JP6201579B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus employing an electrophotographic system.

  In the image forming apparatus, after the peripheral surface of the photosensitive drum is charged by a charger, a part of the peripheral surface is neutralized by a laser beam, thereby forming an electrostatic latent image on the peripheral surface. Then, toner is supplied to the peripheral surface of the photosensitive drum, and the electrostatic latent image is developed into a toner image, whereby the toner image is carried on the peripheral surface of the photosensitive drum.

  A transfer roller to which a transfer bias is applied is disposed at a position facing the peripheral surface of the photosensitive drum. Along with the rotation of the photosensitive drum, the toner image carried on the peripheral surface of the photosensitive drum is attracted to the transfer roller by the action of the transfer bias and transferred to a sheet passing between the photosensitive drum and the transfer roller. As a result, an image is formed on the sheet. The sheet on which the image is formed is discharged out of the image forming apparatus by the rotation of the discharge roller.

  As such an image forming apparatus, a printer that forms an image on both sides of a sheet by switching a sheet conveyance direction between a conveyance path and a reverse conveyance path by switching between forward rotation and reverse rotation of a discharge roller is known. ing.

  In such a printer that can form images on both sides of a sheet, a single motor is used to simplify the configuration. Then, each roller of the conveyance path is rotated by the forward rotation output of a single motor, and the photosensitive drum and the discharge roller are rotated forward to convey the sheet between the photosensitive drum and the transfer roller. Form an image. In addition, each roller in the reverse conveyance path is rotated by the reverse rotation output of a single motor, and the photosensitive drum and the discharge roller are rotated in reverse to join the sheet to the conveyance path to transfer the photosensitive drum and the transfer drum. The sheet is conveyed again between the rollers.

  During reverse rotation of the photosensitive drum, the surface potential of the transfer roller is controlled to be equal to or higher than that of the toner on the photosensitive drum to prevent the toner on the photosensitive drum from transferring to the transfer roller ( For example, see Patent Document 1).

JP 2011-150140 A

  However, in the printer described in Patent Document 1, the surface of the photosensitive drum from the contact portion with the transfer roller during forward rotation to the portion facing the charger is neutralized by the transfer roller, and the surface potential is lowered. In reverse.

  As a result, the toner in the portion where the surface potential of the photosensitive drum has dropped may be transferred to the transfer roller when it again faces the transfer roller as the photosensitive drum rotates backward. This causes a problem that the toner transferred to the transfer roller adheres to the sheet.

  SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an image forming apparatus that can prevent the recording medium from becoming dirty.

(1) In order to achieve the above object, an image forming apparatus of the present invention includes a photosensitive drum configured to carry a developer image, and a transfer configured to transfer the developer image to a recording medium. A member and a charger configured to face the photosensitive drum and charge the photosensitive drum, and rotate the photosensitive drum in the forward rotation direction by the output in the normal rotation direction, and rotate the photosensitive drum in the reverse rotation direction by the output in the reverse rotation direction A driving unit configured to control, a transfer bias applying unit configured to apply a transfer bias to the transfer member, a driving unit, and a control unit configured to control the transfer bias applying unit. The control means includes a normal rotation control mode for rotating the photosensitive drum in the normal rotation direction, a reverse rotation control mode for rotating the photosensitive drum in the reverse rotation direction, and a reverse rotation control mode from the normal rotation control mode. In the switching control mode, a state in which no transfer bias is applied to the transfer member from a state in which a transfer bias having a polarity opposite to that of the developer on the photosensitive drum is applied to the transfer member, or The transfer bias switching control for controlling the transfer bias applying means so that the transfer member is switched to a state in which a transfer bias having the same polarity as the developer on the photosensitive drum and higher than the potential is applied to the transfer member, and the transfer bias switching control are executed. After the photosensitive drum is rotated in the forward direction until the peripheral surface portion of the photosensitive drum that is in contact with the transfer member exceeds the position facing the charger, the driving means is changed from the output in the forward direction to the reverse direction. Rotation direction switching control for controlling to switch to the output of is performed.

  According to such a configuration, the transfer bias is not applied to the transfer member from the state where the transfer bias having the reverse polarity to the developer on the photosensitive drum is applied by executing the transfer bias switching control. The state is switched to a state where a transfer bias having the same polarity as the developer on the photosensitive drum and a potential higher than the potential is applied to the transfer member.

  Then, after the transfer bias switching control is executed, the photosensitive drum is rotated in the forward direction until the peripheral surface portion of the photosensitive drum that is in contact with the transfer member exceeds the position facing the charger at the time when the transfer bias switching control is executed. Rotation direction switching control for controlling the driving means is executed so that the photosensitive drum is rotated in the reverse rotation direction.

  That is, when the control means executes the switching control mode, the surface of the photosensitive drum charged by the charger is between the point of contact with the transfer member on the peripheral surface of the photosensitive drum and the point facing the charger. It is possible to suppress the potential from being lowered by the transfer member.

  For this reason, the control means can execute the reverse rotation control mode, and when the photosensitive drum is rotated in the reverse rotation direction, the entire peripheral surface of the photosensitive drum can be charged by the charger.

  Thus, even when the developer remains on the peripheral surface of the photosensitive drum, the developer is not attracted to the transfer member even if it passes through the contact point between the photosensitive drum and the transfer member.

As a result, it is possible to suppress the developer from adhering to the recording medium from the transfer member when the recording medium is conveyed between the photosensitive drum and the transfer member.
(2) Further, the control means may control the transfer bias applying means so as to switch to a state in which the transfer bias is not applied to the transfer member in the transfer bias switching control.

  According to such a configuration, the surface potential of the transfer member can be brought close to 0V.

  Therefore, since the surface potential of the transfer member can be made lower than the potential of the developer on the peripheral surface of the photosensitive drum, the developer remaining on the peripheral surface of the photosensitive drum is transferred to the transfer member by a potential difference with the transfer member. It is possible to suppress the transfer.

As a result, it is possible to suppress the developer on the peripheral surface of the photosensitive drum rotating in the reverse direction from being transferred to the transfer member and attached to the recording medium.
(3) In addition, in the transfer bias switching control, the control means switches the transfer bias applying means so that the transfer member is switched to a state in which a transfer bias having the same polarity as the developer on the photosensitive drum and having a potential higher than that is applied. You may control.

  According to such a configuration, the surface potential of the transfer member can be made to have the same polarity as the potential of the developer remaining on the peripheral surface of the photosensitive drum and larger than that potential.

