JP2021086095A - Image forming apparatus - Google Patents

Abstract

To prevent an increase in a conveyance load when a leading end of a sheet enters a fixing nip and warping upward of the sheet when a rear end of the sheet exits from a transfer nip, while suppressing, with a conveyance guide, flapping of the sheet being conveyed.SOLUTION: An image forming apparatus comprises a driving unit that drives a conveyance guide member 44 between a pressed position where a guide surface 44b is pressed against a rear face of a sheet P being conveyed and a retracted position where the conveyance guide member is separated toward a pressure roller 42 from the pressed position in a direction intersecting with a sheet conveyance direction. After a leading end of the sheet enters a transfer nip Nt, the driving unit locates the conveyance guide member 44 at the retracted position until the leading end enters a fixing nip Nf, subsequently, when the leading end of the sheet P enters the fixing nip Nf, moves the conveyance guide member 44 to the pressed position, and after the movement, returns the conveyance guide member 44 to the retracted position before a rear end of the sheet P exits from the transfer nip Nt.SELECTED DRAWING: Figure 12C

Description

The present invention relates to an image forming apparatus.

Generally, an electrophotographic image forming apparatus transfers a transfer unit that transfers a toner image supported on the surface of an image carrier onto a sheet (paper, OHP sheet, etc.) and a toner image transferred to the sheet by the transfer unit. It is provided with a fixing portion for heat fixing to the sheet.

The transfer unit has a transfer roller that forms a transfer nip with the image carrier on which the toner image is supported. When the sheet passes through the transfer nip, the transfer roller applies a transfer bias voltage having a polarity opposite to the polarity of the toner from the back surface of the sheet, thereby attracting the toner image from the image carrier to the printed surface of the sheet and transferring the toner image. I do.

The fixing portion includes a fixing roller that is heated to a predetermined temperature and a pressure roller that is pressed against the peripheral surface of the fixing roller to form a fixing nip. When the sheet passes through the fixing nip, the fixing portion heats and pressurizes the toner image on the printed surface of the sheet to fix it on the printed surface.

In this type of image forming apparatus, a transport guide member is arranged in the transport path of the sheet between the transfer portion and the fixing portion (see, for example, Patent Document 1). The transport guide member guides the sheet that has passed through the transfer nip to the fixing nip.

Japanese Unexamined Patent Publication No. 8-254913

By the way, if the regulation of the sheet by the transport guide member is loose, the sheet being transported becomes wavy. When the sheet enters the fixing nip in a wavy state, the wrinkling of the sheet is crushed by the fixing nip and becomes wrinkled.

In order to prevent the occurrence of such wrinkles, it is conceivable to strengthen the regulation by the transport guide member to prevent the sheet being transported from wrinkling.

However, in order to strengthen the regulation by the transport guide member, it is necessary to change the position of the transport guide member and strongly press the guide surface against the back surface of the sheet. As a result, the sheet transport load when the tip of the sheet enters the fixing nip increases, and the rear end of the sheet flutters when passing through the transfer nip. The fluttering of the sheet passing through the transfer nip causes transfer defects such as horizontal streaks. Further, when the transport guide member is strongly pressed against the back surface of the sheet, the tension of the sheet increases, so that the rear end of the sheet jumps up when it comes out of the transfer nip, and the unfixed toner image on the printed surface of the sheet scatters. is there.

The present invention has been made in view of this point, and an object of the present invention is to suppress the waviness of the sheet during transportation by a transfer guide member, and to carry a transfer load when the tip of the sheet enters the fixing nip. The purpose is to prevent the sheet from jumping up when the rear end of the sheet comes out of the transfer nip.

The image forming apparatus according to the present invention forms a transfer nip between an image carrier that rotates with a toner image supported on the surface and the image carrier, and when the sheet passes through the transfer nip, the above is described. A transfer unit including a transfer roller that transfers a toner image on the surface of the image carrier to the printed surface of the sheet, and a fixing portion provided on the downstream side of the transfer unit in the sheet transport direction and pressed against each other to form a fixing nip. A fixing portion having a roller and a pressure roller, the fixing roller is brought into contact with the printed surface of the sheet passing through the fixing nip, and the toner image on the printed surface is heated and pressurized to be fixed to the sheet, and the above. It is provided between the transfer portion and the fixing portion, and includes a transfer guide member that guides the back surface of the sheet during transportation opposite to the printing surface, and the sheet is sandwiched between both the fixing nip and the transfer nip. It has a length that can be transported in the state of being printed.

Then, the transport guide member is divided into a pressing position in which the guide surface is pressed against the back surface of the sheet being conveyed and a retracted position in which the guide surface is separated from the pressing position on the pressure roller side in a direction intersecting the sheet conveying direction. The drive unit further includes a drive unit that drives between the two, and the drive unit keeps the transport guide member in the retracted position until the tip of the sheet enters the transfer nip and then enters the fixing nip. When the tip of the sheet enters the fixing nip, the transport guide member is moved from the retracted position to the pressing position, and after the movement, the transport guide member is moved before the rear end of the sheet pulls out the transfer nip. It is configured to execute position switching control for returning from the pressing position to the retracted position.

