Image forming apparatus

Abstract

To prevent the occurrence of an image defect due to rubbing between a transfer material and a conveyance rib of a conveyance path.SOLUTION: An image forming apparatus comprises: a fixing device 14 that fixes a toner image on a transfer material formed in an image forming unit 6 to the transfer material; an ejection roller pair 19 that ejects the transfer material; a switchback roller pair 21 that conveys the transfer material to a double-sided conveyance path 23; a double-sided flapper 15 that changes a conveyance direction to the ejection roller pair 19 or to the switchback roller pair 21; a conveyance path 18 that is provided between the double-sided flapper 15 and the ejection roller pair 19; a conveyance path 20 that is provided between the double-sided flapper 15 and the switchback roller pair 21; and a driven roller 17 that is arranged adjacent to ends, on the upstream side in the conveyance direction of the transfer material S, of a conveyance guide 18b of the conveyance path 18 on a side of a print surface of the transfer material and a conveyance guide 20a of the conveyance path 20 on a side of a non-print surface of the transfer material, has an outer peripheral surface projecting toward the conveyance path 18 beyond the conveyance guide 18b and projecting toward the conveyance path 20 beyond the conveyance guide 20a, and is rotatably supported.SELECTED DRAWING: Figure 2

Description

Technical Field

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

Background Art

[0002] In electrophotographic image forming apparatuses such as copiers, printers, and facsimiles, a method of heating and pressurizing an unfixed toner image transferred onto a transfer material by a fixing device and fixing it onto the transfer material is generally known. Immediately after being fixed to the transfer material by the fixing device, the toner image has a uniform surface property and the entire image has a uniform gloss. However, when rubbed against the conveyance ribs provided in the conveyance path, the surface of the toner image changes, resulting in image defects such as image streaks and gloss spots.

[0003] As one method for addressing this problem, for example, in Patent Document 1, a configuration is proposed in which a driven roller is disposed on the printed surface side of the transfer material in the conveyance path between the fixing device and the discharge unit where the transfer material is discharged, to prevent the occurrence of image defects. In the image forming apparatus of Patent Document 1, a configuration is adopted in which the driven roller is disposed so as to protrude from the conveyance path to the printed surface side of the transfer material with respect to the conveyance path immediately after passing through the fixing device, particularly the conveyance path from the fixing device to the discharge unit where the conveyance direction of the transfer material changes abruptly. As a result, the toner image immediately after being fixed by the fixing device does not rub against the conveyance ribs that form the wall surface of the conveyance path, thereby preventing the occurrence of image defects. The conveyance speed of the transfer material on the downstream side in the transfer material conveyance direction of the fixing device is faster than the conveyance speed of the transfer material in the fixing device. As a result, even when the transfer material is pulled, by protruding the driven roller from the wall surface of the conveyance path, the driven roller rotates in contact with the transfer material. As a result, the toner image on the transfer material does not rub against the wall surface of the conveyance path, and the occurrence of image defects can be prevented.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

[0005] However, in recent years, as image forming apparatuses have become smaller, the space between the fuser and the discharge section, and the reversal section that reverses the transfer material from the fuser to the double-sided transport path, has become narrower. As a result, while conventionally the printable side of the transfer material was prone to rubbing against the transport ribs of the transport path from the fuser to the discharge section, the miniaturization of image forming apparatuses has made it easier for the transfer material and transport ribs to rub against each other in areas where they did not previously rub against each other, such as the following: Specifically, the transfer material and transport ribs now rub against each other on the non-printable side of the transfer material in the transport path from the fuser to the discharge section, and on the printable side of the transfer material in the transport path from the fuser to the reversal section, leading to the problem of image defects.