  Therefore, an electrostatic force acting as a repulsive force repels each other between the developer on the peripheral surface of the photosensitive drum and the transfer member, and the developer on the peripheral surface of the photosensitive drum is transferred to the transfer member. Can be suppressed.

As a result, it is possible to suppress the developer on the peripheral surface of the photosensitive drum rotating in the reverse direction from being transferred to the transfer member and attached to the recording medium.
(4) Further, in order to transfer the developer image to the other side of the recording medium having the developer image transferred on one side, the recording medium is again transported between the photosensitive drum and the transfer member. Provided with a switchback roller capable of switching the medium conveyance direction, and the switchback roller is configured to rotate in the forward direction by the output in the forward direction of the drive means and to rotate in the reverse direction by the output in the reverse direction of the drive means. May be.

  According to such a configuration, the rotation direction of the switchback roller can be simultaneously switched by switching the output between the forward rotation direction and the reverse rotation direction of the driving means.

  At this time, the photosensitive drum is interlocked so that the rotational direction of the photosensitive drum can be switched simultaneously with the switching of the rotational direction of the switchback roller. The decrease is suppressed.

Therefore, since the photosensitive drum and the switchback roller can be controlled by a single driving unit, the number of parts can be reduced.
(5) Further, in the rotation direction switching control, the control unit may control the driving unit so that the photosensitive drum rotates at least once in the normal rotation direction.

  According to such a configuration, after the transfer bias switching control is executed and the potential of the peripheral surface of the photosensitive drum is no longer lowered by the transfer member, the photosensitive drum rotates at least once in the forward rotation direction. When rotating in the direction, the entire peripheral surface of the photosensitive drum can be reliably charged by the charger.

As a result, it is possible to reliably suppress the developer from adhering to the recording medium from the transfer member.
(6) A developing roller configured to contact the photosensitive drum and configured to supply developer to the electrostatic latent image on the photosensitive drum may be provided.

According to such a configuration, the developer image can be developed by supplying the developer to the electrostatic latent image on the photosensitive drum by the developing roller in contact with the photosensitive drum.
(7) The image forming apparatus further includes a developing bias applying unit configured to apply a developing bias to the developing roller, and the control unit is configured to control the developing bias applying unit, and the developing roller includes a circumferential surface of the photosensitive drum. A developing bias having the same polarity and lower than the potential may be applied.

  According to such a configuration, the developing roller supplies the developer to the electrostatic latent image on the photosensitive drum to develop the developer image regardless of the rotation direction of the photosensitive drum. Transfer of toner from the developing roller to the photosensitive drum is suppressed by the action of electrostatic forces that are repulsive forces repelling each other.

For this reason, it is not necessary to perform control so as to switch the developing bias applied to the developing roller during the output of the driving means, so that the number of control steps can be reduced.
(8) Further, the developing roller may be configured to collect deposits attached to the photosensitive drum when the photosensitive drum rotates in the normal rotation direction.

  According to such a configuration, the developing roller can supply the developer to the electrostatic latent image on the photosensitive drum, and can collect deposits adhering to the photosensitive drum.

Therefore, it is not necessary to separately provide a cleaning mechanism for collecting the developer remaining on the peripheral surface of the photosensitive drum, and the configuration of the image forming apparatus can be simplified.
(9) The charger may be a scorotron charger.

  According to such a configuration, since the charger and the photosensitive drum are not in contact with each other, it is possible to prevent the developer from transferring from the photosensitive drum to the charger or from the charger to the photosensitive drum. Can do.

  In the image forming apparatus of the present invention, it is possible to prevent the recording medium from becoming dirty.

FIG. 1 is a central sectional view showing a printer according to an embodiment of the image forming apparatus of the present invention. FIG. 2 is a block diagram showing an electrical configuration and a drive transmission system in the printer shown in FIG. FIG. 3 is a timing chart for explaining the double-sided image forming process. 4A is an explanatory diagram for explaining sheet conveyance in the double-sided image forming process corresponding to the timing A in FIG. 3, and FIG. 4B is a diagram of the sheet in the double-sided image forming process corresponding to the timing B in FIG. It is explanatory drawing for demonstrating conveyance. FIG. 5C is an explanatory diagram for explaining the conveyance of the sheet in the surface image forming process corresponding to the timing C in FIG. 3, and FIG. 5D is the sheet in the double-sided image forming process corresponding to the timing D in FIG. It is explanatory drawing for demonstrating conveyance. FIG. 6E is an explanatory diagram for explaining sheet conveyance in the double-sided image forming process corresponding to the timing E in FIG. 3, and FIG. 6F is a view of the sheet in the double-sided image forming process corresponding to the timing F in FIG. It is explanatory drawing for demonstrating conveyance. FIG. 7G is an explanatory diagram for explaining the conveyance of the paper in the double-sided image forming process corresponding to the timing G in FIG. 3, and FIG. 7H is the drawing of the paper in the double-sided image forming process corresponding to the timing H in FIG. It is explanatory drawing for demonstrating conveyance. FIG. 8I is an explanatory diagram for explaining the conveyance of the paper in the double-sided image forming process corresponding to the timing I in FIG. 3, and FIG. 8J is the drawing of the paper in the double-sided image forming process corresponding to the timing J in FIG. It is explanatory drawing for demonstrating conveyance.

1. Overall Configuration of Printer As shown in FIG. 1, a printer 1 as an example of an image forming apparatus is an electrophotographic monochrome printer. The printer 1 includes a main body casing 2.

  The main casing 2 has a substantially box shape. The main casing 2 includes, in its internal space, a paper feeding unit 3 for feeding paper P as an example of a recording medium, and an image forming unit 4 for forming an image on the fed paper P. ing.

In the following description, when referring to the direction of the printer 1, the state in which the printer 1 is horizontally placed is used as the upper and lower reference. That is, the upper side of the drawing in FIG. 1 is the upper side, and the lower side of the drawing is the lower side. Further, the right side of FIG. 1 is the front, and the left side of FIG. 1 is the rear. In addition, when the printer 1 is viewed from the front, the left and right reference is used. That is, the front side in FIG. 1 is the left side, and the back side of the page is the right side.
(1) Main Body Casing The main body casing 2 includes a cartridge opening 6 and a front cover 7.

  The cartridge opening 6 is configured to penetrate the front wall of the main body casing 2 in the front-rear direction and allow passage of a process cartridge 16 described later.

