JP6082688B2 - Image forming apparatus - Google Patents

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 printer, a facsimile, or a composite machine using an electrophotographic method, and in particular, the surface potential of an image carrier when a recording medium having a size smaller than the maximum width is used. It is related with the method of suppressing the fluctuation | variation of this.

  In an image forming apparatus using an electrophotographic system such as a copying machine, a printer, a facsimile, etc., a powdery developer (toner) is mainly used, and a photosensitive layer on the surface of a photosensitive drum (image carrier) is predetermined by a charging device. After charging to a surface potential (same polarity as the charging polarity of the toner), an electrostatic latent image is formed on the photosensitive drum by the exposure device. Then, the formed electrostatic latent image is visualized with toner in the developing device, and the toner image is recorded on a recording medium that passes through a nip portion (transfer nip portion) between the photosensitive drum and a transfer member that contacts the photosensitive drum. Generally, a process of performing a fixing process after the image is transferred to the sheet. At this time, the transfer process of the toner image to the recording medium is performed in a state where a transfer bias having a polarity opposite to the charging polarity of the toner is applied to the transfer member.

  In such an image forming apparatus, generally, a plurality of types of sheets having different sizes in the width direction perpendicular to the transport direction are used as recording media. For this reason, after transfer is performed on a sheet having a width smaller than the maximum width through which the sheet can be passed, an area (non-contact area) where the sheet does not come into contact with both ends in the rotation axis direction of the photosensitive drum is generated.

  Since the transfer member to which the transfer bias is applied directly touches the non-contact area, the transfer current flows from the transfer member to the photosensitive drum, and the surface potential of the non-contact area is in contact with the paper. The potentials of the different values are lowered by being pulled to the opposite polarity side than the surface potential of the region (contact region). This is because, in particular, the photosensitive drum characteristic of the photosensitive drum having the organic photosensitive layer (OPC) has a defect that an electron trap is easily formed in the photosensitive layer.

  This electron trap prevents positive charge movement, and is present in the material constituting the photosensitive layer itself, and is accumulated in the material due to light fatigue or the like. When an electron trap is formed in the photosensitive layer, when a charging voltage is applied to the photosensitive layer by the charging device (an electric field is applied), the electrons trapped by the electric field pop out and are applied to the surface of the photosensitive drum. The surface potential of the photosensitive drum is lowered to cancel the positive charge that has been generated, but depending on how much negative electric field (transfer bias) is applied by the transfer member, there is a difference in how easily the electron trap pops out. It also affects the surface potential of the body drum. That is, the surface potential is greatly lowered in the non-sheet passing region where the transfer current flows directly.

  As described above, a small amount of developer adheres to the non-contact area of the photosensitive drum in the development process due to the difference in surface potential between the contact area and the non-contact area of the photosensitive drum. A phenomenon called so-called edge fogging that shifts to the edge of the paper in the width direction, or a density step in the paper width direction when a wide paper is used after a narrow paper is used. There was a problem.

  Therefore, for example, Patent Document 1 has a recording medium width detection unit, and when the detected width of the recording medium is narrower than the maximum width, the area of the post-transfer area in the direction of the rotation axis of the photoconductor. Among them, the surface potential of the non-contact area is raised by reducing the amount of static elimination in the non-contact area (non-sheet passing area) of the narrow width recording medium compared to the contact area (sheet passing area) of the recording medium. An image forming apparatus that reduces the potential difference between the contact area and the non-contact area is disclosed.

JP 2010-210892 A

  However, in the method disclosed in Patent Document 1, it is necessary to control the output of the static eliminator by dividing it into a plurality according to the size in the width direction of the recording medium, and the control becomes complicated. Further, the size in the width direction of the recording medium can be set infinitely by the user's intention, and it is difficult to completely cope with the light emitting area of the static eliminator. Further, as long as the photosensitive drum is irradiated with light by the static eliminator, not a few electron traps are generated, and depending on the conditions, the potential difference in the rotation axis direction of the photosensitive drum may not be completely eliminated.