According to the present invention, while suppressing the waviness of the sheet during transportation by the transfer guide, the transfer load increases when the front end of the sheet enters the fixing nip, and the sheet when the rear end of the sheet exits the transfer nip. Bounce back can be prevented.

FIG. 1 is an overall schematic view showing an image forming apparatus according to an embodiment. FIG. 2 is a schematic cross-sectional view of the fixing device cut in a vertical plane extending in the left-right direction, and is a view showing a state in which the transport guide member is in the pressing position. FIG. 3 is a perspective view seen from the front diagonally right side showing a state in which the fixing frame of the fixing device is removed. FIG. 4 is a perspective view showing a nip pressure switching portion and a drive portion of the transport guide member provided at the front end portion of the fixing device. FIG. 5 is a side view seen from the front side showing the nip pressure switching portion on the front side. FIG. 6 is a perspective view showing a pressure adjusting member and a pressure switching cam that are a part of the nip pressure switching portion on the front side. FIG. 7A is a perspective view showing a pressure switching cam. FIG. 7B is a side view showing the pressure switching cam as viewed from the axial direction. FIG. 8A is a perspective view showing the transport guide member as viewed from the diagonally forward right side. FIG. 8B is a perspective view showing the transport guide member as viewed from the diagonally front left side. FIG. 9A is a perspective view showing a guide drive cam. FIG. 9B is a side view of the guide drive cam as viewed from the axial direction. FIG. 10 is a view corresponding to FIG. 2 showing a state in which the transport guide member is in the retracted position. FIG. 11 is a block diagram showing a configuration of a control system in a drive unit of a transport guide member. FIG. 12A is an explanatory diagram for explaining the content of the position switching control of the transfer guide member, and is a diagram showing a state in which the sheet is sandwiched and conveyed only by the transfer nip. FIG. 12B is an explanatory diagram for explaining the content of the position switching control of the transfer guide member, and is a diagram showing a state in which the sheet is sandwiched and conveyed by both the transfer nip and the fixing nip. FIG. 12C is an explanatory diagram for explaining the content of the position switching control of the transfer guide member, in which the sheet is sandwiched and conveyed by both the transfer nip and the fixing nip, and immediately before the rear end of the sheet is pulled out from the transfer nip. It is a figure which shows the state. FIG. 12D is an explanatory diagram for explaining the content of the position switching control of the transport guide member, and is a diagram showing a state in which the sheet is transported only by the fixing nip.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The present invention is not limited to the following embodiments.

<< Embodiment >>
FIG. 1 shows an image forming apparatus 1 in this embodiment. The image forming apparatus 1 comprises a monochrome laser printer in this embodiment. The image forming apparatus 1 has, as printing modes, a normal printing mode for printing on a normal sheet P (plain paper or the like) having a relatively thin thickness, and a sheet P (envelope) having a thickness thicker than the normal sheet P. Etc.) It has an envelope printing mode for printing. The user can set the print mode to either the normal print mode or the envelope print mode by operating the operation unit 1a (shown only in FIG. 11) provided on the front side surface of the image forming apparatus 1. There is.

[Overall configuration of image forming apparatus 1]
The image forming apparatus 1 includes a sheet supply unit 10, an image forming unit 20, a fixing device (fixing unit) 40, a sheet discharging unit 50, and a housing 60. A plurality of transport roller pairs 11 to 13 that sandwich and transport the sheet P are arranged in the sheet transport path T from the sheet supply section 10 to the sheet discharge section 50. In the sheet transport path T of FIG. 1, the path from the image forming unit 20 to the fixing device 40 is vertically drawn, but is actually slightly tilted to the left toward the upper side (see FIG. 2 and the like). In the following description, "front side" and "rear side" mean the front side and the rear side (front side and back side in the vertical direction of the paper surface in FIG. 1) of the image forming apparatus 1, and "left side" and "right side" are used. , Means the left side and the right side when the image forming apparatus 1 is viewed from the front side.

The sheet supply unit 10 has a sheet cassette 10a in which the sheet P is housed, and a pickup roller 10b that takes out the sheet P in the sheet cassette 10a and sends it to the outside thereof. The sheet P sent out of the cassette from the sheet cassette 10a is supplied to the image forming unit 20 via the transport roller pair 11.

The image-creating portion 20 is arranged at an intermediate portion in the vertical direction of the right end portion in the housing 60. The image forming unit 20 has a photoconductor drum 21, and around the photoconductor drum 21, the charger 23, the exposure device 25, the developer 27, and the like, are arranged in the counterclockwise direction in the figure with reference to the 10 o'clock direction. The transfer unit 29, the cleaning device 31, and the static eliminator 33 are arranged in this order.

At the time of image formation, first, the peripheral surface of the photoconductor drum 21 is charged by the charger 23. After that, the exposure apparatus 25 irradiates the peripheral surface of the photoconductor drum 21 with laser light based on image data (for example, image data received from an external terminal). As a result, an electrostatic latent image corresponding to the image data is formed on the peripheral surface of the photoconductor drum 21. The electrostatic latent image formed on the peripheral surface of the photoconductor drum 21 is developed and visualized by the toner charged by the developing device 27.

