Image forming apparatus

Abstract

To effectively prevent a reduction in image quality due to separation electrical discharge.SOLUTION: An image forming apparatus is provided with a regulation part that extends between a transfer nip part and one end of an endless belt that conveys a sheet while opposing to an upper part of the endless belt. The regulation part includes a horizontal part that extends along a width direction orthogonal to a sheet conveyance direction, and a pair of inclined parts that extend in the width direction from both ends in the width direction of the horizontal part and are inclined in a direction approaching the endless belt as directed from one end closer to the horizontal part to the other end in the width direction. The regulation part comes into contact with both ends of a rear end of the sheet floating from the endless belt to prevent separation electrical discharge.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to an electrophotographic image forming apparatus.

Background Art

[0002] Conventionally, there has been known an image forming apparatus including a process unit having a photosensitive drum and a transfer roller for transferring a developer image onto a sheet, and a fixing unit having a heating roller and a pressure roller for fixing the developer image onto the sheet. In such an image forming apparatus, when the sheet is discharged from the process unit, the rear end side in the sheet conveyance direction may suddenly displace upward. In this case, the image forming surface of the sheet may collide with the internal structure of the apparatus, and the developer image before fixing may peel off, which may reduce the image quality.

[0003] In order to suppress such a reduction in image quality, a configuration in which an entrance conveyance guide is provided near the entrance of the fixing device is described in Patent Document 1.

[0004] In the invention described in Patent Document 1, an entrance conveyance guide member for guiding a recording material to a fixing device, which is provided near the fixing device, is formed of an upper member and a lower member having a predetermined gap, and the gap is made narrower at both end sides than at the central portion side in the longitudinal direction. Thus, even when the rear end of the recording material bounces, only the non-image areas at both ends of the recording material come into contact with the upper member, preventing rubbing of the image and suppressing a reduction in image quality.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, in the image forming apparatus as described above, a transfer bias is applied between the photosensitive drum and the transfer roller, causing the surface of the endless belt to be charged. The sheet is then electrostatically attracted to this charged surface and conveyed. Therefore, when the sheet electrostatically attracted to the endless belt suddenly rises at a sharp angle, peeling discharge occurs between the sheet and the endless belt, and the static electricity disrupts the developer before fixing, resulting in a deterioration of image quality.

[0007] Therefore, even if, as described in Patent Document 1, the image forming surface of the sheet is prevented from colliding with the internal structures of the apparatus, the developer image before fixing is disrupted by this peeling discharge, resulting in a deterioration of image quality.

[0008] The present invention has been made to solve the above-described problems, and an object thereof is to provide an image forming apparatus that effectively suppresses a deterioration of image quality due to peeling discharge.

Means for Solving the Problems

[0009] To achieve this object, an image forming apparatus according to the present invention is an image forming apparatus that forms an image on a sheet by transferring a developer image onto the sheet, and includes a plurality of rollers (17, 18), and an endless belt (19) that is installed between the plurality of rollers and conveys the sheet in a conveyance direction with an upper surface as a conveyance surface, and a transfer unit (13), and a process unit that includes a photosensitive drum (15) provided above the endless belt and forms an image on the sheet at a transfer nip portion between the photosensitive drum and the endless belt, and a first regulating portion (31) that faces the conveyance surface above the conveyance surface between the transfer nip portion in the conveyance direction and a downstream end portion of the conveyance surface in the conveyance direction, the first regulating portion including a first regulating portion capable of contacting the sheet, the first regulating portion including a first horizontal portion (32) extending along a width direction orthogonal to the conveyance direction, and a pair of first inclined portions (33) extending from both ends in the width direction of the first horizontal portion in the width direction and inclining in a direction approaching the endless belt from one end closer to the horizontal portion in the width direction toward the other end, the pair of first inclined portions being capable of contacting both ends in the width direction of the sheet respectively.

[0010] According to this configuration, when the rear end of the sheet discharged from the transfer nip portion floats up, the first inclined portion contacts both ends in the width direction of the rear end of the sheet to suppress the floating of the sheet.

[0011] Further, it is preferable that the first inclined portion is provided at a position overlapping with the rotation axis center of the roller located most downstream in the conveyance direction among the plurality of rollers in the conveyance direction.

[0012] According to this configuration, even when the rear end of the sheet floats up when the sheet separates from the endless belt, the first inclined portion contacts both ends in the width direction of the rear end of the sheet to suppress the floating of the sheet.

[0013] Further, the image forming apparatus according to the present invention further includes a process unit having a drum frame that supports the photosensitive drum, and the first regulating portion may be provided on the drum frame.

[0014] According to this configuration, the first regulating portion is accurately positioned with respect to the conveying surface of the endless belt. As a result, when the rear end of the sheet rises, the first inclined portion accurately contacts both ends in the width direction of the rear end of the sheet.

[0015] Further, the image forming apparatus according to the present invention further includes a transfer unit having a belt frame that rotatably supports the plurality of rollers, and the first regulating portion may be provided on the second housing.