With the lower end of the front cover 7 as a fulcrum, the front cover 7 can swing between a closed position shown by a solid line in FIG. 1 for closing the cartridge opening 6 and an open position shown by a phantom line in FIG. 1 for opening the cartridge opening 6. It is configured.
(2) Paper Feed Unit The paper feed unit 3 is disposed in the lower part of the main body casing 2. The paper feed unit 3 includes a paper feed tray 8 for storing paper P, a pickup roller 9 provided above the front end of the paper feed tray 8, a separation roller 10 and a separation pad 11 facing each other in front of the pickup roller 9. It has.

In addition, the paper feed unit 3 includes a pair of front and rear paper feed rollers 12 facing each other above the separation pad 11, a paper feed path 14 extending substantially rearward and upward from a portion where both the paper feed rollers 12 are opposed, and a paper feed path 14. And a main body side registration roller 24 disposed behind the paper path 14.
(3) Image Forming Unit The image forming unit 4 includes a scanner unit 15, a process cartridge 16, and a fixing unit 17.
(3-1) Scanner Unit The scanner unit 15 is disposed on the upper part of the main body casing 2. As indicated by the solid line, the scanner unit 15 emits a laser beam toward the photosensitive drum 20 described later based on the image data, and exposes the photosensitive drum 20 described later.
(3-2) Process Cartridge The process cartridge 16 is detachably accommodated in the main body casing 2 below the scanner unit 15 and above the paper feed unit 3.

  The process cartridge 16 includes a drum cartridge 18 that is detachably attached to the main body casing 2, and a developing cartridge 19 that is detachably attached to the drum cartridge 18.

  The drum cartridge 18 includes a photosensitive drum 20, a scorotron charger 21 as an example of a charger, and a transfer roller 22 as an example of a transfer member.

  The photosensitive drum 20 has a substantially cylindrical shape extending in the left-right direction, and is rotatably supported by a rear portion of the drum cartridge 18.

  The scorotron charger 21 is supported by the drum cartridge 18 so as to be positioned at an interval above and behind the photosensitive drum 20. The scorotron charger 21 includes a grid 51 and a charging wire 52.

  The grid 51 has a substantially cylindrical shape that is substantially U-shaped in cross-sectional view and extends in the left-right direction and opened rearward and upward. The grid 51 is made of metal.

  The charging wire 52 has a substantially linear shape extending in the left-right direction. The charging wire 52 is stretched along the left-right direction in the grid 51. The charging wire 52 is made of metal.

  Note that the front surface of the grid 51 extending in the direction connecting the rear upper side and the front lower side is extended toward the photosensitive drum 20 at a point facing the scorotron charger 21 on the peripheral surface of the photosensitive drum 20. The intersection of the imaginary line and the peripheral surface of the photosensitive drum 20 is defined as a discharge point E.

  The transfer roller 22 has a substantially cylindrical shape extending in the left-right direction, is rotatably supported by a rear portion of the drum cartridge 18, and is pressed against the lower end portion of the photosensitive drum 20. A point where the photosensitive drum 20 is pressed against the transfer roller 22 is defined as a transfer nip point T.

  The drum cartridge 18 includes a process side registration roller 25.

  The process-side registration roller 25 has a substantially columnar shape extending in the left-right direction, is rotatably supported at the lower end portion of the drum cartridge 18 in the front-rear direction, and is relatively rotatable with respect to the upper end portion of the main body-side registration roller 24. Is touching.

  The developing cartridge 19 contains toner as an example of a developer in its internal space. The developing cartridge 19 includes a developing roller 27, a supply roller 28, and a layer thickness regulating blade 29.

The developing roller 27 has a substantially cylindrical shape extending in the left-right direction, and is supported rotatably at the rear end portion of the developing cartridge 19 so that the rear portion thereof is exposed. The developing roller 27 is in pressure contact with the front end portion of the photosensitive drum 20. The developing roller 27 has a low resistance. Specifically, the resistance value of the developing roller 27 is, for example, 10 ³ Ω or more, preferably 10 ^3.5 Ω or more, for example, 10 ^6.5 Ω or less, preferably 10 ⁶ Ω or less. . A point where the photosensitive drum 20 is in pressure contact with the developing roller 27 is defined as a developing nip point D.

  The supply roller 28 is rotatably supported at the rear end portion of the developing cartridge 19. The supply roller 28 is in contact with the front end portion of the developing roller 27.

The upper end portion of the layer thickness regulating blade 29 is supported by the rear upper end portion of the developing cartridge 19, and the lower end portion thereof is in contact with the front upper end portion of the developing roller 27.
(3-3) Fixing Unit The fixing unit 17 is disposed behind the process cartridge 16. The fixing unit 17 includes a heating roller 30 and a pressure roller 31 that is in pressure contact with the rear lower end portion of the heating roller 30.
(4) Image Forming Operation The toner in the developing cartridge 19 is supplied to the supply roller 28 and further supplied to the developing roller 27, and is frictionally charged positively between the supply roller 28 and the developing roller 27.

  The toner supplied to the developing roller 27 is regulated by the layer thickness regulating blade 29 as the developing roller 27 rotates, and is carried on the surface of the developing roller 27 as a thin layer having a constant thickness.

  On the other hand, the photosensitive drum 20 is rotated at a constant speed clockwise as viewed from the left side. The surface of the photosensitive drum 20 is uniformly positively charged by the discharge from the scorotron charger 21 as the photosensitive drum 20 rotates. Specifically, the portion of the surface of the photosensitive drum 20 that has passed through the discharge point E is uniformly positively charged.

  Further, the scanner unit 15 performs high-speed exposure of a laser beam indicated by a solid line in FIG. 1 on the surface of the uniformly positively charged photosensitive drum 20 based on image data received from a CPU 55 (described later) connected to the printer 1. To expose.

  Thereby, the surface of the photosensitive drum 20 is selectively exposed, and electric charges are selectively removed from the exposed portion. Thus, an electrostatic latent image corresponding to the image to be formed on the paper P is formed on the surface of the photosensitive drum 20.

  When the photosensitive drum 20 further rotates, the positively charged toner carried on the surface of the developing roller 27 is supplied to the electrostatic latent image formed on the surface of the photosensitive drum 20. As a result, the electrostatic latent image on the photosensitive drum 20 is visualized, and a toner image as an example of a developer image by reversal development is carried on the surface of the photosensitive drum 20.

  The paper P accommodated in the paper feed tray 8 is supplied to the facing portion between the separation roller 10 and the separation pad 11 by the rotation of the pickup roller 9, and is fed one by one by them. Thereafter, the paper P that has been squeezed passes through the paper feed path 14 to make a U-turn backward by the rotation of the paper feed rollers 12, and is conveyed between the main body side registration roller 24 and the process side registration roller 25. The

  Next, the sheet P is conveyed into the process cartridge 16 and passes between the rear lower end portion of the developing cartridge 19 and the lower wall of the drum cartridge 18 at a predetermined timing, and the photosensitive drum 20 and the transfer roller 22. Paper is fed between.

