JP4855154B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an electrophotographic image forming apparatus represented by a copying machine or a printer.

  In an electrophotographic image forming apparatus such as a copying machine or a printer, a photosensitive drum is widely used as an image carrier. A general image forming operation using the photosensitive drum is as follows. The surface of the photosensitive drum is uniformly charged with a predetermined potential by a charging device, and by irradiating the LED light or the like of the exposure device there, the potential is partially attenuated to form an electrostatic latent image of an original image. Is done. The electrostatic latent image is developed with a developing device to form a toner image. After the transfer of the toner image onto the paper, the remaining toner is cleaned on the surface of the photosensitive drum by the cleaning device, and the charge removal is performed by irradiating the neutralization light by the neutralization device in preparation for the next image forming operation. Is called.

  Here, if low density printing is continuously performed in the image forming apparatus, the charge amount of the toner contained in the developing device may become abnormally high. This makes it difficult for the toner to move away from the developing roller of the developing device, and the amount of toner moving to the photosensitive drum may be reduced. Then, when printing is performed as it is, a problem of image defects such as a decrease in image density occurs.

  In order to solve such problems and maintain the performance of the developing device, a method for refreshing the developing device by supplying toner from the developing device to the photosensitive drum during the non-transfer period to the paper has been proposed. It can be seen in Patent Document 1 and Patent Document 2. In the image forming apparatuses described in Patent Document 1 and Patent Document 2, toner is forcibly discharged from the developing device on the condition that the toner occupancy (printing density) with respect to the paper surface is low, and the refreshing process of the developing device. To do.

  On the other hand, when an amorphous silicon photoconductor is used as the photoconductor, it is known that discharge products easily adhere to the surface of the photoconductor due to the discharge of the charging device. When this discharge product absorbs moisture, the electrical resistance on the surface of the photoreceptor is lowered, and a problem of image flow that disturbs the electrostatic latent image may occur. For this reason, a small amount of abrasive is mixed in the toner, or a toner particle itself having a shape having a high polishing effect is used, and a polishing roller as a rotating member and a cleaning blade are used in combination. There is known a method of removing residual toner adhering to the surface, cleaning the toner by holding the toner on the surface of the polishing roller, and polishing the discharge product adhering to the surface of the photoreceptor by the toner.

Thus, an example of a cleaning method for removing the discharge products attached to the surface of the photosensitive drum using the polishing roller can be seen in Patent Document 3. The image forming apparatus described in Patent Document 3 performs a refresh process in which toner is supplied from the developing device to the photosensitive drum during the non-transfer period to the paper, and the surface of the photosensitive member is polished and refreshed.
JP 2000-310909 A (page 3-7, FIG. 1-3) Japanese Patent Laying-Open No. 2005-55842 (pages 12-14, FIG. 5) Japanese Patent Laid-Open No. 11-3014 (page 2-3, FIG. 1)

  The image forming apparatus described in Patent Document 2 is a so-called tandem image forming apparatus that includes four image forming units that form toner images of different colors and an intermediate transfer belt (endless belt). During the refresh process, the toner forcibly discharged from the developing device and supplied to the photosensitive drum is collected by the photosensitive drum cleaning device without being transferred to the intermediate transfer belt. Here, in the tandem image forming apparatus provided with the intermediate transfer belt as described in Patent Document 2, when the toner image is primarily transferred from the photosensitive drum to the intermediate transfer belt, the toner is moved to the intermediate transfer belt. A primary transfer bias is applied to the primary transfer portion. In the refresh process, since the toner is not moved to the intermediate transfer belt, a bias voltage having a polarity different from the primary transfer bias is applied to the primary transfer unit.

  In the refresh process, when a bias voltage having a polarity different from the primary transfer bias is applied to the primary transfer portion, the toner is attracted to the photosensitive drum, that is, the intermediate transfer belt is attracted to the photosensitive drum (FIG. 6). reference). As a result, the tension of the intermediate transfer belt may change. Therefore, when a normal image forming operation is executed subsequent to the refreshing process of the developing device and the photosensitive drum, the primary transfer of the toner image to the intermediate transfer belt may be performed while the rotation of the intermediate transfer belt remains defective. There is. As a result, when color printing with a plurality of colors is performed in the tandem image forming apparatus, there is a problem in that color misregistration occurs in the rotation direction of the intermediate transfer belt and image defects occur.