[0006] This invention was made under such circumstances and aims to prevent image defects caused by friction between the transfer material and the transport ribs of the transport path. [Means for solving the problem]

[0007] To solve the above-mentioned problems, the present invention has the following configuration. (1) An image forming unit that forms a toner image on a transfer material; a fixing unit having a heating member and a pressurizing member, which fixes the toner image formed on the transfer material in the image forming unit by clamping the transfer material in a nip portion formed by the heating member and the pressurizing member; a discharge unit that discharges the transfer material on which the toner image has been fixed in the fixing unit; a reversing unit that reverses the transport direction of the transfer material in order to transport the transfer material on which the toner image has been fixed in the fixing unit back to the image forming unit via a double-sided transport path; a rotatably supported switching unit that contacts the leading edge of the transfer material that has passed through the fixing unit and switches the transport direction of the transfer material to the discharge unit or the reversing unit; and a unit provided between the switching unit and the discharge unit that pressurizes the transfer material An image forming apparatus comprising: a first transport path formed by a first transport surface facing the side of the transfer material that is in contact with a heating member and a second transport surface facing the side of the transfer material that is in contact with the pressurizing member; a second transport path provided between the switching unit and the reversing unit, formed by a third transport surface facing the side of the transfer material that is in contact with the heating member and a fourth transport surface facing the side of the transfer material that is in contact with the pressurizing member; and a first rotating member arranged adjacent to the upstream ends of the second transport surface and the third transport surface in the transport direction of the transfer material, having an outer peripheral surface that protrudes further toward the first transport path than the second transport surface and further toward the second transport path than the third transport surface, and being rotatably supported. [Effects of the Invention]

[0008] According to the present invention, it is possible to prevent image defects caused by friction between the transfer material and the transport ribs of the transport path. [Brief explanation of the drawing]

[0009] [Figure 1] Cross-sectional view showing the configuration of the image forming apparatus in Examples 1 and 2. [Figure 2] Cross-sectional view illustrating the configuration of the driven roller in Example 1 [Figure 3] Cross-sectional view illustrating how the transfer material is transported during single-sided printing in Example 1. [Figure 4]Cross-sectional view illustrating how the transfer material is transported during double-sided printing in Example 1. [Figure 5] Cross-sectional view illustrating how the transfer material is transported during double-sided printing in Example 1. [Figure 6] Cross-sectional view illustrating how the transfer material is transported during double-sided printing in Example 1. [Figure 7] Cross-sectional view illustrating how the transfer material is transported during continuous double-sided printing in Example 1. [Figure 8] Cross-sectional view illustrating the configuration of the driven roller in Example 2 [Figure 9] Cross-sectional view illustrating how the transfer material is transported during double-sided printing in Example 2. [Figure 10] Cross-sectional view illustrating how the transfer material is transported during double-sided printing in Example 2. [Figure 11] Cross-sectional view illustrating how the transfer material is transported during double-sided printing in Example 2. [Figure 12] Cross-sectional view illustrating how the transfer material is transported during continuous double-sided printing in Example 2. [Modes for carrying out the invention]

[0010] Embodiments of the present invention will be described in detail below with reference to the drawings. [Examples]

[0011] [Configuration of the image forming apparatus] Figure 1 is a cross-sectional view showing the configuration of an image forming apparatus 1 to which the present invention is applied. In Figure 1, the image forming apparatus 1 includes image forming units 6 (6Y, 6M, 6C, 6K), which are image forming stations corresponding to the toner colors of yellow (Y), magenta (M), cyan (C), and black (K). Each image forming unit 6 has the same configuration except for the toner color, and the Y, M, C, and K appended to the end of the reference numerals of the components constituting each image forming unit 6 indicate that they are components of the image forming unit 6 with toner colors of yellow, magenta, cyan, and black. Hereafter, the Y, M, C, and K appended to the end of reference numerals will be omitted unless they refer to a specific component of the image forming unit 6.