  When the paper P passes through the transfer nip point T between the photosensitive drum 20 and the transfer roller 22, the toner image carried on the photosensitive drum 20 is transferred to the paper P.

  In the process cartridge 16, the toner image transferred to the paper P is thermally fixed to the paper P by being heated and pressed while the paper P passes between the heating roller 30 and the pressure roller 31. Is done.

  The paper P on which the toner image has been fixed makes a U-turn forward along the upper front surface of the chute 33 which will be described later, and passes through the paper discharge port 34 disposed at the rear upper end portion of the main body casing 2. The paper is discharged onto a paper discharge tray 36 disposed above 15.

A transport path from when the paper P fed by the pickup roller 9 from the paper feed tray 8 passes through the image forming unit 4 to the transport unit 37 to be described later is referred to as a primary transport path 40.
2. Configuration for Forming Images on Both Sides of Paper The printer 1 conveys the paper P from the rear to the front of the main casing 2 in order to form images on both the one side and the other side of the paper P. The image forming unit 4 is configured to be re-conveyed toward the image forming unit 4.

  The main body casing 2 includes a switching unit 37, a chute 33, a pair of first reverse conveyance rollers 38, and a pair of second reverse conveyance rollers 39.

  The switching unit 37 is disposed in the vicinity of the paper discharge port 34 at the rear upper end of the main casing 2. The switching unit 37 includes a switchback roller 42 and a driven roller 43.

  The switchback roller 42 can switch the conveyance direction of the paper P between a forward rotation direction indicated by a broken line in FIG. 1 toward the paper discharge tray 36 and a reverse rotation direction indicated by a one-dot chain line in FIG. It is configured. The switchback roller 42 rotates counterclockwise when viewed from the left side when the transport direction of the paper P is the forward rotation direction, and rotates clockwise when viewed from the left side when the transport direction of the paper P is the reverse direction.

  The driven roller 43 is in contact with the lower end portion of the switchback roller 42. Accordingly, the driven roller 43 rotates so as to follow the rotation of the switchback roller 42.

  The chute 33 is disposed at the rear end portion in the main body casing 2. The chute 33 has a shape that includes a substantially triangular prism that has a vertex on the upper side and that extends from side to side, and a substantially flat plate that has a substantially rectangular shape when viewed from the front.

  The upper front surface of the chute 33 is curved so as to be slightly recessed from the front upper side to the rear lower side. The upper front surface of the chute 33 guides the conveyance of the sheet P on which the image is formed by the image forming unit 4, that is, the sheet P conveyed in the normal rotation direction, as described above.

  The rear surface of the chute 33 is curved so that the center portion slightly bulges rearward with respect to the upper end portion and the lower end portion.

  The pair of first reverse conveying rollers 38 is disposed in a rear portion between the paper feeding unit 3 and the image forming unit 4 in the main body casing 2.

  The pair of second reverse conveyance rollers 39 is a front portion between the paper feed unit 3 and the image forming unit 4 in the main body casing 2 and is disposed in front of the first reverse conveyance roller 38 with a space therebetween. ing.

  In order to switch the conveyance direction of the paper P from the normal rotation direction to the reverse rotation direction, the rear end portion of the paper P conveyed in the normal rotation direction, that is, the upstream end portion of the primary conveyance path 40 of the paper P is changed. After being conveyed to the vicinity of the switching unit 37, the rotation direction of the switchback roller 42 is switched under the control of the CPU 55 described later.

  As a result, after an image is formed on one side of the paper P, the conveyance direction is switched from the normal rotation direction to the reverse rotation direction by the switching unit 37, and the paper P passes behind the chute 33 and passes through a pair of first reversals. The conveyance roller 38 and the pair of second reverse conveyance rollers 39 are conveyed in the order from the rear to the front between the paper feeding unit 3 and the image forming unit 4.

  The paper P is U-turned forward from the main body side registration roller 24 and the process side registration roller 25 and rearward from the paper feed path 14 so that the main body side registration roller 24 and the process side registration roller are turned. It is conveyed so as to merge with the roller 25, that is, the primary conveyance path 40. The sheet P transported to the primary transport path 40 is transported again between the photosensitive drum 20 and the transfer roller 22, an image is formed on the other side, and the sheet P is ejected onto the sheet ejection tray 36.

  The paper P switched back by the switchback roller 42 of the switching unit 37 passes from the switching unit 37 to the rear of the chute 33 and passes through a pair of first reverse conveyance rollers 38 and a pair of second reverse conveyance rollers 39. A transport path from the transport to the primary transport path 40 is referred to as a secondary transport path 44.

Thus, an image is formed on both sides of the paper P by forming an image on the other side of the paper P by the image forming unit 4.
3. Printer Drive and Electrical Configuration As shown in FIG. 2, the printer 1 further includes a motor 46 as an example of a drive unit.

  The motor 46 is a motor that can be switched between an output in the forward direction and an output in the reverse direction. The motor 46 is configured to transmit a rotational driving force to the photosensitive drum 20 and the switchback roller 42.

  The motor 46 is configured so that each of the rollers in the primary transport path 40, specifically, the pickup roller 9, the separation roller 10, and the paper feed so that the paper P is discharged to the paper discharge tray 36 by the output in the normal rotation direction. Each of the roller 12, the main body side registration roller 24, and the pressure roller 31 is rotated.

  Further, the motor 46 inputs a drive to a coupling (not shown) of the process cartridge 16 by an output in the forward rotation direction. As a result, the motor 46 rotates the developing roller 27 and the supply roller 28 of the process cartridge 16. The rotation direction of the developing roller 27 is counterclockwise as viewed from the left side, as indicated by a solid line in FIG.

  The photosensitive drum 20 and the switchback roller 42 are rotated in the forward direction by the output in the forward direction of the motor 46. Specifically, the forward rotation direction of the photosensitive drum 20 is clockwise when viewed from the left side as shown by a solid line in FIG. 1, and the forward rotation direction of the switchback roller 42 is as shown by a solid line in FIG. Left side counterclockwise.

  At this time, the transfer roller 22 rotates counterclockwise as viewed from the left side as shown by the solid line in FIG. 1 because the gear (not shown) of the transfer roller 22 and the gear (not shown) of the photosensitive drum 20 are engaged with each other. .