  In view of the above problems, the present invention provides an image forming apparatus capable of suppressing the occurrence of surface potential unevenness in the rotation axis direction of an image carrier when a recording medium having a size smaller than the maximum width is used by a simple method. For the purpose.

  In order to achieve the above object, the first configuration of the present invention includes an image carrier, a charging device, an exposure device, a developing device, a transfer member, a charge eliminating device, a charging bias power source, a transfer bias power source, And an image forming apparatus. A photosensitive layer is formed on the surface of the image carrier. The charging device charges the surface of the image carrier with the same polarity as the charging polarity of the toner. The exposure device exposes and scans the surface of the image carrier uniformly charged by the charging device to form an electrostatic latent image on the image carrier. The developing device is disposed on the downstream side of the charging device with respect to the rotation direction of the image carrier, and forms a toner image on the image carrier by attaching toner to the electrostatic latent image formed on the image carrier. The transfer member is disposed on the downstream side of the developing device with respect to the rotation direction of the image carrier so as to contact the image carrier to form a transfer nip portion, and the toner image is transferred onto a recording medium passing through the transfer nip portion. To do. The static eliminator is disposed downstream of the transfer member and upstream of the charging device with respect to the rotation direction of the image carrier, and removes residual charges on the surface of the image carrier to a predetermined potential or less. The charging bias power source applies a charging bias to the charging device. The transfer bias power source applies a transfer bias having a polarity opposite to the charging polarity of the toner or a transfer reverse bias having the same polarity as the charging polarity of the toner to the transfer member. When the width direction size of the recording medium that passes through the transfer nip is smaller than the image formable area of the image carrier, the static eliminator is installed after the trailing end of the recording medium passes through the transfer nip. A charging step of charging the surface of the image carrier to the same polarity as the charging polarity of the toner using at least one of a charging device or a transfer member in the off state, and a discharging step of turning on the discharging device after the charging step; A surface potential equalization mode having

  According to the first configuration of the present invention, when the width direction size of the recording medium passing through the transfer nip is smaller than the image formable area of the image carrier, the surface of the image carrier with the static eliminator turned off. By performing a charging process in which the toner is charged to the same polarity as the charging polarity of the toner, the surface potential of the area in contact with the recording medium becomes the same as that in the area not in contact with the recording medium. Thereafter, the surface charge of the image bearing member can be uniformly discharged by performing a charge removal step for turning on the charge removal device. Therefore, since the surface potential unevenness in the rotation axis direction of the image carrier is eliminated at the time of the next image formation, regardless of the width direction size of the recording medium to be used, it is possible to suppress the density step and the edge fogging and to improve the quality. Image quality can be maintained.

1 is a schematic diagram illustrating an overall configuration of an image forming apparatus 100 according to an embodiment of the present invention. Partial enlarged view of the image forming unit 9 in FIG. A block diagram showing an example of a control path used in the image forming apparatus 100 The top view which shows a mode that the paper from which the width direction size passes the photoconductive drum 14 of the image forming apparatus 100. FIG. Graph showing the behavior of the surface potential in the contact area and non-contact area by executing the surface potential equalization mode 7 is a flowchart showing the execution procedure of the surface potential uniformization mode in the image forming apparatus 100 of the present invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic diagram illustrating the overall configuration of an image forming apparatus 100 according to an embodiment of the present invention, and FIG. 2 is a partially enlarged view of the periphery of the image forming unit 9 in FIG. In FIG. 1, the right side is illustrated as the front side of the image forming apparatus 100. As shown in FIG. 1, an image forming apparatus 100 (in this case, a monochrome printer) includes a paper feed cassette 2 that accommodates sheets stacked on the lower part of the apparatus body 1. Above the sheet feeding cassette 2, a sheet conveyance path 4 is formed that extends substantially horizontally from the front to the rear of the apparatus main body 1 and further extends upward to reach the paper discharge unit 3 formed on the upper surface of the apparatus main body 1. A pickup roller 5, a feed roller 6, an intermediate conveyance roller 7, a registration roller pair 8, an image forming unit 9, a fixing device 10, and a discharge roller pair 11 are arranged in this order along the paper conveyance path 4 from the upstream side. ing. Further, in the image forming apparatus 100, a control unit 90 that controls the operations of the above-described rollers, the image forming unit 9, the fixing device 10, and the like is disposed.

