JP2014029385A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device configured to stably control potential of a photoreceptor drum during consecutive paper passing, while preventing a rear face of a sheet from being stained with toner deposited on a transfer roller.SOLUTION: The number of rotations per unit time of a transfer roller 34 is set to be twice the number of rotations per unit time of a photoreceptor drum. The photoreceptor drum 31 and the transfer roller 34 are synchronously rotated. After image formation, cleaning cycle of the transfer roller 34 is executed. In the cleaning cycle, a voltage of the same polarity as that of toner is applied to the transfer roller 34 for a time equivalent to two turns of the photoreceptor drum 31 and four turns of the transfer roller 34. The toner is moved from the transfer roller 34 to the photoreceptor drum 31, accordingly. Uniform electric charges are applied in a circumferential direction of the photoreceptor drum 31 and the transfer roller 34.

Description

  The present invention relates to an image forming apparatus that forms an image on a sheet.

  Conventionally, in an image forming apparatus that forms an image on a sheet, a toner image is formed on a photosensitive drum, and the toner image is transferred to the sheet at a transfer nip portion formed between the photosensitive drum and a transfer roller. The The image forming apparatus further includes a fixing unit, and the sheet on which the toner image is transferred is subjected to a fixing process in the fixing unit and then discharged.

  When images are continuously formed on the sheet at the transfer nip portion, the surfaces of the photosensitive drum and the transfer roller directly contact each other between the sheets corresponding to the gap between the sheets. At this time, toner may adhere to the surface of the transfer roller. When a large amount of toner adheres to the surface of the transfer roller, the back surface of the sheet conveyed to the transfer nip portion is soiled with toner.

  The technique described in Patent Document 1 discloses a technique in which a transfer bias having a polarity opposite to that at the time of printing is applied to a transfer roller in correspondence with the interval between sheets at the time of continuous sheet passing. Such reverse polarity bias attracts the toner from the transfer roller toward the photosensitive drum, and thus prevents the toner from adhering to the transfer roller.

JP 2009-020326 A

  In the technique described in Patent Document 1, there is a case where the unevenness of the potential occurs in the photosensitive drum when the reverse polarity bias as described above is applied between the sheets. In some cases, such potential unevenness can be eliminated by the static eliminator. However, the potential irregularity can be eliminated only by passing the surface of the photosensitive drum where the potential irregularity has occurred once through the region where the static eliminator is opposed. There were cases where it was difficult. In particular, in the case of continuous paper passing, after the surface of the photosensitive drum corresponding to the position between the papers passes through the transfer nip portion, the surface is again charged in the same rotation to form a toner image. Often done. As a result, the above-described potential unevenness also affects the electrostatic latent image, and an image defect may occur in the toner image.

  The present invention has been made in view of the above problems, and the potential of the photosensitive drum during continuous paper feeding is stably controlled, and the back surface of the sheet is prevented from being stained by toner adhering to the transfer roller. An object of the present invention is to provide an image forming apparatus.

  An image forming apparatus according to an aspect of the present invention is an image that carries a toner image that is driven to rotate, an electrostatic latent image is formed on a peripheral surface, and the electrostatic latent image is manifested by toner having a predetermined polarity. A carrier, charging means for charging the peripheral surface of the image carrier, and abutting the peripheral surface of the image carrier, forming a transfer nip between the image carrier and being driven to rotate; A transfer roller for transferring the toner image onto the sheet; a transfer bias applying means for applying a voltage to the transfer roller; a bias control means for controlling the transfer bias applying means; and the image carrier and the transfer roller. Driving means for synchronously driving the image carrier and the transfer roller so that the number of rotations of the transfer roller per unit time is an integral multiple of the number of rotations of the image carrier per unit time; In a continuous printing condition in which toner images are continuously transferred to a plurality of sheets, the transfer between the toner images and a sheet between one sheet and the subsequent sheet passes through the transfer nip portion. The bias control unit controls the transfer bias applying unit to apply a voltage having a polarity opposite to the polarity of the toner to the transfer roller, and after the continuous transfer of the toner image, The driving means further includes a cleaning cycle for rotationally driving the image carrier and the transfer roller, and the bias control means corresponds to a time that is an integral multiple of the rotational period of the image carrier in the cleaning cycle. A voltage having the same polarity as that of the toner is applied to the transfer roller.

  According to this configuration, under continuous printing conditions in which toner images are continuously transferred to a plurality of sheets, a bias is applied when the toner images are transferred and when a sheet formed between the plurality of sheets passes through the transfer nip portion. The control unit controls the transfer bias applying unit to apply a voltage having a polarity opposite to the polarity of the toner to the transfer roller. For this reason, compared to the case where a voltage having the same polarity as that of the toner is applied to the transfer roller between the sheets, it is possible to suppress the potential fluctuation on the surface of the image carrier. On the other hand, since a voltage having a polarity opposite to that of the toner is applied to the transfer roller between the sheets, the toner may adhere to the surface of the transfer roller. Even in such a case, according to the above configuration, the cleaning cycle is provided after the continuous transfer of the toner image. The bias control unit applies a voltage having the same polarity as that of the toner to the transfer roller in the cleaning cycle. For this reason, the toner adhered on the transfer roller is moved to the image carrier side, and the transfer roller is suitably cleaned. Further, the bias controller applies a voltage having the same polarity as that of the toner to the transfer roller corresponding to a time that is an integral multiple of the rotation period of the image carrier. The number of rotations per unit time of the transfer roller is an integer multiple of the number of rotations per unit time of the image carrier. For this reason, uniform charges are imparted in the circumferential direction on the surfaces of the image carrier and the transfer roller. Therefore, potential fluctuations on the surfaces of the image carrier and the transfer roller are suppressed. As a result, it is possible to execute image formation in the state where the potential of the surface of the image carrier is stably controlled during the next printing.

