JP2005031587A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus that can perform excellent image formation by preventing characteristic defects concerned with image formation of an image carrier and application defects of a transfer bias. <P>SOLUTION: A transfer means of transferring images formed on image carriers 3 (3a to 3d) to a transfer medium is equipped with transfer members 6 (6a to 6d) which press the transfer medium against the image carriers 3 to apply a high voltage and high-voltage power units A, B, C, and D which apply the high voltage to the transfer members. Further, the transfer means is placed in operation to apply the high voltage to the transfer members 6 even when the transfer medium is not present between the image carriers 3 and transfer members 6, and a control part is provided which controls the timing wherein the transfer means is operated in response to the rotation starting operation of the image carriers 3 according to the surface potentials of the image carriers 3. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a copying machine, a facsimile machine, or a printer, and relates to an image forming apparatus that transfers an image formed on an image carrier onto a sheet, and more particularly, a transfer for transferring an image onto a sheet. The present invention relates to voltage control technology.

  Conventionally, an image forming apparatus forms an image on a rotating image carrier based on input image data from an installed image reading apparatus or input image data transmitted from an external terminal, and develops the image. The image is formed by the means, the visualized image is transferred to a sheet, and the transferred image is then fixed on the sheet. In performing such image formation, when a visualized image formed on an image carrier is transferred to a sheet, a transfer bias is charged (applied) from the back surface of the sheet.

  As this transfer bias, a very high voltage of about several KV is required. If there is no sheet between the image carrier and the transfer means, charging the transfer bias adversely affects the characteristics of the photosensitive drum. There is a problem that the image quality deteriorates during image formation. Therefore, it has been attempted to control so that the transfer bias is charged only when the sheet exists between the image carrier and the transfer unit in accordance with the sheet conveyance timing.

  However, it is very difficult to accurately match the transfer bias to the sheet conveyance timing. If the transfer bias charge is delayed, leading edge transfer omission occurs, and the transfer bias charge is too fast. This gives a history to the image carrier and causes image formation defects. Therefore, when there is no sheet between the image carrier and the transfer means, the transfer bias voltage is stabilized by applying a voltage lower than the transfer bias applied during normal transfer as the paper-to-paper voltage, An adverse effect on the image bearing member has been reduced to perform good image formation.

  Further, at the time of non-image formation where no transfer voltage is applied (between sheets), the transfer roller is charged to the negative side due to the influence of the surface potential of the image carrier charged to the negative side. Under such conditions, even when a transfer voltage is suddenly applied to the transfer roller during image formation, a good rise cannot be obtained and a sufficient potential cannot be obtained.

  Therefore, when there is no sheet between the image carrier and the transfer means, a voltage lower than the transfer bias applied during normal transfer is applied as the paper-to-paper voltage, thereby stabilizing the timing of rising of the transfer bias and at the same time. A transfer voltage control technique has been proposed in which adverse image effects such as damage to the carrier can be reduced and good image formation can be performed (see, for example, Patent Documents 1, 2, and 3).

JP-A-2-39181 (Specification, page 3, upper right column, line 11 to page 4, upper left column, line 8, line 3) Japanese Patent Laid-Open No. 5-150577 (paragraphs “0021” to “0023”, FIGS. 1 and 2) JP-A-10-142893 (paragraphs “0044” to “0047”, FIG. 1)

  By the way, when a voltage lower than the transfer bias is applied as the paper-to-paper voltage, an electrostatic latent image is formed between the charging means for charging the image carrier and the transfer means such as a transfer roller for charging the transfer bias based on the image data. Since a writing unit for forming an image and a developing unit for visualizing the formed electrostatic latent image are disposed, the charging unit and the transfer unit are separated from each other by a predetermined distance.

  Therefore, when the image forming unit starts operation from the stopped state, a predetermined time has elapsed after the image carrier is charged so that the inter-paper voltage is charged in accordance with the charged portion of the image carrier. Control is performed so that the voltage between the sheets is charged by the transfer unit (after the charging point reaches the transfer point with a predetermined delay from the time when the image carrier is charged).

  However, when the operation of the image forming unit is stopped instantaneously due to a sheet conveyance jam or the like, a charge already exists at the transfer point of the image carrier due to the charging operation of the charging unit. In particular, when the jam processing or the like is completed in a short time, the electric charge does not escape much from the image carrier and the potential is high.

  Therefore, as described above, when the transfer means is activated and the inter-sheet voltage is charged after a predetermined time has elapsed after the image carrier is charged (after a predetermined time delay), the image carrier remains. Due to the influence of the charged electric charge, there arises a problem that the transfer bias voltage applied by the transfer means cannot be stably supplied. In such a case, it goes without saying that the image quality deteriorates.

  The present invention has been made in view of such circumstances, and by controlling the charging timing of the transfer bias between papers at the start of the operation of the image forming unit, it is possible to apply characteristic imperfections related to image formation of the image carrier and application of the transfer bias. An object of the present invention is to provide an image forming apparatus capable of preventing defects and forming a good image.

  In the present invention, means for solving the above-described problems are configured as follows.

  (1) A transfer member in which a transfer unit that transfers an image formed on a charged and rotationally driven image carrier onto a transfer medium applies a high voltage by pressing the transfer medium against the image carrier And a high voltage power supply device for applying a high voltage to the transfer member, and the high voltage is applied to the transfer member even when the transfer medium is not present between the image carrier and the transfer member. In order to apply the voltage, a control unit that operates the transfer unit is provided, and the control unit determines the timing from the start of rotation of the image carrier to the operation of the transfer unit according to the surface potential of the image carrier. A control unit for controlling is provided.