  The paper feed cassette 2 is provided with a paper stacking plate 12 that is rotatably supported with respect to the paper feed cassette 2 by a rotation fulcrum 12a provided at the rear end in the paper transport direction. The paper (recording medium) stacked on the paper 12 is pressed by the pickup roller 5. In addition, a retard roller 13 is disposed on the front side of the paper feed cassette 2 so as to be in pressure contact with the feed roller 6. The paper is rolled by the roller 6 and the retard roller 13 so that only the uppermost sheet is conveyed.

  Then, the paper rolled by the feed roller 6 and the retard roller 13 is conveyed to the registration roller pair 8 by changing the conveyance direction to the rear of the apparatus by the intermediate conveyance roller 7, and the timing is adjusted by the registration roller pair 8. Are supplied to the image forming unit 9.

  The image forming unit 9 forms a predetermined toner image on a sheet by an electrophotographic process. The image forming unit 9 is a photosensitive drum 14 that is an image carrier that is rotatably supported in the clockwise direction in FIG. A charging device 15, a developing device 16, a cleaning device 17, a transfer roller 18 disposed so as to face the photosensitive drum 14 across the paper conveyance path 4, and the photosensitive drum 14. 1 is composed of an exposure device (LSU) 19 and a charge removal device 25 (not shown in FIG. 1). A toner container 20 for supplying toner to the developing device 16 is disposed above the developing device 16.

  In the present embodiment, the photosensitive drum 14 is an organic photoreceptor in which an organic photosensitive layer (OPC) is laminated on the outer peripheral surface of a conductive substrate (cylinder) such as aluminum.

  The charging device 15 includes a charging roller 41 that contacts the photosensitive drum 14 and applies a charging bias to the drum surface and a charging roller cleaning brush 43 for cleaning the charging roller 41 in the charging housing. . The charging roller 41 is made of conductive rubber and is disposed so as to contact the photosensitive drum 14.

  When the photosensitive drum 14 rotates in the clockwise direction in FIG. 2, the charging roller 41 that contacts the surface of the photosensitive drum 14 is rotated in the counterclockwise direction in FIG. At this time, the surface of the photosensitive drum 14 is uniformly charged by applying a predetermined voltage to the charging roller 41. Further, as the charging roller 41 rotates, the charging cleaning roller 43 that contacts the charging roller 41 is driven to rotate in the clockwise direction in FIG. 2 to remove foreign matter attached to the surface of the charging roller 41.

  The developing device 16 supplies toner to the electrostatic latent image formed on the photosensitive drum 14 by the developing roller 16a. The toner is supplied to the developing device 16 from the toner container 20 (see FIG. 1) via an intermediate hopper (not shown). Here, a one-component developer (hereinafter also simply referred to as toner) composed only of magnetic toner components is accommodated in the developing device 16.

  The transfer roller 18 is disposed on the downstream side of the developing device 16 with respect to the rotation direction of the photosensitive drum 14 so as to come into contact with the photosensitive drum 14 to form a transfer nip portion N, and is formed on the surface of the photosensitive drum 14. The toner image thus transferred is transferred onto the sheet P when the sheet P conveyed through the sheet conveyance path 4 passes through the transfer nip N.

  The cleaning device 17 includes a rubbing roller 45, a cleaning blade 47, and a toner collection roller 50. The rubbing roller 45 is in pressure contact with the photosensitive drum 14 at a predetermined pressure, and is rotated in the same direction on the contact surface with the photosensitive drum 14 by a drum cleaning motor (not shown). Residual toner on the surface of the photosensitive drum 14 is removed and the photosensitive layer on the surface of the photosensitive drum 14 is rubbed and polished using the residual toner. The toner supplied from the developing device 16 is a toner containing an abrasive (abrasive toner). The polishing toner not only adheres to the electrostatic latent image on the photosensitive drum 14 to form a toner image, but also polishes the surface of the photosensitive drum 14 using residual toner that has not been transferred by the transfer roller 18. Used to do.