  The present invention has been made in view of the above points, and includes a plurality of image forming units arranged in a line along the intermediate transfer member, and primarily transfers a toner image from the image carrier to the intermediate transfer member. In an image forming apparatus that performs secondary transfer from an intermediate transfer body to a sheet, even when a refresh process is performed on the surface of a developing device and / or an image carrier, color printing with a plurality of colors is performed following the refresh process. An object of the present invention is to provide an image forming apparatus that can prevent color misregistration and can perform high-quality image formation.

  In order to solve the above problems, the present invention includes a plurality of image forming units arranged in a line along an intermediate transfer member, and a toner image formed on the surface of the image carrier of each image forming unit. First transfer from the body to the intermediate transfer body, secondary transfer from the intermediate transfer body to the paper, and during the non-transfer period to the paper, toner is supplied from the developing device to the image carrier, and the developing device and / or Alternatively, in the image forming apparatus that performs a refresh process for refreshing the surface of the image carrier, the image carrier is applied during the refresh process by applying a bias voltage having a polarity different from that during the primary transfer of the toner image to the primary transfer unit. After the toner supplied to the toner is passed through the primary transfer portion without being transferred to the intermediate transfer member, the bias voltage is switched to the same polarity as in the primary transfer, and image formation is performed after a predetermined time has elapsed. It was decided to start the work.

  In the image forming apparatus having the above-described configuration, the intermediate transfer located at the primary transfer portion when the bias voltage is switched to the same polarity as that at the time of the primary transfer after the refresh process is performed after the refresh step. The body part is a time longer than the time required to pass through the primary transfer part of the image forming part that is the most downstream in the rotation direction of the intermediate transfer member among the plurality of image forming parts.

  According to the configuration of the present invention, a plurality of image forming units arranged in a line along the intermediate transfer member is provided, and a toner image formed on the image carrier surface of each image forming unit is transferred from the image carrier to the intermediate transfer member. In the non-transfer period, the toner is supplied from the developing device to the image carrier, and the surface of the developing device and / or the image carrier is transferred. In an image forming apparatus that executes a refresh process for refreshing toner, a bias voltage having a polarity different from that at the time of primary transfer of a toner image is applied to the primary transfer section, whereby the toner supplied to the image carrier during the refresh process is transferred to the intermediate transfer section. After passing through the primary transfer portion without transferring to the body, the bias voltage is switched to the same polarity as at the time of primary transfer, and after a predetermined time has elapsed, the image forming operation is started. When Mesh step, the tension of the intermediate transfer body in the form as is drawn to the image bearing member, in a state of normal is recovered, it is possible to start an image forming operation. Thereby, even when performing color printing with a plurality of colors following the refreshing step of the developing device and / or the image carrier surface, it is possible to prevent occurrence of color misregistration. Therefore, an image forming apparatus capable of performing high-quality image formation can be obtained.

  In addition, after the refresh step, the predetermined time is changed to the same polarity as that at the time of primary transfer after the refresh step, and the location of the intermediate transfer member positioned at the primary transfer portion at the time of switching is changed between the plurality of image forming portions. Of these, the time required to pass through the primary transfer portion of the image forming portion on the most downstream side in the rotation direction of the intermediate transfer member is longer than the time required to pass through the intermediate transfer member. It is possible to recover the tension of the transfer member normally and keep the time required for the refreshing process as short as possible. As a result, when performing color printing with a plurality of colors following the refresh process of the developing device, it is possible to prevent the occurrence of color misregistration and also prevent the deterioration of the high-speed printing performance of the image forming apparatus. .

  Hereinafter, embodiments of the present invention will be described with reference to FIGS.