[0016] According to this configuration, the first regulating portion is positioned more accurately with respect to the conveying surface of the endless belt. As a result, when the rear end of the sheet rises, the first inclined portion more accurately contacts both end portions in the width direction of the rear end of the sheet.

[0017] Further, the image forming apparatus according to the present invention further includes a fixing device that heats and fixes the image formed on the sheet, the fixing device including a heating roller and a pressure roller that contacts the heating roller to form a fixing nip portion that nips the sheet. And it is preferable that the other end of the first inclined portion is provided below a virtual straight line connecting the fixing nip portion and the transfer nip portion.

[0018] According to this configuration, the first inclined portion prevents the sheet that contacts the first inclined portion from bridging linearly between the transfer nip portion and the fixing nip portion, and can bend the sheet.

[0019] Further, the image forming apparatus according to the present invention can further form an image on a sheet having a first width in the width direction and a sheet having a second width larger than the first width. And it is preferable that the distance in the width direction between the other ends of each of the pair of first inclined portions is larger than the first width, and the distance in the width direction of the first horizontal portion is smaller than the first width.

[0020] According to this configuration, the first inclined portion contacts both ends in the width direction of the sheet having the first width.

[0021] Further, the image forming apparatus according to the present invention further includes a fixing device (50) that heats and fixes the image formed on the sheet, and a second restricting portion (41) provided between a first restricting portion in the conveyance direction and the fixing nip portion in the conveyance path of the sheet. The fixing device has a heating roller and a pressure roller that contacts the heating roller at the fixing nip portion to nip the sheet. The second restricting portion includes a second horizontal portion (42) extending along the width direction orthogonal to the conveyance direction, and a pair of second inclined portions (43) extending from both ends of the second horizontal portion in the width direction, respectively, and inclined downward from one end closer to the second horizontal portion in the width direction toward the other end, and being capable of contacting both ends of the sheet in the width direction, respectively. The distance in the width direction between the other ends of the pair of second inclined portions is preferably larger than the second width, and the distance in the width direction of the second horizontal portion is preferably smaller than the second width.

[0022] According to this configuration, the second inclined portion contacts both end portions in the width direction of the sheet having a second width larger than the first width.

[0023] Further, the distance in the width direction of the second horizontal portion is preferably larger than the distance in the width direction between the other ends of the first inclined portions.

[0024] According to this configuration, the second restricting portion restricts the image forming surface of the sheet contacting the first restricting portion from contacting the second restricting portion.

[0025] Further, it is preferable that the other end of the second inclined portion is provided below an imaginary straight line connecting the fixing nip portion and the lower end of the first inclined portion.

[0026] According to this configuration, the second restricting portion restricts the image forming surface of the sheet contacting the second inclined portion from contacting the second restricting portion.

[0027] Further, it is preferable that the angle formed by the second inclined portion and the horizontal plane is larger than the angle formed by the first inclined portion and the horizontal plane.

[0028] According to this configuration, the second restricting portion further restricts the image forming surface of the sheet contacting the first inclined portion from contacting the second restricting portion. Further, even if the sheet is curved, the second restricting portion restricts the image forming surface of the sheet from contacting the second restricting portion.

Advantages of the Invention

[0029] According to the present invention, it is possible to effectively suppress the deterioration of image quality due to peeling discharge.

Brief Description of the Drawings

[0030]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Embodiments for Carrying Out the Invention

[0031] Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[0032] In the following description, the vertical direction is defined based on the state of FIG. 1 in which the image forming apparatus 1 is installed for use. The front-rear direction is defined with the direction in which the driven roller 18 is located with respect to the driving roller 17 in FIG. 1 as the front, and the left-right direction is defined when viewing the image forming apparatus 1 from the front. The vertical direction, the front-rear direction, and the left-right direction are orthogonal to each other. In the present embodiment, the vertical direction is the vertical direction, the front-rear direction and the left-right direction are the horizontal directions, and the left-right direction is an example of the width direction.

[0033] The image forming apparatus 1 is a color printer, and includes a housing 2 having an opening 5 located on the upper side, a top cover 6 that opens and closes the opening 5, an image forming unit 11 for forming an image on a sheet, a feeder unit 4, a transfer unit 13, and a fixing unit 50.

[0034] The top cover 6 is supported by the housing 2 so as to be swingable about its rear end as a fulcrum. A paper discharge tray 3 is disposed on the upper surface of the top cover 6. The paper discharge tray 3 supports the sheets discharged from the sheet discharge port 57 after printing.

[0035] The image forming unit 11 is housed inside the housing 2 and forms an image on a sheet. The feeder unit 4 is configured to supply a sheet to the image forming unit 11. The transfer unit 13 supplies the sheet supplied from the feeder unit 4 to the image forming unit 11. The fixing unit 50 fixes the image formed on the sheet by the image forming unit 11 to the sheet.

[0036] The image forming apparatus 1 turns the conveyance direction of the sheet that has completed image formation upward by approximately 180° by means of an intermediate conveyance roller 55 disposed downstream in the conveyance direction from the fixing unit 50 and a discharge chute (not shown), and then discharges the sheet from the sheet discharge port 57 to the paper discharge tray 3 by a discharge roller 56.