  In this configuration, since the timing from the start of rotation of the image carrier to the operation of the transfer means is controlled according to the potential of the surface of the image carrier, the transfer operation can always be stabilized.

  When the image forming operation is stopped for a long time, the surface potential of the image carrier is low and almost close to 0 potential. In that case, a high voltage is applied to an uncharged portion of the image carrier by starting the operation of the transfer means (application of a high voltage) at a predetermined timing from the start of rotation of the image carrier. Thus, the transfer means can be started in a normal state without damaging the image carrier, and a transfer bias (transfer voltage) can be stably applied.

  Also, when an emergency stop occurs when a trouble such as a jam occurs, or when image formation is finished and image formation is started again immediately after stopping, the surface potential of the image carrier may be high. Sometimes, by starting the operation of the transfer unit simultaneously with the rotation of the image carrier, the transfer unit can be quickly started up in a normal state without causing idle time, and the transfer operation can be stabilized.

  (2) When the control unit determines that the stop time of the image carrier is long and the surface potential of the image carrier is lower than a predetermined value, a predetermined time has elapsed from the start of rotation of the image carrier. The operation of the transfer means is started later, and when it is determined that the stop time of the image carrier is short and the surface potential of the image carrier is higher than the predetermined value, the rotation of the image carrier is started. At the same time, the operation of the transfer means is started.

  In this configuration, when it is determined that the stop time of the image carrier is long and the surface potential of the image carrier is lower than a predetermined value, the transfer means after the predetermined time has elapsed from the start of rotation of the image carrier. By starting the operation, it is possible to avoid applying a high voltage to the uncharged portion of the image carrier, and the transfer means is in a normal state without damaging the image carrier. To stabilize the transfer operation.

  When the stop time of the image carrier is short and it is determined that the surface potential of the image carrier is higher than the predetermined value, the operation of the transfer unit is started simultaneously with the start of rotation of the image carrier. Thus, the transfer means can be quickly activated in a normal state without causing idle time, and the transfer operation can be stabilized.

  (3) The predetermined time is a time until a portion on the charged image carrier reaches a position corresponding to the transfer unit.

  In this configuration, a high voltage is applied to an uncharged portion of the image carrier by activating the transfer device after the charged portion on the image carrier reaches the transfer position corresponding to the transfer device. The transfer means can be started in a normal state and the transfer bias can be stably applied without damaging the image carrier.

  (4) The predetermined value is set to 13% to 16% of a rated surface potential of the image carrier.

  In this configuration, when the rated surface potential of the image carrier is, for example, −700 V, the predetermined value is approximately −100 V (−91 V to −112 V). Therefore, when the surface potential of the image carrier is lower than the predetermined value, the operation of the transfer means is started after a lapse of a predetermined time from the start of rotation of the image carrier, so that the uncharged portion of the image carrier High voltage can be avoided from being applied to the image carrier, and the transfer means can be activated in a normal state without damaging the image carrier, thereby stabilizing the transfer operation.

  When the stop time of the image carrier is short and the surface potential of the image carrier is higher than a predetermined value, idle time is generated by starting the operation of the transfer means simultaneously with the start of rotation of the image carrier. In addition, the transfer means can be quickly activated in a normal state to stabilize the transfer operation.

  (5) When it is determined that the stop time of the image carrier is short, the transfer medium has a trouble such as a jam of the transfer medium.

  In an electrophotographic image forming apparatus that performs reversal development, the image carrier is always charged by a charging means so as to have a predetermined potential while the image carrier is in operation. Therefore, even when the image carrier is stopped in normal operation, the surface potential of the image carrier may not be lowered so much when the stop time is short. Since the image carrier is started immediately after the jam processing, the time during which the image carrier is stopped is relatively short and the surface potential of the image carrier is often high. By operating, the transfer means can be activated in a normal state, and the transfer bias can be stably applied.

(6) The image forming apparatus further includes a surface potential meter for measuring the surface potential of the image carrier.
The control unit performs a control operation according to a result of a surface potential of the image carrier measured by the surface potential meter.

  In this configuration, the surface potential of the image carrier can be accurately measured by providing a surface electrometer, and the transfer means can be operated immediately by measuring before or simultaneously with the rotation of the image carrier. There is an effect that it is possible to surely determine whether to operate with timing.

  (7) The surface electrometer may be disposed between a charging point and a transfer point on the image carrier.

  In this configuration, since the surface potential meter is provided upstream of the transfer point (which is a high voltage application point by the transfer means), the surface potential of the image carrier upstream of the transfer point is increased. Thus, the transfer means can be operated, so that the transfer bias can be stably applied.

  (8) The surface electrometer is disposed between a charging point and a developing point in the image carrier.

  In this configuration, the transfer bias is further increased depending on the surface potential of the image carrier until the charged portion of the image carrier reaches the transfer point without being affected by development after exposure. It can be applied with good stability.

  (9) The surface electrometer is disposed between a charging point and an exposure point in the image carrier.

  In this configuration, since the charged portion of the image carrier reaches the transfer point without being affected by exposure and development, the portion with the highest surface potential of the image carrier can be detected. Further, it can be applied more stably.