  First, an image output operation of an image forming apparatus according to an embodiment of the present invention will be described while explaining an outline of the structure with reference to FIGS. 1 and 2. FIG. 1 is a schematic vertical sectional front view of the image forming apparatus, and FIG. 2 is a schematic partially enlarged view showing the arrangement of image forming units of the image forming apparatus of FIG. This image forming apparatus is of a color printing type in which a toner image is transferred onto a sheet using an intermediate transfer belt.

  As shown in FIG. 1, a paper cassette 3 is disposed below the inside of the main body 2 of the image forming apparatus 1. The paper cassette 3 stores and accommodates paper P such as cut paper before printing. The sheets P are separated and sent one by one toward the upper left of the sheet cassette 3 in FIG. The paper cassette 3 can be pulled out horizontally from the front side of the main body 2.

  A first paper transport unit 4 is provided inside the main body 2 and on the left side of the paper cassette 3. The first paper transport unit 4 is formed substantially vertically along the left side surface of the main body 2. The first paper transport unit 4 receives the paper P sent out from the paper cassette 3 and transports the paper P vertically upward along the left side surface of the main body 2 to the secondary transfer unit 40.

  A manual paper feed unit 5 is provided above the paper cassette 3 and on the right side, which is the side opposite to the left side of the main body 2 where the first paper transport unit 4 is formed. . In the manual sheet feeding portion 5, paper P having a size not contained in the paper cassette 3, a paper to be fed one by one such as a thick paper and an OHP sheet are placed.

  A second paper transport unit 6 is provided on the left side of the manual paper feed unit 5. The second paper transport unit 6 is located immediately above the paper cassette 3, extends substantially horizontally from the manual paper feed unit 5 to the first paper transport unit 4, and joins the first paper transport unit 4. Then, the second paper transport unit 6 receives the paper P or the like sent from the manual paper feed unit 5 and transports it to the first paper transport unit 4 substantially horizontally.

  On the other hand, the image forming apparatus 1 receives document image data from an external computer (not shown). The information of the image data is sent to a laser irradiation unit 7 that is an exposure unit disposed above the second paper transport unit 6. The laser irradiation unit 7 irradiates the image forming unit 20 with laser light L controlled based on the image data.

  A total of four image forming units 20 are provided above the laser irradiation unit 7, and an intermediate transfer belt 8 using an intermediate transfer member in the form of an endless belt is provided above each image forming unit 20. The intermediate transfer belt 8 is wound around and supported by a plurality of rollers, and is rotated clockwise in FIG. 1 by a driving device (not shown).

  As shown in FIG. 1, the four image forming units 20 are of a so-called tandem system that are arranged in a line from the upstream side to the downstream side in the rotational direction along the rotational direction of the intermediate transfer belt 8. The four image forming units 20 are, in order from the upstream side, a magenta image forming unit 20M, a cyan image forming unit 20C, a yellow image forming unit 20Y, and a black image forming unit 20B. As shown in FIG. 2, the four image forming units 20 are arranged at an interval of 90 mm. These image forming units 20 are replenished with a developer (toner) by a developer supply container and a conveying unit (not shown) corresponding to each color. In the following description, the identification symbols “M”, “C”, “Y”, and “B” are omitted unless particularly limited.

  In each image forming unit 20, an electrostatic latent image of a document image is formed by the laser light L irradiated by the laser irradiation unit 7 serving as an exposure unit, and the toner image is developed from the electrostatic latent image. The toner image is primarily transferred to the surface of the intermediate transfer belt 8 by the primary transfer unit 30 provided above each image forming unit 20. As the intermediate transfer belt 8 rotates, the toner image of each image forming unit 20 is transferred to the intermediate transfer belt 8 at a predetermined timing, so that the surface of the intermediate transfer belt 8 is magenta, cyan, yellow, or black. A color toner image is formed by superimposing the color toner images.

  A secondary transfer unit 40 including a secondary transfer roller 41 is disposed at a position where the intermediate transfer belt 8 is suspended on the sheet conveyance path. The color toner image on the surface of the intermediate transfer belt 8 is a secondary formed by the intermediate transfer belt 8 and the secondary transfer roller 41 being pressed against the paper P sent in synchronization by the first paper transport unit 4. Transferred at the transfer nip.