  In the above-described configuration, it is preferable that the driving unit rotationally drives the image carrier and the transfer roller in mechanical synchronization.

  According to this configuration, the image carrier and the transfer roller are rotationally driven mechanically synchronously. Therefore, the above control according to the rotation time corresponding to the integer rotation of the image carrier is realized with higher accuracy.

  In the above-described configuration, the bias control unit corresponds to a time that is an integral multiple of the rotation period of the image carrier after a voltage having the same polarity as the polarity of the toner is applied to the transfer roller. It is desirable to apply a voltage having a polarity opposite to that of the polarity to the transfer roller.

  According to this configuration, even when toner charged to the opposite polarity to the normal polarity is attached to the surface of the transfer roller, the toner is collected on the image carrier side. At this time, a voltage having a polarity opposite to the polarity of the toner is applied to the transfer roller corresponding to the rotation time corresponding to an integer number of rotations of the image carrier, so that the potential fluctuation occurs on the surfaces of the image carrier and the transfer roller. It is suppressed from occurring.

  In the above-described configuration, it is preferable that the bias control unit applies a voltage having a smaller absolute value to the transfer roller when the interval between the sheets passes through the transfer nip portion than when the toner image is transferred.

  According to this configuration, since the absolute value of the voltage applied to the transfer roller during the interval between sheets is smaller than that during the transfer, toner adhesion to the transfer roller is suppressed.

  In the above configuration, it is desirable that the charging operation by the charging unit is stopped in the cleaning cycle.

  According to this configuration, in the cleaning cycle, it is possible to suppress the members other than the transfer roller from applying charges to the image carrier. This further suppresses potential fluctuations on the surfaces of the image carrier and the transfer roller.

  In the above configuration, the image forming apparatus further includes a rotation time calculating unit that calculates a rotation time of the image bearing member or the charging roller, and the bias control unit or the driving unit is configured according to a calculation result of the rotation time calculating unit. It is desirable to execute the application of the voltage or the rotational drive in the cleaning cycle.

  According to this configuration, according to the calculation result of the rotation time calculation unit, the above control according to the rotation time corresponding to the integer rotation of the image carrier is more accurately realized.

  According to the present invention, there is provided an image forming apparatus in which the potential of the photosensitive drum during continuous paper feeding is stably controlled and the back surface of the sheet is prevented from being soiled by the toner attached to the transfer roller.

1 is a perspective view illustrating an appearance of an image forming apparatus according to an embodiment of the present invention. 1 is a cross-sectional view showing an internal structure of an image forming apparatus according to an embodiment of the present invention. FIG. 2 is a schematic diagram illustrating a main part of an image forming unit according to an embodiment of the present invention. 2 is an electrical block diagram of a control unit of the image forming apparatus according to the embodiment of the present disclosure. FIG. 5 is a timing chart including an image forming operation in the image forming apparatus according to the embodiment of the present disclosure.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a perspective view showing an appearance of an image forming apparatus 1 according to an embodiment of the present invention. FIG. 2 is a side sectional view showing the internal structure of the image forming apparatus 1. FIG. 3 is a schematic diagram illustrating a main part of the image forming unit 30 of the image forming apparatus 1. 3 corresponds to a diagram obtained by inverting the left-right direction of FIG. Here, a monochrome printer is illustrated as the image forming apparatus 1, but the image forming apparatus may be a copying machine, a facsimile machine, or a multifunction machine having these functions, and an image forming apparatus that forms a color image. It may be.

  The image forming apparatus 1 includes a main body housing 10 having a substantially rectangular parallelepiped housing structure, and a paper feeding unit 20, an image forming unit 30, a fixing unit 40, and a toner container 50 housed in the main body housing 10. .

  A front cover 11 is provided on the front side of the main body housing 10, and a rear cover 12 is provided on the rear side. When the front cover 11 is opened, the toner container 50 is exposed to the front side. Thereby, the user can take out the toner container 50 from the front side of the main body housing 10 when the toner runs out. The rear cover 12 is a cover that is opened at the time of sheet jam or maintenance. The units of the image forming unit 30 and the fixing unit 40 can be taken out from the rear side of the main body housing 10 by opening the rear cover 12. Further, on the side surface of the main body housing 10, a left cover 12L (FIG. 1) and a right cover 12R (not shown in FIG. 1) opposite to the left cover 12L extend in the vertical direction. It is arranged. An intake port 12La for taking in air into the main body housing 10 is disposed at the front portion of the left cover 12L. Further, on the upper surface of the main body housing 10, a paper discharge unit 13 for discharging the sheet after image formation is provided. Various devices for executing image formation are installed in an internal space S (FIG. 2) defined by the front cover 11, the rear cover 12, the left cover 12L, the right cover 12R, and the paper discharge unit 13. The

