JP5999846B2 - 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 of the recording medium passing through the transfer nip is smaller than the image formable area of the image carrier, the image forming apparatus uses a static eliminator after the trailing end of the recording medium passes through the transfer nip. A toner band forming step of forming a toner band in a region where the recording medium is in contact with the surface of the image carrier before removing the residual charge on the surface of the image carrier; and a transfer bias is applied to the transfer member to transfer the toner band to the transfer member. A surface potential equalization mode having a toner attaching step for attaching to the outer peripheral surface of the toner and a toner returning step for applying a transfer reverse bias to the transfer member to return the toner attached to the transfer member to the image carrier again. is there.

  According to the first configuration of the present invention, when the size in the width direction of the recording medium passing through the transfer nip portion is smaller than the image formable area of the image carrier, the surface potential equalization mode is executed to perform the image carrier. In the area where the recording medium is in contact with the recording medium, most of the charge having the same polarity as the charging polarity of the toner applied to the transfer member in the toner returning process is consumed for returning the toner to the image carrier side. In the non-contact area, a stronger electric field acts on the image carrier than in the area where the recording medium is in contact. As a result, since the elimination of electron traps in the photosensitive layer is promoted in the area where the recording medium is not in contact as compared with the area where the recording medium is in contact, the electron in the area where the recording medium is not in contact is promoted. Traps can be reduced to the same extent. 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 FIG. 4 is a plan view showing a state in which sheets P and P ′ having different sizes in the width direction pass through the photosensitive drum 14 and the transfer roller 18 of the image forming apparatus 100, and shows a state in which a toner band T is formed in a contact region R1. 4 is a diagram illustrating a state in which the toner band T formed in the contact region R1 is attached to the outer peripheral surface of the transfer roller 18 from the state of FIG. 5 is a diagram illustrating a state in which the toner attached to the outer peripheral surface of the transfer roller 18 is returned to the contact region R1 of the photosensitive drum 14 from the state of FIG. 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.

  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 and the transfer roller 18 (transfer nip portion) 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 paper P ′ is smaller than the maximum width in the width direction, a toner band (toner solid portion) is formed in the contact region R1 of the photosensitive drum 14 after the transfer operation as shown in FIG. A toner band forming step for forming T, a toner attaching step for attaching the toner band T formed in the contact region R1 to the outer peripheral surface of the transfer roller 18 as shown in FIG. 5, and a transfer as shown in FIG. And a toner returning step for returning the toner adhering to the roller 18 to the contact region R1 of the photosensitive drum 14 again. After the surface potential is made uniform, the static eliminating device 25 causes the photosensitive drum 14 The residual charge is removed and all operations are terminated.

  FIG. 7 is a flowchart illustrating an example of an execution procedure of the surface potential equalization mode. The surface potential equalization mode of the image forming apparatus 100 will be described in detail along the steps of FIG. 7 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. 4, 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 sheets that are not the maximum size pass through the transfer nip portion continuously (YES in step S3), the surface potential of the photosensitive drum 14 is lower in the non-contact area R2 than in the contact area R1.

  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 image forming unit 9, and the contact area on the photosensitive drum 14 as shown in FIG. A toner band T is formed on R1 (step S5). Specifically, after the surface of the photosensitive drum 14 is uniformly charged by the charging device 15, the exposure device 19 performs exposure by irradiating the entire region in the width direction of the contact region R 1 of the photosensitive drum 14 with laser light. Then, a toner band T is formed by attaching toner to the exposed portion by the developing device 16 (toner band forming step). The length of the toner band T in the circumferential direction of the photosensitive drum 14 is set to a length equal to or longer than one rotation of the transfer roller 18.

  Next, as shown in FIG. 5, the toner band T formed on the photosensitive drum 14 is attached to the outer peripheral surface of the transfer roller 18 (step S6). Specifically, the toner band T is applied to the outer peripheral surface of the transfer roller 18 by continuously applying a transfer bias having a polarity opposite to the charging polarity of the toner to the transfer roller 18 for a time that the photosensitive drum 14 makes one turn. Of these, it is adhered to the entire circumference of the portion facing the contact region R1 (toner adhesion step). The voltage value of the transfer bias applied to the transfer roller 18 in the toner adhering step is the same as the voltage value applied when the toner image is transferred onto the paper, or the amount of toner applied to the paper by the amount of no paper interposed in the transfer nip portion. The voltage is lower than the voltage applied during image transfer.