  (10) The control unit is configured to set a voltage applied to the transfer medium by the transfer unit during an operation other than image transfer lower than a voltage applied to the transfer medium during image transfer. To do.

  In this configuration, the high voltage applied before or always before transferring the image on the image carrier is set lower than the transfer bias applied when the image formed on the image carrier is transferred, to the image carrier. Good image formation can be performed over a long period of time without causing damage.

  As is clear from the above description, the present invention has the following effects.

  (1) Since the timing from the start of rotation of the image carrier to the operation of the transfer means is controlled according to the potential of the surface of the image carrier, the transfer operation can always be stabilized and the image quality can be stabilized. Can be achieved.

  When the image forming operation is stopped for a long time, the surface potential of the image carrier is low and almost close to 0 potential. In that case, a high voltage is applied to an uncharged portion of the image carrier by starting the operation of the transfer means (application of a high voltage) at a predetermined timing from the start of rotation of the image carrier. Thus, the transfer means can be started in a normal state without damaging the image carrier, and a transfer bias (transfer voltage) can be stably applied.

  Also, when an emergency stop occurs when a trouble such as a jam occurs, or when image formation is finished and image formation is started again immediately after stopping, the surface potential of the image carrier may be high. Sometimes, by starting the operation of the transfer unit simultaneously with the rotation of the image carrier, the transfer unit can be quickly started up in a normal state without causing idle time, and the transfer operation can be stabilized. Thereby, the image quality can be stabilized.

  (2) When it is determined that the stop time of the image carrier is long and the surface potential of the image carrier is lower than a predetermined value, the operation of the transfer means after a predetermined time has elapsed from the start of rotation of the image carrier By starting the transfer unit, it is possible to avoid applying a high voltage to the uncharged portion of the image carrier, and start the transfer unit in a normal state without damaging the image carrier. Thus, the transfer operation can be stabilized, and the image quality can be stabilized.

  When the stop time of the image carrier is short and it is determined that the surface potential of the image carrier is higher than the predetermined value, the operation of the transfer unit is started simultaneously with the start of rotation of the image carrier. As a result, the transfer means can be quickly activated in a normal state without causing idle time, the transfer operation can be stabilized, and the image quality can be stabilized.

  (3) A high voltage is applied to an uncharged portion of the image carrier by activating the transfer unit after the charged portion on the image carrier arrives at the transfer position corresponding to the transfer unit. The transfer means can be started in a normal state without causing damage to the image carrier, and the transfer bias can be stably applied, and the image quality can be stabilized. .

  (4) When the rated surface potential of the image carrier is −700 V, for example, the predetermined value is approximately 100 V (91 V to 112 V). Therefore, when the surface potential of the image carrier is lower than the predetermined value, the operation of the transfer means is started after a lapse of a predetermined time from the start of rotation of the image carrier, so that the uncharged portion of the image carrier High voltage can be avoided, the transfer means can be started in a normal state without damaging the image carrier, and the transfer operation can be stabilized. Can be stabilized.

  When the stop time of the image carrier is short and the surface potential of the image carrier is higher than a predetermined value, idle time is generated by starting the operation of the transfer means simultaneously with the start of rotation of the image carrier. In addition, the transfer means can be started quickly in a normal state, the transfer operation can be stabilized, and the image quality can be stabilized.

  (5) In an electrophotographic image forming apparatus that performs reversal development, the image carrier is always charged by the charging means so as to have a predetermined potential while the image carrier is in operation. Even if the image carrier stops, if the stop time is short, the surface potential of the image carrier may not decrease so much. Since the image carrier is stopped for a short time in order to immediately start the image forming operation, the surface potential of the image carrier is often high, so the transfer means is operated simultaneously with the rotation of the image carrier. The transfer means can be activated in a normal state to stably apply the transfer bias, and the image quality can be stabilized.

  (6) Since the surface potential meter is provided, it is possible to accurately measure the surface potential of the image carrier, and by measuring before or simultaneously with the start of the rotation of the image carrier, it is possible to determine whether the transfer means is operated immediately. It is possible to reliably determine whether to take it and operate it. As a result, the image quality can be stabilized.

  (7) Since the surface potential meter is provided on the upstream side of the transfer point, the transfer operation can be started with respect to the portion where the surface potential of the image carrier is high, and the transfer bias can be stably applied. Image quality can be stabilized.

  (8) Since the surface potential meter is disposed between the charging point and the development point, the transfer operation can be started according to the height of the surface potential of the image carrier without being affected by the development. The transfer bias can be applied more stably, and the image quality can be stabilized.

  (9) Since a surface electrometer is disposed between the charging point and the exposure point, the portion where the surface potential of the image carrier is the highest can be detected without being affected by exposure and development. The transfer bias can be applied even more stably, and the image quality can be stabilized.

  (10) Damage to the image carrier by making the high voltage applied before or always before transferring the image on the image carrier lower than the transfer bias applied when transferring the image formed on the image carrier. Good image formation can be performed over a long period of time without giving.

  Hereinafter, an image forming apparatus according to an embodiment of the present invention will be described in detail with reference to the drawings.

Embodiment 1
<Multicolor image forming device>
FIG. 1 is an explanatory diagram showing a configuration of a multicolor image forming apparatus 100 as an image forming apparatus of the present invention. The image forming apparatus 100 forms multicolor and single color images on a predetermined sheet (transfer medium) in accordance with image data transmitted from the outside. The developing unit 2, the photosensitive drum 3, the charger 5, the cleaner unit 4, the transfer conveyance belt unit 8, the fixing unit 12, the sheet conveyance path S, the sheet feed tray 10, the sheet discharge trays 15 and 33, and the like. .