  The paper feed unit 20 includes a paper feed cassette 21 that accommodates sheets to be subjected to image forming processing (FIG. 2). A part of the sheet feeding cassette 21 protrudes further forward from the front surface of the main body housing 10. An upper surface of a portion of the sheet cassette 21 accommodated in the main body housing 10 is covered with a sheet cassette top plate 21U. The sheet feeding cassette 21 is provided with a sheet accommodating space for accommodating the bundle of sheets, a lift plate for lifting the bundle of sheets for feeding. A sheet feeding portion 21 </ b> A is provided at an upper portion on the rear end side of the sheet feeding cassette 21. The sheet feeding unit 21A is provided with a sheet feeding roller 21B for feeding the uppermost sheet of the sheet bundle in the sheet feeding cassette 21 one by one.

  The image forming unit 30 performs an image forming process for forming a toner image on a sheet fed from the paper feeding unit 20. The image forming unit 30 includes a photosensitive drum 31 (image carrier), a charging roller 32 (charging unit), an exposure device (not shown in FIG. 2), and a developing unit disposed around the photosensitive drum 31. A device 33, a transfer roller 34, and a cleaning device 35. The image forming unit 30 is disposed between the left cover 12L and the right cover 12R.

  The photosensitive drum 31 includes a rotation shaft and a cylindrical surface (circumferential surface) that rotates around the rotation shaft. An electrostatic latent image is formed on the cylindrical surface, and a toner image in which the electrostatic latent image is made visible by toner having a predetermined polarity is carried on the cylindrical surface. As the photosensitive drum 31, a photosensitive drum using an amorphous silicon (a-Si) material can be used. In FIG. 3, the photosensitive drum 31 is rotationally driven in the direction of arrow D1 by a motor 501 described later. In the present embodiment, the outer diameter of the photosensitive drum 31 is 30 mm.

  The charging roller 32 uniformly charges the surface of the photosensitive drum 31. The charging roller 32 is a roll member whose surface is made of a rubber material. A charging bias is applied to the charging roller 321 by a charging bias applying unit 502 described later. The charging roller 321 is rotated while being driven by the photosensitive drum 31 by contacting the circumferential surface of the photosensitive drum 31. The peripheral surface of the photosensitive drum 31 is uniformly charged by the charging roller 321 to which the charging voltage is applied.

  The cleaning device 35 has a cleaning blade 351 (FIG. 3), cleans the toner adhering to the peripheral surface of the photosensitive drum 31 after the toner image is transferred, and conveys the toner to a collecting device (not shown). The cleaning blade 351 is a plate-like member made of a rubber material. The front end of the cleaning blade 351 is a free end and is in contact with the cylindrical surface of the photosensitive drum 31. The toner remaining on the cylindrical surface of the photosensitive drum 31 is removed by the tip of the cleaning blade 351. The cleaning device 35 includes a housing wall 352 that is a part of the housing of the cleaning device 35 (FIG. 3). A seal member 353 is disposed on the housing wall 352. The seal member 353 is disposed so as to protrude from the housing wall portion 352, and the distal end portion of the seal member 353 comes into contact with the cylindrical surface of the photosensitive drum 31. The seal member 353 prevents the toner from leaking from the inside of the cleaning device 35. In addition, an unillustrated optical static eliminator is disposed downstream of the cleaning device 35 in the rotation direction of the photosensitive drum 31. The optical static eliminator has a function of aligning the potential on the surface of the photosensitive drum 31.

  The exposure apparatus has a laser light source and optical equipment such as a mirror and a lens, and irradiates the peripheral surface of the photosensitive drum 31 with light modulated based on image data given from an external device such as a personal computer. An electrostatic latent image is formed.

  The developing device 33 supplies toner to the peripheral surface of the photosensitive drum 31 in order to develop the electrostatic latent image on the photosensitive drum 31 to form a toner image. For this reason, the developing device 33 contains toner. In this embodiment, the toner has a positively charged polarity. The developing device 33 includes a developing roller 331 that carries toner to be supplied to the photosensitive drum 31, a first conveying screw 332 and a second conveying screw 333 that circulate and convey the developer inside a developing housing (not shown). including. The developing roller 331 is driven to rotate in the direction of arrow D3 in FIG. Further, a development bias obtained by superimposing a rectangular wave AC bias on a DC bias having the same polarity (plus) as the toner is applied to the developing roller 331 by a developing bias applying unit 503 described later.

  The transfer roller 34 is a roller for transferring a toner image formed on the peripheral surface of the photosensitive drum 31 onto a sheet. The transfer roller 34 is in contact with the cylindrical surface of the photosensitive drum 31 and forms a transfer nip portion TP (FIG. 3) with the photosensitive drum 31. The transfer roller 34 is driven to rotate in the direction of arrow D4 in FIG. Further, a transfer bias having a reverse polarity (minus) to that of toner is applied to the transfer roller 34 by a transfer bias applying unit 504 described later. Further, during the cleaning cycle described later, a bias having the same polarity or reverse polarity as that of the toner is applied. In the present embodiment, the outer diameter of the transfer roller 34 is 15.4 mm.