The developed toner image is transferred to the sheet P by the transfer unit 29. The transfer unit 29 has a transfer roller 30 (an example of a transfer member) that forms a transfer nip Nt with the photoconductor drum 21, and is applied to the transfer roller 30 when the sheet P passes through the transfer nip Nt. Due to the transfer bias, the toner image on the photoconductor drum 21 is moved to the printing surface of the sheet P and transferred.

After the toner image is transferred, the residual toner remaining on the peripheral surface of the photoconductor drum 21 is recovered by the cleaning device 31, and the electric charge on the peripheral surface of the photoconductor drum 21 is eliminated by the static eliminator 33. On the other hand, the sheet P on which the toner image is transferred is sent out to the fixing device 40 on the downstream side by the transfer roller 30 and the photoconductor drum 21.

The fixing device 40 has a fixing roller 41 and a pressure roller 42 that are pressed against each other to form a fixing nip Nf. The fixing roller 41 has a built-in heater 41a. In the fixing device 40, when the sheet P supplied from the image forming unit 20 passes through the fixing nip Nf, the toner image is heated and pressed by the fixing roller 41 and the pressure roller 42 to be fixed on the sheet P. Then, the sheet P on which the toner image is fixed by the fixing device 40 is sent out to the downstream side by the fixing roller 41 and the pressure roller 42. The sheet P sent out from the fixing device 40 is discharged to the sheet discharging portion 50 formed on the upper surface of the housing 60 via a plurality of transport roller pairs 12 and 13.

[Explanation of fixing mode]
The fixing device 40 has a high pressure mode in which the nip pressure between the fixing roller 41 and the pressurizing roller 42 (hereinafter referred to as the fixing nip pressure) becomes a high pressure (first pressure), and the fixing nip pressure is a low pressure (lower than the first pressure). It has two fixing modes, a low pressure mode that becomes the second pressure). The fixing mode of the fixing device 40 is set by the controller 106 (shown only in FIG. 11). The controller 106 comprises a microcomputer having a CPU, ROM and RAM. When the controller 106 determines that the normal print mode is set as the print mode based on the operation signal from the operation unit 1a, the controller 106 sets the fixing mode of the fixing device 40 to the high pressure mode. When the controller 106 determines that the envelope printing mode is set as the printing mode based on the operation signal from the operation unit 1a, the controller 106 sets the fixing mode of the fixing device 40 to the low pressure mode. This reduces the nip pressure acting on the sheet P of an envelope or the like, which is thicker than plain paper, and prevents the sheet P from wrinkling.

[Detailed configuration of fixing device 40]
As shown in FIG. 2, the fixing roller 41 and the pressure roller 42 are housed in a rectangular box-shaped fixing frame 43. The fixing roller 41 and the pressure roller 42 extend in the front-rear direction of the image forming apparatus 1 in the fixing frame 43. In the present embodiment, the fixing roller 41 is a drive roller driven by a drive motor, and the pressure roller 42 is a driven roller. The pressure roller 42 may be used as a driving roller, and the fixing roller 41 may be used as a driven roller.

The fixing frame 43 has a metal bottom plate 431 and a resin case 432 arranged so as to cover the upper side of the bottom plate 431. The resin case 432 is formed in a rectangular box shape that opens downward, and the lower side of the resin case 432 is closed by a bottom plate 431. The bottom plate 431 is formed with a sheet receiving port 431a for receiving the sheet P supplied from the photoconductor drum 21 and the transfer roller 30. The guide plate portion 44a of the transport guide member 44 penetrates through the sheet receiving port 431a. The guide plate portion 44a guides the sheet P between the fixing roller 41 and the pressure roller 42. A through hole 431b for allowing the inclined plate portion 44g of the transport guide member 44 to escape is formed on the right side of the sheet receiving port 431a in the bottom plate 431. A sheet discharge port 432a for discharging the sheet P that has passed through the fixing roller 41 and the pressure roller 42 to the outside of the fixing frame 43 is formed on the upper wall of the resin case 432.

On the right side of the pressurizing roller 42 in the fixing frame 43, there is a nip pressure switching portion 45 that switches the fixing nip Nf between high pressure and low pressure, and a drive that drives the transport guide member 44 between a pressing position and a retracting position, which will be described later. A unit 100 is provided.

[Structure of nip pressure switching unit 45]
As shown in FIG. 3, nip pressure switching portions 45 are provided at both end portions in the front-rear direction of the pressure roller 42. Since the front and rear nip pressure switching portions 45 have the same configuration, only the configuration of the front nip pressure switching portion 45 will be described below with reference to FIGS. 4 to 6.

The nip pressure switching unit 45 includes a roller support member 451 that supports the pressure roller 42, an urging spring 458 that urges the roller support member 451 toward the fixing roller 41, and a pressure adjusting member 452.