  The image data handled in the image forming apparatus 100 corresponds to a color image using each color of black (K), cyan (C), magenta (M), and yellow (Y). Accordingly, the exposure unit 1 (1a, 1b, 1c, 1d), the developing device 2 (2a, 2b, 2c, 2d), the photosensitive drum 3 (3a, 3b, 3c, 3d), and the charger 5 (5a, 5b, 5c, 5d), four transfer rollers 6 (6a, 6b, 6c, 6d), and four cleaner units 4 (4a, 4b, 4c, 3d), respectively, so as to form four types of latent images corresponding to the respective colors. Four image stations are configured, each having a set to black, b set to cyan, c set to magenta, and d set to yellow.

  A surface potential meter 23 (23a-23d) for measuring the surface potential of the photosensitive drum 3 (3a-3d) is positioned between the charging point and the transfer point, that is, the charger 5 (5a-5d). ) And the transfer roller 6 (6a to 6d). As will be described later, the transfer bias application timing is controlled according to the surface potential of the photosensitive drum 3 (3a to 3d) measured by the surface potential meter 23 (23a to 23d) without causing an idle time. A stable transfer bias can be applied.

  The photosensitive drum 3 is disposed (attached) almost at the center of the image forming apparatus 100, and a charger 5 as a charging unit is a charger-type charger or a contact-type roller type as shown in the figure. A brush type or the like is used to uniformly charge the surface of the photosensitive drum 3 to a predetermined potential. Further, the exposure unit 1 uses a laser scanning unit (LSU) provided with light emitting elements arranged in an array, for example, an EL or LED writing head, a laser irradiation unit, and a reflection mirror.

  The exposure unit 1 has a function of forming an electrostatic latent image corresponding to image data on the surface thereof by exposing the charged photosensitive drum 3 according to input image data. The developing device 2 visualizes the electrostatic latent images formed on the respective photosensitive drums with toners of K, C, M, and Y colors. The cleaner unit 4 removes and collects toner remaining on the surface of the photosensitive drum after development and image transfer.

  The transfer conveyance belt unit 8 disposed below the photosensitive drum 3 includes a transfer belt 7, a transfer belt drive roller 71, a transfer belt tension roller 72, a transfer belt driven roller 73, a transfer belt support roller 74, and a transfer roller 6 ( 6a, 6b, 6c, 6d) and a transfer belt cleaning unit 9.

  The transfer belt drive roller 71, the transfer belt tension roller 72, the transfer roller 6, the transfer belt driven roller 73, the transfer belt support roller 74, and the like stretch the transfer belt 7 and rotate the transfer belt 7 in the direction of arrow B. The transfer roller 6 is rotatably supported by a frame (not shown) inside the transfer belt unit 8, and transfers the toner image on the photosensitive drum 3 to a sheet conveyed by being attracted to the transfer belt 7. To do.

  The transfer belt 7 is provided so as to come into contact with the respective photosensitive drums 3, and the color toner images formed on the photosensitive drums 3 are sequentially superimposed on the sheet and transferred, thereby transferring the color toner. It has a function of forming a toner image (multicolor toner image). This transfer belt is formed endlessly using a film having a thickness of about 100 μm to 150 μm.

  Transfer of the toner image from the photosensitive drum 3 to the sheet is performed by the transfer roller 6 that is in contact with the back side of the transfer belt 7. A high voltage (a high voltage having a polarity (+) opposite to the toner charging polarity (−)) is applied to the transfer roller 6 in order to transfer the toner image.

  The transfer roller 6 is a roller whose base is a metal (for example, stainless steel) shaft having a diameter of 8 to 10 mm and whose surface is covered with a conductive elastic material (for example, EPDM, urethane foam, or the like). By this conductive elastic material, a high voltage can be uniformly applied to the sheet. In this embodiment, the transfer roller 6 is used as the transfer electrode, but a brush or the like is also used.

  Further, the toner adhering to the transfer belt 7 due to contact with the photosensitive drum 3 causes the back surface of the recording paper to become dirty, and is removed and collected by the transfer belt cleaning unit 9. The transfer belt cleaning unit 9 is provided with a cleaning blade as a cleaning member that contacts the transfer belt 7, for example, and the transfer belt 7 that contacts the cleaning blade is supported by a transfer belt support roller 74 from the back side.

  The sheet feeding tray 10 is a tray for storing sheets used for image formation, and is provided below the image forming unit of the image forming apparatus 100. A paper discharge tray 15 provided on the upper part of the image forming apparatus 100 is a tray for placing printed sheets face down, and is provided on a side portion of the image forming apparatus. The paper tray 33 is a tray on which an image-formed sheet is placed face up.

  Further, the image forming apparatus 100 is provided with a paper transport path S for sending the sheets on the paper feed tray 10 to the paper discharge tray 15 via the transfer transport unit 8 and the fixing unit 12. Further, in the vicinity of the sheet conveyance path S from the sheet feed tray 10 to the sheet discharge tray 15 and the sheet discharge tray 33, a pickup roller 16, a registration roller 14, a fixing unit 12, a conveyance direction switching guide 34, and a conveyance for conveying a sheet. A roller 25 and the like are provided.