  After the secondary transfer, the toner remaining on the surface of the intermediate transfer belt 8 is cleaned by the cleaning device 9 for the intermediate transfer belt 8 provided on the upstream side of the intermediate transfer belt 8 in the rotation direction of the magenta image forming unit 20M. To be recovered.

  A fixing unit 10 is provided above the secondary transfer unit 40. The paper P carrying the unfixed toner image in the secondary transfer unit 40 is sent to the fixing unit 10 where the toner image is heated and pressed by the heat roller and the pressure roller to be fixed.

  A branch portion 11 is provided above the fixing portion 10. The sheet P discharged from the fixing unit 10 is discharged from the branching unit 11 to the sheet discharging unit 12 provided on the upper part of the image forming apparatus 1 when double-sided printing is not performed.

  The discharge port portion from which the paper P is discharged from the branch portion 11 toward the paper discharge portion 12 functions as the switchback portion 13. When performing duplex printing, the switchback unit 13 switches the transport direction of the paper P discharged from the fixing unit 10. Then, the sheet P is sent downward through the branching unit 11, the left side of the fixing unit 10, and the left side of the secondary transfer unit 40, and again passes through the first sheet transport unit 4 to the secondary transfer unit 40. Sent.

  Next, a detailed configuration around the image forming unit 20 of the image forming apparatus 1 will be described with reference to FIG. FIG. 3 is a partially enlarged vertical sectional view showing the periphery of the image forming unit. Since the four color image forming units 20 have the same structure, the identification symbols “M”, “C”, “Y”, and “B” are omitted as described above.

  As shown in FIG. 3, the image forming unit 20 includes a photosensitive drum 21 that is an image carrier at the center thereof. In the vicinity of the photosensitive drum 21, a charging device 50, a developing device 60, a charge removal device 70, and a drum cleaning device 80 are arranged in this order along the rotation direction. The primary transfer unit 30 is provided between the developing device 60 and the charge removal device 70 along the rotation direction of the photosensitive drum 21.

  The photoconductor drum 21 extends in the paper width direction perpendicular to the paper conveyance direction in the image forming apparatus 1, that is, the depth direction of the paper surface in FIG. The photosensitive drum 21 is an inorganic photosensitive drum in which an amorphous silicon photosensitive layer, which is an inorganic photoconductive material, is provided on the outer side of a conductive roller-shaped substrate made of aluminum or the like by vacuum deposition or the like, and has a diameter of 30 mm. It is. In order to improve the development performance by forming an electrostatic latent image in a suitable state, the thickness of the photosensitive layer is more preferably 25 μm or less, and further preferably 20 μm or less. The photosensitive drum 21 is rotated by a driving device (not shown) so that the peripheral speed thereof is substantially the same as the paper conveyance speed (150 mm / s).

  The charging device 50 includes a charging roller 52 that contacts the photosensitive drum 21 inside the housing 51. The charging roller 52 is in pressure contact with the photosensitive drum 21 with a predetermined pressure, and rotates according to the rotation of the photosensitive drum 21. By this charging roller 52, the surface of the photosensitive drum 21 is uniformly charged with a predetermined polarity and potential. In the housing 51, a cleaning brush 53 is provided at a position separating the charging roller 52 from the photosensitive drum 21.

  The developing device 60 includes a stirring screw 62, a supply screw 63, a magnetic roller 64, a spike cutting blade 65, and a developing roller 66 inside a housing 61. In the developing device 60, a two-component developer composed of toner and a magnetic carrier is used.

  The agitation screw 62 and the supply screw 63 are arranged in the lower part of the developing device 60 in FIG. The developer is accommodated in a developer supply container (not shown), conveyed to the location of the developing device 60, and replenished to the location of the stirring screw 62. The agitating screw 62 agitates the developer composed of the toner and the magnetic carrier and charges the developer to a predetermined level. The supply roller 63 supplies the charged developer to the magnetic roller 64 provided above the supply developer.

