JP5401887B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an electrophotographic image forming apparatus provided with an intermediate transfer belt, and more particularly to an improvement around a secondary transfer portion that is transferred onto a sheet via the intermediate transfer belt.

In an electrophotographic image forming apparatus provided with an intermediate transfer belt, when secondary transfer of a toner image from an intermediate transfer belt to a sheet, in order to realize more stable transfer, for example, an intermediate portion near the entrance side of a transfer nip The transfer material conveyance guide for guiding the sheet toward the transfer belt is composed of upper and lower guide members, and the lower guide member is formed in a mountain shape with a surface facing the intermediate transfer belt and a surface along the intermediate transfer belt. A boundary portion that is the top of the chevron shape is between a transfer member and a pressing member provided so as to press the intermediate transfer belt outward from the transfer counter roller (driven roller) upstream in the rotational direction of the intermediate transfer belt. There is an image forming apparatus provided so as to be positioned on an extension line of an intermediate transfer belt stretched on the intermediate transfer belt or on the intermediate transfer belt side with respect to the extension line.
According to this image forming apparatus, by eliminating the gap in the vicinity of the transfer nip formed between the paper and the intermediate transfer belt, it is possible to prevent image quality deterioration such as color shift and toner dust, Further, it is possible to make the sheet adhere to the intermediate transfer belt by the boundary portion, which is suitable for preventing toner dust (see, for example, Patent Document 1).

As another example, among the pair of upper and lower guide plates that form a transport path for transporting the transfer material (paper) toward the transfer position (transfer nip), an elastic guide member is provided at the tip of the upper guide plate. There is an image forming apparatus provided so that the protruding length is variable.
According to this image forming apparatus, when the rear end of the transfer material is separated from the registration roller disposed upstream of the pair of upper and lower guide plates, the rear end of the transfer material jumps up and the transfer material vibrates. It is said that the occurrence of toner dust and transfer deviation can be suppressed by the guide member restraining (see, for example, Patent Document 2).
Japanese Patent No. 3813378 JP 2008-3447 A

However, in the image forming apparatus described in Patent Document 1, since the leading end of the sheet fed from the registration roller directly collides with the intermediate transfer belt, the toner image on the intermediate transfer belt is generated by flapping of the sheet immediately after the collision. There was a possibility of disturbance. Further, immediately before the trailing edge of the sheet is inserted into the transfer nip, the posture of the trailing edge of the sheet becomes unstable, and the toner image on the intermediate transfer belt may be disturbed due to the flapping of the sheet.
As is clear from FIG. 3 in Patent Document 2, the image forming apparatus described in Patent Document 2 has an intermediate transfer belt positioned on an imaginary straight line of the paper guide surface of the lower guide plate (reference numeral 30). As described above, since the leading edge of the paper fed from the registration roller directly collides with the intermediate transfer belt, the toner image on the intermediate transfer belt can be disturbed by flapping of the paper immediately after the collision. There was sex.
In the image forming apparatus described in Patent Document 2, a guide member (reference numeral 32) is slidably provided on the upper guide plate (reference numeral 31), and the amount of protrusion of the guide member is temporarily increased. In addition, the leading edge of the paper directly collides with the intermediate transfer belt by the guide of the lower guide plate, or the paper is largely bent by the upper guide plate from the arrangement state of the lower guide plate, There was a risk that stable secondary transfer could not be performed.
Therefore, the present invention has been made in view of such a situation, and an object thereof is to provide an image forming apparatus that can eliminate the above-described fear and obtain a high-quality image without image distortion.

In order to achieve the above technical problem, the image forming apparatus according to the present invention employs the following technical means.
That is, the image forming apparatus according to claim 1 includes the transfer counter roller around which the intermediate transfer belt is hung, and the upstream side of the transfer counter roller in the rotation direction of the intermediate transfer belt and in the vicinity of the transfer counter roller. A secondary transfer portion having a transfer guide roller around which an intermediate transfer belt is hung, and a rotatable transfer member engaged with the transfer counter roller via the intermediate transfer belt; and the secondary transfer portion And a registration roller pair that feeds the paper toward the secondary transfer portion, and is configured to transfer the toner image formed on the image carrier onto the paper via the intermediate transfer belt. an image forming apparatus that is, the sheet conveying path is gradually narrowed toward the sheet inlet side of the transfer nip is engaged portion between the transfer member and the transfer opposing roller via the intermediate transfer belt Is urchin obliquely, comprising a pair of paper guide member for guiding the sheet, the pair of paper guide member, the transfer provided member side, is formed flat paper guide surface sheet conveying a the paper guide surface with the entrance of the previous SL transfer nip on a virtual straight line obtained by extending direction are arranged so as to be located, the registration roller to a virtual straight line obtained by extending the sheet guide surface to the sheet conveying direction and opposite side A lower paper guide member disposed so that a pair of paper feeding portions are positioned; and the transfer nip provided on the intermediate transfer belt side than the transfer nip side end position of the lower paper guide member. by positioning the end on the side, and a upper paper guide member disposed so as to cover the lower paper guide member, the upper paper guide member, the end of the transfer nip side, said paper guide Characterized in that are provided so as to be positioned on a virtual straight line obtained by extending the sheet conveyance direction.