  The conveyance rollers 25 are small rollers for promoting and assisting conveyance of the sheet, and a plurality of conveyance rollers 25 are provided along the sheet conveyance path S. The pickup roller 16 is a drawing roller that is provided at the end of the paper feed tray 10 and supplies sheets from the paper feed tray 10 to the paper transport path S one by one.

  The conveyance direction switching guide 34 is rotatably provided on the side cover 35, and separates the sheet from the middle of the conveyance path S and discharges the sheet to the sheet discharge tray 33 when the state shown by the solid line changes to the state shown by the broken line. be able to. In the state indicated by the solid line, the sheet passes through a conveyance unit S ′ (a part of the sheet conveyance path S) formed between the fixing unit 12, the side cover 35, and the conveyance switching guide 34, and the sheet discharge roller 25. And then discharged to the upper discharge tray 15.

  Further, the registration roller 14 rotates the photosensitive drum 3 so that the sheet conveyed through the paper conveyance path S is temporarily held and the toner image on the photosensitive drum 3 can be transferred onto the sheet in a satisfactory multiple manner. At the same time, the sheet is conveyed in a timely manner. That is, the registration roller 14 conveys the sheet so that the leading edge of the toner image on each photosensitive drum 3 is aligned with the leading edge of the image forming range on the sheet based on a detection signal output from a pre-registration detection switch (not shown). .

  The fixing unit 12 includes a heat roller 31, a pressure roller 32, and the like. The heat roller 31 and the pressure roller 32 rotate with the sheet interposed therebetween. The heat roller 31 is set to a predetermined fixing temperature by the control unit based on a signal from a temperature detector (not shown), and is transferred to the sheet by thermocompression bonding the sheet together with the pressure roller 33. The resulting multicolor toner image is melted, mixed, and pressed to thermally fix the sheet.

  The sheet after the fixing of the multicolor toner image is conveyed to the reverse paper discharge path of the paper transport path S by the paper discharge rollers 25... And reversed (with the multicolor toner image facing downward). The paper is discharged onto the paper discharge tray 15.

  An image quality sensor 21 that measures density patches for image adjustment formed by transferring an image onto the surface of the transfer belt 7 is provided below the transfer belt 7. Based on the density patch density measurement result by the image quality sensor 21, the control conditions (surface potential of the photosensitive drum, developing bias voltage, transfer charge voltage, power of the semiconductor laser light source, etc.) of the process unit of the image forming apparatus 100 are adjusted. The density patch for adjustment is set to have a predetermined density.

  In addition, an environmental sensor 22 is installed in the image forming unit 100 to detect the temperature and humidity inside the apparatus and correct the previous control conditions. In addition, the previous control conditions are corrected based on the number of images formed and the usage time of replaceable consumables.

<Control of transfer voltage>
The above-mentioned transfer belt 7 serving as a transfer carrier is set to a relatively low value of 10 10 to 10 12 Ω · cm. As shown in FIG. 2, the transfer rollers 6 a to 6 d that are in contact with the back side of the transfer belt 7 at the positions of the image forming stations are provided with a transfer high-voltage power supply (contained in the transfer high-voltage power supply unit 24). High voltage applying means A, B, C, and D are connected to each other, and the voltage output from each of the transfer high-voltage power supplies A, B, C, and D is controlled by the control unit 200 (see FIG. 3). The transfer roller (transfer member) 6 (6a to 6d) and the transfer high-voltage power supply (high voltage applying means) A, B, C, D constitute the transfer means of the present invention.

  The control unit 200 includes a CPU, a ROM, and a RAM. As shown in FIG. 3, an image data input unit 201, a sheet detector (resist) 23, and an environment detector 22 are connected to the input side of the control unit 200. On the output side, the image processing unit 202, the memory 203, the writing unit 204, the charging unit 205, the developing unit 206, the transfer unit 207, the fixing unit 208, the transport mechanism unit 211, and the transfer belt 7 are transferred to the photosensitive drums 3a to 3d. A separation mechanism 212 for performing a separation operation is connected.

  The transfer unit 207 includes a transfer belt 7, transfer rollers 6a to 6d, and a transfer high-voltage power supply unit 24 that accommodates the transfer high-voltage power supplies A to D. The inter-paper voltage is supplied from the transfer rollers 6a to 6d to the photosensitive drums 3a to 3d via the transfer belt 7. At the time of transfer, the rising timing of the transfer operation with respect to the start of rotation of the photosensitive drums 3a to 3d is controlled according to the surface potential of the photosensitive drums 3a to 3d at that time, and is necessary for the transfer operation with respect to the sheet. A high voltage transfer voltage (transfer bias) is supplied. The inter-sheet voltage is controlled within a range of +50 to +300 V, and is changed and set based on a table prepared in advance according to various conditions.

  The above-described high-voltage power supplies A to D for transfer are composed of, for example, a high-voltage transformer (step-up transformer), a primary drive circuit, a PWM oscillator, and the like, and supply output voltages required by the PWM oscillator to the transfer rollers 6a to 6d. The primary voltage supplied from the DC + 24V power source of the main power source of the image forming apparatus 100 is oscillated by the PWM oscillator as a secondary voltage within a range of about 0 to 4 KV by a high voltage transformer (step-up transformer) or the like. 6 is supplied.

  Further, as shown in FIG. 2, the high-voltage power supplies A to D for transfer include a high voltage including a high-voltage transformer (step-up transformer) for supplying the single transfer high-voltage power supply unit 24 to the transfer roller 6 of each image station. Each circuit is formed separately, and each transfer roller 6a, 6b, 6c, 6d can be charged separately with a transfer bias.