[0012] The image forming unit 6 includes a photosensitive drum 7 which is an image carrier, a charging roller 8 for charging the surface of the photosensitive drum 7 to a uniform potential, and a developing roller 10 for forming a toner image by attaching toner to an electrostatic latent image formed on the photosensitive drum 7. Further, the scanner unit 9 irradiates the photosensitive drum 7 with a laser beam based on image information to form an electrostatic latent image on the photosensitive drum 7. And, at a position facing each other through the intermediate transfer belt 11 of the photosensitive drum 7 of each image forming unit 6, a primary transfer roller 12 is disposed, and the toner images formed on each photosensitive drum 7 are sequentially and superposed and transferred onto the intermediate transfer belt 11. Then, the toner image transferred onto the intermediate transfer belt 11 moves to a secondary transfer nip portion where the intermediate transfer belt 11 and the secondary transfer roller 13 are in contact with each other.

[0013] As shown in FIG. 1, at the lower part in the drawing of the image forming apparatus 1, a paper cassette 2 for accommodating the transfer material S is retractably stored, and a feeding unit 3 is provided at the end of the paper cassette 2. Also, on the right side in the drawing of the image forming apparatus 1, a manual feeding unit 4 for feeding the transfer material S placed on the manual feed tray is provided. The feeding unit 3 and the manual feeding unit 4 separate the transfer material S accommodated in the paper cassette 2 and the transfer material S placed on the manual feed tray one by one, and feed them to the registration roller 5. A conveyance sensor 30a is disposed on the downstream side in the conveyance direction of the transfer material S of the registration roller 5. The conveyance sensor 30a detects that the transfer material S is being normally conveyed by being pushed up from the position shown by the solid line in the drawing to the position shown by the dotted line by the transfer material S conveyed through the conveyance path.

[0014] In synchronization with the image forming operation in the above-described image forming unit 6, the transfer material S fed from the paper cassette 2 or the like is conveyed to the secondary transfer roller 13, and at the secondary transfer nip portion, the toner image formed on the intermediate transfer belt 11 is transferred to the transfer material S. The transfer material S onto which the toner image has been transferred is conveyed to the fixing device 14 which is a fixing unit. The fixing device 14 includes a heating unit 14a which is a heating member for heating the toner image transferred to the transfer material S, and a pressure roller 14b which is a pressure member that presses against the heating unit 14a. The transfer material S is conveyed to the fixing device 14, and is heated and pressed when passing through the fixing nip portion formed by the heating unit 14a and the pressure roller 14b, and the toner image transferred to the transfer material S is fixed to the transfer material S. A conveyance sensor 30b is disposed on the downstream side in the conveyance direction of the transfer material S in the fixing device 14. The conveyance sensor 30b detects that the transfer material S is being normally conveyed by being pushed up from the position shown by the solid line in the figure to the position shown by the dotted line in the figure by the transfer material S conveyed through the conveyance path.

[0015] The transfer material S that has passed through the fixing device 14 is conveyed to the conveyance path 18 which is the first conveyance path, or the conveyance path 20 which is the second conveyance path, according to the state shown by the solid line or the broken line in the figure of the duplex flapper 15 which is a switching unit. The duplex flapper 15 is a switching unit that is rotatably supported, contacts the leading end of the transfer material S that has passed through the fixing device 14, and switches the conveyance direction of the transfer material S to the discharge roller pair 19 or the switchback roller pair 21 which will be described later. When the rotatably supported duplex flapper 15 is in the state shown by the solid line in the figure, the transfer material S is conveyed to the conveyance path 18 where the driven roller 16 which is the second rotating member is disposed on the printing surface side and the driven roller 17 which is the first rotating member is disposed on the non-printing surface side. Then, the transfer material S conveyed to the conveyance path 18 is conveyed to the discharge roller pair 19 which is a discharge unit, and the discharge roller pair 19 conveys the conveyed transfer material S while sandwiching it, and discharges the transfer material S to the stacking unit 22.