The image forming apparatus according to claim 2, in claim 1, wherein the upper paper guide member, the end of the transfer nip side, with the center of rotation of the transfer member with the center of rotation of said transfer opposed roller of the transfer guide roller It is provided so that it may be located on the side of the triangle formed connecting the rotation center.
The image forming apparatus according to claim 3, in claim 1, wherein the upper paper guide member, the end of the transfer nip side, with the center of rotation of the transfer member with the center of rotation of said transfer opposed roller of the transfer guide roller It is provided so that it may be located in the inside of the triangle formed connecting the rotation center.

An image forming apparatus according to a fourth aspect is the image forming apparatus according to any one of the first to third aspects, wherein the transfer guide roller is smaller in diameter than the transfer counter roller.
An image forming apparatus according to a fifth aspect is the image forming apparatus according to any one of the first to fourth aspects, wherein the image forming apparatus is faster than a peripheral speed of a transfer roller disposed opposite to the intermediate transfer roller with the transfer member interposed therebetween. The peripheral speed of the registration roller pair is faster.

An image forming apparatus according to a sixth aspect is the image forming apparatus according to any one of the first to fifth aspects, wherein the transfer member is electrically grounded, and the position of the rotation center is the transfer counter roller and the transfer guide. It is characterized in that it is disposed on the upstream side in the sheet conveying direction with respect to the intermediate transfer belt stretched around the roller from the virtual perpendicular line lowered from the rotation center of the transfer counter roller.
An image forming apparatus according to a seventh aspect is the image forming apparatus according to any one of the first to sixth aspects, wherein the upper sheet guide member includes a guide main body portion and the guide main body portion extending from the guide main body portion. A low-rigidity guide portion having a low bending rigidity in the transfer belt direction is provided, and the low-rigidity guide portion is arranged on the transfer nip side.

An image forming apparatus according to an eighth aspect is the image forming apparatus according to the seventh aspect , wherein the guide main body portion includes an elastic portion on one end side, and the low-rigidity guide portion extends from the elastic portion. .
An image forming apparatus according to a ninth aspect is the image forming apparatus according to the eighth aspect , wherein the low-rigidity guide portion is formed of a member separate from the elastic portion.
An image forming apparatus according to a tenth aspect is the image forming apparatus according to the eighth aspect , wherein the low-rigidity guide portion is formed such that a thickness of a tip portion of the elastic portion is thin so that rigidity can be lowered.
An image forming apparatus according to an eleventh aspect is the image forming apparatus according to the seventh aspect , wherein the low-rigidity guide portion is formed of a separate member from the guide main body portion.
The image forming apparatus according to a twelfth aspect of the present invention is the image forming apparatus according to the seventh aspect , wherein the low-rigidity guide portion is formed of the same member as the guide main body portion, and the thickness of the tip portion of the guide main body portion is reduced so that the rigidity can be reduced. It is formed.

According to the present invention, it is possible to provide an image forming apparatus that can stably transfer a toner image onto a sheet and obtain a high-quality image without image distortion.

Next, an embodiment of an image forming apparatus according to the present invention will be described with reference to the accompanying drawings. The main part of the present invention is around the secondary transfer part that is transferred onto the sheet via the intermediate transfer belt. Before describing the main part in detail, the outline of the image forming apparatus according to the present embodiment will be described. explain.
As shown in FIG. 1, the image forming apparatus 1 according to the present embodiment is a color image forming apparatus capable of forming a full-color image using a tandem method, and includes four image forming units 10 ( Y (yellow), M (magenta), C (cyan), and Bk (black)) are laid on the plurality of rollers 21, 22, 23, 25, 32, and 33 on the endless intermediate transfer belt 20 It is installed side by side along the running side.
Each image forming unit 10 has the same configuration except for the color of the toner to be handled. In FIG. 1, only the leftmost yellow image forming unit 10Y has a reference numeral attached to a device constituting the unit. .
The image forming unit 10 is opposed to a photosensitive drum 11 as an image carrier, a charging unit 12, a developing device 13, a cleaning device 14, and a photosensitive drum 11 disposed around the photosensitive drum 11. Thus, a transfer roller (transfer member) 15 constituting a primary transfer portion is provided inside the intermediate transfer belt 20. In addition, the image forming apparatus 1 according to the present embodiment includes an exposure unit for each image forming unit 10. For example, an exposure unit 16 such as an LED is disposed between the charging unit 12 and the developing device 13. Has been. In addition, as an exposure means, it is also possible to use an exposure apparatus common to each image forming unit 10, for example, a laser writing apparatus.