  The linear velocity of the rubbing roller 45 is controlled to be faster (1.2 times here) than the linear velocity of the photosensitive drum 14. Examples of the rubbing roller 45 include a structure in which a foam layer made of EPDM rubber and having an Asker C hardness of 55 ° is formed as a roller body around a metal shaft.

  The material of the roller body is not limited to EPDM rubber, but may be a rubber or foamed rubber body of another material, and those having an Asker C hardness of 10 to 90 ° are preferably used. In addition, Asker C is one of durometers (spring type hardness testers) defined in the Japan Rubber Association standard, and is a measuring instrument for measuring hardness. Asker C hardness refers to the hardness measured with the above measuring instrument, and the larger the value, the harder the material.

  A cleaning blade 47 is fixed in contact with the photoconductive drum 14 on the surface of the photoconductive drum 14 downstream of the contact surface with the rubbing roller 45 in the rotation direction. As the cleaning blade 47, for example, a polyurethane rubber blade having a JIS hardness of 78 ° is used, and is attached at a predetermined angle with respect to the tangential direction of the photosensitive member at the contact point. The material, hardness, and dimensions of the cleaning blade 47, the amount of biting into the photosensitive drum 14, the pressure contact force, and the like are appropriately set according to the specifications of the photosensitive drum 14. The JIS hardness refers to the hardness specified by Japanese Industrial Standards (JIS).

  The toner collecting roller 50 collects toner and the like attached to the rubbing roller 45 by rotating in reverse with the rubbing roller 45 while contacting the surface of the rubbing roller 45. The toner collected by the toner collection roller 50 is scraped off from the surface of the toner collection roller 50 by a scraper (not shown). The residual toner removed from the surface of the photosensitive drum 14 by the cleaning blade 47 and the waste toner scraped off from the surface of the toner collection roller 50 are discharged to the outside of the cleaning device 17 by a collection spiral (not shown).

  The static eliminator 25 is disposed downstream of the cleaning device 17 and upstream of the charging device 15 with respect to the rotation direction of the photosensitive drum 14. The static eliminator 25 has a light emitting element such as a light emitting diode (LED), and removes residual charges on the surface of the photosensitive drum 14 to a predetermined potential or less by irradiating the photosensitive drum 14 with static eliminating light.

  When image data is input from a host device such as a personal computer, first, the charging device 15 uniformly charges the surface of the photosensitive drum 14. Next, an electrostatic latent image based on the image data input on the photosensitive drum 14 is formed by the laser beam from the exposure device (LSU) 19. Further, the developing device 16 attaches toner to the electrostatic latent image and forms a toner image on the surface of the photosensitive drum 14.

  The toner image formed on the surface of the photosensitive drum 14 is transferred to the paper P supplied to the nip portion (transfer nip portion N) between the photosensitive drum 14 and the transfer roller 18 by the transfer roller 18. At this time, a negative voltage (transfer bias) having a polarity opposite to the charging polarity (here, positive polarity) of the toner is applied to the transfer roller 18 by a transfer bias power source 54 (see FIG. 3). Due to the movement of the charged toner from the photosensitive drum 14 to the paper P, a current flows between the photosensitive drum 14 and the transfer roller 18. This current is controlled to a predetermined value (constant current control) and the paper P The amount of toner attached to the toner is stabilized.

  The paper P on which the toner image has been transferred is separated from the photosensitive drum 14 and conveyed toward the fixing device 10. The fixing device 10 is disposed on the downstream side of the image forming unit 9 with respect to the paper transport direction. The paper P on which the toner image is transferred in the image forming unit 9 is heated by a heating roller 22 provided in the fixing device 10, The toner image transferred to the paper P is fixed by being heated and pressed by the pressure roller 23 pressed against the heating roller 22. The paper P on which image formation has been performed in the image forming unit 9 and the fixing device 10 is discharged to the paper discharge unit 3 by the discharge roller pair 11.