The roller support member 451 has a long shape in the vertical direction. A bearing member 459 that supports the end of the pressure roller 42 is attached to an intermediate portion of the roller support member 451 in the vertical direction. A through hole 451a penetrating in the front-rear direction is formed at the lower end of the roller support member 451. The through hole 451a is formed coaxially with the through hole 452c formed at the lower end of the pressure adjusting member 452. The inner peripheral surfaces of both through holes 451a and 452c are fitted into a common support pin 455 (shown only in FIG. 5). The support pin 455 is fixed to a standing plate (not shown) erected on the bottom plate 431.

As shown in FIG. 6, the pressure adjusting member 452 has a cam roller support portion 452b and a rotating portion 452a having the through hole 452c. The rotating portion 452a has a U-shape that opens to the left when viewed from above. The through hole 452c penetrates the left end of the rotating portion 452a in the front-rear direction. The rotating portion 452a is rotatably supported by the support pin 455 penetrating the through hole 452c.

The cam roller support portion 452b extends upward from the right end portion of the rotating portion 452a. The upper end of the cam roller support portion 452b is formed in a U shape that opens to the right when viewed from above. A cam roller 457 is provided between the front and rear facing walls at the upper end of the cam roller support portion 452b. The cam roller 457 is rotatably supported by support pins 456 supported at both ends by the front and rear facing walls. The outer peripheral surface of the cam roller 457 is in contact with the outer peripheral surface of the pressure switching cam 454.

As shown in FIGS. 4 and 5, an urging spring 458 is provided between the left side surface of the upper end portion of the cam roller support portion 452b and the upper end portion of the roller support member 451. The urging spring 458 is composed of a compression coil spring whose axis extends in the left-right direction. The left end of the urging spring 458 is in contact with the upper end of the roller support member 451 via an L-shaped washer 460. The urging spring 458 is arranged in a compressed state between the upper end portion of the roller support member 451 and the upper end portion (cam roller support portion 452b) of the pressure adjusting member 452. The outer peripheral surface of the cam roller 457 is constantly pressed against the outer peripheral surface of the pressure switching cam 454 by the urging force of the urging spring 458.

The pressure switching cam 454 is fixed to a rotating shaft 453 extending in the front-rear direction on the right side of the pressure roller 42. Both ends of the rotary shaft 453 are rotatably supported via bearings by a fixing member (not shown) erected on the bottom plate 431. When the fixing device 40 is attached to the image forming device 1, the rotating shaft 453 is connected to a motor 105 (shown only in FIG. 11) in the image forming device 1 via a gear train (not shown). The motor 105 is controlled by a controller 106 provided in the image forming apparatus 1.

As shown in FIGS. 7A and 7B, the inner peripheral surface 454a of the pressure switching cam 454 is formed in a D-shaped cross section, and the inner peripheral surface 454a is fitted onto the D-cut shaft portion of the rotating shaft 453. Arc-shaped protrusions 454b are formed on both side surfaces of the pressure switching cam 454 in the axial direction. The pressure switching cam 454 is firmly fixed to the peripheral surface of the rotary shaft 453 by abutting the inner peripheral surface of the arc-shaped protrusion 454b on the outer peripheral surface of the rotary shaft 453. The outer peripheral surface of the pressure switching cam 454 is formed with a minimum radius portion 454c that minimizes the distance from the axis and a maximum radius portion 454d that maximizes the distance from the axis. The maximum radius portion 454d is formed over a certain range extending in the circumferential direction when viewed from the axial direction of the pressure switching cam 454 (a range of approximately 150 ° when viewed from the central angle θ1). The downstream end of the maximum radius portion 454d in the clockwise direction (D1 direction in FIG. 7B) and the minimum radius portion 454c are adjacent to each other with a large step in the radial direction. An intermediate radius portion 454e is formed between the upstream end of the maximum radius portion 454d in the clockwise direction (D1 direction) and the minimum radius portion 454c. The intermediate radius portion 454e is formed so that the radius becomes smaller toward the side closer to the minimum radius portion 454c.

[Operation of nip pressure switching unit 45]
Next, the operation of the nip pressure switching unit 45 will be described with reference to FIG. FIG. 5 shows a state in which the maximum radius portion 454d of the pressure switching cam 454 abuts on the cam roller 457, and in this state, the nip pressure between the fixing roller 41 and the pressure roller 42 is set to a high pressure. When the rotating shaft 453 rotates in the clockwise direction (D2 direction) in the figure and the minimum radius portion 454c of the pressure switching cam 454 comes into contact with the cam roller 457, the pressure adjusting member 452 moves with the support pin 455 as a fulcrum. It rotates in the clockwise direction (D3 direction). Along with this, the total length of the urging spring 458 increases and the urging force of the urging spring 458 decreases. As a result, the nip pressure between the fixing roller 41 and the pressure roller 42 is switched from high pressure to low pressure. On the contrary, when the nip pressure is switched from low pressure to high pressure, the rotary shaft 453 is rotated in the counterclockwise direction until the maximum radius portion 454d of the pressure switching cam 454 abuts on the pressure roller 42. Just do it.