  The magnetic roller 64 is disposed immediately above the supply roller 63. The developer received from the supply roller 63 and attached to the surface of the magnetic roller 64 forms a magnetic brush on the surface. The magnetic brush is set to a predetermined length in the radial direction by the panning blade 65 provided in the vicinity of the magnetic roller 64 according to the rotation of the magnetic roller 64. With this magnetic brush, the magnetic roller 64 supplies only the toner of the developer to the developing roller 66 and collects the toner that has not been used for development. After development, the length in the axial direction of the magnetic roller 64 is longer than that of the developing roller 66 in order to reliably collect the toner remaining on the surface of the developing roller 66 without leaving it.

  The developing roller 66 is disposed immediately above the magnetic roller 64 and facing the magnetic roller 64 with a predetermined gap. The developing roller 66 is also arranged facing the photosensitive drum 21 with a predetermined gap. Only the toner is supplied to the surface of the developing roller 66 by a magnetic brush made of developer on the surface of the magnetic roller 64. Then, a thin toner layer having an appropriate thickness is formed on the surface of the developing roller 66.

  A predetermined bias voltage is applied to the developing roller 66 and the magnetic roller 64. Since a bias voltage lower than that of the magnetic roller 64 is applied to the developing roller 66, a thin layer of only toner is formed on the surface of the developing roller 66 due to the potential difference. A potential difference of 100 to 350 V is appropriate. The developing roller 66 develops the electrostatic latent image formed on the surface of the photosensitive drum 21 with a thin toner layer on the surface thereof. At this time, a bias voltage in which direct current and alternating current are superimposed is applied between the developing roller 66 and the photosensitive drum 21. In order to prevent toner scattering, an AC voltage is applied immediately before development. The DC component of the bias voltage is 150 V or less, the AC component is 500 to 2000 V, and the frequency is 1 to 4 kHz.

  Toner that is not used for development and remains on the surface of the developing roller 66 is collected by the magnetic roller 64 at a location facing the magnetic roller 64.

  Here, as a method for efficiently promoting toner replacement, a peripheral speed difference is provided between the developing roller 66 and the magnetic roller 64. By setting this difference in peripheral speed to 1.0 to 2.0 times, a uniform toner layer with an appropriate toner concentration can be obtained and toner can be collected smoothly.

As the carrier contained in the developer, a coated ferrite carrier having a volume resistivity of 10 ¹⁰ Ωcm, a saturation magnetization of 65 emu / g, and an average particle size of 45 μm was used. As the developer, a carrier having a resistance of 10 ⁶ Ωcm to 10 ¹³ Ωcm having a role of supplying and collecting toner is used. The carrier supplies toner necessary for development to the developing roller 66 between the developing roller 66 and the magnetic roller 64, and after development, the toner firmly and electrostatically adhered to the developing roller 66 is magnetically brushed. It is necessary to peel off with. At this time, in order to increase the number of contacts with the toner, it is preferable to use a carrier having a small diameter of 50 μm or less.

  The primary transfer unit 30 is provided with a primary transfer roller 31 that comes into contact with the photosensitive drum 21 via the intermediate transfer belt 8. The primary transfer roller 31 does not have a driving device, and rotates according to the rotation of the intermediate transfer belt 8 by contacting the intermediate transfer belt 8. The primary transfer roller 31 is applied with a primary transfer bias of minus 500 to minus 1000 V, which has a polarity different from the charging polarity of the photosensitive drum 21 or toner, as necessary.

  As shown in FIG. 1, the intermediate transfer belt 8 is wound around and supported by a plurality of rollers. The intermediate transfer belt 8 is constituted by an elastic belt in which a rubber layer is laminated on the surface of a base layer made of a synthetic resin. As materials, polyvinylidene fluoride (PVDF) and chloroprene rubber (CR) are used for the base layer, silicon and urethane, and polytetrafluoroethylene (PTFE) and urethane are used for the coating.

  The neutralization device 70 is disposed further downstream of the primary transfer unit 30 along the rotation direction of the photosensitive drum 21. The static eliminator 70 includes an LED (light emitting diode) 71 and a reflector 72. The LED 71 is attached to the upper surface of the housing 81 of the cleaning device 80. Instead of the LED 71, an EL (electroluminescence) light source, a fluorescent lamp, or the like can be used. The reflector 72 is provided above the LED 71 so as to cover the LED 71. The neutralization device 70 irradiates the photosensitive drum 21 with the neutralization light from the LED 71, thereby removing the charged charges on the surface.