  Next, as shown in FIG. 6, the toner adhering to the transfer roller 18 is returned again to the contact region R1 of the photosensitive drum 14 (step S7). Specifically, by applying a positive voltage (transfer reverse bias) having the same polarity as the charging polarity of the toner to the transfer roller 18, the toner attached to the transfer roller 18 is caused to fly toward the photosensitive drum 14 (toner return). Process). The reverse transfer bias applied to the transfer roller 18 in the toner returning process is, for example, a bias having the same voltage and reverse polarity as the transfer bias applied to the transfer roller 18 in the toner attaching process.

  The reverse transfer bias is continuously applied for the time that the photosensitive drum 14 makes two revolutions (step S8). As a result, the toner adhering to the transfer roller 18 whose outer peripheral length is smaller than that of the photoconductor drum 14 is gradually returned to the photoconductor drum 14 side and adheres to the entire contact area R1 of the photoconductor drum 14.

  At this time, in the contact region R1, most of the positive charge applied to the transfer roller 18 is consumed to return the toner to the photosensitive drum 14, so that the non-contact region R2 has a stronger positive charge than the contact region R1. This electric field acts on the photosensitive drum 14. As a result, in the non-contact region R2, the elimination of electron traps in the photosensitive layer is promoted compared to the contact region R1, so that the electron traps in the contact region R1 and the non-contact region R2 can be reduced to the same extent. . The toner returned to the photosensitive drum 14 side is removed by the cleaning device 17. Thereafter, the static eliminator 25 and the charging device 15 are turned on (step S9), and then the charging device 15 is turned off (step S10), and the photosensitive drum 14 is rotated once (step S11). The residual charge is removed.

  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 S9), and then the charging device 15 is turned off (step S10). (Step S11), 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, when a sheet having a width direction size smaller than the image formable region Rmax is passed, the surface potential unevenness in the rotation axis direction of the photosensitive drum 14 is eliminated at the start of the next image formation. Therefore, high quality image quality can be maintained regardless of the width direction size of the paper. Further, the surface potential unevenness can be effectively eliminated by simple control in which the toner is moved from the photosensitive drum 14 side to the transfer roller 18 side and returned to the photosensitive drum 14 side again.

  Further, the surface potential equalization mode is executed only when a predetermined number or more sheets of paper that are not the maximum size are continuously passed, and only when the final sheet is a single print or continuous print. Execution of the potential equalization mode is suppressed to the minimum necessary, and a decrease in image formation 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, the control shown in FIG. 7 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 paper with a small paper width to a paper with a large paper width. 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, and the surface potential equalization mode can be executed by applying a transfer bias or a transfer reverse bias to the transfer belt.

  However, when a transfer belt is used, it is necessary to form a long toner band T in order to attach toner to the entire belt circumferential direction, and the amount of toner consumption other than printing increases. Therefore, a configuration using the transfer roller 18 that can reduce toner consumption in the surface potential uniformization mode is preferable.

  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 T Toner band

Claims (7)

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 a transfer nip portion, and a toner image is transferred onto a recording medium passing through the transfer nip portion. A transfer member to be
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 size in the width direction of the recording medium passing through the transfer nip is smaller than the image formable area of the image carrier, the charge removal device removes the trailing edge of the recording medium after passing through the transfer nip. A toner band forming step of forming a toner band in a region where the recording medium on the surface of the image carrier is in contact before removing residual charges on the surface of the image carrier, and applying the transfer bias to the transfer member to apply the toner to the toner A toner attaching step of attaching a band to the outer peripheral surface of the transfer member; and a toner returning step of applying the transfer reverse bias to the transfer member to return the toner attached to the transfer member to the image carrier side again. An image forming apparatus capable of executing a surface potential uniformization mode.   2. The image formation according to claim 1, wherein a length of the toner band formed in the toner band forming step in a circumferential direction of the image carrier is equal to or longer than a length of one rotation of the transfer member. apparatus.   3. The image forming apparatus according to claim 1, wherein in the toner returning step, the reverse transfer bias is continuously applied for a period of time that the image carrier rotates twice.   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. The image forming apparatus according to any one of claims 1 to 3.   5. The image forming apparatus according to claim 4, wherein the surface potential equalization mode is executed after the trailing edge of the recording medium passes through the transfer nip portion during single printing or final printing of continuous printing. .   The image forming apparatus according to claim 1, wherein the transfer member is a transfer roller.   The image forming apparatus according to claim 1, wherein the image carrier is an organic photoreceptor having an organic photosensitive layer formed on a surface thereof.

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