In the image forming apparatus 1, a transfer roller 31 that is a second transfer unit is disposed opposite to a transfer counter roller 32 that is one of the support rollers of the intermediate transfer belt 20. The transfer roller 31 is provided with a pair of registration rollers 54 on the right side in the drawing and an endless conveying belt 55 on the left side in the drawing.
A fixing device 40 is provided on the left side of the conveyance belt 55 in the drawing. The fixing device 40 is a belt fixing device having a fixing belt 43 stretched between a heating roller 41 and a fixing roller 42, and a pressure roller 44 is pressed against the fixing roller 42 with the fixing belt 43 interposed therebetween.
A sheet feed tray 50 for storing the sheet bundle P is disposed in the lower part of the image forming apparatus 1, and has a pickup roller 51 and a separation feeding roller 52 as sheet feeding means. In FIG. 1, a transport roller 53 is disposed at an appropriate position in the paper transport path indicated by a one-dot chain line. Further, although not shown, a paper guide is provided as appropriate. In addition, a manual paper feed unit and a paper reversing unit can be provided as necessary. It is also possible to mount an image reading device (scanner), an automatic document feeder (ADF), or the like.

An image forming operation in the image forming apparatus 1 according to the present embodiment configured as described above will be briefly described.
The photosensitive drum 11 of the image forming unit 10 is rotationally driven counterclockwise in the figure by a driving unit (not shown), and the surface of the photosensitive drum 11 is uniformly charged to a predetermined polarity by the charging unit 12. The charged photosensitive member surface is irradiated with scanning light from the exposure unit 16, thereby forming an electrostatic latent image on the photosensitive drum 11 surface. At this time, the image information exposed to each photosensitive drum 11 is single-color image information obtained by separating a desired full-color image into color information of yellow, magenta, cyan, and black. Each color toner is applied from the developing device 13 to the electrostatic latent image formed in this manner, and visualized as a toner image.
Further, the intermediate transfer belt 20 is driven to run in the clockwise direction in the drawing, and the respective color toner images are sequentially transferred from the photosensitive drum 11 to the intermediate transfer belt 20 by the action of the primary transfer roller 15 in each image forming unit 10. In this way, the intermediate transfer belt 20 carries a full-color toner image on its surface.

Note that a single color image can be formed using any one of the image forming units 10, or a two-color or three-color image can be formed using a plurality of image forming units. In the case of monochrome printing, image formation is performed using the rightmost black unit 10Bk in the drawing among the four image forming units.
The residual toner adhering to the surface of the photosensitive drum 11 after transferring the toner image is removed from the surface of the photosensitive drum by the cleaning device 14, and then the surface is subjected to the action of a static eliminator (not shown) so that the surface potential is initially set. To prepare for the next image formation.
On the other hand, the sheet is fed from the sheet feeding tray 50 and is sent out toward the secondary transfer position by the registration roller pair 54 in timing with the toner image carried on the intermediate transfer belt 20. The toner image on the surface of the intermediate transfer belt is collectively transferred onto the sheet by the transfer roller 31 as the second transfer unit. The sheet on which the toner image has been transferred is sent to the fixing device 40 by the conveying belt 55, and when passing through the fixing device 40, the toner image is fused and fixed to the sheet by heat and pressure. The paper on which the toner image is fixed is discharged to a paper discharge tray (not shown).