  The fixing unit 40 performs a fixing process for fixing the transferred toner image on the sheet. The fixing unit 40 includes a fixing roller 41 having a heating source therein, and a pressure roller 42 that is pressed against the fixing roller 41 and forms a fixing nip portion with the fixing roller 41. When the sheet on which the toner image has been transferred is passed through the fixing nip portion, the toner image is fixed on the sheet by heating by the fixing roller 41 and pressing by the pressure roller 42.

  The toner container 50 stores toner to be supplied to the developing device 33. The toner container 50 includes a container main body 51 serving as a main storage location of toner, a cylindrical portion 52 projecting from a lower portion on one side surface of the container main body 51, and a lid member 53 covering the other side surface of the container main body 51. And a rotating member 54 that is contained in the container and conveys toner. The toner stored in the toner container 50 is supplied into the developing device 33 from the toner discharge port 521 provided on the lower surface of the front end of the cylindrical portion 52 when the rotating member 54 is driven to rotate. Further, the container top plate 50H covering the upper side of the toner container 50 is located below the paper discharge unit 13 (see FIG. 2).

  In the main body housing 10, a main conveyance path 22F and a reverse conveyance path 22B are provided to convey the sheet. The main conveyance path 22 </ b> F passes through the image feeding unit 21 and the fixing unit 40 from the sheet feeding unit 21 </ b> A of the paper feeding unit 20, and the paper discharge port 14 provided to face the paper discharge unit 13 on the upper surface of the main body housing 10. It extends to. The reverse conveyance path 22B is a conveyance path for returning a sheet printed on one side to the upstream side of the image forming unit 30 in the main conveyance path 22F when performing duplex printing on the sheet.

  The main conveyance path 22F is extended so as to pass through a transfer nip portion TP (FIG. 3) formed by the photosensitive drum 31 and the transfer roller 34 from the lower side to the upper side. In addition, a registration roller pair 23 is disposed on the upstream side of the transfer nip portion TP in the main conveyance path 22F. After the sheet is temporarily stopped by the registration roller pair 23 and skew correction is performed, the sheet is sent out to the transfer nip portion at a predetermined timing for image transfer. A plurality of transport rollers for transporting the sheet are disposed at appropriate positions on the main transport path 22F and the reverse transport path 22B. For example, a pair of paper discharge rollers 24 is disposed in the vicinity of the paper discharge port 14.

  The reverse conveyance path 22 </ b> B is formed between the outer side surface of the reverse unit 25 and the inner surface of the rear cover 12 of the main body housing 10. Note that one roller of the transfer roller 34 and the registration roller pair 23 is mounted on the inner surface of the reversing unit 25. The rear cover 12 and the reversing unit 25 can be rotated about the axis of the fulcrum 121 provided at the lower end thereof. When a sheet jam occurs in the reverse conveyance path 22B, the rear cover 12 is opened. When a sheet jam occurs in the main conveyance path 22F, or when the unit of the photosensitive drum 31 and the developing device 33 are taken out, the reversing unit 25 is opened in addition to the rear cover 12.

  Referring to FIG. 3, the image forming apparatus 1 further includes a motor 501 (driving unit), a charging bias applying unit 502, a developing bias applying unit 503, a transfer bias applying unit 504 (transfer bias applying unit), A synchronization transmission unit 505 (drive means).

  The motor 501 generates a rotational driving force transmitted to the image forming unit 30. The motor 501 is connected to the rotating shaft of the photosensitive drum 31 by a drive transmission unit (not shown). The synchronization transmission unit 505 is a gear portion that connects the photosensitive drum 31 and the transfer roller 34. In the synchronous transmission unit 505, the number of gear teeth of the photosensitive drum 31 and the number of gear teeth of the transfer roller 34 have a 2: 1 relationship. That is, the number of rotations per unit time of the transfer roller 34 is set to twice (integer multiple) the number of rotations of the photosensitive drum 31 per unit time. A driving unit configured by the motor 501 and the synchronization transmission unit 505 is connected to the photosensitive drum 31 and the transfer roller 34, and rotationally drives the photosensitive drum 31 and the transfer roller 34 in mechanical synchronization. As a result, when the photosensitive drum 31 rotates once, the transfer roller 34 rotates twice. As described above, in this embodiment, the outer diameter of the transfer roller 34 is 15.4 mm, and the outer diameter of the photosensitive drum 31 is 30 mm. Accordingly, the surface speed of the transfer roller 34 is set slightly slower than the surface speed of the photosensitive drum 31 in the transfer nip portion TP.

  The motor 501 is connected to the charging roller 32 and the developing roller 331 (developing device 33) by a drive transmission unit (not shown). That is, in the present embodiment, the photosensitive drum 31, the charging roller 32, the developing roller 331, and the transfer roller 34 are rotationally driven mechanically and synchronously by the motor 501 and a drive transmission unit (not shown). In other embodiments, the photosensitive drum 31 and the transfer roller 34 may be rotationally driven mechanically synchronously, and the charging roller 32 and the developing roller 331 may be rotationally driven by other driving means. The motor 501 rotationally drives the photosensitive drum 31, the charging roller 32, the developing roller 331, and the transfer roller 34 in the directions of arrows D1, D2, D3, and D4 in FIG. 3 according to a control signal from the drive control unit 510 described later. Let

  The charging bias application unit 502 is connected to the charging roller 32 and applies a charging voltage to the charging roller 32. In the present embodiment, the charging bias application unit 502 charges the surface of the photosensitive drum 31 to a positive potential.