  Further, the motor 46 is configured so that each roller in the secondary conveyance path 44, specifically, the first reverse conveyance roller 38, the second reversal roller 38, and the second reversal conveyance roller 44 so that the paper P is drawn into the main body casing 2 by the output in the reverse rotation direction. The reverse conveying roller 39 is rotated.

  When the output of the motor 46 is in the reverse direction, the drive input to the coupling (not shown) of the process cartridge 16 is stopped, and the rotation of the developing roller 27 and the supply roller 28 is stopped.

  Due to the output of the motor 46 in the reverse direction, the photosensitive drum 20 and the switchback roller 42 rotate in the reverse direction. Specifically, the reverse direction of the photosensitive drum 20 is counterclockwise when viewed from the left side as shown by the phantom line in FIG. 1, and the reverse direction of the switchback roller 42 is shown by the phantom line in FIG. The left side view is clockwise.

  At this time, the transfer roller 22 rotates clockwise as viewed from the left side as indicated by the phantom line in FIG. 1 because the gear (not shown) of the transfer roller 22 is engaged with the gear (not shown) of the photosensitive drum 20. To do.

  The printer 1 also includes a charging bias circuit 47, a transfer bias circuit 48 as an example of a transfer bias applying unit, and a developing bias circuit 49 as an example of a developing bias applying unit.

  The charging bias circuit 47 is configured to apply a charging bias to the scorotron charger 21. More specifically, the charging bias circuit 47 is configured to apply a wire bias to the charging wire 52 and apply a grid bias to the grid 51.

  The transfer bias circuit 48 is configured to apply a transfer bias to the transfer roller 22.

  The developing bias circuit 49 is configured to apply a developing bias to the developing roller 27.

  Furthermore, the printer 1 includes a CPU 55 as an example of a control unit.

The CPU 55 is connected to the motor 46, the charging bias circuit 47, the transfer bias circuit 48, and the developing bias circuit 49 so that control signals can be input.
4). Surface Potential Control in Double-Sided Image Forming Process With reference to FIGS. 3 to 8, surface potential control in the double-sided image forming process by the CPU 55 will be described.

  The main casing 2 includes a first sensor 57 and a second sensor 58 as shown in FIG.

  The first sensor 57 is disposed between the main body side registration roller 24 and the transfer roller 22 in the primary conveyance path 40.

  The second sensor 58 is disposed between the pressure roller 31 and the chute 33 in the primary conveyance path 40.

  Each of the first sensor 57 and the second sensor 58 includes a swingable actuator. When the paper P comes in contact, the actuator tilts and an ON signal is output, and the paper P is separated. Thus, the actuator is raised and an OFF signal is output. The ON / OFF detection signal is configured to be transmitted to the CPU 55.

  In order to form an image on one side of the paper P, the CPU 55 uses a normal rotation control mode in which the photosensitive drum 20 is rotated in the normal rotation direction by an output in the normal rotation direction of the motor 46 and an output in the reverse rotation direction of the motor 46. Then, a reverse rotation control mode for rotating the photosensitive drum 20 in the reverse rotation direction and a switching control mode for switching from the normal rotation control mode to the reverse rotation control mode are executed.

  First, the CPU 55 executes the normal rotation control mode.

  In order to execute the forward rotation control mode, first, after the mode is detected, a startup process is executed.

  During the start-up process, the CPU 55 controls the charging bias circuit 47 to apply a charging bias to the scorotron charger 21. By applying a charging bias to the scorotron charger 21, the scorotron charger 21 is discharged, and the peripheral surface of the photosensitive drum 20 is charged to a constant positive potential (for example, + 800V).

  Next, the CPU 55 controls the developing bias circuit 49 to apply a positive bias (for example, +300 V) to the developing roller 27. That is, the CPU 55 controls the developing bias circuit 49 to apply a developing bias having the same polarity as that of the peripheral surface of the photosensitive drum 20 to the developing roller 27 and lower than the potential.

  After the start-up process is completed, the motor 46 is driven in the forward direction under the control of the CPU 55. The photosensitive drum 20 and the switchback roller 42 are rotated in the forward direction by the output of the motor 46 in the forward direction.

  Then, the paper P in the paper feed tray 8 of the paper feed unit 3 is conveyed between the main body side registration roller 24 and the process side registration roller 25.

  The CPU 55 controls the transfer bias circuit 48 to apply a negative transfer bias (for example, −1000 V) to the transfer roller 22 after a predetermined time has elapsed since the paper P was conveyed from the paper feed tray 8 of the paper feed unit 3. To do.

  When the peripheral surface of the photosensitive drum 20 is exposed to the laser beam from the scanner unit 15, the electric charge disappears from the exposed portion. Accordingly, the peripheral surface of the photosensitive drum 20 is selectively exposed to form an electrostatic latent image on the peripheral surface of the photosensitive drum 20 based on the presence or absence of electric charges. Then, the surface potential of the exposed portion is lower than the developing bias applied to the developing roller 27 by selectively removing the charge. For this reason, the positive toner carried on the peripheral surface of the developing roller 27 is configured to be attracted and transferred to the exposed portion by electrostatic force when facing the exposed portion of the peripheral surface of the photosensitive drum 20. Yes.

  In this way, development of the electrostatic latent image with toner is achieved, and a toner image is formed on the peripheral surface of the photosensitive drum 20.

  Then, as shown in FIG. 4A, the paper P is supplied to the primary conveyance path 40 and conveyed between the main body side registration roller 24 and the process side registration roller 25.

  Thereby, as shown in the timing A in FIG. 3, the first sensor 57 is turned on.

  Next, when the paper P is further transported in the primary transport path 40, the toner image on the photosensitive drum 20 faces the transfer roller 22 with the paper P interposed therebetween, as shown in FIG. 4B.

  At this time, as shown at timing B in FIG. 3, the sheet P is in a transfer nip state. Then, since the toner image on the photosensitive drum 20 is positively charged, it is attracted by the electrostatic force to the transfer roller 22 to which a negative transfer bias is applied. As a result, the toner image is transferred from the peripheral surface of the photosensitive drum 20 to the paper P.

  That is, the surface potential of the portion of the peripheral surface of the photosensitive drum 20 that exceeds the transfer nip point T decreases. That is, the potential is reduced (for example, 0 V) from the portion beyond the transfer nip point T to the discharge point E on the peripheral surface of the photosensitive drum 20. Note that, schematically, a hatching pattern is illustrated in a portion of the peripheral surface of the photosensitive drum 20 that is uniformly positively charged by the discharge from the scorotron charger 21, and a hatching pattern is not illustrated in a portion in which the potential is decreased. Shall.