Next, the area around the secondary transfer portion, which is the main part of the present invention, will be described in detail.
FIG. 2 is a perspective view showing the configuration of the intermediate transfer belt 20. FIG. 3 is an enlarged view showing the secondary transfer unit 30 in detail.
As shown in FIG. 2, the intermediate transfer belt 20 is stretched over a drive roller 21, a driven roller 22, an external stretching roller 23, a transfer counter roller 32 and a transfer guide roller 33, and the drive roller 21 is driven by a motor 26. Is driven in the direction of arrow A in the figure.
The driven roller 22 applies tension to the intermediate transfer belt 20 by a load applying unit (not shown), and generates a friction conveying force between the driving roller 21 and the intermediate transfer belt 20.
As described above, the photosensitive drums 11 of the respective color image forming units are arranged in parallel on the upper running side of the intermediate transfer belt 20 which is a belt stretching surface formed by the driving roller 21 and the driven roller 22. A transfer roller 15 as a primary transfer unit is disposed in the loop of the intermediate transfer belt 20 so as to face the body drum 11.
A transfer roller 31 is disposed below the transfer counter roller 32 and is pressed against the transfer counter roller 32 with the intermediate transfer belt 20 interposed therebetween.

A predetermined high voltage is applied to the transfer roller 31 from a transfer power source (not shown), and an electrostatic bias is applied between the transfer roller 31 and the transfer counter roller 32, and the transfer roller 31 includes the transfer counter roller 32 and the intermediate transfer belt 20. At the same time, the sheet is conveyed with a predetermined pressing force.
Further, the transfer roller 31 is electrically grounded as described above, and as shown in FIG. 3, the position of the rotation center C1 of the transfer roller 31 is transferred from the rotation center C2 of the transfer counter roller 32 to the transfer counter. It is positioned on the upstream side in the paper transport direction with respect to the perpendicular C2Sb drawn down to the intermediate transfer belt 20 stretched between a roller 32 and a transfer guide roller 33 described later.
Further, a transfer guide roller 33 is disposed on the back side of the intermediate transfer belt 20 and in the vicinity of the upstream side of the transfer counter roller 32 in the belt rotation direction.
The transfer roller 31, the transfer counter roller 32, the transfer guide roller 33, and the intermediate transfer belt 20 described above constitute the secondary transfer unit 30. As can be seen from FIG. 1, the transfer guide roller 33 has a smaller diameter than the transfer counter roller 32, and the outer peripheral surfaces of the drive roller 21 and the transfer counter roller 32 so as to push the intermediate transfer belt 20 from the inner side to the outer side of the belt. It is arranged slightly outside the virtual straight line connecting. As a result, a portion of the intermediate transfer belt 20 stretched between the transfer counter roller 32 and the transfer guide roller 33 is gently inclined as shown in FIG.

Although not shown in FIG. 2, belt cleaning means 24 (see FIG. 1) for cleaning the intermediate transfer belt 20 is disposed outside the belt between the transfer counter roller 32 and the external stretching roller 23. A backing roller 25 (see FIG. 1) corresponding to the cleaning blade of the belt cleaning means 24 is disposed inside the belt.
The sheet P is conveyed in the direction of arrow B in FIG. 2, and after the posture of the sheet is corrected by the registration roller pair 54, the sheet P is conveyed between the transfer roller 31 and the intermediate transfer belt 20. Is electrostatically transferred onto the paper.
In the present embodiment, the secondary transfer unit 30 includes a transfer roller 31, a transfer counter roller 32, and a transfer guide roller 33.
The registration roller pair 54 that feeds the paper P toward the secondary transfer portion is disposed on the paper entrance side to the secondary transfer portion 30. The registration roller pair 54 sandwiches the paper P with a predetermined pressing force and conveys the paper P in the arrow B direction.
Between the registration roller pair 54 and the secondary transfer unit 30, the sheet P fed from the registration roller pair 54 is a transfer nip that is an engagement portion between the transfer roller 31 and the transfer counter roller 32 via the intermediate transfer belt. A paper conveyance guide means 60 is provided for guiding the conveyance toward N.

In the present embodiment, the paper transport guide means 60 is composed of a pair of upper and lower paper guide members 61 and 62 disposed on the transfer nip N side, and a pair of upper and lower sheets disposed on the registration roller pair 54 side. The guide members 63 and 64 are configured.
Of these, first, the lower paper guide member 62 is a substantially belt-like member in a front view formed with a flat paper guide surface 62a, as shown in FIG. In FIG. 3, the surface 62a is horizontally arranged so that the entrance of the transfer nip N is positioned on an imaginary straight line L extending in the paper transport direction. Further, the sheet P delivery portion of the registration roller pair 54 is positioned on a virtual straight line L obtained by extending the flat sheet guide surface 62a in the direction opposite to the sheet conveyance direction. The lower guide member 64 is disposed at a position slightly below the virtual straight line L.
As shown in FIG. 6, the upper sheet guide member 61 includes a guide main body 61c having an elastic portion 61a on one end side, and an intermediate transfer belt extending from the elastic portion 61a and extending beyond the guide main body 61c (elastic portion 61a). A low-rigidity guide portion 61b having a low bending rigidity in the 20 directions is formed in a band plate shape.
The elastic portion 61a and the guide main body portion 61c may be integrally formed of the same member or may be formed of separate members. The low-rigidity guide portion 61b is formed of an elastic portion 61a as shown in FIG. Alternatively, as shown in FIG. 7, the open end of the elastic portion 61a may be formed in a single blade shape, and the low-rigidity guide portion 61b may be formed integrally with the elastic portion 61a. Alternatively, the guide body 61c may be configured without providing the elastic portion 61a, and the upper sheet guide member 61 may be configured by the guide body 61c and the low-rigidity guide 61b.