[0016] On the other hand, when the double-sided flapper 15 is in the state shown by the dashed line in the figure, the transfer material S is transported to a transport path 20 on which a second driven roller 17 is positioned on the printing surface side. The transfer material S transported to the transport path 20 is then transported to a switchback roller pair 21, which is a reversal section located at the top of the discharge roller pair 19 in the figure. The transfer material S transported to the switchback roller pair 21 is reversed and transported by the switchback roller pair 21 and then transported to the double-sided transport path 23. The double-sided transport path 23 is located on the right side of the fixing device 14 in the figure and is formed from the top to the bottom of the image forming apparatus 1 in the figure. A transport sensor 30c is positioned in the middle of the double-sided transport path 23, and the transport sensor 30c detects that the transfer material S is being transported correctly by changing from the state shown by the solid line to the state shown by the dotted line in the figure depending on the transported transfer material S. The transfer material S, having passed through the double-sided transport path 23, is again guided to the register roller 5, which then transports it to the secondary transfer roller 13. The toner image on the intermediate transfer belt 11 is transferred to the secondary transfer roller 13, and the toner image is fixed to the transfer material S by the fixing device 14. After that, the transfer material S with the fixed toner image is transported to the transport path 18 and discharged to the loading section 22 by the discharge roller pair 19.

[0017] [Configuration of the driven roller] Next, the first driven roller, driven roller 16, and the second driven roller, driven roller 17, of this embodiment will be described. Figure 2 is a cross-sectional view showing the configuration of the image forming apparatus 1 downstream of the fixing apparatus 14 described in Figure 1 in the direction of transport of the transfer material S. As described above, a double-sided flapper 15 is arranged downstream of the fixing apparatus 14 in the direction of transport of the transfer material S, so that the transfer material S transported from the fixing apparatus 14 is transported to the transport path 18 or the transport path 20.

[0018] Two transport paths are provided on the downstream side of the double-sided flapper 15 in the transport direction of the transfer material S. One is a first transport path 18 that guides the transfer material S transported from the fixing device 14 to the discharge roller pair 19, and the other is a second transport path 20 that guides the transfer material S transported from the fixing device 14 to the switchback roller pair 21. In Figure 2, the double-sided flapper 15 is set to a state where the opening to the transport path 20 is closed, and the transfer material S transported from the fixing device 14 is transported to the transport path 18. When the transfer material S transported from the fixing device 14 is transported to the transport path 20, the tip of the double-sided flapper 15 faces the driven roller 16, and the opening to the transport path 18 is closed (see Figure 4).

[0019] In the transport path 18 between the fixing device 14 and the discharge roller pair 19, a driven roller 16 is positioned on the side that has passed the heating unit 14a side of the fixing device 14 (hereinafter referred to as the printing side of the transfer material S), with its outer circumferential surface protruding from the transport surface of the transport path 18 and rotatably supported. In addition, a driven roller 17 is positioned between the transport path 18 and the transport path 20, with its outer circumferential surface protruding from the transport surface of both the transport path 18 and the transport surface of the transport path 20 and rotatably supported.

[0020] A transport guide 18a, which is the first transport surface constituting the transport path 18, is provided between the driven roller 16 and the discharge roller pair 19, and a transport guide 18b, which is the second transport surface constituting the transport path 18, is provided between the driven roller 17 and the discharge roller pair 19. Similarly, a transport guide 20a, which is the third transport surface constituting the transport path 20, is provided between the driven roller 17 and the switchback roller pair 21, and a transport guide 20b, which is the fourth transport surface constituting the transport path 20, is provided on the transport surface opposite the transport guide 20a. As shown in Figure 2, the driven roller 16 is positioned adjacent to the transport guide 18a near the outlet of the fixing device 14 where the transfer material S is transported. The driven roller 17 is also positioned adjacent to the transport guide 18b, the upstream end of the transport guide 20a in the transport direction of the transfer material S, and the double-sided flapper 15, as shown in Figure 2.

[0021] [Operation of driven rollers during single-sided printing] Next, the operation of the transfer material S and the driven roller 16 during single-sided printing will be described. Figure 3 is a cross-sectional view showing how the transfer material S is transported from the fixing device 14 to the discharge roller pair 19 during single-sided printing. During single-sided printing, the transfer material S is transported from the fixing device 14 to the discharge roller pair 19 via the transport path 18 and discharged to the loading section 22. Also, during single-sided printing, the double-sided flapper 15 is set to a state where the opening to the transport path 20 is closed, and the transported transfer material S is transported towards the transport path 18 side.