[0037] <Feeder Unit 4> As shown in FIG. 1, the feeder unit 4 includes a paper feed tray 7, a pickup roller 8, a pair of paper feed rollers 9, and a pair of registration rollers 10. The feeder unit 4 conveys the sheet placed on the paper feed tray 7 to the image forming unit 11 located at approximately the center in the front-rear direction within the housing 2 by making a U-turn at the front side within the housing 2.

[0038] The paper feed tray 7 is housed in the lowermost part of the housing 2. The paper feed tray 7 supports a plurality of sheets and is detachable from the housing 2. The pickup roller 8 is disposed above the front end of the paper feed tray 7 and feeds out the sheet placed on the paper feed tray 7 toward the paper feed roller 9. The pair of paper feed rollers 9 is located downstream of the pickup roller 8 in the conveyance direction and feeds the sheet fed out from the paper feed tray 7 by the pickup roller 8 in the conveyance direction. The pair of registration rollers 10 is located downstream of the paper feed roller 9 in the conveyance direction and conveys the sheet fed by the paper feed roller 9 toward the image forming unit 11. The skew of the sheet conveyed to the registration roller 10 is corrected by the leading edge hitting the nip portion of the pair of registration rollers 10.

[0039] Note that the image forming apparatus 1 can form an image on a plurality of types of sheets having different thicknesses, rigidities, or sizes. The sheet types are mainly classified into thin paper with a small thickness and thick paper with a thickness greater than that of the thin paper. Since thick paper has higher rigidity than thin paper, it has a stronger texture. Also, thick paper generally has a smaller length in the width direction than thin paper in many cases. Examples of thin paper include a sheet of A4 size of 210 mm × 297 mm and a sheet of letter size with an example size of 216 mm × 279 mm. Examples of thick paper include a postcard size sheet with an example size of 100 mm × 148 mm and glossy paper coated with a layer having gloss on the image forming surface. In the following description, the thick paper sheet is referred to as thick paper S1, and the thin paper sheet is referred to as thin paper S2. The thick paper S1 is an example of a sheet having a first width in the present invention, and the thin paper S2 is an example of a sheet having a second width in the present invention.

[0040] <image forming unit 11> As shown in FIG. 1, the image forming unit 11 includes four process units 24 and four LED arrays 12. The process unit 24 is of a so-called direct tandem type capable of color printing, and includes a developing cartridge 25 and a drum unit 22.

[0041] <Developing cartridge 25> As shown in FIG. 1, the four developing cartridges 25 are arranged in four corresponding to the four colors of developer (toner) of black, yellow, magenta, and cyan in order from the upstream side in the sheet conveyance direction. The four developing cartridges 25K, 25Y, 25M, and 25C are arranged at intervals along the sheet conveyance direction. The developing cartridge 25 supplies the developer image of each color to the photosensitive drum 15.

[0042] The developing cartridge 25 is detachably disposed in the housing 2 below the LED array 12 and can be replaced by, for example, the user.

[0043] The developing cartridge 25C includes a developer storage chamber 27 in which the developer is stored, an agitator 23, a supply roller 28, a developing roller 16, and a layer thickness regulating blade 29.

[0044] The agitator 23 is disposed in the developer storage chamber 27. The agitator 23 conveys the developer stored in the developer storage chamber 27 to the supply roller 28 by its rotation. The supply roller 28 supplies the developer conveyed by the agitator 23 to the developing roller 16 by its rotation. The developing roller 16 carries the developer supplied from the supply roller 28 on its surface. The surface of the developing roller 16 is in contact with the surface of the photosensitive drum 15, and the developing roller 16 supplies the developer carried on the surface of the developing roller 16 to the surface of the photosensitive drum 15 by rotating. The layer thickness regulating blade 29 adjusts so that the thickness of the developer carried on the surface of the developing roller 16 becomes uniform at a predetermined thickness. The layer thickness regulating blade 29 protrudes from the rear diagonally above the developing roller 16 toward the developing roller 16, and the tip is in contact with the surface of the developing roller 16.

[0045] <Drum unit 22> The drum unit 22 includes a photosensitive drum 15, a charger 26, and a drum frame 30 that supports the photosensitive drum 15.

[0046] The photosensitive drum 15 carries an image to be transferred onto a sheet, and is a cylindrical photoreceptor formed with a positively chargeable photosensitive layer whose outermost layer is made of polycarbonate or the like. The photosensitive drum 15 transfers an electrostatic latent image formed on its surface by contacting with the sheet, onto the sheet. The photosensitive drum 15 is disposed above the transfer unit 13, and its surface faces the conveyance surface 19B of the endless belt 19.

[0047] The charger 26 charges the surface of the photosensitive drum 15, and faces the surface of the photosensitive drum 15 with a predetermined interval so as not to contact the photosensitive drum 15, diagonally above the rear side of the photosensitive drum 15.

[0048] Note that the charger 26 is a scorotron type charger that uniformly charges the surface of the photosensitive drum 15 with positive charges by performing corona discharge from a charging wire made of tungsten or the like.