  The development bias application unit 503 is connected to the development roller 331 and applies a development bias to the development roller 331. In the present embodiment, the developing bias applying unit 503 applies a developing bias in which an AC bias is superimposed on a DC bias having the same polarity (plus) as the toner to the developing roller 331. A toner image is formed on the surface of the photosensitive drum 31 by the potential difference between the potential of the electrostatic latent image formed on the surface of the photosensitive drum 31 and the developing bias applied to the developing roller 331.

  The transfer bias application unit 504 is connected to the transfer roller 34 and applies a transfer bias (transfer voltage) to the transfer roller 34. In the present embodiment, during printing, the transfer bias applying unit 504 applies a transfer bias having a reverse polarity (minus) to the toner to the transfer roller 34. A transfer electric field is formed from the surface of the positively charged photosensitive drum 31 toward the transfer roller 34 to which a negative voltage is applied. As a result, the toner image is transferred from the surface of the photosensitive drum 31 onto the sheet conveyed to the transfer nip portion TP. The transfer bias applying unit 504 applies a voltage having the same polarity or opposite polarity as that of the toner during the cleaning cycle of the transfer roller 34.

  Next, an image forming and cleaning cycle in the image forming unit 30 according to the present embodiment will be described with reference to FIGS. FIG. 4 is a functional block diagram of the control unit 500. Referring to FIG. 4, the image forming apparatus 1 includes a control unit 500 that comprehensively controls the operation of each unit of the image forming apparatus 1. The controller 500 includes a CPU (Central Processing Unit), a ROM (Read Only Memory) that stores a control program, a RAM (Random Access Memory) that is used as a work area of the CPU, and the like. Further, the motor 501, the charging bias applying unit 502, the developing bias applying unit 503, and the transfer bias applying unit 504 are electrically connected to the control unit 500.

  The control unit 500 includes a drive control unit 510, a bias control unit 511 (bias control unit), and a drive time accumulation unit 512 (rotation time calculation unit) when the CPU executes a control program stored in the ROM. To work.

  The drive control unit 510 controls the motor 501 according to the image forming operation on the sheet, and rotationally drives the photosensitive drum 31 and the like. In particular, when images are continuously formed on a plurality of sheets, the drive control unit 510 controls the motor 501 to continuously rotate the photosensitive drum 31 and the like. In addition, the drive control unit 510 continuously rotates and drives the photosensitive drum 31 and the like in a cleaning cycle described later.

  The bias control unit 511 controls the charging bias application unit 502 to apply a predetermined charging voltage to the charging roller 32 when executing the image forming operation. The bias control unit 511 controls the development bias application unit 503 to apply a predetermined development bias to the development roller 331. Further, the bias control unit 511 controls the transfer bias application unit 504 to apply a predetermined transfer bias to the transfer roller 34. In addition, the bias control unit 511 controls the transfer bias application unit 504 and applies a cleaning bias or a reverse cleaning bias to the transfer roller 34 in a cleaning cycle described later.

  The drive time accumulating unit 512 (rotation time calculation unit) accumulates the rotation drive time T of the photoconductive drum 31 and temporarily stores it in order to calculate the rotation speed of the photoconductive drum 31. The drive time accumulating unit 512 accumulates the rotation drive time of the photosensitive drum 31 during the image forming operation. Further, in the present embodiment, the drive time accumulating unit 512 accumulates the rotational drive time T of the photosensitive drum 31 during the cleaning cycle described later from the time when image formation (transfer) to the sheet is completed. The rotational drive time T is reset to zero when the cleaning cycle is completed.

  Next, an image forming operation and a subsequent cleaning cycle in the image forming unit 30 according to the present embodiment will be described. FIG. 5 is a timing chart under continuous printing conditions in which toner images are continuously transferred to a plurality of sheets. In FIG. 5, images are formed on two sheets in succession.

  When execution of image formation is instructed from an operation screen (not shown) provided in the image forming apparatus 1 or a personal computer connected to the image forming apparatus 1, the motor 501 is controlled by the drive control unit 510. Then, rotation of the photosensitive drum 31, the charging roller 32, the developing roller 331, and the transfer roller 34 is started (time t1). At this time, the bias control unit 511 controls the charging bias application unit 502 to apply a charging voltage to the charging roller 32. At the same time, the bias control unit 511 controls the transfer bias application unit 504 to apply a voltage having the same polarity (plus) as the toner to the transfer roller 34. Further, the drive time accumulating unit 512 starts counting the drive time of the motor 501 (photosensitive drum 31).

  When the count time of the drive time accumulation unit 512 reaches t2, the bias control unit 511 controls the transfer bias application unit 504 to apply a transfer voltage having a polarity (minus) opposite to that of the toner to the transfer roller 34. The time t2 corresponds to t2 = t1 + ΔT1. ΔT1 is a time obtained by dividing the distance from the charging roller 32 to the transfer roller 34 on the circumference of the photosensitive drum 31 by the rotational speed (circumferential speed, linear speed) of the photosensitive drum 31.