The upper sheet guide member 61 has a sheet conveyance path gradually narrowing toward the sheet entrance side of the transfer nip N that is an engagement portion between the transfer counter roller 32 and the transfer roller 31 via the intermediate transfer belt 20. More specifically, as shown in FIG. 3, the end h of the low-rigidity guide portion 61b extends the paper guide surface 62a of the lower paper guide member 62 in the paper transport direction, as shown in FIG. On the imaginary straight line L, on the side of a triangle (illustrated by phantom lines) connecting the rotation center C1 of the transfer roller 31, the rotation center C2 of the transfer counter roller 32, and the rotation center C3 of the transfer guide roller 33 ( Or it is obliquely arranged so as to be located inside the triangle).
With the above configuration, the conveyance direction of the paper P sent out from the registration roller pair 54 is regulated toward the transfer roller 31, and the leading edge of the paper P does not directly collide with the intermediate transfer belt 20. The toner image on the intermediate transfer belt 20 can be prevented from being disturbed, stable toner transfer can be realized, and a high-quality image without image distortion can be obtained.
In addition, by positioning the end h of the low-rigidity guide portion 61b closer to the transfer roller 31 (on the side opposite to the intermediate transfer belt 20) than the virtual straight line L, the above-described regulation effect of the paper P (paper P) The transport direction is restricted in the direction of the transfer roller 31). However, in this case, since the conveyance path of the sheet P is bent, it is not preferable that the position of the end h is far away from the virtual straight line L (positioned below the virtual straight line L). The vicinity of L is preferable.

In the present embodiment, the position of the end h of the low-rigidity guide portion 61b connects the rotation center C1 of the transfer roller 31, the rotation center C2 of the transfer counter roller 32, and the rotation center C3 of the transfer guide roller 33. By being positioned on the side of the triangle (illustrated by an imaginary line) (or inward of the triangle), after the sheet P is sandwiched between the transfer roller 31 and the transfer counter roller 32, the registration roller pair 54 and the transfer roller 31 When the sheet is bent due to the difference in peripheral speed, the sheet P is bent in a convex shape toward the intermediate transfer belt 20 to prevent the sheet P and the intermediate transfer belt 20 from contacting each other, so that the toner image on the intermediate transfer belt 20 is disturbed. It is possible to prevent this, realize stable toner transfer, and obtain a high-quality image without image distortion.
Note that the position of the end h of the low-rigidity guide portion 61b is positioned inward of the above-described triangle, thereby increasing the effect of preventing the above-described bending of the paper. If it is positioned, the end h may come into close contact with the intermediate transfer belt 20 or the transfer roller 31. Therefore, the position of the end h is set so as to be far away from the side of the triangle. It is preferable to avoid it.

In this embodiment, the transfer roller 31 is electrically grounded, and the position of the rotation center C1 of the transfer roller 31 is shifted from the rotation center C2 of the transfer counter roller 32 to the transfer counter roller 32 and the transfer guide. The sheet P and the intermediate transfer belt 20 are given a predetermined pressing force by the transfer roller 31 by being positioned upstream of the virtual perpendicular line C2Sb lowered to the intermediate transfer belt 20 stretched around the roller 33 in the sheet conveyance direction. Since the paper P is bitten into the transfer nip N to which the transfer bias is applied after being conveyed while maintaining stable contact in the applied state, it is possible to avoid image defects due to discharge in a minute gap. Therefore, it is possible to prevent the toner image on the intermediate transfer belt from being disturbed, realize stable toner transfer, and obtain a high-quality image without image disturbance.
Furthermore, in the present embodiment, the transfer nip N side of the upper sheet guide member 61 is configured by the elastic portion 61a, and further, the elastic portion 61a has a low rigidity lower than that of the elastic portion 61a on the transfer nip N side. A rigid guide portion 61b is extended.
With this configuration, the rebound of the paper P when the front end of the paper P collides with the elastic portion 61a of the upper paper guide member 61 or when the paper P is bent and pressed against the elastic portion 61a can be reduced and attenuated. Further, the rear end of the sheet P when the rear end of the sheet P moves away from the upper sheet guide member 61 can reduce or suppress the collision with the intermediate transfer belt 20, and the conveyance of the sheet P can be realized stably. The elastic body constituting the elastic portion 61a and the low-rigidity guide portion 61b is a material that can realize appropriate bending and paper regulation at the same time, and the material itself is not particularly limited.