[0049] <LED array 12> The LED array 12 is provided with a plurality of LEDs at its tip. The LED array 12 is held by the top cover 6 so as to be suspended from the top cover 6. In a state where the top cover 6 is closed, the LED array 12 is disposed facing the surface of the photosensitive drum 15 above the photosensitive drum 15. Specifically, the LED array 12 moves between an exposure position where the photosensitive drum 15 is exposed and a retracted position farther above than the exposure position, by opening and closing the top cover 6.

[0050] The LED array 12 exposes the surface of the corresponding photosensitive drum 15 by turning on and off a plurality of LEDs based on image data, and forms an electrostatic latent image on its surface.

[0051] <Transfer unit 13> As shown in FIGS. 1 and 2, the transfer unit 13 includes a driving roller 17, a driven roller 18, an endless belt 19, a transfer roller 20, and a belt frame 14.

[0052] The driving roller 17 rotates when driving is input from a motor (not shown). The driven roller 18 is rotatably disposed at a position isolated forward of the driving roller 17. The endless belt 19 is stretched so as to span between the driving roller 17 and the driven roller 18. The endless belt 19 rotates along with the driving roller 17 and the driven roller 18 when the driving roller 17 rotates. The belt frame 14 supports the driving roller 17, the driven roller 18, the endless belt 19, and the transfer roller 20. Note that the driving roller 17 and the driven roller 18 are an example of a plurality of rollers in the present invention.

[0053] As shown in FIG. 2, the endless belt 19 has a conveyance surface 19B. The conveyance surface 19B is an upper surface on the outer surface of the endless belt 19 that supports the sheet from below. The transfer unit 13 conveys the sheet conveyed from the feeder unit 4 toward the fixing unit 50 while placing it on the conveyance surface 19B of the endless belt 19.

[0054] The transfer roller 20 is disposed to face the photosensitive drum 15 in the vertical direction with the endless belt 19 interposed therebetween. As shown in FIG. 2, the transfer roller 20 forms a transfer nip portion 21 at a position where the photosensitive drum 15 and the endless belt 19 come into contact by sandwiching the endless belt 19 between the transfer roller 20 and the photosensitive drum 15. The photosensitive drum 15 and the transfer roller 20 sandwich and press-contact the sheet at the transfer nip portion 21. The transfer roller 20 transfers the developer adhering to the surface of the photosensitive drum 15 to the image formation surface of the sheet by applying a voltage of a polarity opposite to the charge charged on the photosensitive drum 15 to the sheet from the opposite side of the image formation surface at the transfer nip portion 21.

[0055] Note that the photosensitive drum 15 and the transfer roller 20 are arranged such that the axial direction of their rotation axes coincides with the direction orthogonal to the sheet conveyance direction and the thickness direction of the conveyed sheet. Therefore, each of the photosensitive drums 15 and the transfer roller 20 is configured to be arranged along the direction orthogonal to its rotation axis.

[0056] <Fixing device 50> The fixing device 50 heats and melts the developer transferred to the sheet discharged from the image forming unit 11 and fixes it to the sheet. The fixing device 50 is arranged downstream of the drum unit 22 in the sheet conveyance direction. The fixing device 50 is detachably assembled to the housing 2.

[0057] The fixing device 50 includes a heating roller 51, a pressure roller 52, and a fixing frame 54. The heating roller 51 is arranged above the image forming surface of the sheet supported on the conveyance surface 19B of the endless belt 19, and applies a conveyance force to the sheet while heating the developer transferred to the sheet. The pressure roller 52 is arranged on the opposite side of the heating roller 51 with the sheet sandwiched therebetween in the vertical direction, and presses the sheet against the heating roller 51. The heating roller 51 and the pressure roller 52 form a fixing nip portion 53 by being in pressure contact with each other, and sandwich the sheet at the fixing nip portion 53. The fixing device 50 applies heat to the sheet at the fixing nip portion 53 to melt the developer and fix the developer to the sheet. The fixing frame 54 houses the heating roller 51 and the pressure roller 52.

[0058] <First regulating portion 31> FIG. 2 is a side cross-sectional view near the drum unit 22 and the fixing unit 50. Further, FIG. 9 is a side cross-sectional view in which the vicinity of the first regulating portion 31 and the second regulating portion 41 is further enlarged. As shown in FIGS. 2 and 9, the drum frame 30 includes a first regulating portion 31 having a rib shape. The first regulating portion 31 is located between the transfer nip portion 21A located most downstream in the sheet conveyance direction and one end portion 19A of the endless belt 19 of the endless belt 19 and above the endless belt 19, and faces the conveyance surface 19B of the endless belt 19 in the vertical direction. One end portion 19A of the endless belt 19 is a rear end portion on the conveyance surface 19B of the endless belt 19, and specifically, is located substantially directly above the center of the rotation axis of the driving roller 17.