[Structure of transport guide member 44]
Next, the transport guide member 44 will be described with reference to FIGS. 2, 8A, and 8B. The transport guide member 44 has a guide plate portion 44a extending in the front-rear direction, a front slide plate portion 44c protruding to the right from the lower end portion of the guide plate portion 44a, and a rear slide plate portion 44d. The transport guide member 44 is formed by cutting a single sheet metal member into a predetermined shape and bending it.

The guide plate portion 44a is slightly inclined to the left toward the upper side. The left side surface of the guide plate portion 44a constitutes a guide surface 44b that guides the seat P to the fixing nip Nf. The lower end of the guide plate portion 44a is bent diagonally upward to the right in a V shape. The guide plate portion 44a is always urged to the right by the urging spring 103. The urging spring 103 is composed of, for example, a tension coil spring. The urging spring 103 is fixed to a fixing member (not shown) provided in the fixing frame 43.

The front slide plate portion 44c and the rear slide plate portion 44d are slidably in contact with the lower surface of the bottom plate 431 (see FIG. 2) of the fixing frame 43 in the left-right direction.

As shown in FIGS. 8A and 8B, the front slide plate portion 44c has a pair of slide holes 44e arranged at intervals in the left-right direction. Each slide hole 44e is formed in an elongated shape extending in the left-right direction. The rear slide plate portion 44d has a pair of slide holes 44f arranged in the front-rear direction. Each slide hole 44f is formed in a long hole shape extending in the left-right direction. A header pin (not shown) having a screw portion formed at the tip thereof is inserted into each slide hole 44e of the front slide plate portion 44c and each slide hole 44f of the rear slide plate portion 44d. The screw portion of each header pin is screwed into a screw hole formed in the bottom plate 431 of the fixing frame 43. Then, the front slide plate portion 44c and the rear slide plate portion 44d are held by these header pins on the lower surface of the bottom plate 431 so as not to fall off, and are guided so as to be slidable in the left-right direction.

An inclined plate portion 44g is formed at the right end portion of the front slide plate portion 44c. The inclined plate portion 44g is formed by cutting out a part of the front slide plate portion 44c in a U shape and folding it upward. The inclined plate portion 44g is inclined to the left side toward the upper side. The transport guide member 44 is driven in the left-right direction by the drive unit 100 via the inclined plate portion 44 g.

[Structure of drive unit 100]
As shown in FIGS. 2 and 4, the drive unit 100 includes a rotary shaft 453 that forms a part of the nip pressure switching unit 45, a guide drive cam 101 fixed to the front end portion of the rotary shaft 453, and a vertical direction. It has a drive shaft 102 extending to.

A protruding plate 102a (shown only in FIG. 2) is connected to an intermediate portion of the drive shaft 102 in the vertical direction. The projecting plate 102a projects horizontally outward in the radial direction from the peripheral surface of the drive shaft 102. The projecting plate 102a is always urged upward by the urging spring 104 fixed to the fixing frame 43. The drive shaft 102 is always urged upward by the urging spring 104 via the projecting plate 102a. Then, the upper end surface of the drive shaft 102 is pressed against the outer peripheral surface of the guide drive cam 101 by the urging force of the urging spring 104.
The lower end of the drive shaft 102 is in contact with the upper surface of the inclined plate portion 44g of the transport guide member 44. Since the transport guide member 44 is urged to the right side by the urging force of the urging spring 103, the lower end portion of the drive shaft 102 and the upper surface of the inclined plate portion 44g are always in contact with each other.
When the drive shaft 102 is lowered, the inclined plate portion 44g of the transport guide member 44 is pressed to the left by the lower end portion of the drive shaft 102, the transport guide member 44 is moved to the left side, the drive shaft 102 is raised, and the lower end portion thereof is raised. When it escapes to the upper side, the transport guide member 44 moves to the right side by the urging force of the urging spring 103.

The vertical movement of the drive shaft 102 is caused by the rotation of the guide drive cam 101. As shown in FIG. 4, the guide drive cam 101 is fixed to the rear side of the pressure switching cam 454 on the rotary shaft 453 at intervals.

As shown in FIGS. 9A and 9B, the inner peripheral surface 101a of the guide drive cam 101 is formed in a D-shaped cross section and is fitted onto the D-cut shaft portion of the rotating shaft 453.

Arc-shaped protrusions 101b are formed on both side surfaces of the guide drive cam 101 in the axial direction. The guide drive cam 101 is firmly fixed to the peripheral surface of the rotary shaft 453 by abutting the inner peripheral surface of the arc-shaped protrusion 101b on the outer peripheral surface of the rotary shaft 453.

The outer peripheral surface of the guide drive cam 101 has a minimum radius portion 101c that minimizes the distance from the axis and a maximum radius portion 101d that maximizes the distance from the axis.