  After the transfer, the residual toner on the surface of the photosensitive drum 14 is removed by the cleaning device 17, and the residual charge on the surface of the photosensitive drum 14 is neutralized by the neutralization device 25. The charging device 15 is simultaneously charged for a while after the neutralization device 25 is turned on, and thereafter, only the neutralization by the neutralization device 25 is performed with the charging device 15 turned off. As a result, after the toner image is transferred to the paper P, unnecessary toner adhesion to the photosensitive drum 14 is suppressed. The photosensitive drum 14 is charged again by the charging device 15 and image formation is performed in the same manner.

  FIG. 3 is a block diagram illustrating an example of a control path used in the image forming apparatus 100. It should be noted that since various control of each part of the apparatus is performed when the image forming apparatus 100 is used, the control path of the entire image forming apparatus 100 becomes complicated. Therefore, here, a portion of the control path that is necessary for the implementation of the present invention will be mainly described.

  The bias control circuit 51 is connected to the charging bias power source 52, the developing bias power source 53, and the transfer bias power source 54, and operates these power sources 52 to 54 according to output signals from the control unit 90. To 54 apply a predetermined bias to the charging device 15, the developing roller 16 a, and the transfer roller 18 in accordance with a control signal from the bias control circuit 51.

  The operation unit 70 is provided with a liquid crystal display unit 71 and LEDs 72 that indicate various states, and displays the state of the image forming apparatus 100 and displays the image forming status and the number of copies to be printed. Various settings of the image forming apparatus 100 are performed from a printer driver of a personal computer.

  In addition, the operation unit 70 is provided with a stop / clear button used when image formation is stopped, a reset button used when various settings of the image forming apparatus 100 are set to a default state, and the like.

  The control unit 90 includes a central processing unit (CPU) 91 as a central processing unit, a read only memory (ROM) 92 that is a read-only storage unit, a random access memory (RAM) 93 that is a read / write storage unit, A plurality of (two in this case) I's for temporarily transmitting control signals to the temporary storage unit 94 for storing image data and the like, and for receiving input signals from the operation unit 70. / F (interface) 95 is provided at least. Further, the control unit 90 can be arranged at an arbitrary location inside the apparatus main body.

  In addition, the control unit 90 transmits a control signal from the CPU 91 to the respective units and apparatuses in the image forming apparatus 100 through the I / F 95. In addition, a signal indicating the state and an input signal are transmitted from each part or device to the CPU 91 through the I / F 95. Examples of each part and device controlled by the control unit 90 include the charging device 15 of the image forming unit 9, the transfer roller 18, the neutralization device 25, the fixing device 10, the bias control circuit 51, and the operation unit 70.

  The ROM 92 stores a control program for the image forming apparatus 100, data necessary for control, and the like that are not changed during use of the image forming apparatus 100. The RAM 93 stores necessary data generated during the control of the image forming apparatus 100, data temporarily required for controlling the image forming apparatus 100, and the like.

  Next, the charging bias applied to the charging device 15, the transfer bias applied to the transfer roller 18, and the on / off control of the charge removal device 25 in the image forming apparatus 100 of the present invention will be described in detail. FIG. 4 is a plan view illustrating a state in which sheets having different sizes in the width direction pass through the photosensitive drum 14 (transfer nip portion N) of the image forming apparatus 100.

  As shown in FIG. 4, when the paper P having the maximum size in the width direction passes through the photosensitive drum 14, the paper P contacts the entire image formable region Rmax of the photosensitive drum 14. On the other hand, when a sheet P ′ whose size in the width direction is smaller than the sheet P passes through the photosensitive drum 14, only the central portion in the rotation axis direction (left and right direction in FIG. 4) of the photosensitive drum 14 becomes the sheet P ′. A contact area (contact area) R1 is formed, and an area (non-contact area) R2 that does not contact the paper is generated at both ends.

  As described above, since the transfer roller 18 to which the transfer bias is applied directly touches the non-contact region R2 of the photoconductive drum 14, a transfer current flows from the transfer roller 18 to the photoconductive drum 14, and the non-contact region R2. The surface potential of the region R2 is pulled and lowered to the opposite polarity side than the surface potential of the contact region R1.