[0059] FIG. 3 is a perspective view of the drum unit 22. Further, FIG. 5 is a front view of the drum unit 22 as viewed from the fixing unit 50 side. As shown in FIGS. 3 and 5, the first regulating portion 31 includes a first horizontal portion 32 and a pair of first inclined portions 33. The first horizontal portion 32 extends in the left-right direction. The pair of first inclined portions 33 extend in the left-right direction from both ends of the first horizontal portion 32 in the left-right direction, respectively. Further, the pair of first inclined portions 33 are inclined in a direction approaching the endless belt 19 from the center in the left-right direction toward both ends. Specifically, the first inclined portion 33R located on the right side of the pair of first inclined portions 33 has its left end connected to the right end of the first horizontal portion 32 and is inclined so as to gradually approach the endless belt 19 from the left end to the right end. The first inclined portion 33L located on the left side of the pair of first inclined portions 33 has its right end connected to the left end of the first horizontal portion 32 and is inclined so as to gradually approach the endless belt 19 from the right end to the left end. Here, the lower end 34 of the first inclined portion 33 indicates the right end 34R of the first inclined portion 33R and the left end 34L of the first inclined portion 33L.

[0060] Further, as shown in FIGS. 2 and 9, a part of the pair of first inclined portions 33 is provided directly above the one end portion 19A of the endless belt 19 in the sheet conveyance direction.

[0061] Then, as shown in FIGS. 2 and 9, the lower end 34 of the first inclined portion 33 is located below the virtual straight line L1 connecting the transfer nip portion 21A and the fixing nip portion 53, and is at the position closest to the endless belt 19 in the vertical direction within the first regulating portion 31.

[0062] As shown in FIGS. 3 and 5, the lateral distance W1 between the left end 34L of the first inclined portion 33L and the right end 34R of the first inclined portion 33R is larger than the width of the cardboard S1 in the lateral direction, and the lateral distance of the first horizontal portion 32 is smaller than the width of the cardboard S1. As the cardboard S1, a postcard can be exemplified, and the width in that case is 100 mm.

[0063] Here, the image forming apparatus 1 performs sheet conveyance according to a so-called center reference in which the center of the sheet conveyance path in the lateral direction coincides with the center of the sheet in the lateral direction. Then, the first horizontal portion 32 is disposed at a position such that the center of the sheet conveyed by the endless belt 19 in the lateral direction substantially coincides with the center of the first horizontal portion 32 in the lateral direction.

[0064] <Second regulating portion 41> FIG. 4 is a perspective view of the fixing device 50. FIG. 6 is a front view of the fixing device 50 as viewed from the downstream side of the sheet conveyance path at the sheet entrance. As shown in FIGS. 4 and 6, the fixing frame 54 includes a second regulating portion 41 having a rib shape. The second regulating portion 41 is disposed above the entrance side of the fixing device 50, that is, above the conveyance path of the sheet from the transfer nip portion 21A to the fixing device 50. The second regulating portion 41 includes a second horizontal portion 42 and a pair of second inclined portions 43. The second horizontal portion 42 extends along the left-right direction. The pair of second inclined portions 43 extend from both ends in the left-right direction of the second horizontal portion 42 in the left-right direction, respectively. Further, the second inclined portion 43 is inclined downward from the center in the left-right direction toward both ends. Specifically, the second inclined portion 43R located on the right side of the pair of second inclined portions 43 has one end connected to the right end of the second horizontal portion 42 and is inclined downward to the right from the left end to the right end. The second inclined portion 43L located on the left side of the pair of second inclined portions 43 has the right end connected to the left end of the second horizontal portion 42 and is inclined downward to the left from the right end to the left end. Here, the lower end 44 of the second inclined portion 43 indicates the right end 44R of the second inclined portion 43R and the left end 44L of the second inclined portion 43L.

[0065] And, as shown in FIGS. 2 and 9, the lower end of the second inclined portion 43 is located below the virtual straight line L2 connecting between the lower end 34 of the first inclined portion 33 and the fixing nip portion 53.

[0066] Note that, as shown in FIGS. 4 and 6, the distance W2 between the left end 44L of the second inclined portion 43L and the right end 44R of the second inclined portion 43R is larger than the width of the thin paper S2 in the left-right direction, and the distance in the left-right direction of the second horizontal portion 42 is smaller than the width of the thin paper S2. Also, the distance in the left-right direction of the second horizontal portion 42 is larger than the width of the thick paper S1. And, the distance in the left-right direction of the second horizontal portion 42 is larger than the distance W1 between the left end 34L of the first inclined portion 33L and the right end 34R of the first inclined portion 33R.

[0067] Also, as shown in FIG. 6, the angle θ formed by the second inclined portion 43 and the horizontal plane 2 is larger than the angle θ formed by the first inclined portion 33 and the horizontal plane 1 is larger.