[0022] The leading edge of the transfer material S transported from the fixing device 14 moves towards the discharge roller pair 19, contacting the double-sided flapper 15 and the transport guide 18b, and is then held between the discharge roller pair 19. When the transported transfer material S is held between the fixing nip section of the fixing device 14 and the discharge roller pair 19, it is conceivable that the difference in transport speed between the fixing nip section and the discharge roller pair 19 may cause the transfer material S to deflect too much within the transport path 18, leading to jamming. Therefore, in this embodiment, the transport speed of the transfer material S at the discharge roller pair 19 is set faster than the transport speed of the transfer material S at the fixing nip section of the fixing device 14 to prevent deflection of the transfer material S. Then, the transfer material S, which is held between the fixing nip portion of the fixing device 14 and the discharge roller pair 19, is pulled by the discharge roller pair 19 and conveyed without deflection between the discharge roller pair 19 and the driven roller 16, as shown in Figure 3.

[0023] As shown in Figure 3, the printing surface side of the transfer material S is in contact with the driven roller 16, and the driven roller 16 rotates in conjunction with the transport of the transfer material S. The driven roller 16 has an outer surface that protrudes beyond the transport guide 18a of the transport path 18 and is pulled by the discharge roller pair 19, so the printing surface side of the transfer material S does not come into contact with the transport guide 18a. Therefore, it is possible to prevent image defects caused by friction between the printing surface of the transfer material S and the transport guide 18a.

[0024] [Operation of driven rollers during double-sided printing] (Operation during the first printing of the transfer material) Next, the operation of the transfer material S and driven rollers 16 and 17 during double-sided printing will be explained. First, the operation of the transfer material S and driven rollers 17 during the printing of the first side will be explained. Figure 4 is a cross-sectional view showing how the transfer material S is transported from the fuser 14 to the switchback roller pair 21 during the printing of the first side of double-sided printing. During the printing of the first side of double-sided printing, the transfer material S is transported from the fuser 14 to the switchback roller pair 21 via the transport path 20. Once the transfer material S reaches the switchback roller pair 21, it is held by the switchback roller pair 21 and transported in the same transport direction until the timing to switch the transport direction. After that, the transfer material S is transported in reverse direction by the switchback roller pair 21 and transported to the double-sided transport path 23. Also, during the printing of the first side of double-sided printing, the double-sided flapper 15 is set to a state where the opening to the transport path 18 is closed, and the transported transfer material S is set to be transported towards the transport path 20 side.

[0025] The leading edge of the transfer material S transported from the fixing device 14 moves towards the switchback roller pair 21 while in contact with the transport guide 20b, and is then gripped by the switchback roller pair 21. When the transported transfer material S is gripped between the fixing nip section of the fixing device 14 and the switchback roller pair 21, it is conceivable that the transfer material S may bend significantly and jam in the transport path 20 due to the difference in transport speed between the fixing nip section and the switchback roller pair 21. Therefore, in this embodiment, the transport speed of the transfer material S at the switchback roller pair 21 is set faster than the transport speed of the transfer material S at the fixing nip section of the fixing device 14 to prevent bending of the transfer material S. This eliminates bending of the transfer material S in the transport path 20. Then, the transfer material S, which is held between the fixing nip of the fixing device 14 and the switchback roller pair 21, is pulled by the switchback roller pair 21 and conveyed without deflection between the switchback roller pair 21 and the driven roller 17, as shown in Figure 4.