  Therefore, in the present invention, when the sheet P ′ has a width direction size narrower than the maximum width, the charging step of charging the surface of the photosensitive drum 14 with the charging roller 41 in a state in which the static eliminator 25 is turned off after the transfer is completed. And a neutralization step of neutralizing the surface charge by turning on the neutralization device 25 after the charging step, and performing all the operations after the surface potential of the photosensitive drum 14 is uniformized. finish.

  FIG. 5 is a graph showing the behavior of the surface potential in the contact area and the non-contact area when the surface potential equalization mode is executed, and FIG. 6 is a flowchart showing an example of the execution procedure of the surface potential uniformity mode. The surface potential equalization mode of the image forming apparatus 100 will be described in detail along the steps in FIG. 6 with reference to FIGS.

  When the toner image formed on the photosensitive drum 14 in the image forming unit 9 is transferred onto the sheet and the trailing end of the sheet passes through the transfer nip (step S1), the control unit 90 determines whether the sheet width is the maximum size. Is determined (step S2). The paper width may be determined based on a paper size input from the printer driver, or may be determined based on a detection result of a conveyance detection sensor (not shown) arranged in the image forming apparatus 100. .

  When the paper width is not the maximum size (NO in step S2), as shown in FIG. 5, a contact area R1 and a non-contact area R2 are generated on the outer peripheral surface of the photosensitive drum 14. Next, the control unit 90 determines whether a predetermined number of sheets that are not the maximum size are continuous (step S3). When a predetermined number of non-maximum size sheets pass through the transfer nip portion continuously (YES in step S3), the photosensitive drum in the non-contact area R2 (broken line in FIG. 5) is compared with the contact area R1 (solid line in FIG. 5). The surface potential of 14 is low.

  Therefore, the control unit 90 determines whether the sheet that has passed through the transfer nip is the final sheet for single printing or continuous printing (step S4). In the case of the final sheet of single-shot printing or continuous printing (YES in step S4), the control unit 90 transmits a control signal to the bias control circuit 51, and turns on the charging device 15 with the static eliminator 25 turned off. (Step S5). The state in which the static eliminator 25 is turned off and the charging device 15 is turned on needs to be continued for at least the time for which the photosensitive drum 14 makes one revolution, and as shown in FIG. It is preferable. During this time, the surface potential of the non-contact region R2 becomes the same as that of the contact region R1 (charging process).

  Thereafter, when the photosensitive drum 14 makes two revolutions (YES in step S6), the static eliminator 25 is turned on (step S7). As shown in FIG. 6, the state in which the static elimination device 25 is turned on and the charging device 15 is turned on is continued while the photosensitive drum 14 makes one revolution, and the surface potentials of the contact region R1 and the non-contact region R2 slightly decrease during this time. (First charge eliminating step).

  Then, when the photosensitive drum 14 makes one revolution (YES in step S6), the charging device 15 is turned off (step S7). The state in which the static eliminator 25 is turned on and the charging device 15 is turned off is continued while the photosensitive drum 14 makes one turn (step S8), and the contact area R1 and the non-contact area R2 of the photosensitive drum 14 have a uniform surface charge. It will be in the state neutralized (2nd static elimination process).

  On the other hand, when the paper width is the maximum size (YES in step S2), the non-contact region R2 does not occur on the outer peripheral surface of the photosensitive drum 14. In addition, when a predetermined number of sheets that are not the maximum size are not continuous (NO in step S3), the potential difference between the contact area R1 and the non-contact area R2 is not so large as to cause edge fogging or density step. For this reason, the neutralizing device 25 and the charging device 15 are turned on without executing the surface potential equalization mode (step S11), and then the charging device 15 is turned off (step S12). By doing so, the surface charge can be uniformly eliminated.

  Further, even when a predetermined number of non-maximum size sheets are continuously printed, if continuous printing is continued, the image forming efficiency is reduced if the job is interrupted and the surface potential equalization mode is executed. . Therefore, the surface potential uniformization mode is not executed during continuous printing (NO in step S4), and the surface potential uniformization mode is executed at the end of the job.