The minimum radius portion 101c is formed over a certain range extending in the circumferential direction when viewed from the axial direction of the guide drive cam 101 (a range of approximately 180 ° when viewed from the central angle θ2). The maximum radius portion 101d is formed at a position 90 ° away from the downstream end of the minimum radius portion 101c in the clockwise direction (D4 direction in FIG. 9B) in the clockwise direction. An intermediate radius portion 101e is formed between the downstream end of the minimum radius portion 101c in the clockwise direction and the maximum radius portion 101d. The intermediate radius portion 101e is formed so that the radius increases toward the maximum radius portion 101d side.

[Change in position of transport guide member 44 due to rotation of guide drive cam 101]
The transport guide member 44 is configured to be movable between a pressing position (position in FIG. 2) and a retracting position (position in FIG. 10) according to the rotation position of the guide driving cam 101.

As shown in FIG. 2, the transport guide member 44 is located at the pressing position in a state where the maximum radius portion 101d of the guide drive cam 101 is in contact with the upper end surface of the drive shaft 102. At the pressing position, the guide surface 44b of the transport guide member 44 is located on the fixing roller 41 side (left side) of the nip line L. Here, the nip line L is a line segment connecting the fixing nip Nf and the transfer nip Nt when viewed from the axial direction of the fixing roller 41. Since the guide surface 44b is located closer to the fixing roller 41 than the nip line L, the guide surface 44b can be strongly pressed against the back surface of the sheet P.

When the rotary shaft 453 is rotated in the clockwise direction (D5 direction) of FIG. 2 from the state shown in FIG. 2, the radius from the axis of the guide drive cam 101 to the upper end surface of the drive shaft 102 gradually decreases. To do. As a result, the drive shaft 102 is pushed upward by the urging force of the urging spring 104, and the transport guide member 44 moves to the right side accordingly. Then, when the drive shaft 102 is rotated by approximately 90 ° from the state of FIG. 2, as shown in FIG. 10, the minimum radius portion 101c of the guide drive cam 101 comes into contact with the upper end surface of the drive shaft 102, and the drive shaft 102 is the most. To rise. Then, the transport guide member 44 moves to the retracted position as the drive shaft 102 rises. In the retracted position, the guide surface 44b of the transport guide member 44 is located on the pressure roller 42 side (right side) of the nip line L. By moving the guide surface 44b closer to the pressure roller 42 than the nip line L, the pressing force of the guide surface 44b against the seat P is reduced.

As shown in FIGS. 2 and 10, in the process of switching the transport guide member 44 between the pressing position and the retracting position, the cam roller 457 is in contact with the maximum radius portion 454d of the pressure switching cam 454, so that the fixing nip pressure is increased. Maintained at high pressure.
When the pressure switching cam 454 is rotated 90 ° clockwise (D6 direction) from the state shown in FIG. 10 in order to switch the fixing nip pressure from high pressure to low pressure, the guide drive cam 101 also rotates 90 ° in the same direction. However, since the minimum radius portion 101c of the guide drive cam 101 is formed over 180 ° around the axis, the upper end surface of the drive shaft 102 maintains the contact state with the minimum radius portion 101c.
Therefore, even if the fixing nip is switched from high pressure to low pressure, the transport guide member 44 does not move and is maintained in the retracted position. Since the minimum radius portion 454c corresponding to the low pressure of the fixing nip pressure in the pressure switching cam 454 does not have a length in the circumferential direction, the position of the transport guide member 44 can be switched when the fixing nip pressure is low. The transport guide member 44 is maintained in the retracted position. As described above, in the present embodiment, the drive unit 100 switches the position between the pressing position and the retracting position of the transport guide member 44 only when the fixing nip pressure is in a high pressure state.

[Details of position switching control of transport guide member 44]
Next, the details of the position switching control of the transport guide member 44 will be described. The position switching control is executed by the controller 106 which is a part of the drive unit 100.

As shown in FIG. 11, the controller 106 is electrically connected to a seat detection sensor 107 (see FIG. 10) provided on the downstream side of the transfer nip Nt and a motor 105 for driving the rotary shaft 453. The sheet detection sensor 107 is composed of, for example, a reflection type optical sensor, detects the sheet P, and transmits the detection signal to the controller 106. The controller 106 calculates the transport position of the seat P based on the signal from the seat detection sensor 107.

The controller 106 drives the rotary shaft 453 by the motor 105 based on the calculated transfer position of the seat P to execute the position switching of the transfer guide member 44.

Specifically, the controller 106 causes the transport guide member 44 to stand by at the retracted position as shown in FIG. 10 before the tip of the seat P enters the transfer nip Nt.

After that, even after the tip of the sheet P enters the transfer nip Nt and the sheet P is sandwiched and conveyed only by the transfer nip Nt as shown in FIG. 12A, the controller 106 keeps the transfer guide member 44 in the retracted position. Keep in.

After that, when the tip of the sheet P enters the fixing nip Nf and the sheet P is sandwiched and conveyed by both the fixing nip Nf and the transfer nip Nt as shown in FIG. 12B, the controller 106 moves the transfer guide member 44. Is moved from the retracted position to the pressed position.