[0026] As shown in Figure 4, the printing surface side of the transfer material S is in contact with the driven roller 17, and the driven roller 17 rotates in conjunction with the transport of the transfer material S. The driven roller 17 has an outer surface that protrudes beyond the transport guide 20a of the transport path 20 and is pulled by the switchback roller pair 21, so the printing surface side of the transfer material S does not come into contact with the transport guide 20a. Therefore, it is possible to prevent image defects caused by friction between the printing surface of the transfer material S and the transport guide 20a.

[0027] (Operation during second-side printing of transfer material) Next, the operation of the transfer material S and driven rollers 16 and 17 during the printing of the second side of double-sided printing will be explained. Figure 5 is a cross-sectional view showing how the transfer material S is transported from the fuser 14 to the discharge roller pair 19 during the printing of the second side of double-sided printing, and shows that the leading edge of the transfer material S in the transport direction has not yet reached the discharge roller pair 19. Figure 6 is a cross-sectional view showing how the transfer material S is transported from the fuser 14 to the discharge roller pair 19 during the printing of the second side of double-sided printing, and shows that the transfer material S is being held between the discharge roller pair 19. In addition, the double-sided flapper 15 is set so that the opening to the transport path 20 is closed, and the transported transfer material S is transported towards the transport path 18 side.

[0028] As shown in Figure 5, when the transfer material S is conveyed from the fixing nip section of the fixing device 14 toward the discharge roller pair 19, it is conveyed along the conveying surface of the conveying path 18 that is opposite to the conveying guide 18a. At this time, the printing surface side of the first side of the transfer material S (the side that has passed the pressure roller 14b of the fixing nip section) comes into contact with the driven roller 17, and the driven roller 17 rotates in conjunction with the conveying of the transfer material S. Since the outer surface of the driven roller 17 protrudes beyond the conveying guide 18b of the conveying path 18, the printing surface side of the first side of the transfer material S does not come into contact with the conveying guide 18b. Therefore, it is possible to prevent image defects caused by the printing surface of the first side of the transfer material S rubbing against the conveying guide 18b.

[0029] Subsequently, the leading edge of the transfer material S transported from the fixing device 14 moves toward the discharge roller pair 19 and is held between them. The transfer material S, held between the fixing nip of the fixing device 14 and the discharge roller pair 19, is pulled by the discharge roller pair 19 and transported without deflection between the discharge roller pair 19 and the driven roller 16, as shown in Figure 6. As shown in Figure 6, the second printing surface of the transfer material S is in contact with the driven roller 16, and the driven roller 16 rotates in conjunction with the transport of the transfer material S. The driven roller 16's outer surface protrudes beyond the transport guide 18a of the transport path 18 and is pulled by the discharge roller pair 19, so the second printing surface of the transfer material S does not come into contact with the transport guide 18a. Similarly, the first printing surface of the transfer material S also does not come into contact with the transport guide 18b. As a result, it is possible to prevent image defects caused by friction between the first and second printing surfaces of the transfer material S and the transport guides 18a and 18b of the transport path 18.

[0030] [Operation of the driven roller when double-sided printing of the transfer material is performed continuously] Next, the operation of the transfer material S and driven rollers 16 and 17 when double-sided printing is performed continuously will be explained. Figure 7 is a cross-sectional view showing how the transfer material S2, which has been printed on the first side, is transported to the transport path 20, reversed by the switchback roller pair 21 and transported to the double-sided transport path 23, and how the transfer material S1, which has been printed on the second side, is transported from the fixing nip section of the fixing device 14 to the discharge roller pair 19. In Figure 7, the transfer material S2 is transported to the transport path 20 first, and then the transfer material S1 is transported to the transport path 18. Therefore, in Figure 7, the double-sided flapper 15 is set to a state where the opening to the transport path 20 is closed, and the transported transfer material S1 is set to be transported to the transport path 18 side.