In FIG. 4, after the sheet P sent out from the registration roller pair 54 is sandwiched between the transfer roller 31 and the transfer counter roller 32, the sheet P is bent due to a difference in peripheral speed between the registration roller pair 54 and the transfer roller 31. Indicates the state.
Generally, the peripheral speed of the registration roller pair 54 is set several percent faster than the peripheral speed of the transfer roller 31. For this reason, after the sheet P is sandwiched between the transfer roller 31 and the transfer counter roller 32, the sheet P is bent between the secondary transfer unit 30 and the registration roller pair 54.
In the present embodiment, as described above, the upper sheet guide member 61 and the lower sheet guide member 62 disposed on the transfer nip N side, and the pair of upper and lower guide members disposed on the registration roller pair 54 side. 63, 64.
Further, the end of the upper sheet guide member 61, that is, the position of the end h of the low-rigidity guide portion 61b is positioned on a virtual straight line L that extends the sheet guide surface 62a of the lower sheet guide member 62 in the sheet conveyance direction. In addition, the position of the end h is located on a triangular side connecting the rotation centers of the transfer roller 31, the transfer counter roller 32, and the transfer guide roller 33.
With this configuration, the distance Lb between the transfer nip N and the end h of the low-rigidity guide portion 61b is shortened. Therefore, the length of the sheet P spanned between them is shortened. And the deformation of the paper P toward the transfer counter roller 32 is suppressed. As a result, the deflection of the sheet P due to the difference in peripheral speed between the registration roller pair 54 and the transfer roller 31 is extremely unlikely to occur between the transfer nip N and the end h of the low-rigidity guide portion 61b. The fear of contacting the belt 20 can be eliminated.
In this way, the sheet P can be prevented from being bent in a convex shape toward the intermediate transfer belt 20 to prevent the sheet P and the intermediate transfer belt 20 from coming into contact with each other, so that the toner image on the intermediate transfer belt 20 can be prevented from being disturbed and stabilized. Transfer of toner can be realized, and a high-quality image without image disturbance can be obtained.

FIG. 5 shows a state in which the sheet P immediately before the rear end of the sheet P is separated from the upper sheet guide member 61 is bent.
In the present embodiment, as described above, the upper sheet guide member 61 and the lower sheet guide member 62 disposed on the transfer nip N side, and the pair of upper and lower guide members disposed on the registration roller pair 54 side. 63, 64, and the position of the end of the upper sheet guide member 61, that is, the end h of the low-rigidity guide portion 61b extends the sheet guide surface 62a of the lower sheet guide member 62 in the sheet conveyance direction. It is configured such that the position of the end h is positioned on the triangle side connecting the rotation centers of the transfer roller 31, the transfer counter roller 32, and the transfer guide roller 33.
With this configuration, as described above, it is possible to prevent the sheet P from being bent in a convex shape toward the intermediate transfer belt 20 and contact between the sheet P and the intermediate transfer belt 20. Disturbance can be prevented, stable toner transfer can be realized, and a high-quality image without image disturbance can be obtained.
However, in the case of a highly rigid sheet, when the rear end of the sheet P moves away from the upper sheet guide member 61, the elastic energy stored by the bending is released at a stretch, and the rear end of the sheet P collides with the intermediate transfer belt 20 and becomes intermediate. The toner image on the transfer belt 20 may be disturbed.
Therefore, in the present embodiment, as described above, the low-rigidity guide portion 61 b whose bending rigidity in the direction of the intermediate transfer belt 20 is lower than that of the upper paper guide member 61 is provided.

