rolls 2M, 2C, 2K).

  Further, when the time t6 elapses from the input time point of the reference signal, the charging potential continuously increases in the above-described direct proportional state with respect to the charging roll 2Y of the image forming unit 10M located downstream in the paper transport direction. A suitable charging bias is applied. Thereafter, when a predetermined time elapses from the input point of the reference signal, the respective charging potentials of the charging rolls 2C and 2K in the image forming units 10C and 10K located further downstream in the sheet transport direction increase in the same state. The charging bias is sequentially applied.

  As a result, the photosensitive drum 1 of each image forming unit 10 is charged so that the charging potential continuously increases in accordance with the application of the continuously increasing charging bias. As described above, toner images of the same color are sequentially formed on the respective photosensitive drums 1 that are charged so as to increase continuously, through a transfer process of a toner image formed in advance.

  When the toner images of different colors are successively formed on the plurality of photosensitive drums 1 as described above, charging is performed so that the charging potential is increased. The charge history generated in the transfer process can be eliminated or reduced by the charging process in which the charging potential is increased when the subsequent toner image is formed. As a result, in the finally obtained full-color image, for example, an image defect such as a transfer memory ghost in which a toner image of a color formed in advance is visually recognized does not occur.

  As for the control device 50, for example, when the full color print mode is selected, as shown in FIG. 10, the four color (Y, M, C, K) toner images constituting the full color image are displayed. In addition to continuously increasing the charging potential of each unit of each photosensitive drum, the charging potential of each image carrier on which each color toner image is formed corresponds to the order of transfer of each toner image. Thus, the charging bias (charging bias at the start of application) of the second and subsequent charging means (charging rolls 2M, 2C, 2K) can be set so as to increase in a stepwise manner.

In this case, the stepwise increase rate is determined by setting the final voltage (V _E4 , V _E5 , V _E6 ) of the charged potential on the photosensitive drums 10Y, 10M, and 10C on which the toner images are formed in advance to the subsequent photosensitive. The drums 10M, 10C, and 10K start to be applied as they are to the charging rolls 2M, 2C, and 2K as starting reference voltages for the charging bias. After that, the respective starting reference voltages are applied so as to be continuously increased at the same increasing rate, and finally, the respective charging potentials of the photosensitive drums 10M, 10C, and 10K become the final ultimate voltages (V _E5 , V _E6). , V _E7 ).

FIG. 3 is an explanatory diagram illustrating a main part of the color printer according to the first embodiment. FIG. 2 is an explanatory diagram illustrating a configuration of a control system having a control function for changing a charging bias in the first embodiment. It is explanatory drawing which shows the example of a change pattern of the charging bias at the time of full color mode selection. It is explanatory drawing which shows the example of a change pattern of the charging bias at the time of monochrome mode selection. It is explanatory drawing which shows typically transition of the surface potential of a photosensitive drum. It is explanatory drawing which shows the other example of the change pattern of the charging bias at the time of full color mode selection. FIG. 10 is an explanatory diagram illustrating a main part of a color printer according to a second embodiment. FIG. 10 is an explanatory diagram showing a configuration of a control system having a control function for changing a charging bias in the second embodiment. It is explanatory drawing which shows the example of a change pattern of the charging bias at the time of full color mode selection. It is explanatory drawing which shows the other example of the change pattern of the charging bias at the time of full color mode selection.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Photosensitive drum (image carrier), 2 ... Charge roll (charging means), 3 ... ROS (image exposure means), 4 ... Developing apparatus (developing means) 5 ... Transfer roll (transfer means), 10 ... Image forming unit (Imaging device), 12 ... charging power source, 13 ... paper (transferred member), 21 ... intermediate transfer belt (transferred member), 50 ... control device (control means).

Claims (6)

An image bearing member; a charging unit that charges the image bearing member by applying a charging bias; an image exposing unit that forms an electrostatic latent image on the charged image bearing member based on image information; and In an image forming apparatus comprising: a developing unit that develops an image with toner to form a toner image; and a transfer unit that applies a transfer bias to transfer the toner image to a transfer member.
An image forming apparatus comprising: a control unit that changes a charging bias of the charging unit so that a charging potential of the image carrier increases during image formation. Forming a one-color toner image on the image carrier and forming a single-color image composed of the one-color toner image;
The control means changes the charging bias so that the charging potential of the image carrier corresponding to the toner image forming area constituting the monochrome image continuously increases when the monochrome image is formed by the function. The image forming apparatus according to claim 1. A function of sequentially forming a plurality of color toner images on the image carrier and forming a multicolor image composed of the plurality of color toner images;
In addition, when the multi-color image is formed by the function, the control unit is configured so that each charging potential of the image carrier corresponding to each toner image forming area constituting the multi-color image continuously increases. The image forming apparatus according to claim 1, wherein the charging bias is changed. A function of sequentially forming a plurality of color toner images on the image carrier and forming a multicolor image composed of the plurality of color toner images;
In addition, when the multi-color image is formed by the function, the control means is configured such that each charging potential of the image carrier corresponding to each toner image forming area constituting the multi-color image is stepped between the forming areas. The image forming apparatus according to claim 1, wherein the charging bias is changed so as to increase. A plurality of image forming apparatuses having the image carrier, charging means, image exposing means, developing means and transfer means are provided, and toner images of different colors are respectively provided on the image carriers of the respective image forming apparatuses. Having the function of forming and forming a multicolor image composed of the respective toner images,
In addition, when the multi-color image is formed by the function, the control means is configured so that the charging potential of each image carrier on which each toner image constituting the multi-color image is continuously increased increases. The image forming apparatus according to claim 1, wherein the charging bias of each of the charging units is changed.   When the multi-color image is formed by the function, the control means is configured so that the charging potential of each image carrier on which each toner image constituting the multi-color image is formed corresponds to the order of transfer of each toner image. The image forming apparatus according to claim 5, wherein the charging bias of the charging unit of each of the image forming devices is changed so that the relationship increases.

Images