[0068] <Operation and Effect of this Embodiment> The image forming apparatus 1 conveys the sheet on which the image is formed at the transfer nip portion 21A toward the fixing device 50 by the endless belt 19. The sheet assumes a stable posture when the front end and the rear end are respectively sandwiched between the fixing nip portion 53 and the transfer nip portion 21A. When the rear end of the sheet is discharged from the transfer nip portion 21A, the sheet is in a state where only the front end is sandwiched by the fixing nip portion 53. At this time, the rear end of the sheet that is not sandwiched may float from the conveyance surface 19B due to the impact applied to the sheet during sheet discharge or the force applied to the sheet at the fixing nip portion 53. When the rear end of the sheet thus floats from the conveyance surface 19B, peeling discharge occurs between the conveyance surface 19B and the floating sheet, and there is a risk that the developer image before fixing may be disturbed by the generated static electricity or its impact, leading to a deterioration in image quality.

[0069] The pair of first inclined portions 33 of the first restricting portion 31 are disposed between the transfer nip portion 21A, which is the position where the sheet floats from the endless belt 19, and one end portion 19A of the endless belt 19, and face the conveyance surface 19B above the conveyance surface 19B of the endless belt 19.

[0070] According to this configuration, when the rear end of the sheet floats from the conveyance surface 19B, the pair of first inclined portions 33 can contact both ends in the left - right direction of the floating rear end of the sheet and directly hold down the rear end of the sheet, suppressing the floating of the rear end of the sheet and effectively suppressing peeling discharge.

[0071] When the sheet discharged from the transfer nip portion 21A is further conveyed, the rear end separates from the conveyance surface 19B at the one - end portion 19A of the endless belt 19 of the endless belt 19. Also in this case, there is a risk that the rear end of the sheet without the support of the endless belt 19 floats and peeling discharge occurs.

[0072] A part of the pair of first inclined portions 33 of the first regulating portion 31 is disposed at a position overlapping with a substantially directly upper portion of the endless belt end portion 19A of the endless belt 19 in the sheet conveying direction.

[0073] According to this configuration, even when the rear end of the sheet leaves the conveying surface 19B, the pair of first inclined portions 33 can contact both ends of the rear end of the sheet in the left - right direction and directly hold down the rear end of the sheet, suppressing the lifting of the rear end of the sheet and effectively suppressing the peeling discharge.

[0074] Also, the pair of first inclined portions 33 are inclined so as to gradually approach the conveying surface 19B of the endless belt 19 from the center toward both ends in the left - right direction.

[0075] According to this configuration, when the first inclined portion 33 contacts both ends of the sheet in the left - right direction, it is possible to regulate the contact of the image forming surface on the center side of the sheet in the left - right direction with the first regulating portion 31, and it is possible to suppress a decrease in image quality due to image rubbing of the pre - fixing image.

[0076] By the way, when the fixing nip portion 53 is disposed above the conveying surface 19B of the endless belt 19, when forming an image on a sheet having high rigidity and stiffness like thick paper, peeling discharge due to the lifting of the rear end of the sheet is likely to occur significantly. A stiff sheet bridges between the transfer nip portion 21A and the fixing nip portion 53 in a substantially straight line without bending like thin paper, so the rear end of the sheet is likely to lift up greatly inclined with respect to the conveying surface 19B of the endless belt 19. When the rear end of the sheet lifts up greatly in this way, the impact due to peeling discharge becomes large, so especially in a stiff sheet, a decrease in image quality due to peeling discharge is likely to become significant. In this embodiment, the thick paper S1 is taken as an example of a stiff sheet.

[0077] In this embodiment, the lateral distance W1 between the left end 34L of the first inclined portion 33L and the right end 34R of the first inclined portion 33R is larger than the width of the thick paper S1 in the lateral direction, and the lateral distance of the first horizontal portion 32 is smaller than the width of the thick paper S1.

[0078] According to this configuration, the first inclined portion 33 abuts against both ends in the left - right direction of the cardboard S1, and by suppressing the lifting of the rear end of the cardboard S1, peeling discharge can be effectively suppressed.

[0079] Also, as shown in FIG. 9, the lower end 34 of the first inclined portion 33 is disposed below the virtual straight line L1 connecting the transfer nip portion 21A and the fixing nip portion 53.

[0080] FIG. 7 is a schematic side cross - sectional view showing the state in which the cardboard S1 is conveyed from the image forming unit 11 to the fixing device 50. According to this configuration, as shown in FIG. 7, when the first regulating portion 31 is in contact with the cardboard S1, the first regulating portion 31 prevents the cardboard S1 from bridging straight between the transfer nip portion 21A and the fixing nip portion 53, and can bend the cardboard S1. When the cardboard S1 bends, the impact when the rear end of the cardboard S1 lifts is mitigated, further suppressing peeling discharge.

[0081] Also, the first regulating portion 31 is disposed in the drum unit 22 that is in contact with the transfer unit 13 in the transfer nip portion 21A.

[0082] According to this configuration, the first regulating portion 31 is accurately positioned with respect to the conveying surface 19B of the endless belt 19. Thereby, when the rear end of the sheet lifts, the first inclined portion 33 can accurately abut against both end portions in the left - right direction of the sheet, and can effectively suppress peeling discharge.

[0083] By the way, when the sheet conveyed by the endless belt 19 is the thin paper S2 having a width larger in the left - right direction than the left - right distance W1 between the right end 34R of the first inclined portion 33R and the left end 34L of the first inclined portion 33L, when the thin paper S2 lifts from the conveying surface 19B of the endless belt 19, the first regulating portion 31 comes into contact with the image forming surface of the thin paper S2, and there is a risk that image rubbing will occur on the pre - fixing image.