  The transfer voltage (transfer bias charge (TC)) during the image forming operation for transferring the image from the photosensitive drum 3 to the sheet varies depending on environmental conditions such as humidity and temperature detected by the environmental sensor 22 and the like, fatigue level, and the like. Is controlled based on the characteristics of the photosensitive drum 3 (degree of deterioration), the change in characteristics due to the sag of the developer (toner), the type of sheet used, and the like, and generally within the range of +1.5 to +4 KV ( High voltage).

  Incidentally, as shown in FIG. 4, the surface potential of the photosensitive drum 3 is charged to the negative side by a charger 5 connected to a high voltage power supply for charging, and the developing roller and developer of the developing device 2 are charged. Is negatively charged. Accordingly, when there is no sheet between the photosensitive drum 3 and the transfer belt 7 and the sheet is in direct contact (when no power is supplied), the transfer roller 6 is also charged to the negative side.

  From this state, when the transfer roller 6 is suddenly energized in order to obtain a necessary transfer potential, when there is no sheet between the photosensitive drum 3 and the transfer belt 7, the photosensitive roller 3 Due to the surface potential of the body drum 6 (for example, −500 V to −700 V) (FIG. 5A), the charged portion in which the photosensitive drums 3 a to 3 d are charged by the charger 5 rotates to the transfer position. After that, it appears that a negative output voltage is applied from the high-voltage transformer (step-up transformer) (FIG. 5 (b)), and when the transfer bias is applied, oscillation cannot be performed normally. As shown in the broken line in (c), almost no output is obtained, or the timing is delayed (delay time d), and the timing and output are not as set as in the two-dot chain line. Raised, it can not be stably supplied to the transfer bias. Therefore, a symptom of missing transfer occurs particularly at the leading edge of the sheet, and the image quality is significantly lowered.

  Therefore, in this embodiment, as described above, the rising timing of the transfer operation with respect to the start of rotation of the photosensitive drums 3a to 3d is controlled according to the surface potential of the photosensitive drums 3a to 3d at that time. I have to.

  The surface potential meter 23 for measuring the surface potential of the photosensitive drums 3a to 3d may be disposed at least between the charging point and the transfer point on the photosensitive drum 3, and preferably between the charging point and the developing point. More preferably, as shown in FIG. 4, it may be disposed between the charging point and the exposure point. By disposing the surface potential meter 23 at such a position, the transfer means can be operated reliably from the portion where the surface potential of the photosensitive drum 3 is high, so that the transfer bias can be stably applied. it can.

  By the way, when the charging operation is not performed on the photosensitive drums 3a to 3d, the surface potential of the photosensitive drums 3a to 3d is approximately 0V. On the other hand, when the photosensitive drums 3a to 3d are charged to -700 V and the energization to the high voltage power source is OFF (the high voltage power source is OFF), the surface potential of the photosensitive drums 3a to 3d, the transfer belt 7 and the transfer belt 7 are transferred. The output from the high-voltage power supply is in the range of −50 to −300 V (actually measured value) through the impedance of the rollers 6a to 6d and the internal impedance of the high-voltage power supply.

  When an emergency stop occurs when a jam or the like occurs, the drive motor, charger, and high-voltage power supply are instantaneously turned OFF, and the surfaces of the photosensitive drums 3a to 3d are stopped at the rated surface potential (for example, -700V). It will be in the state. FIG. 6 is a graph showing attenuation characteristics of the surface potential when the surface potentials of the photosensitive drums 3a to 3d are left in a state of being charged to −700V.

  As can be seen from this graph, the surface potential of the photosensitive drums 3a to 3d becomes a potential (−200 V) of less than 30% when about 1 minute has passed since the emergency stop. Therefore, when the return operation such as the jam processing is completed early after the emergency stop, the surface potentials of the photosensitive drums 3a to 3d are in a considerably high state. Further, it can be seen from this graph that the surface potential of the photosensitive drums 3a to 3d attenuates to approximately 0V after a few minutes have passed.

  In consideration of such attenuation characteristics, when the photosensitive drums 3a to 3d have a long stop time (for example, 90 sec or more) and the surface potentials of the photosensitive drums 3a to 3d are lower than a predetermined value Vk. The transfer unit 207 is activated after a predetermined time t from the start of rotation of the photosensitive drums 3a to 3d, and a transfer bias is applied to each of the transfer rollers 6a to 6d.

  As described above, by controlling the timing of applying the transfer bias according to the surface potential of the photosensitive drums 3a to 3d, high voltage is not applied to the uncharged portions of the photosensitive drums 3a to 3d. It is possible to avoid damaging the photosensitive drums 3a to 3d, and the transfer unit 207 is activated in a normal state, and a transfer bias is stably applied to the transfer rollers 6a to 6d. be able to. As for the transfer bias output start of the transfer high-voltage power supply (A to D), a stable output cannot be obtained unless the voltage of the output portion is 0 V or more, but it is predetermined from the start of rotation of the photosensitive drums 3a to 3d. After the elapse of time t, the voltage of the output unit has returned to 0V.