  The cleaning device 80 includes a cleaning roller 82 as a cleaning member, a cleaning blade 83, a scraper 84, and a discharge screw 85 inside the housing 81.

  The cleaning roller 82 and the cleaning blade 83 have substantially the same axial length as the photosensitive drum 21 and are provided so as to contact the photosensitive drum 21. After the toner image on the surface of the photoconductive drum 21 is primarily transferred to the intermediate transfer belt 8, the cleaning roller 82 and the cleaning blade 83 remove the adhered matters such as toner remaining on the surface of the photoconductive drum 21 and perform cleaning. The scraper 84 is provided in contact with the cleaning roller 82 from above in FIG. 2, and removes excess toner adhering to the surface of the cleaning roller 82 to make the toner layer uniform. With this toner layer, the cleaning roller 82 polishes the surface of the photosensitive drum 21. The toner removed from the surface of the photosensitive drum 21 is sent to the discharge screw 85 according to the action of gravity and the rotation of the cleaning roller 82, and is conveyed to the outside of the housing 81 by the discharge screw 85.

  The image forming apparatus 1 as described above refreshes the developing device 60 by supplying an appropriate amount of toner from the developing device 60 to the photosensitive drum 21 within the non-transfer period to the paper P in order to maintain the performance of the developing device 60. A refresh process is performed. Similarly, the image forming apparatus 1 cleans deposits such as discharge products that adhere to the surface of the photosensitive drum 21, so that an appropriate amount is transferred from the developing device 60 to the photosensitive drum 21 within the non-transfer period to the paper P. A refreshing process for supplying the toner and refreshing the surface of the photosensitive drum 21 is executed. In these refresh processes, toner is supplied from the developing device 60 to the surface of the photosensitive drum 21 in a linear shape having a predetermined width extending in the drum axial direction, that is, a line of 100 mm in the drum rotating direction.

  Next, the operation during the refresh process of the developing device 60 and the surface of the photosensitive drum 21 will be described with reference to FIGS. 4 to 6 in addition to FIG. 4 is a flowchart showing the operation during the refresh process, FIG. 5 is a control chart of the photosensitive drum and the primary transfer unit, and FIG. 6 is a schematic front view showing the state of the primary transfer unit during the refresh process. Here, the refresh process of the developing device 60 will be described as an example, but the same operation is performed in the refresh process of the surface of the photosensitive drum 21 as well.

  As shown in FIG. 4, the image forming apparatus 1 refreshes the developing device within a non-transfer period to the paper P within a non-transfer period on the condition that the toner occupancy (print density) with respect to the paper surface becomes low during normal operation. The process is started (step # 101). When the refreshing process of the developing device is started, as shown in FIG. 5, the power of the drum motor that rotationally drives the photosensitive drum 21 is turned on, and the rotation of the photosensitive drum 21 is started.

  In step # 102 shown in FIG. 4, the developing device refresh toner is supplied to the photosensitive drum 21. In step # 103, the primary transfer unit 30 applies a refresh bias voltage as shown in FIG. This is a bias voltage of plus 500 V, which has a polarity different from that during the primary transfer of the toner image and the same polarity as the charging polarity of the photosensitive drum 21 and the toner. As a result, the toner for developing device refreshing passes through the primary transfer unit 30 while being adhered to the surface of the photosensitive drum 21 without being transferred to the intermediate transfer belt 8. At this time, the primary transfer unit 30 is configured such that the intermediate transfer belt 8 is attracted to the photosensitive drum 21 as shown in FIG.

  Thereafter, the process proceeds to step # 104, and the toner for refreshing the developing device is collected by the cleaning roller 82 and the cleaning blade 83 of the cleaning device 80. When the recovery of the developing device refresh toner in step # 104 is completed, in step # 105, the bias voltage application of the primary transfer unit 30 is turned off (0 V) as shown in FIG. finish.