  Further, when the count time of the drive time accumulating unit 512 reaches t3, an electrostatic latent image is formed on the surface of the charged photosensitive drum 31 by the exposure device. The electrostatic latent image is conveyed to a position where the developing roller 331 opposes as the photosensitive drum 31 rotates, and is visualized as a toner image by the toner supplied by the developing roller 331.

  Further, when the count time of the drive time accumulating unit 512 reaches t4, the toner image is transferred onto the sheet by the transfer voltage applied to the transfer roller 34. The time t4 corresponds to t4 = t3 + ΔT2. ΔT2 is a time obtained by dividing the distance from the exposure position of the exposure apparatus on the circumference of the photosensitive drum 31 to the transfer roller 34 by the rotational speed of the photosensitive drum 31. Prior to the transfer, the sheet is conveyed to the transfer nip portion TP (FIG. 3) corresponding to the transfer of the toner image started at time t4.

  In this embodiment, the transfer of the toner image to the first sheet is continued until time t5. That is, the period from time t4 to t5 corresponds to the sheet passing range of the first sheet. Note that the exposure apparatus starts creating an electrostatic latent image corresponding to the second sheet near time t5.

  Further, the portion (transfer nip portion TP) where the transfer roller 34 faces the photosensitive drum 31 corresponds to the interval between time t5 and time t6. In the present embodiment, a voltage having a polarity opposite to the polarity of the toner is applied to the transfer roller 34 not only at the time of transfer (time t4 to t5) when the toner image is transferred to the sheet but also between the sheets.

  Further, the toner image is transferred to the second sheet from time t6 to time t8. Prior to time t8, at time t7, the exposure apparatus finishes creating the electrostatic latent image on the second sheet (t8 = t7 + ΔT2). At time t8, the transfer of the toner image to the two sheets is completed. Thereafter, as described above, the toner image transferred to the sheet is fixed on the sheet by heating by the fixing roller 41 and pressing by the pressure roller 42 (FIG. 2).

  As described above, in this embodiment, the toner image is transferred (time t4 to t5, time t6 to t8) and the interval between the sheets formed between the plurality of sheets passes through the transfer nip portion TP (from time t5). At t <b> 6), the bias control unit 511 controls the transfer bias application unit 504 to apply a voltage having a polarity (minus) opposite to the polarity of the toner to the transfer roller 34. For this reason, as compared with the case where a voltage having the same polarity (plus) as that of the toner is applied to the transfer roller 34 between the sheets, it is possible to suppress the fluctuation of the potential on the surface of the photosensitive drum 31. On the other hand, since a voltage having a polarity opposite to that of the toner is applied to the transfer roller 34 between the sheets, the toner may adhere to the surface of the transfer roller. Even in such a case, the present embodiment includes the following cleaning cycle.

  In the present embodiment, the cleaning cycle starts from time t8 when the transfer of the toner image to the second sheet is completed. The drive control unit 510 continues to control the motor 501 after the transfer at the time t <b> 8 to rotate the photosensitive drum 31, the charging roller 32, the developing roller 331, and the transfer roller 34. In another embodiment, when the developing roller 331 is rotated by another driving unit, the developing roller 331 may be stopped. In the cleaning cycle, the application of the charging bias and the developing bias by the charging bias applying unit 502 and the developing bias applying unit 503 is stopped.

  The drive control unit 510 rotates the photosensitive drum 31 and the transfer roller 34 from time t8 to t9. The time from t8 to t9 corresponds to the time for which the transfer roller 34 makes four rotations (four rounds). During this time, the bias control unit 511 controls the transfer bias application unit 504 to generate a voltage (cleaning bias) having the same polarity (plus) as the polarity of the toner corresponding to the rotation time of four rotations of the transfer roller 34. Apply to the transfer roller 34. As a result, in the transfer nip portion TP, an electric field is formed such that toner having a positive polarity is moved from the transfer roller 34 toward the photosensitive drum 31. Therefore, the toner adhering to the surface of the transfer roller 34 is suitably cleaned. At this time, as described above, the time during which the voltage having the same polarity as the toner is applied to the transfer roller 34 corresponds to the time for which the transfer roller 34 rotates four times. In the present embodiment, four rotations of the transfer roller 34 correspond to two rotations of the photosensitive drum 31 as described above. That is, between time t8 and time t9, the transfer roller 34 rotates four times and the photosensitive drum 31 rotates two times. In other words, the region on the circumference of the transfer roller 34 located at the transfer nip portion TP at time t8 is also located at the transfer nip portion TP at time t9. Similarly, the area on the circumference of the photosensitive drum 31 located at the transfer nip portion TP at the time t8 is also located at the transfer nip portion TP at the time t9.

  Further, the drive control unit 510 rotates the photosensitive drum 31 and the transfer roller 34 from time t9 to time t10. The time from t9 to t10 corresponds to the time for which the transfer roller 34 makes two rotations (two revolutions). During this time, the bias control unit 511 controls the transfer bias application unit 504 so as to correspond to the rotation time of two rotations of the transfer roller 34 (negative cleaning voltage) with respect to the polarity of the toner (reverse cleaning bias). Is applied to the transfer roller 34. As a result, an electric field is formed in the transfer nip portion TP so that the reverse polarity toner having a negative polarity is moved from the transfer roller 34 toward the photosensitive drum 31. Therefore, the reverse polarity toner adhering to the surface of the transfer roller 34 is suitably cleaned. At this time, the time for which the voltage having the opposite polarity to the toner is applied to the transfer roller 34 corresponds to the time for which the transfer roller 34 rotates twice. Two rotations of the transfer roller 34 correspond to one rotation of the photosensitive drum 31. That is, between time t9 and time t10, the transfer roller 34 rotates twice and the photosensitive drum 31 rotates once. In other words, the region on the circumference of the transfer roller 34 located at the transfer nip portion TP at the time t9 is also located at the transfer nip portion TP at the time t10. Similarly, the region on the circumference of the photosensitive drum 31 located at the transfer nip portion TP at the time t9 is also located at the transfer nip portion TP at the time t10.