  On the other hand, when the photosensitive drums 3a to 3d have a short stop time (for example, within 90 seconds) and the surface potential of the photosensitive drums 3a to 3d is higher than the predetermined value Vk, the rotation of the photosensitive drums 3a to 3d is started. By simultaneously starting the transfer unit 207, the transfer unit 207 can be started in a normal state, and a transfer bias can be stably applied to the transfer rollers 6a to 6d.

  The predetermined value Vk can be set, for example, in a range of 13% to 16% of the rated surface potential of the photosensitive drums 3a to 3d. That is, when the rated surface potential of the photosensitive drums 3a to 3d is -700V, the predetermined value Vk can be set in the range of -91V to -112V. The predetermined time t is the time until the charged portions on the photosensitive drums 3a to 3d charged by the charger 5 are rotated to the transfer position, as described above.

  Further, when an emergency stop occurs when a trouble such as a jam occurs, or when the image formation is finished and the image formation is started again immediately after the stop, the surface potential of the photosensitive drums 3a to 3d may be high. In this case, the transfer bias can be stably applied to the transfer rollers 6a to 6d by starting the transfer unit 207 simultaneously with the rotation of the photosensitive drums 3a to 3d.

  In consideration of the attenuation characteristics of the surface potentials of the photosensitive drums 3a to 3d as shown in FIG. 6, the time from the emergency stop to the restart after returning (the time corresponds to the elapsed time in FIG. 6). And the operation of the transfer unit 207 may be controlled based on the measured time.

  Incidentally, FIG. 7 is an explanatory diagram of a conventional transfer bias application timing. As shown in the figure, when the rotation of the photosensitive drums 3a to 3d is started after the rotation of the normal photosensitive drums 3a to 3d is stopped, a predetermined time t is used to avoid damage to the photosensitive drums 3a to 3d. Similarly, when the application timing of the transfer bias is delayed and restarted in the case of an emergency stop, similarly, the transfer bias is applied for a predetermined time t after the rotation of the photosensitive drums 3a to 3d is started. The timing is delayed.

  However, when restarting after an emergency stop, the surface potential of the photosensitive drums 3a to 3d has not decreased so much (see FIG. 6), so there is no need to delay the application timing of the transfer bias. As described above, when the application timing is delayed, the delay time extends the total image formation time.

  Therefore, in the present invention, as shown in FIG. 8, when the photosensitive drums 3a to 3d are restarted after an emergency stop (the stop time is within 90 seconds, for example), the surface potentials of the photosensitive drums 3a to 3d are If the value is higher than the predetermined value Vk, the transfer unit 207 is activated simultaneously with the start of rotation of the photosensitive drums 3a to 3d (without delaying the predetermined time t), thereby generating the idle time as described above. As described above, the transfer unit 207 is activated in a normal state, and a transfer bias can be stably applied to the transfer rollers 6a to 6d.

<< Embodiment 2 >>
The control of the transfer voltage according to the control system block diagram shown in FIG. 3 is not limited to the multicolor image forming apparatus 100 but can be similarly applied to a single color image forming apparatus.

<Single-color image forming device>
FIG. 9 shows a schematic configuration of a monochrome digital copying machine 300 as an image forming apparatus of the present invention. An automatic document feeder 112 is provided on a document table 111 made of transparent glass on the upper surface of the monochrome digital copying machine 300. The automatic document feeder 112 is a device that automatically feeds a plurality of documents set on a document setting tray onto the document table 111 one by one.

  The document reading unit 110 is disposed below the document table 111 and scans and reads an image of the document placed on the document table 111. The document scanning unit 110 includes a first scanning unit 113, a second scanning unit 114, and an optical unit. It has a lens 115 and a CCD line sensor 116 which is a photoelectric conversion element. The first scanning unit 113 includes an exposure lamp unit that exposes the surface of the document, a first mirror that reflects a reflected light image from the document in a predetermined direction, and the like. The second scanning unit 114 includes a second mirror and a third mirror that guide the reflected light from the original reflected from the first mirror to the CCD line sensor 116 that is a photoelectric conversion element. The optical lens 115 focuses reflected light from the original on the CCD line sensor 116.

  The reading unit 110 reads an image of a document automatically conveyed by the automatic document conveying device 112 at a predetermined exposure position by an operation related to the automatic document conveying device 112.

  The document image read by the reading unit 110 is sent as image data to an image data input unit (not shown), and after predetermined image processing is performed on the image data, it is temporarily stored in the memory of the image processing unit. In response to the output instruction, the image in the memory is read out and transferred to the laser writing unit 227 which is an optical writing device of the image forming unit 210.

  The laser writing unit 227 includes a semiconductor laser light source that emits laser light according to image data read from the memory or image data transferred from an external device, a polygon mirror that deflects the laser light at a constant angular velocity, and a deflection at a constant angular velocity. The laser beam is composed of an f-θ lens that corrects the laser beam so that it is deflected on the photosensitive drum 222 at a constant angular velocity. In this embodiment, a laser writing unit is used as the writing device, but a solid scanning optical writing head unit using a light emitting element array such as an LED or an EL may be used.

  In addition, the image forming unit 210 supplies toner to the electrostatic latent image formed on the photosensitive drum 222 and a charger 223 that charges the photosensitive drum 222 to a predetermined potential around the photosensitive drum 222. A developing device 224 for visualizing, a transfer device (transfer roller or the like) 225 for transferring a toner image formed on the surface of the photosensitive drum 222 to a recording paper, a cleaning device 226 for collecting excess toner, and discharging the recording paper for photosensitivity. A static eliminator (e.g., a static elimination needle) 229 for facilitating peeling from the body drum 222, a surface potential meter 230 for measuring the surface potential of the photosensitive drum 222, and the like are provided. Based on the surface potential of the photosensitive drum 222 detected by the surface potential meter 230, the control unit 211 controls the timing from the start of rotation of the photosensitive drum 222 until the transfer device 225 as a transfer unit is operated.