[0084] Therefore, in the present embodiment, the lower end 44 of the second inclined portion 43 is disposed below the virtual straight line L2 connecting the lower end 34 of the first inclined portion 33 and the fixing nip portion 53.

[0085] FIG. 8 is a schematic side cross-sectional view showing a state in which the thin paper S2 is conveyed from the image forming unit 11 to the fixing device 50. According to this configuration, as shown in FIG. 8, the second inclined portion 43 contacts the thin paper S2 before the image forming surface of the thin paper S2 contacts the first inclined portion 33. Thereby, the second restricting portion 41 restricts the lifting of the thin paper S2 to suppress peeling discharge, and restricts the image forming surface of the thin paper S2 from contacting the first restricting portion 31, and also suppresses a decrease in image quality due to image rubbing of the pre-fixing image of the thin paper S2 at the same time.

[0086] Further, as shown in FIG. 9, the lower end 44 of the second inclined portion 43 is provided below the virtual straight line L2 connecting the lower end 34 of the first inclined portion 33 and the fixing nip portion 53. That is, the lower end 44 of the second inclined portion 43 is disposed below the virtual straight line L1 connecting the transfer nip portion 21A and the fixing nip portion 53.

[0087] According to this configuration, as shown in FIG. 8, when the thin paper S2 abuts on the second inclined portion 43, the second restricting portion 41 prevents the thin paper S2 from bridging straight between the transfer nip portion 21A and the fixing nip portion 53, and can bend the thin paper S2. When the thin paper S2 bends, the impact when the rear end of the thin paper S2 rises is mitigated, further suppressing peeling discharge.

[0088] Further, the second inclined portion 43 is inclined downward from the center toward both ends in the left-right direction.

[0089] According to this configuration, when the second inclined portion 43 abuts on both ends in the left-right direction of the sheet, the second restricting portion 41 restricts the image forming surface on the center side in the left-right direction of the sheet from contacting the second restricting portion 41, and can suppress a decrease in image quality due to image rubbing of the pre-fixing image.

[0090] Further, as shown in FIG. 6, the angle θ formed by the second inclined portion 43 and the horizontal plane 2is the angle θ formed between the first inclined portion 33 and the horizontal plane 1 is larger than that.

[0091] According to this configuration, the pair of second inclined portions 43 widen the space between the left and right inclined portions 43L and 43R, thereby restricting the image formation surface of the cardboard S1 conveyed to the fixing device 50 through the space from contacting the second regulating portion 41.

[0092] By the way, when the sheet is curved upward in a convex shape, even if both ends in the left - right direction of the sheet are in contact with the second inclined portion 43, there is a possibility that the image formation surface on the central side in the left - right direction of the sheet may contact the second regulating portion 41. And the thin paper S2 is weaker in stiffness than the cardboard S1 and tends to be curved easily.

[0093] The second inclined portion 43 forms an angle θ 2 with the horizontal plane, which 1 is larger than the angle θ. Thus, even if the thin paper S2 is curved upward in a convex shape, it further restricts the image formation surface of the thin paper S2 from contacting the second regulating portion 41, and can suppress the deterioration of the image quality due to image rubbing of the pre - fixing image.

[0094] As described above, the embodiments have been explained, but the present invention is not limited to the above - described embodiments. Regarding the specific configuration, it can be appropriately changed without departing from the gist of the invention as follows.

[0095] For example, in the above - described embodiment, the first regulating portion 31 is provided on the drum frame 30, but it is not limited thereto. For example, the first regulating portion 31 may be provided on the belt frame 14 of the transfer unit 13. In this case, since the endless belt 19 for conveying the sheet and the first regulating portion 31 are both supported by the belt frame 14, the first regulating portion 31 can be accurately positioned with respect to the conveying surface 19B. Thereby, when the rear end of the sheet floats up, the first inclined portion can more accurately contact both end portions in the width direction of the rear end of the sheet.

[0096] Further, the first inclined portion 33 is not limited to the shape of the above-described embodiment, and may be inclined in a direction approaching the transport surface 19B from the center in the left-right direction toward both ends. Also, the shapes of the left and right first inclined portions 33L and 33R do not have to be the same.

[0097] Further, the second inclined portion 43 is not limited to the shape of the above-described embodiment, and may be inclined downward from the center in the width direction toward both ends. Also, the shapes of the left and right second inclined portions 43L and 43R do not have to be the same.

[0098] Also, in the above-described embodiment, an exposure device using LEDs is exemplified as the LED array 12, but it is not limited thereto. For example, the exposure device may be an exposure device composed of a laser light source, a polygon mirror, an fθ lens, a reflecting mirror, and the like. In this case, a laser beam is emitted from the laser light source based on the image data. The laser beam is deflected by the polygon mirror, passes through the fθ lens, and then the optical path is folded back by the reflecting mirror, and the optical path is further bent downward by the reflecting mirror, so that the surface of the photosensitive drum 15 is irradiated, and an electrostatic latent image is formed on the photosensitive drum 15.