With this configuration, when the trailing edge of the sheet P moves away from the upper sheet guide member 61, the bending of the sheet P absorbs elastic energy while being relaxed by the deformation of the low-rigidity guide portion 61b. It is possible to suppress the collision of the end with the intermediate transfer belt 20, prevent the toner image on the intermediate transfer belt 20 from being disturbed, realize stable toner transfer, and obtain a high-quality image without image disturbance. . In addition, in this Embodiment, what provided the low-rigidity guide part 61b is illustrated, However, The presence or absence of the arrangement | positioning is not specifically limited.
As described above, in the image forming apparatus according to the present embodiment, the upper sheet guide member 61 and the lower sheet guide member 62 are disposed so as to gradually narrow toward the sheet entrance side of the transfer nip N. The lower paper guide member 62 is disposed so that the entrance of the transfer nip N is positioned on a virtual straight line L obtained by extending the paper guide surface 62a in the paper conveyance direction, and the upper paper guide member 61 is disposed on the lower paper guide. An end (h) is positioned closer to the transfer nip N side than the end position of the member 62 on the transfer nip N side, and is disposed so as to cover the lower paper guide member 62, and intermediate transfer is performed in the conveyance direction of the paper P. The belt 20 is not positioned, and the end of the upper sheet guide member 61 from the end of the lower sheet guide member 62, that is, the end h of the low-rigidity guide portion 61b is closer to the transfer nip N so that the guide outlet is narrowed. The Since the paper P is prevented from directly colliding with the intermediate transfer belt and at the same time the paper bounce is suppressed, the paper conveyance to the transfer nip N is stable, the transfer of the toner image onto the paper P is stable, and there is no image distortion. It is possible to provide an image forming apparatus capable of obtaining a quality image.

The image forming apparatus according to the present embodiment has been described above. However, the above-described embodiment shows an example of a preferred embodiment of the present invention, and the present invention is not limited thereto. Various modifications can be made without departing from the scope of the invention.
For example, in the present embodiment, as shown in FIG. 3, the upper sheet guide member 61 is arranged such that the end of the upper sheet guide member 61, that is, the end h of the low-rigidity guide portion 61b is the lower sheet guide member. 62 on a virtual straight line L obtained by extending the sheet guide surface 62a in the sheet conveyance direction, and the rotation center C1 of the transfer roller 31, the rotation center C2 of the transfer counter roller 32, and the rotation center C3 of the transfer guide roller 33. However, the end h is located closer to the transfer nip N than the end of the lower paper guide member 62 on the transfer nip N side. Any configuration may be used as long as it is positioned so as to cover the lower paper guide member 62 and prevent the paper P from directly colliding with the intermediate transfer belt 20 and at the same time suppressing the paper bounce.
Accordingly, the upper sheet guide member 61 is provided such that the end (h) of the upper sheet guide member 61 on the transfer nip N side is positioned on an imaginary straight line L obtained by extending the sheet guide surface 62a in the sheet conveyance direction. Or it is provided so as to be positioned on the transfer roller 31 side with respect to the imaginary straight line L, provided so as to be positioned on the side of the triangle described above, or provided on the inner side of the triangle described above. There may be any configuration as long as it is, or a combination of these.

In this embodiment, the transfer roller 31 is exemplified as the transfer member, but an endless transfer belt may be used instead of this. In this case, the center of the rotation roller that is in pressure contact with the transfer counter roller 32 among the rotation rollers on which the transfer belt is stretched is defined as one vertex of the triangle.
Furthermore, the configuration of each part of the image forming apparatus is also arbitrary, and the arrangement order of the color image forming units in the tandem system is arbitrary. In addition to the tandem type, a configuration in which a plurality of developing devices are arranged around a single photosensitive member, or a configuration using a revolver type developing device is also possible. Further, the present invention can be applied to a full-color machine using three color toners and a multi-color machine using two color toners. Of course, the image forming apparatus is not limited to a printer, and may be a copier, a facsimile machine, or a multifunction machine having a plurality of functions.

1 is a schematic configuration diagram of an image forming apparatus according to an embodiment. FIG. 3 is a perspective view illustrating a configuration of an intermediate transfer belt. It is an enlarged view around a secondary transfer part. FIG. 4 is an enlarged view around a secondary transfer portion showing a state where a sheet is bent. FIG. 4 is an enlarged view around a secondary transfer portion showing a state in which a rear end of a sheet is bent. It is a side view of the upper sheet guide member according to the present embodiment. It is the side view which showed the other aspect of the upper paper guide member.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Image forming apparatus 10 ... Image forming unit 11 ... Photosensitive drum 15 ... Primary transfer roller 20 ... Intermediate transfer belt 21 ... Drive roller 22 ... Driven roller 23 ... External stretch roller 26 ... Motor 30 ... Secondary transfer part 31 ... Secondary transfer roller 32 ... Transfer counter roller 33 ... Transfer guide roller 40 ... Fixing device 50 ... Paper feed tray 54 ... Registration roller pair 55 ... Conveyance belt 60 ... Paper conveyance guide means 61 ... Upper paper guide member 61a ... Elastic part 61b ... Low rigidity guide part 61c ... Guide body part 62 ... Lower paper guide member 62a ... Paper guide surface 63, 64 ... Guide member N ... Transfer nip h ... End of low rigidity guide part (End of transfer paper nip side of upper paper guide member ) L: Virtual straight line with the paper guide surface extended in the paper transport direction C1: Transfer roller rotation center C2: Rotation center of transfer counter roller C3 ... Rotation center of transfer guide roller