  Then, the recording sheet onto which the image has been transferred by the image forming unit 210 is then sent to the fixing unit 217 and the image is fixed on the recording sheet. On the discharge side of the image forming unit 210, in addition to the fixing unit 217, a switchback path 221 that reverses the front and back of the recording paper in order to form an image again on the back side of the recording paper, and the recording paper on which the image is formed A post-processing device 260 having a lifting tray 261 is also provided.

  The recording paper on which the toner image is fixed by the fixing unit 217 is guided to the post-processing device 260 by the paper discharge roller 219 through the switchback path 221 as necessary, and is subjected to predetermined post-processing. After that, it is discharged. The surface potential meter 230 is disposed at a position between the charging point and the transfer point, that is, between the charger 223 and the transfer device 225.

  The paper feed unit is disposed below the image forming unit 210, and includes a manual feed tray 254, a duplex unit 255, a paper tray 251, and a paper feed tray 252 provided in the multistage paper feed unit 270. 253. Further, a transport unit 250 is provided for transporting paper fed from the trays 251, 252, 253, and 254 to a transfer position in the image forming unit 210 where a transfer unit is disposed.

  The duplex unit 255 communicates with a switchback path 221 that reverses the recording paper, and is used when image formation is performed on both sides of the recording paper. The duplex unit 255 can be replaced with a normal paper cassette, and the duplex unit 255 can be replaced with a normal paper cassette.

  In the present invention, the image forming apparatus is not limited to the configuration shown in FIG. 1 or FIG. 9, but at least an image formed on an image carrier charged with a surface potential by a charger is used as a transfer medium. If the transfer means for transferring is an image forming apparatus provided with a transfer member that applies a high voltage by pressing the transfer medium against the image carrier, a high-voltage power supply device that applies a high voltage to the transfer member, and The present invention can be applied regardless of the configuration or form.

1 is a configuration diagram of a multicolor image forming apparatus according to Embodiment 1 of the present invention. FIG. It is the principal part structure explanatory drawing. It is the same control system block diagram. It is explanatory drawing of the charged state of the image carrier. 6 is a graph showing the surface potential of the image carrier and the potential of the transfer roller. FIG. 10 is an explanatory diagram showing a change in surface potential of an image carrier and a timing of applying a transfer bias in the past. FIG. 6 is an explanatory diagram showing a change in surface potential of an image carrier and an application timing of a transfer bias according to an embodiment of the present invention. It is a graph which shows the attenuation state of the surface potential of the image carrier. It is a block diagram of the monochrome image forming apparatus which concerns on Embodiment 2 of this invention.

Explanation of symbols

3 (3a to 3d)-Image carrier 6 (6a to 6d)-Transfer member (transfer means)
A, B, C, D-High voltage power supply (transfer means)
23 (23a to 23d) -surface potential meter 100,300-image forming apparatus

Claims (10)

A transfer means for transferring an image formed on a charged and rotationally driven image carrier to a transfer medium, a transfer member that presses the transfer medium against the image carrier and applies a high voltage; and A high voltage power supply device for applying a high voltage to the transfer member, and applying a high voltage to the transfer member even when the transfer medium is not present between the image carrier and the transfer member. For this purpose, a control unit for operating the transfer means is provided,
The image forming apparatus according to claim 1, wherein the control unit includes a control unit that controls a timing from the start of rotation of the image carrier to the operation of the transfer unit in accordance with a surface potential of the image carrier.   The controller, when it is determined that the stop time of the image carrier is long and the surface potential of the image carrier is lower than a predetermined value, the transfer is performed after a lapse of a predetermined time from the start of rotation of the image carrier. When the image carrier is stopped and the surface potential of the image carrier is determined to be higher than the predetermined value, the transfer is started simultaneously with the start of rotation of the image carrier. 2. The image forming apparatus according to claim 1, wherein the operation of the means is started.   The image forming apparatus according to claim 2, wherein the predetermined time is a time until a portion on the charged image carrier reaches a position corresponding to the transfer unit.   The image forming apparatus according to claim 2, wherein the predetermined value is set to 13% to 16% of a rated surface potential of the image carrier.   5. The image forming apparatus according to claim 2, wherein a trouble such as a jam of the transfer medium is included when it is determined that the stop time of the image carrier is short. The image forming apparatus further includes a surface potential meter for measuring the surface potential of the image carrier,
The controller is
6. The image forming apparatus according to claim 1, wherein a control operation is performed based on a result of a surface potential of the image carrier measured by the surface potential meter.   The image forming apparatus according to claim 6, wherein the surface potentiometer is disposed between a charging point and a transfer point in the image carrier.   The image forming apparatus according to claim 6, wherein the surface electrometer is disposed between a charging point and a developing point in the image carrier.   The image forming apparatus according to claim 6, wherein the surface potential meter is disposed between a charging point and an exposure point on the image carrier.   The control unit sets a voltage applied to the transfer medium by the transfer unit during an operation other than during image transfer, lower than a voltage applied to the transfer medium during image transfer. The image forming apparatus according to any one of 1 to 9.

Images