  Subsequently, when proceeding to step # 106, in preparation for the start of the next normal image forming operation, the primary transfer unit 30 applies the bias voltage to the same polarity as that at the time of primary transfer, as shown in FIG. . The primary transfer bias is −500 to −1000 V as described above. In step # 107, the image forming unit 20 enters a standby state. The standby period is 1.8 seconds as shown in step # 107 of FIG. 4 and FIG. The reason for setting this standby period will be described in detail later.

  After 1.8 seconds, the process proceeds to step # 108, where the image forming unit 20 starts the next image forming operation and enters a normal operation state.

  Next, in the refreshing process of the developing device 60, after the process is completed, the elapsed time from when the transfer bias of the primary transfer unit 30 is switched to the same polarity as during the primary transfer until the start of the image forming operation is set. This will be described with reference to FIG. FIG. 7 is a graph showing the relationship between the elapsed time after switching the bias voltage and the maximum color shift amount.

  Regarding the elapsed time after switching the bias voltage, in the refreshing process, toner was supplied from the developing device 60 to the surface of the photosensitive drum 21 in a linear shape having a predetermined width extending in the drum axis direction, that is, 100 mm in the drum rotation direction. Then, after switching the bias voltage to the same polarity as in the primary transfer, as shown in FIG. 7, the elapsed time is changed between 0 to 3.5 seconds, and a predetermined image is displayed by the four image forming units 20. The maximum color shift amount after the formation was measured.

  According to FIG. 7, it can be seen that the maximum color misregistration amount is 3 dots or more until the elapsed time is 1.7 seconds. When the elapsed time is 1.8 seconds or longer, the maximum color misregistration amount is 1 dot or 2 dots, and it can be seen that the color misregistration amount is kept low.

  Next, considering this 1.8 seconds, as shown in FIG. 2, the interval between the image forming units 20 is 90 mm, and the most upstream magenta primary transfer unit 30M is the most downstream. The distance to a certain black primary transfer portion 30B is 270 mm.

  Since the sheet conveyance speed, that is, the peripheral speed of the intermediate transfer belt 8 is 150 mm / s as described above, the passing time of the intermediate transfer belt 8 from the magenta primary transfer portion 30M to the black primary transfer portion 30B. Becomes 1.8 seconds (= 270/150). Therefore, after the bias voltage is switched to the same polarity as that at the time of the primary transfer, the position of the intermediate transfer belt 8 positioned in the magenta primary transfer unit 30M at the time of switching is the belt rotation of the four image forming units. The predetermined elapsed time was 1.8 seconds, which is the time required to pass through the primary transfer portion 30B of the black image forming portion 20B located on the most downstream side in the direction.

  In this way, a plurality of image forming units 20 arranged in a line along the intermediate transfer belt 8 are provided, and a toner image formed on the surface of the photosensitive drum 21 which is an image carrier of each image forming unit 20 is The toner image is primarily transferred from the photosensitive drum 21 to the intermediate transfer belt 8 and secondarily transferred from the intermediate transfer belt 8 to the paper P. During the non-transfer period to the paper P, toner is transferred from the developing device 60 to the photosensitive drum 21. In the image forming apparatus 1 that supplies and executes the refresh process for refreshing the surface of the developing device 60 and / or the photosensitive drum 21, a bias voltage having a polarity different from that at the time of the primary transfer of the toner image is applied to the primary transfer unit 30. As a result, the toner supplied to the photosensitive drum 21 during the refresh process passes through the primary transfer unit 30 without being transferred to the intermediate transfer belt 8, and The bias voltage is switched to the same polarity as that at the time of primary transfer, and the image forming operation is started after a lapse of a predetermined time. Therefore, during the refresh process, the tension of the intermediate transfer belt 8 shaped so as to be attracted to the photosensitive drum 21 is The image forming operation can be started in a state in which the image is normally recovered. Thereby, even when color printing with a plurality of colors is performed following the refreshing process of the surface of the developing device 60 and / or the photosensitive drum 21, the occurrence of color misregistration can be prevented. Therefore, it is possible to obtain the image forming apparatus 1 capable of performing high-quality image formation.