  Next, when the paper P is further transported in the primary transport path 40, as shown in FIG. 5C, the rear end of the paper P (the upstream end in the transport direction in the primary transport path 40) is the photosensitive drum 20. Passes through a transfer nip point T between the transfer roller 22 and the transfer roller 22, and the tip of the transfer nip point T reaches the switching unit 37.

  As a result, the first sensor 57 is turned off and the second sensor 58 is turned on as shown at timing C in FIG.

  Then, the CPU 55 executes a switching control mode for switching from the normal rotation control mode to the reverse rotation control mode at this timing.

  In the switching control mode, the CPU 55 controls the transfer bias circuit 48 after a predetermined time elapses after the transfer nip point T between the photosensitive drum 20 and the transfer roller 22 of the paper P is exceeded based on the OFF of the first sensor 57. Thus, transfer bias switching control for switching to a state in which a negative transfer bias is not applied to the transfer roller 22 is executed. As a result, the surface potential of the transfer roller 22 becomes a minute negative potential.

  On the other hand, since the charging bias is always applied to the scorotron charger 21, the potential of the toner on the peripheral surface of the photosensitive drum 20 is uniformly positively charged by the discharge from the scorotron charger 21. The surface potential of 20 is almost the same. As a result, the toner remaining on the photosensitive drum 20 is positively charged, whereas the surface potential of the transfer roller 22 is charged to a minute negative potential. Movement of toner to the toner is suppressed.

  Next, when the sheet P is transported slightly in the primary transport path 40, the photosensitive drum 20 is slightly rotated in the forward rotation direction as shown in FIG. 5D.

  At this time, the application of the negative transfer bias to the transfer roller 22 is canceled by the transfer bias circuit 48 as shown at timing D in FIG. 3, so that the surface on the peripheral surface of the photosensitive drum 20 is shown in FIG. 5D. The portion beyond the transfer nip point T becomes substantially the same as the surface potential of the photosensitive drum 20 that is uniformly positively charged by the discharge from the scorotron charger 21. In other words, the portion of the peripheral surface of the photosensitive drum 20 beyond the transfer nip point T is not neutralized by the transfer roller, and is kept charged by the discharge from the scorotron charger 21.

  Next, as shown in FIG. 6E, the sheet P passes through the fixing unit 17, and the switching unit 37 positions the rear end (the upstream end in the transport direction on the primary transport path 40) near the switching unit 37. It is conveyed to. Then, after the transfer bias switching control is executed, the photosensitive drum 20 makes one or more rotations in the forward rotation direction, specifically, two to three rotations. Therefore, the transfer nip point T on the circumferential surface of the photosensitive drum 20 at the time when the transfer bias switching control is executed exceeds the discharge point E due to the rotation of the photosensitive drum 20 in the normal rotation direction. As a result, the entire peripheral surface of the photosensitive drum 20 is uniformly positively charged by the discharge from the scorotron charger 21.

  At this time, as shown at timing E in FIG. 3, the second sensor 58 is turned off.

  Further, in the switching control mode, the CPU 55 executes rotation direction switching control for switching the motor 46 from the output in the normal rotation direction to the output in the reverse rotation direction after a predetermined time has elapsed based on the OFF of the second sensor 58. Thereby, as shown in FIG. 6F, the rotation directions of the photosensitive drum 20 and the switchback roller 42 are switched from the normal rotation direction to the reverse rotation direction.

  At this time, as shown at timing F in FIG. 3, the output of the motor 46 is in the reverse direction.

  Then, after the switching control mode is executed, the CPU 55 executes the reverse rotation control mode.

  In the reverse rotation control mode, the photosensitive drum 20 and the switchback roller 42 rotate in the reverse rotation direction by the output of the motor 46 in the reverse rotation direction.

  As a result, as shown in FIG. 7G, the paper P is reversed and conveyed toward the secondary conveyance path 44.

  At this time, as indicated by the timing G in FIG. 3, the motor 46 maintains the output in the reverse rotation direction.

  Next, as shown in FIG. 7H, the paper P is further transported through the secondary transport path 44 and passes between the second reverse transport rollers 39.

  At this time, as shown at a timing H in FIG. 3, the motor 46 is switched from the output in the reverse direction to the output in the normal direction. Thereby, the rotation directions of the photosensitive drum 20 and the switchback roller 42 are switched from the reverse rotation direction to the normal rotation direction.

  Next, as shown in FIG. 8I, the paper P passes between the photosensitive drum 20 and the transfer roller 22 so that an image is formed in the other direction.

  At this time, as indicated by the timing I in FIG. 3, the CPU 55 controls the transfer bias circuit 48 to control the transfer roller after a predetermined time has elapsed based on switching of the output of the motor 46 from the reverse rotation direction to the normal rotation direction. A transfer bias is applied to 22. Further, the first sensor 57 and the second sensor 58 are turned ON in order.

  Next, as shown in FIG. 8J, the sheet P on which images are formed on one side and the other side of the sheet P is discharged through the discharge port 34 by the rotation of the pressure roller 31 and the switchback roller 42. It is discharged to the tray 36.

  At this time, as indicated by timing J in FIG. 3, the first sensor 57 and the second sensor 58 are sequentially turned OFF.

  After the paper P is discharged, the motor 46, the charging bias circuit 47, and the developing bias circuit 49 are stopped under the control of the CPU 55.

  Thus, the image forming process on both sides of the paper P is completed.

  Note that the printer 1 employs a cleanerless system, and when the motor 46 outputs in the forward direction, the adhered matter that adheres to the photosensitive drum 20 after the toner image is transferred to the paper P is the same as that of the photosensitive drum 20. When the roller passes through the developing nip point D with the developing roller 27, it is attracted to the surface of the developing roller 27 and collected.

  Specifically, the deposit that adheres to the photosensitive drum 20 passes through the discharge point E, so that its potential becomes higher than the surface potential of the developing roller 27. As a result, the adhering matter adhering to the photosensitive drum 20 moves to the surface of the developing roller 27 by the action of electrostatic force when passing through the developing nip point D as the photosensitive drum 20 rotates, Collected inside.

Further, the developing roller 27 can scrape off deposits adhering to the photosensitive drum 20 from the photosensitive drum 20 by a mechanical frictional force and collect them in the developing cartridge 19.
6). Operation and Effect (1) According to the printer 1 of the present invention, as shown in FIGS. 2 and 3, the transfer roller 22 has a polarity opposite to that of the toner on the photosensitive drum 20 by executing the transfer bias switching control. The state where the transfer bias is applied is switched to the state where the transfer bias is not applied to the transfer roller 22.