Then, before the rear end of the seat P comes out of the transfer nip Nt, the controller 106 returns the transport guide member 44 to the retracted position as shown in FIG. 12C. The timing for returning the transport guide member 44 to the retracted position is preferably immediately before the rear end of the sheet P comes out of the transfer nip Nt. Immediately before can be defined as, for example, when the rear end position of the sheet P coincides with the center position of the transfer nip Nt in the paper transport direction, but the present invention is not limited to this. Since the transfer nip Nt has a slight width in the transport direction due to the elastic deformation of the transfer roller 30, the central position of the transfer nip Nt in the paper transport direction can be determined in consideration of the compression deformation of the transfer roller 30 and the like. Good.

After that, after the rear end of the sheet P escapes from the transfer nip Nt and the sheet P is conveyed only by the fixing nip Nf as shown in FIG. 12D, the controller 106 moves the transfer guide member 44 to the retracted position. maintain. In FIGS. 12A to 12D, the movement amount δ is shown so that the movement between the pressing position and the retracting position of the transport guide member 44 can be easily visually recognized.

[Action and effect of this embodiment]
As described above, in the present embodiment, the transport guide member 44 has a pressing position in which the guide surface 44b is pressed against the back surface of the sheet P being transported, and a direction intersecting the sheet transport direction (left-right direction in the present embodiment). The drive unit 100 is provided to be driven between the pressing position and the retracting position separated from the pressing position on the pressure roller 42 side. The drive unit 100 retracts the transport guide member 44 until the tip of the seat P enters the transfer nip Nt and then enters the fixing nip Nf, and the tip of the seat P enters the fixing nip Nf and both nips. In the transport state, the transport guide member 44 is moved from the retracted position to the pressing position, and after the movement, the transport guide member 44 is returned from the pressed position to the retracted position before the rear end of the sheet pulls out the transfer nip. Execute position switching control.

According to this configuration, since the sheet P is located at the retracted position in the state where the sheet P is sandwiched and conveyed only by the transfer nip Nt, the transfer load increases due to the friction between the transfer guide member 44 and the sheet P, and the transfer is performed. It is possible to prevent defects (horizontal streaks, etc.) from occurring. Further, in a state where the sheet P is sandwiched and conveyed by both the fixing nip Nf and the transfer nip Nt, the transfer guide member 44 is driven to the pressing position by the controller 106. As a result, the back surface of the sheet P can be firmly pressed against the guide surface 44b of the transport guide member 44 to bring the sheet P into a stretched state. Therefore, it is possible to prevent the sheet P during transportation from being wavy. Further, after the sheet P is sandwiched and conveyed to both the fixing nip Nf and the transfer nip Nt, the transfer guide member 44 is retracted by the controller 106 before the rear end of the sheet P pulls out the transfer nip Nt. Returned to position. Therefore, when the rear end of the sheet P pulls out the transfer nip Nt, the tension of the sheet P can be relaxed to prevent the rear end of the sheet P from jumping up. As a result, it is possible to prevent the unfixed toner image from being scattered due to the jumping up of the rear end of the sheet P.

Further, in the present embodiment, the fixing device 40 has a nip pressure switching unit 45 capable of switching the fixing nip pressure between high pressure and low pressure. The drive unit 100 executes the position switching control when the fixing nip pressure is set to a high pressure by the nip pressure switching unit 45, and when the fixing nip pressure is set to a low pressure by the nip pressure switching unit 45, the drive unit 100 executes the position switching control. , The transport guide member 44 is configured to be maintained in the retracted position regardless of the transport position of the sheet P.

According to this configuration, it is possible to avoid unnecessarily executing the position switching control and complicating the cam configuration. That is, waviness of the sheet P is likely to occur on a normal sheet P (plain paper) or the like, which is thinner than an envelope or the like, and the fixing nip pressure is set to a high pressure when printing on the normal sheet P. In the present embodiment, paying attention to this point, by executing the position switching control of the transport guide member 44 only when the fixing nip pressure is set to high pressure, the normal sheet P in which waviness is likely to occur is being transported. Can prevent the occurrence of waviness by pressing the guide surface 44b of the transport guide member 44 against the sheet P as much as possible.
Further, in the present embodiment, the nip pressure switching unit 45 has a rotating shaft 453 and a pressure switching cam 454 rotationally and integrally fixed to the shaft 453, and is fixed by the rotation of the pressure switching cam 454. Selectively switch the pressure between high pressure and low pressure. On the other hand, the drive unit 100 has a rotary shaft 453 and a guide drive cam 101 rotationally and integrally fixed to the rotary shaft 453, and the transfer guide member 44 is pressed to a pressing position by the rotation of the guide drive cam 101. Drive to and from the retracted position. As described above, in the present embodiment, the rotary shaft 453 of the nip pressure switching unit 45 and the rotary shaft 453 of the drive unit 100 are also used.

According to this configuration, the number of parts can be reduced and the product cost can be reduced by using the rotary shaft 453 common to the nip pressure switching unit 45 and the drive unit 100.