  According to the above control, by performing the charging step of turning on the charging device 15 with the static eliminator 25 turned off, even when a sheet whose width direction size is smaller than the image formable region Rmax is passed. Since the surface potential unevenness in the rotation axis direction of the photosensitive drum 14 is eliminated at the start of the next image formation, a good image quality can be maintained regardless of the size in the width direction of the paper. Further, surface potential unevenness can be effectively eliminated by simple control that simply turns on or off the charging device 15 and the charge removal device 25.

  It is possible to immediately remove the charge on the surface of the photosensitive drum 14 (second charge removal step) by turning on the charge removal device 25 and turning off the charge device 15 immediately after the charging step. However, the charge device 15 is turned on. By providing the first charge removal step of removing the charge by the charge removal device 25 in the state, the energetically unstable charge is less likely to remain inside the photosensitive layer. As a result, the charge behavior in the photosensitive layer can be stabilized to eliminate charge unevenness.

  Furthermore, the surface potential equalization mode is executed only when a predetermined number of non-maximum size sheets are continuously passed and is the last sheet of single printing or continuous printing. Execution of the uniform mode is suppressed to the minimum necessary, and a decrease in image forming efficiency can be suppressed.

  In addition, this invention is not limited to the said embodiment, A various change is possible in the range which does not deviate from the meaning of this invention. For example, in the above configuration, the photosensitive drum 14 is charged by applying a charging bias to the charging device 15 in a state in which the charge removal device 25 is turned off, but the transfer roller 18 has the same polarity as the toner charging polarity. It is also possible to charge the photosensitive drum 14 by applying a positive voltage (transfer reverse bias). When the transfer bias power supply 54 cannot be applied with a high voltage like the charging bias power supply 52, the charging time of the photosensitive drum 14 by the transfer roller 18 may be set longer than that of the charging device 15.

  Further, if the charging step is performed using both the charging device 15 and the transfer roller 18, the non-contact region R2 and the contact region R1 of the photosensitive drum 14 have the same potential in a shorter time, and therefore the surface The execution time of the potential equalization mode can be shortened.

  Further, the control shown in FIG. 6 is an example, and the surface potential equalization mode is executed not only at the end of the job but also when switching from a sheet having a small sheet width to a large sheet is performed. Alternatively, the sheet width may be detected every time a sheet is passed, and the surface potential equalization mode may be executed when a sheet that is not the maximum size is passed.

  In the above-described embodiment, the transfer roller 18 is used as a transfer member that is disposed so as to be in contact with the photosensitive drum 14 and forms the transfer nip portion N. However, an endless transfer belt is used instead of the transfer roller 18. Can be brought into contact with the photosensitive drum 14 to form a transfer nip portion.

  Further, instead of the contact charging type charging device 15 using the charging roller 41 as shown in FIG. 2, a corona charging type charging device provided with a corona wire and a grid may be used. Further, instead of the one-component developing type developing device 16, a two-component developing type developing device using a two-component developer containing toner and a magnetic carrier may be used.

  Furthermore, the image forming apparatus of the present invention is not limited to the monochrome printer as shown in FIG. 1, and may be other image forming apparatuses such as monochrome and color copiers, digital multifunction peripherals, color printers, and facsimiles. .

  The present invention can be used in an image forming apparatus including an image carrier such as a photosensitive drum, a charging device that charges the image carrier, and a static eliminator that removes residual charges from the image carrier. By using the present invention, it is possible to eliminate the occurrence of surface potential unevenness in the direction of the rotation axis of the image carrier when using a recording medium having a size smaller than the maximum width by a simple method. It is possible to provide an image forming apparatus capable of effectively suppressing the above.