  As described above, in the cleaning cycle according to the present embodiment, the bias controller 511 corresponds to a cycle of two rotations or one rotation of the photosensitive drum 31 (a rotation time of four rotations or two rotations of the transfer roller 34). A voltage having the same or opposite polarity as the polarity of the toner is applied to the transfer roller 34. The photosensitive drum 31 and the transfer roller 34 are rotationally driven in synchronization by a motor 501 and a synchronous drive unit 505 (FIG. 3). As a result, in the cleaning cycle as described above, the position on the circumference of the photosensitive drum 31 or the transfer roller 34 in the transfer nip portion TP when the application of the predetermined voltage is started is when the application of the voltage is completed. Also located at the transfer nip TP. Therefore, uniform charges are applied to the surfaces of the photosensitive drum 31 and the transfer roller 34 in the circumferential direction.

  The voltage application time in the cleaning cycle is not limited to the above. In other words, the bias control unit 511 corresponds to a time that is an integral multiple of the time required for one rotation of the photosensitive drum 31, in other words, corresponding to a time that is an integral multiple of the rotation period of the photosensitive drum 31. A voltage having the same or opposite polarity as the polarity may be applied to the transfer roller 34.

  In addition, after the cleaning cycle is executed, the bias control unit 511 controls the charging bias application unit 502 to apply the charging voltage to the charging roller 32 at time t1 even at the start of the next printing. At the same time, the bias control unit 511 controls the transfer bias application unit 504 to apply a voltage having the same polarity (plus) as the toner to the transfer roller 34. At time t1, the charging voltage by the charging roller 32 cannot be applied to the region from the charging roller 32 to the transfer roller 34 on the periphery of the photosensitive drum 31. Therefore, as described above, when a voltage having the same polarity (plus) as that of the toner is applied to the transfer roller 34 in advance, the toner moves from the region to the transfer roller 34 and the surface of the transfer roller 34 is contaminated with the toner. Is prevented.

  As described above, according to the above-described embodiment, the transfer nip portion TP is formed between the sheets formed between the plurality of sheets and when the toner images are transferred under continuous printing conditions in which the toner images are continuously transferred to the plurality of sheets. When passing, the bias control unit 511 controls the transfer bias application unit 504 to apply a voltage having a polarity opposite to the polarity of the toner to the transfer roller 34. For this reason, as compared with the case where a voltage having the same polarity as the toner is applied to the transfer roller 34 between the sheets, it is possible to suppress the potential fluctuation on the surface of the photosensitive drum 31. On the other hand, since a voltage having a polarity opposite to that of the toner is applied to the transfer roller 34 between the sheets, the toner may adhere to the surface of the transfer roller 34. Even in such a case, according to the above configuration, the cleaning cycle is provided after the continuous transfer of the toner image. Then, the bias controller 511 applies a voltage having the same polarity as that of the toner to the transfer roller 34 in the cleaning cycle. For this reason, the toner adhered on the transfer roller 34 is moved to the photosensitive drum 31 side, and the transfer roller 34 is suitably cleaned. Further, the bias controller 511 applies a voltage having the same polarity as the polarity of the toner to the transfer roller 34 corresponding to a time that is an integral multiple of the rotation period of the photosensitive drum 31. For this reason, uniform charges are applied in the circumferential direction on the surfaces of the photosensitive drum 31 and the transfer roller 34. Therefore, partial potential fluctuations in the circumferential direction of the photosensitive drum 31 and the transfer roller 34 are suppressed. As a result, it is possible to start image formation in the state where the surface potential of the photosensitive drum 31 is stably controlled during the next printing.

  Further, according to the above-described embodiment, the photosensitive drum 31 and the transfer roller 34 are rotationally driven in mechanical synchronization. For this reason, the above control according to the rotation time corresponding to the integer rotation of the photosensitive drum 31 is realized with higher accuracy.

  Further, according to the above embodiment, the bias control unit 511 corresponds to a time that is an integral multiple of the rotation period of the photosensitive drum 31 after a voltage having the same polarity as the polarity of the toner is applied to the transfer roller 34. A voltage having a polarity opposite to that of the toner is applied to the transfer roller 34. For this reason, even when the toner charged to the surface opposite to the normal polarity is attached to the surface of the transfer roller 34, the toner is collected on the photosensitive drum 31 side. At this time, a voltage having a polarity opposite to the polarity of the toner is applied to the transfer roller 34 in accordance with the rotation time corresponding to the integer rotation of the photoconductor drum 31, and thus the circumferential direction of the photoconductor drum 31 and the transfer roller 34. In this case, the fluctuation of the potential is suppressed.