  That is, the surface potential of the transfer roller 22 can be brought close to 0V.

  Therefore, the surface potential of the transfer roller 22 can be made lower than the potential of the toner on the peripheral surface of the photosensitive drum 20, so that the toner remaining on the peripheral surface of the photosensitive drum 20 is transferred by the potential difference with the transfer roller 22. Transfer to the roller 22 can be suppressed.

  After the transfer bias switching control is executed, the photosensitive drum 20 is moved in the forward direction until the transfer nip point T on the peripheral surface of the photosensitive drum 20 at the time when the transfer bias switching control is executed exceeds the discharge point E. After the rotation, the rotation direction switching control for controlling the motor 46 is executed so that the photosensitive drum 20 is rotated in the reverse rotation direction.

  That is, when the CPU 55 executes the switching control mode, the surface of the peripheral surface of the photosensitive drum 20 charged by the scorotron charger 21 between the transfer nip point T and the discharge point E on the peripheral surface of the photosensitive drum 20 is displayed. It is possible to suppress the potential from being lowered by the transfer roller 22.

  Therefore, the CPU 55 can execute the reverse rotation control mode and make the entire peripheral surface of the photosensitive drum 20 charged by the scorotron charger 21 while rotating the photosensitive drum 20 in the reverse rotation direction.

  As a result, even when toner remains on the peripheral surface of the photosensitive drum 20, the toner is not attracted to the transfer roller 22 even after passing through the transfer nip point T on the photosensitive drum 20.

As a result, it is possible to prevent toner from adhering to the paper P from the transfer roller 22 when the paper P is conveyed between the photosensitive drum 20 and the transfer roller 22.
(2) Also, according to the printer 1, as shown in FIGS. 2 and 3, the rotation direction of the switchback roller 42 is simultaneously switched by switching the output of the motor 46 between the forward rotation direction and the reverse rotation direction.

  At this time, simultaneously with switching of the rotation direction of the switchback roller 42, the rotation is performed so that the rotation direction of the photosensitive drum 20 is also switched. However, the photosensitive drum 20 is driven by the transfer roller 22 regardless of the linkage with the switchback roller 42. A decrease in potential on the peripheral surface is suppressed.

Therefore, since the photosensitive drum 20 and the switchback roller 42 can be controlled by the single motor 46, the number of parts can be reduced.
(3) Further, according to the printer 1, as shown in FIGS. 3 and 6E, after the transfer bias switching control is executed and the potential on the peripheral surface of the photosensitive drum 20 is not lowered by the transfer roller 22, the photosensitive drum 20 Since the drum 20 rotates at least once in the forward rotation direction, specifically, two to three rotations, when the photosensitive drum 20 rotates in the reverse rotation direction, the entire surface of the photosensitive drum 20 is reliably secured to the scorotron charger 21. It can be in a charged state.

As a result, it is possible to reliably suppress the toner from adhering to the paper P from the transfer roller 22.
(4) Further, according to the printer 1, as shown in FIG. 1, the developing roller 27 that contacts the photosensitive drum 20 supplies toner to the electrostatic latent image on the photosensitive drum 20 to develop the toner image. be able to.
(5) According to the printer 1, as shown in FIGS. 2 and 3, the developing roller 27 supplies toner to the electrostatic latent image on the photosensitive drum 20, regardless of the rotation direction of the photosensitive drum 20. Then, the toner image is developed, and the transfer of toner from the developing roller 27 to the photosensitive drum 20 is suppressed by the action of electrostatic force which is a repulsive force repelling the peripheral surface of the photosensitive drum 20.

Therefore, since it is not necessary to control to switch the developing bias applied to the developing roller 27 during the output of the motor 46, the control man-hour can be reduced.
(6) According to the printer 1, as shown in FIG. 1, the developing roller 27 supplies toner to the electrostatic latent image on the photosensitive drum 20 and collects deposits adhering to the photosensitive drum 20. can do.

  Therefore, it is not necessary to separately provide a cleaning mechanism such as a cleaning brush and a cleaning roller for collecting toner remaining on the peripheral surface of the photosensitive drum 20, and the configuration of the printer 1 can be simplified.

  Further, if the cleaning mechanism is not provided, toner does not transfer from the cleaning mechanism to the peripheral surface of the photosensitive drum 20 when the motor 46 outputs in the reverse direction.

Therefore, the toner adhesion amount on the peripheral surface of the photosensitive drum 20 can be reduced, and toner transfer from the peripheral surface of the photosensitive drum 20 to the transfer roller 22 can be further prevented.
(7) According to the printer 1, as shown in FIG. 1, since the scorotron charger 21 and the photosensitive drum 20 are not in contact with each other, toner is transferred from the photosensitive drum 20 to the scorotron charger 21. In addition, the transfer of toner from the scorotron charger 21 to the photosensitive drum 20 can be suppressed.
7). In the above-described embodiment, in the transfer bias switching control, the CPU 55 controls the transfer bias circuit 48 to cancel the application of the transfer bias to the transfer roller 22, but the transfer roller 22 has a peripheral surface of the photosensitive drum 20. A transfer bias having the same polarity as that of the electrode and having a potential equal to or higher than that potential may be applied.

  Specifically, as indicated by a broken line in FIG. 3, the transfer bias at this time has the same polarity as the toner remaining on the peripheral surface of the photosensitive drum 20 and a potential higher than that potential (for example, +1500 V). Is set.

  Therefore, due to the rotation of the photosensitive drum 20, when the toner remaining on the peripheral surface of the photosensitive drum 20 passes through the transfer nip point T, a repulsive force repels between the toner and the transfer roller 22. Electrostatic force works.

Thereby, transfer of toner from the peripheral surface of the photosensitive drum 20 to the transfer roller 22 is prevented, and a decrease in the surface potential of the peripheral surface of the photosensitive drum 20 is suppressed.
8). Effects of Modifications According to such modifications, the transfer roller 22 has a polarity opposite to that of the toner on the photosensitive drum 20 by executing the transfer bias switching control as shown in FIGS. The state where the transfer bias is applied is switched to a state where the transfer roller 22 is applied with a transfer bias having the same polarity as the toner on the photosensitive drum 20 and a potential higher than that.