9 Image forming unit 10 Fixing device 14 Photosensitive drum (image carrier)
DESCRIPTION OF SYMBOLS 15 Charging device 16 Developing device 16a Developing roller 17 Cleaning device 18 Transfer roller (transfer member)
DESCRIPTION OF SYMBOLS 19 Exposure apparatus 25 Static elimination apparatus 41 Charging roller 52 Charging bias power supply 54 Transfer bias power supply 90 Control part 100 Image forming apparatus N Transfer nip part

Claims (6)

An image carrier having a photosensitive layer formed on the surface;
A charging device that charges the surface of the image carrier to the same polarity as the charging polarity of the toner;
An exposure device that exposes and scans the surface of the image carrier uniformly charged by the charging device to form an electrostatic latent image on the image carrier;
A developing device that is disposed downstream of the charging device with respect to the rotation direction of the image carrier, and forms a toner image on the image carrier by attaching toner to the electrostatic latent image;
A transfer nip portion is formed on the downstream side of the developing device with respect to the rotation direction of the image carrier to form the transfer nip portion, and the toner image is transferred to a recording medium passing through the transfer nip portion. A transfer member;
A static eliminator that is disposed downstream of the transfer member and upstream of the charging device with respect to the rotation direction of the image carrier, and removes residual charges on the surface of the image carrier to a predetermined potential or less;
A charging bias power source for applying a charging bias to the charging device;
A transfer bias power source for applying a transfer bias having a reverse polarity to the charge polarity of the toner or a transfer reverse bias having the same polarity as the charge polarity of the toner to the transfer member;
In an image forming apparatus comprising:
If the width direction size of the recording medium passing through the transfer nip is smaller than the image formable area of the image carrier, the static eliminator is turned off after the trailing edge of the recording medium passes through the transfer nip. In this state, at least one of the charging device and the transfer member is used to charge the surface of the image carrier to the same polarity as the charging polarity of the toner, and the neutralization device is turned on after the charging step. and discharging process, Ri executable der surface potential equalizing mode having,
The static elimination step includes a first static elimination step in which the static elimination device is turned on while charging the surface of the image carrier by the charging device or the transfer member, and the static elimination device following the first static elimination step. an image forming apparatus comprising a second charge eliminating step of stopping while turning on the charging of the image bearing member surface, the Rukoto which have a a. 2. The image forming apparatus according to claim 1, wherein each of the first static elimination step and the second static elimination step is continuously performed for a time required for one rotation of the image carrier . An image carrier having a photosensitive layer formed on the surface;
A charging device that charges the surface of the image carrier to the same polarity as the charging polarity of the toner;
An exposure device that exposes and scans the surface of the image carrier uniformly charged by the charging device to form an electrostatic latent image on the image carrier;
A developing device that is disposed downstream of the charging device with respect to the rotation direction of the image carrier, and forms a toner image on the image carrier by attaching toner to the electrostatic latent image;
A transfer nip portion is formed on the downstream side of the developing device with respect to the rotation direction of the image carrier to form the transfer nip portion, and the toner image is transferred to a recording medium passing through the transfer nip portion. A transfer member;
A static eliminator that is disposed downstream of the transfer member and upstream of the charging device with respect to the rotation direction of the image carrier, and removes residual charges on the surface of the image carrier to a predetermined potential or less;
A charging bias power source for applying a charging bias to the charging device;
A transfer bias power source for applying a transfer bias having a reverse polarity to the charge polarity of the toner or a transfer reverse bias having the same polarity as the charge polarity of the toner to the transfer member;
In an image forming apparatus comprising:
If the width direction size of the recording medium passing through the transfer nip is smaller than the image formable area of the image carrier, the static eliminator is turned off after the trailing edge of the recording medium passes through the transfer nip. In this state, at least one of the charging device and the transfer member is used to charge the surface of the image carrier to the same polarity as the charging polarity of the toner, and the neutralization device is turned on after the charging step. A surface potential equalization mode having a charge eliminating step,
The surface potential equalization mode is executed when a printing operation using a recording medium having a smaller size in the width direction than the image formable area of the image carrier is continuously performed for a predetermined number or more. be that images forming device. 4. The image forming apparatus according to claim 3, wherein the surface potential equalization mode is executed after the trailing edge of the recording medium passes through the transfer nip portion at the time of single printing or at the time of final printing of continuous printing. . 5. The image forming apparatus according to claim 1 , wherein the charging step is continuously performed for a time required for the image carrier to make one round . It said image bearing member, an image forming apparatus according to any one of claims 1 to 5, characterized in that an organic photoreceptor organic photosensitive layer formed on the surface.

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