  Further, according to the above embodiment, the charging operation by the charging roller 32 is stopped in the cleaning cycle. Accordingly, it is possible to suppress the members other than the transfer roller 34 from applying a charge to the photosensitive drum 31 in the cleaning cycle. For this reason, fluctuations in potential in the circumferential direction of the photosensitive drum 31 and the transfer roller 34 are further suppressed.

  Further, according to the above embodiment, the drive time accumulation unit 512 that calculates the rotation time of the photosensitive drum 31 is provided, and the bias control unit 511 or the drive control unit 510 corresponds to the calculation result of the drive time accumulation unit 512. Thus, voltage application or rotation driving in the cleaning cycle is executed. For this reason, according to the calculation result of the drive time accumulating unit 512, the above control according to the rotation time corresponding to the integer rotation of the photosensitive drum 31 is realized with higher accuracy. Note that the drive time accumulating unit 512 may be configured to calculate the rotation time of the transfer roller 34.

  The image forming apparatus 1 according to the embodiment of the present invention has been described above. However, the present invention is not limited to this, and for example, the following modified embodiment can be taken.

  (1) In the above embodiment, the cleaning cycle has been described in a mode in which a period for applying a voltage having the same polarity as the polarity of the toner to the transfer roller 34 and a period for applying a voltage having the opposite polarity are provided. The present invention is not limited to this. An aspect in which any of the above voltages is applied may be used.

  (2) In the above embodiment, the same voltage as that at the time of transfer (time t4 to t5, time t6 to t7) is applied to the transfer roller 34 in a period corresponding to the interval between the papers (time t5 to t6). However, the present invention is not limited to this. A voltage having a polarity opposite to that of the toner and having an absolute value smaller than that at the time of transfer may be applied to the transfer roller 34 during a period corresponding to the interval between the sheets. In this case, the toner is suppressed from adhering to the transfer roller 34 between the sheets.

  (3) Further, the transfer roller 34 and the photoconductive drum 31 are not limited to those that are rotationally driven mechanically synchronously. That is, driving means are connected to the transfer roller 34 and the photosensitive drum 31, respectively, and the number of rotations per unit time of the photosensitive drum 31 is set to the number of rotations of the photosensitive drum 31 by a control means with a high frequency pulse. Each may be driven to rotate so as to be an integral multiple of.

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 30 Image forming part 31 Photosensitive drum (image carrier)
32 Charging roller (charging means)
33 Developing device 331 Developing roller 34 Transfer roller 35 Cleaning device 351 Blade 352 Housing wall 353 Seal member 500 Control unit 501 Motor (driving means)
502 charging bias application unit 503 development bias application unit 504 transfer bias application unit (transfer bias application unit)
505 Synchronization transmission unit (drive means)
510 Drive control unit 511 Bias control unit (bias control means)
512 Drive time accumulating unit (rotation time calculating unit)

Claims (6)

An image carrier that carries a toner image that is rotationally driven and has an electrostatic latent image formed on a peripheral surface thereof, and the electrostatic latent image is manifested by toner having a predetermined polarity;
Charging means for charging the peripheral surface of the image carrier;
A transfer roller that is in contact with the peripheral surface of the image carrier, forms a transfer nip portion with the image carrier, is driven to rotate, and transfers the toner image to a sheet;
Transfer bias applying means for applying a voltage to the transfer roller;
Bias control means for controlling the transfer bias applying means;
The image carrier and the transfer roller are connected to the image carrier and the transfer roller so that the number of rotations per unit time of the transfer roller is an integral multiple of the number of rotations per unit time of the image carrier. Driving means for rotationally driving in synchronization with
In continuous printing conditions where toner images are continuously transferred to a plurality of sheets,
The bias control unit controls the transfer bias applying unit to transfer the toner image when the toner image is transferred and when a sheet between one sheet and the succeeding sheet passes through the transfer nip portion. Apply a voltage of opposite polarity to the polarity to the transfer roller,
After the transfer of the continuous toner image is completed, the driving unit further includes a cleaning cycle in which the image carrier and the transfer roller are rotationally driven,
The bias control unit applies a voltage having the same polarity as the polarity of the toner to the transfer roller corresponding to a time that is an integral multiple of the rotation cycle of the image carrier in the cleaning cycle. Forming equipment.   The image forming apparatus according to claim 1, wherein the driving unit rotationally drives the image carrier and the transfer roller in a mechanically synchronized manner.   The bias control means has a polarity opposite to the polarity of the toner corresponding to a time that is an integral multiple of the rotation period of the image carrier after a voltage having the same polarity as the polarity of the toner is applied to the transfer roller. The image forming apparatus according to claim 1, wherein a voltage is applied to the transfer roller.   The bias control means applies a voltage having a smaller absolute value to the transfer roller when the interval between the sheets passes through the transfer nip portion than when the toner image is transferred. The image forming apparatus according to claim 1.   The image forming apparatus according to claim 1, wherein the charging operation by the charging unit is stopped in the cleaning cycle. A rotation time calculation unit for calculating a rotation time of the image carrier or the charging roller;
6. The bias control unit or the drive unit according to claim 1, wherein the voltage application or the rotation drive in the cleaning cycle is executed according to a calculation result of the rotation time calculation unit. 2. The image forming apparatus according to item 1.

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