  Therefore, an electrostatic force acting as a repulsive force repels each other between the toner on the peripheral surface of the photosensitive drum 20 and the transfer roller 22, and the toner on the peripheral surface of the photosensitive drum 20 is transferred to the transfer roller 22. Can be suppressed.

  After the transfer bias switching control is executed, the photosensitive drum 20 is moved in the forward direction until the transfer nip point T on the peripheral surface of the photosensitive drum 20 at the time when the transfer bias switching control is executed exceeds the discharge point E. After the rotation, the rotation direction switching control for controlling the motor 46 is executed so that the photosensitive drum 20 is rotated in the reverse rotation direction.

  That is, when the CPU 55 executes the switching control mode, the surface of the peripheral surface of the photosensitive drum 20 charged by the scorotron charger 21 between the transfer nip point T and the discharge point E on the peripheral surface of the photosensitive drum 20 is displayed. It is possible to suppress the potential from being lowered by the transfer roller 22.

  Therefore, the CPU 55 can execute the reverse rotation control mode and make the entire peripheral surface of the photosensitive drum 20 charged by the scorotron charger 21 while rotating the photosensitive drum 20 in the reverse rotation direction.

  As a result, even when toner remains on the peripheral surface of the photosensitive drum 20, the toner is not attracted to the transfer roller 22 even after passing through the transfer nip point T on the photosensitive drum 20.

As a result, it is possible to prevent toner from adhering to the paper P from the transfer roller 22 when the paper P is conveyed between the photosensitive drum 20 and the transfer roller 22.
9. Other Embodiments The printer 1 described above is an embodiment of the image forming apparatus of the present invention, and the present invention is not limited to the above-described embodiment.

  For example, instead of the transfer roller 22 described above, a non-contact type transfer member such as a corotron type may be applied.

  Further, for example, a contact-type charger such as a charging roller can be used instead of the above-described scorotron charger 21.

DESCRIPTION OF SYMBOLS 1 Printer 20 Photosensitive drum 21 Scorotron type charger 22 Transfer roller 27 Developing roller 42 Switchback roller 46 Motor 48 Transfer bias circuit 49 Development bias circuit 55 CPU

Claims (8)

A photosensitive drum configured to carry a developer image;
A transfer member configured to transfer a developer image to a recording medium;
A charger configured to face the photosensitive drum and charge the photosensitive drum;
Drive means configured to rotate the photosensitive drum in the forward rotation direction by an output in the forward rotation direction, and to rotate the photosensitive drum in the reverse rotation direction by an output in the reverse rotation direction;
A transfer bias applying means configured to apply a transfer bias to the transfer member;
Charging bias applying means configured to apply a charging bias to the charger;
The drive means, and a control means configured to control the transfer bias application means,
The control means includes
A normal rotation control mode to rotate the photosensitive drum in the forward direction,
A reverse rotation control mode for rotating the photosensitive drum in the reverse direction,
A switching control mode that can be switched from the normal rotation control mode to the reverse rotation control mode can be executed,
In the switching control mode, the charging bias applying means is controlled to apply the charging bias to the charger.
From a state in which a transfer bias having a polarity opposite to that of the developer on the photosensitive drum is applied to the transfer member, a state in which no transfer bias is applied to the transfer member, or the same as the developer on the photosensitive drum. Transfer bias switching control for controlling the transfer bias applying means so as to switch to a state in which a transfer bias of polarity and higher than the potential is applied;
Until said peripheral surface portion of the photosensitive drum in contact with the transfer member at the time the transfer bias switching control is executed exceeds a position facing the said charger, the photosensitive drum after rotating the forward direction a rotation direction switching control for controlling the drive means to switch to the output of the reverse rotation direction from the output of the normal rotation direction,
An image forming apparatus characterized in that In order to transfer the developer image to the other side of the recording medium having the developer image transferred on one side, the recording medium is transported again between the photosensitive drum and the transfer member. Provided with a switchback roller that can switch the medium transport direction,
The switchback roller is rotated in the forward direction by the output of the normal rotation direction of the drive means, characterized in that it is configured to rotate in the reverse direction by the output of the reverse rotation direction of the drive means The image forming apparatus according to claim 1 . 3. The image according to claim 1, wherein the control unit controls the driving unit to rotate the photosensitive drum at least once in the forward rotation direction in the rotation direction switching control. 4. Forming equipment. 4. The developing roller according to claim 1, further comprising a developing roller configured to contact the photosensitive drum and configured to supply a developer to the electrostatic latent image on the photosensitive drum . The image forming apparatus according to claim 1. A developing bias applying means configured to apply a developing bias to the developing roller;
The control means is configured to control the developing bias applying means, the peripheral surface of the same polarity of the photosensitive drum to said developing roller and, and applying the following developing bias potential thereof, wherein Item 5. The image forming apparatus according to Item 4 . Said developing roller, when the photosensitive drum is rotated in the forward direction, the is characterized in that is configured to collect the deposits adhering on the photosensitive drum, according to claim 4 or 5 the image forming apparatus according to. The image forming apparatus according to claim 1, wherein the charger is a scorotron charger.   A photosensitive drum configured to carry a developer image;
  A transfer member configured to transfer a developer image to a recording medium;
  A charger configured to face the photosensitive drum and charge the photosensitive drum;
  Drive means configured to rotate the photosensitive drum in the forward rotation direction by an output in the forward rotation direction, and to rotate the photosensitive drum in the reverse rotation direction by an output in the reverse rotation direction;
  A transfer bias applying means configured to apply a transfer bias to the transfer member;
  Charging bias applying means configured to apply a charging bias to the charger;
  Control means configured to control the driving means and the transfer bias applying means;
With
  The control means includes
    A normal rotation control mode for rotating the photosensitive drum in the normal rotation direction;
    A reverse rotation control mode for rotating the photosensitive drum in the reverse rotation direction;
    A switching control mode that can be switched from the normal rotation control mode to the reverse rotation control mode can be executed,
  In the switching control mode, the charging bias applying means is controlled to apply the charging bias to the charger.
    From a state in which a transfer bias having a polarity opposite to that of the developer on the photosensitive drum is applied to the transfer member, a state in which a transfer bias that generates a repulsive force repelling the developer on the photosensitive drum is applied to the transfer member. Transfer bias switching control for controlling the transfer bias applying means to switch;
    After rotating the photosensitive drum in the forward rotation direction until the peripheral surface portion of the photosensitive drum that is in contact with the transfer member exceeds the position facing the charger at the time when the transfer bias switching control is executed. Rotation direction switching control for controlling the drive means to switch from the output in the forward direction to the output in the reverse direction;
An image forming apparatus characterized in that

Images