<< Other Embodiments >>
In the above embodiment, the method of directly transferring the toner image formed on the photoconductor drum 21 to the sheet P by the transfer roller 30 has been described, but the method is not limited to this, and an intermediate transfer method may be adopted. In the intermediate transfer method, the toner image on the surface of the photoconductor drum 21 is once transferred to the intermediate transfer belt. Then, the toner image transferred (supported) on the intermediate transfer belt is moved to the sheet P by the secondary transfer roller and transferred. The secondary transfer roller forms a transfer nip with the intermediate transfer belt. In this case, the intermediate transfer belt corresponds to the image carrier, and the secondary transfer roller corresponds to the transfer member.

In the above embodiment, a monochrome printer has been described as an example of the image forming apparatus 1, but the present invention is not limited to this. The image forming apparatus 1 may be a copier, a facsimile, a multifunction device (MFP), or the like. Further, the image forming apparatus 1 may be a tandem type color printer.

Further, in the above embodiment, the drive unit 100 converts the rotation of the guide drive cam 101 into the linear movement of the transport guide member 44, but the present invention is not limited to this. The drive unit 100 may drive the transfer guide member 44 by, for example, a solenoid having a movable iron core that moves linearly.

Further, in the above embodiment, the guide surface 44b is located closer to the fixing roller 41 than the nip line L at the pressing position of the transport guide member 44, and the guide surface 44b is located closer to the fixing roller 41 than the nip line L at the retracted position. The example of being located on the side has been described, but it is not limited to this. For example, the guide surface 44b may be located closer to the fixing roller 41 than the nip line L at both the pressing position and the retracting position.

Further, in the above embodiment, the guide surface 44b of the transport guide member 44 is formed in a flat surface shape, but the present invention is not limited to this. The guide surface 44b of the transport guide member 44 may be formed of an arcuate projecting surface or a concave surface when viewed from the axial direction.

Further, in the above embodiment, an example in which the rotary fixing member is composed of the fixing roller 41 and the rotary pressure member is composed of the pressure roller 42 has been described, but the present invention is not limited to this. The rotary fixing member may be composed of an endless fixing belt, or the rotary pressurizing member may be composed of an endless pressure belt.

As described above, the present invention is useful for an image forming apparatus, and is particularly useful when applied to a printer, a facsimile, a multifunction device (MFP), or the like.

P: Sheet Nf: Fixing nip Nt: Transfer nip 1: Image forming device 29: Transfer part 30: Transfer roller 41: Fixing roller 42: Pressurizing roller 44: Transfer guide member 44b: Guide surface 45: Nip pressure switching part 100: Drive unit 101: Guide drive cam 453: Shaft 454: Pressure switching cam

Claims (3)

A transfer nip is formed between the image carrier that rotates with the toner image supported on the surface and the image carrier, and when the sheet passes through the transfer nip, the toner image on the surface of the image carrier is displayed. It has a transfer unit including a transfer member to be transferred to the printed surface of the sheet, and a rotary fixing member and a rotary pressurizing member provided on the downstream side of the transfer portion in the sheet transport direction and pressed against each other to form a fixing nip. Then, the rotary fixing member is brought into contact with the printed surface of the sheet passing through the fixing nip, and the toner image on the printed surface is heated and pressurized to be fixed to the sheet, and the transfer portion and the transfer portion. A state in which the sheet is provided between the fixing portions and is provided with a transport guide member that guides the back surface of the sheet being transported on the side opposite to the printed surface, and the sheet is sandwiched between both the fixing nip and the transfer nip. An image forming apparatus having a length that can be conveyed by
Between the pressing position where the guide surface is pressed against the back surface of the sheet being conveyed and the retracting position where the transfer guide member is separated from the pressing position on the pressure roller side in the direction intersecting the sheet conveying direction. Equipped with a drive unit to drive
After the tip of the sheet enters the transfer nip, the drive unit positions the transport guide member in the retracted position until it enters the fixing nip, and then the tip of the sheet enters the fixing nip. Then, the transport guide member is moved from the retracted position to the pressing position, and after the movement, the transport guide member is returned from the pressing position to the retracted position before the rear end of the sheet pulls out the transfer nip. An image forming apparatus configured to perform position switching control. In the image forming apparatus according to claim 1,
The fixing portion further includes a nip pressure switching portion capable of switching the nip pressure in the fixing nip between a first pressure and a second pressure reduced from the first pressure.
The drive unit executes the position switching control of the transport guide member when the nip pressure is set to the first pressure by the nip pressure switching unit, and the nip pressure is increased by the nip pressure switching unit. An image forming apparatus configured to maintain the transport guide member in the retracted position regardless of the transport position of the sheet when the second pressure is set. In the image forming apparatus according to claim 2,
The nip pressure switching unit has a rotatable shaft and a pressure switching cam that is rotationally fixed to the shaft, and the rotation of the pressure switching cam causes the nip pressure to be changed to the first pressure and the above. It is configured to be selectively switchable to the second pressure,
The drive unit has a rotatable shaft and a guide drive cam that is rotationally fixed to the shaft, and the rotation of the guide drive cam pushes the transport guide member to the pressing position and the retract. It is configured to be driveable to and from the position,
An image forming apparatus in which the shaft of the nip pressure switching unit is also used as the shaft of the drive unit.

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