[0099] Also, in the above-described embodiment, a fixing device 50 having a heating roller 51 and a pressure roller 52 is exemplified as the fixing unit, but it is not limited thereto. For example, the fixing unit may be a fixing device including an endless belt and a nip member disposed inside the endless belt and sandwiching the endless belt between the nip member and the pressure roller 52.

[0100] Also, in the above-described embodiment, the image forming apparatus 1 for forming a color image is exemplified, but it is not limited thereto. For example, the image forming apparatus 1 may be an image forming apparatus 1 for forming a monochrome image. Also, the image forming apparatus 1 may be a copying machine or a multifunction machine including a document reading apparatus such as a flatbed scanner in addition to a laser printer.

[0101] Also, the present invention only needs to conform to the gist of the invention described in the claims, and is not limited to the above-described embodiments.

Explanation of Reference Numerals

[0102] 1 Image forming apparatus 2 Housing 13 Transfer unit 15 Photosensitive drum 17 Driving roller 18 Driven roller 19 Endless belt 19B Conveying surface 21 Transfer nip portion 24 Process unit 31 First regulating portion 32 First horizontal portion 33 First inclined portion 41 Second regulating portion 42 Second horizontal portion 43 Second inclined portion 50 Fixer

Claims

1. An image forming apparatus for forming an image on a sheet by transferring a developer image onto the sheet, comprising: a transfer unit (13) including a plurality of rollers (17, 18) and an endless belt (19) that is stretched between the plurality of rollers and transports the sheet in a transport direction with an upper surface thereof as a transport surface; a process unit that includes a photosensitive drum (15) provided above the endless belt and that forms an image on the sheet at a transfer nip portion between the photosensitive drum and the endless belt; The transfer nip portion and the conveying surface a first regulating portion (31) that faces the conveying surface above the conveying surface between the conveying surface and the downstream end in the conveying direction and is capable of contacting the sheet; The first restricting portion is A first horizontal portion (32) extending along a width direction perpendicular to the conveying direction; An image forming apparatus characterized in having a pair of first inclined portions (33) extending in the width direction from both ends of the first horizontal portion in the width direction, and inclined in a direction approaching the endless belt as it moves from one end close to the first horizontal portion in the width direction to the other end, and the first inclined portions being capable of abutting both ends of the sheet in the width direction.

2. 2 . The image forming apparatus according to claim 1 , wherein the first inclined portion is provided at a position overlapping in the transport direction with a rotation axis center of a roller that is located most downstream in the transport direction among the plurality of rollers.

3. The process unit is The photosensitive drum further includes a drum frame for supporting the photosensitive drum.

3. The image forming apparatus according to claim 1, wherein the first regulating portion is provided on the drum frame.

4. The transfer unit further includes a belt frame that rotatably supports the rollers, 3. The image forming apparatus according to claim 1, wherein the first regulating portion is provided on the belt frame.

5. A fixing device that heats and fixes an image formed on the sheet, a fixing unit including a heating roller and a pressure roller that contacts the heating roller to form a fixing nip portion that nips the sheet; 5. The image forming apparatus according to claim 1, wherein the other end of the first inclined portion is provided below a virtual straight line connecting the fixing nip portion and the transfer nip portion.

6. The image forming apparatus is capable of forming an image on a sheet having a first width in the width direction and a sheet having a second width larger than the first width, An image forming device as described in any one of claims 1 to 5, characterized in that the widthwise distance between the other ends of each of the pair of first inclined portions is greater than the first width, and the widthwise distance of the first horizontal portion is smaller than the first width.

7. A fixing device (50) for heat-fixing the image formed on the sheet; a second regulating portion (41) provided in a conveying path of the sheet between the first regulating portion and the fixing nip portion in the conveying direction, The fixing device includes: a heating roller and a pressure roller that contacts the heating roller at a fixing nip portion and nips the sheet; The second restricting portion is A second horizontal portion (42) extending along a width direction perpendicular to the conveying direction; a pair of second inclined portions (43) extending in the width direction from both ends of the second horizontal portion in the width direction, inclined downward from one end close to the second horizontal portion in the width direction toward the other end, the second inclined portions being capable of abutting against both ends of the sheet in the width direction, a distance in the width direction between the other ends of each of the pair of second inclined portions is greater than the second width, 7. The image forming apparatus according to claim 6, wherein a distance in the width direction of the second horizontal portion is smaller than the second width.

8. 8. The image forming apparatus according to claim 7, wherein a distance in the width direction of the second horizontal portion is greater than a distance in the width direction between the other ends of the first inclined portions.

9. 9. The image forming apparatus according to claim 7, wherein the other end of the second inclined portion is provided below a virtual straight line connecting the fixing nip portion and a lower end of the first inclined portion.

10. 10. The image forming apparatus according to claim 7, wherein an angle that the second inclined portion makes with a horizontal plane is larger than an angle that the first inclined portion makes with a horizontal plane.

Images