Claims (12)

A transfer counter roller around which the intermediate transfer belt is hung, and a transfer guide roller around which the intermediate transfer belt is hung on the upstream side of the transfer counter roller in the rotation direction of the intermediate transfer belt and in the vicinity of the transfer counter roller. A secondary transfer portion having a rotatable transfer member engaged with the transfer counter roller via the intermediate transfer belt, and the secondary transfer portion disposed on the entrance side of the secondary transfer portion. An image forming apparatus configured to transfer a toner image formed on an image carrier onto a sheet via the intermediate transfer belt ,
The intermediate transfer belt sheet transport path toward the sheet inlet side of the transfer nip the a engaging portion between the transfer opposing roller and the transfer member via is obliquely gradually so that the width becomes narrow, the paper guide A pair of paper guide members
The pair of paper guide members are:
Provided on the transfer member, it is arranged such that the entrance of the pre-Symbol transfer nip flat paper guide surface is formed the paper guide surface imaginary straight line obtained by extending the sheet conveying direction is located And a lower paper guide member disposed so that a paper feed portion of the pair of registration rollers is positioned on a virtual straight line obtained by extending the paper guide surface in the direction opposite to the paper conveyance direction ;
It is provided on the intermediate transfer belt side, and is disposed so as to cover the lower paper guide member with its end positioned closer to the transfer nip side than the end position of the lower paper guide member on the transfer nip side. An upper sheet guide member ,
The image forming apparatus, wherein the upper sheet guide member is provided such that an end on the transfer nip side is positioned on a virtual straight line obtained by extending the sheet guide surface in the sheet conveyance direction . The upper sheet guide member has an end on the transfer nip side positioned on a side of a triangle formed by connecting the rotation center of the transfer member, the rotation center of the transfer counter roller, and the rotation center of the transfer guide roller. The image forming apparatus according to claim 1, wherein the image forming apparatus is provided. The upper paper guide member has an end on the transfer nip side positioned inward of a triangle formed by connecting the rotation center of the transfer member, the rotation center of the transfer counter roller, and the rotation center of the transfer guide roller. The image forming apparatus according to claim 1, wherein the image forming apparatus is provided. The image forming apparatus according to claim 1, wherein the transfer guide roller has a smaller diameter than the transfer counter roller. 5. The peripheral speed of the pair of registration rollers is faster than a peripheral speed of a transfer roller disposed opposite to the intermediate transfer roller with the transfer member interposed therebetween. Image forming apparatus. The transfer member is electrically grounded, and the position of the rotation center is from the rotation center of the transfer counter roller toward the intermediate transfer belt stretched between the transfer counter roller and the transfer guide roller. 6. The image forming apparatus according to claim 1 , wherein the image forming apparatus is disposed at a position upstream of the lowered virtual perpendicular in the paper conveyance direction . The upper sheet guide member includes a guide main body portion and a low-rigidity guide portion that extends from the guide main body portion and has a lower bending rigidity toward the intermediate transfer belt than the guide main body portion. The image forming apparatus according to claim 1, wherein the low-rigidity guide portion is on the transfer nip side. The image forming apparatus according to claim 7, wherein the guide main body portion includes an elastic portion on one end side, and the low-rigidity guide portion extends from the elastic portion. The image forming apparatus according to claim 8, wherein the low-rigidity guide part is configured by a member separate from the elastic part. The image forming apparatus according to claim 8, wherein the low-rigidity guide portion is formed by reducing a plate thickness of a tip portion of the elastic portion so that rigidity can be lowered. The image forming apparatus according to claim 7, wherein the low-rigidity guide portion is configured by a member separate from the guide main body portion. The said low-rigidity guide part is comprised by the same member as the said guide main-body part, and the board | plate thickness of the front-end | tip part of the said guide main-body part is formed thinly so that rigidity can be reduced. Image forming apparatus.

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