  Further, after the refresh step, the predetermined time is changed to the same polarity as that at the time of primary transfer after the refresh step, and the location of the intermediate transfer belt 8 positioned at the primary transfer portion at the time of switching is changed to a plurality of image forming portions. 20 is more than the time required to pass through the primary transfer portion 30B of the black image forming portion 20B on the most downstream side in the belt rotation direction, so that it is drawn to the photosensitive drum 21 during the refresh process. Further, the tension of the intermediate transfer belt 8 can be recovered normally, and the time required for the refresh process can be kept as short as possible. Thereby, when performing color printing with a plurality of colors following the refresh process of the developing device 60, in addition to preventing the occurrence of color misregistration, it is possible to prevent the high-speed printing performance of the image forming apparatus 1 from being deteriorated. become.

  Although the embodiments of the present invention have been described above, the scope of the present invention is not limited to these embodiments, and various modifications can be made without departing from the spirit of the invention.

  For example, in the above-described embodiment, the developing device 60 includes the developing roller 66 disposed facing the photosensitive drum 21 and the magnetic roller 64 disposed facing the developing roller 66, and the surface of the magnetic roller 64 using a two-component developer. A magnetic brush is formed on the surface of the developing roller 66 by the magnetic brush, and then the electrostatic latent image formed on the surface of the photosensitive drum 21 by the toner thin layer is developed into a toner image. However, the developing method is not limited to this, and other developing methods such as a developing method using a one-component developer may be used.

  In the above embodiment, the numerical values used as the interval between the adjacent image forming units 20 and the supply length of the developing device refreshing toner (the width of the line extending in the drum axis direction) are only one embodiment, and It is not limited, and other lengths may be used.

  The present invention relates to an image forming apparatus that includes an intermediate transfer member and a plurality of image forming units arranged in a line along the intermediate transfer member, and executes a refreshing step of the developing device and / or the surface of the image carrier. Is available.

1 is a schematic vertical sectional front view of an image forming apparatus according to an embodiment of the present invention. FIG. 2 is a schematic partial enlarged view showing an arrangement of image forming units in FIG. 1. FIG. 2 is a partially enlarged vertical sectional view showing the periphery of the image forming unit in FIG. 1. 6 is a flowchart showing an operation during a refresh process of the developing device. 3 is a control chart of a photosensitive drum and a primary transfer unit. It is a model front view which shows the state of the primary transfer part at the time of a refresh process. It is a graph which shows the relationship between the elapsed time after bias voltage switching, and the maximum color shift amount.

Explanation of symbols

1 Image forming device 8 Intermediate transfer belt (intermediate transfer member)
20 Image forming unit 21 Photosensitive drum (image carrier)
DESCRIPTION OF SYMBOLS 30 Primary transfer part 31 Primary transfer roller 60 Developing device 64 Magnetic roller 66 Developing roller 80 Cleaning device

Claims (1)

A plurality of image forming units arranged in a line along the intermediate transfer member, and a toner image formed on the surface of the image carrier of each image forming unit is primarily transferred from the image carrier to the intermediate transfer member, and the intermediate transfer Secondary transfer from the body to the sheet, and during the non-transfer period to the sheet, a toner is supplied from the developing device to the image carrier to perform a refresh process for refreshing the surface of the developing device and / or the image carrier. In the image forming apparatus to
By applying a bias voltage having a polarity different from that at the time of primary transfer of the toner image to the primary transfer unit, the toner supplied to the image carrier during the refresh process is not transferred to the intermediate transfer unit without transferring it to the intermediate transfer unit. The bias voltage is switched to the same polarity as in the primary transfer, and after a predetermined time has passed, the image forming apparatus starts an image forming operation .
After the refresh step, the bias voltage is switched to the same polarity as that at the time of primary transfer after the predetermined time, and the location of the intermediate transfer member positioned at the primary transfer portion at the time of switching is the plurality of image formations. An image forming apparatus characterized in that the time is longer than the time required to pass through the primary transfer portion of the image forming portion that is the most downstream in the rotation direction of the intermediate transfer member .

Images