[0031] As shown in Figure 7, the first printing surface of the transfer material S2, which is reverse-conveyed by the switchback roller pair 21, is conveyed to the double-sided conveying path 23 while in contact with the driven roller 17. Meanwhile, the transfer material S1 being conveyed within the conveying path 18 is conveyed toward the discharge roller pair 19 while the conveying surface facing the conveying guide 18a of the conveying path 18 is in contact with the driven roller 17. At this time, both the transfer materials S1 and S2 are in contact with the driven roller 17. The transfer material S2 being conveyed toward the double-sided conveying path 23 is in contact with the driven roller 17, causing the driven roller 17 to rotate clockwise in the figure. On the other hand, the transfer material S1 being conveyed toward the discharge roller pair 19 is in contact with the driven roller 17, causing the driven roller 17 to rotate clockwise in the figure. In this way, the driven roller 17 is driven to rotate clockwise in the figure by the transfer materials S1 and S2, so the transfer materials S1 and S2 do not rub against the driven roller 17. Therefore, image defects due to friction with the driven roller 17 do not occur with the transfer materials S1 and S2.

[0032] As explained above, the transfer material S is transported in contact with the driven rollers 16 and 17, causing the driven rollers 16 and 17 to rotate, and the transfer material S does not rub against the transport guides, which are the transport surfaces of the transport paths 18 and 20. Therefore, it is possible to prevent image defects such as rib marks and gloss spots that occur in the toner image on the transfer material S due to contact with the transport guides.

[0033] As described above, this embodiment makes it possible to prevent image defects caused by friction between the transfer material and the transport ribs of the transport path. [Examples]

[0034] Example 2 describes a driven roller with a different configuration from Example 1. The image forming apparatus in this example is the same as in Example 1, and the same components and parts are referred to with the same reference numerals as in Example 1, so their explanation is omitted here.

[0035] [Configuration of the driven roller] Figure 8 is a cross-sectional view showing the configuration of the image forming apparatus 1 downstream of the transfer material S in the transport direction of the fixing device 14 in this embodiment. In Figure 8, due to the miniaturization of the image forming apparatus 1, the distance between the fixing device 14 and the discharge roller pair 19, and the distance between the fixing device 14 and the switchback roller pair 21 are shorter compared to Figure 2 of Embodiment 1, and the distance of the transport paths 18 and 20 is also shorter. In addition, the arrangement position and configuration of the driven roller 16 are the same as in Embodiment 1, but the configuration of the driven roller 17 is different from that of Embodiment 1. That is, in Embodiment 1, the driven roller 17 and the double-sided flapper 15 were arranged adjacent to each other. In Embodiment 2, the configuration is different from Embodiment 1 in that the rotation center of the double-sided flapper 15 and the rotation center of the driven roller 17 are configured to be coaxial. Also, the outer circumferential surface of the driven roller 17 protrudes from the surface of the double-sided flapper 15 facing the transport path side.

[0036] [Operation of driven rollers during double-sided printing] (Operation during the first printing of the transfer material) Next, the operation of the transfer material S and driven rollers 16 and 17 during double-sided printing will be explained. First, the operation of the transfer material S and driven rollers 17 during the printing of the first side will be explained. Figure 9 is a cross-sectional view showing how the transfer material S is transported from the fuser 14 to the switchback roller pair 21 during the printing of the first side of double-sided printing. During the printing of the first side of double-sided printing, the transfer material S is transported from the fuser 14 to the switchback roller pair 21 via the transport path 20, and then reversed and transported by the switchback roller pair 21 to the double-sided transport path 23. Also, during the printing of the first side of double-sided printing, the double-sided flapper 15 is set to a state where the opening to the transport path 18 is closed, and the transported transfer material S is transported to the transport path 20 side.

[0037] The leading edge of the transfer material S transported from the fixing device 14 moves towards the switchback roller pair 21 while in contact with the transport guide 20b, and is then held between the switchback roller pair 21. In this embodiment as well, to prevent deflection of the transfer material S, the transport speed of the transfer material S on the switchback roller pair 21 is set faster than the transport speed of the transfer material S on the fixing nip of the fixing device 14. As a result, the transfer material S held between the fixing nip of the fixing device 14 and the switchback roller pair 21 is pulled by the switchback roller pair 21, and as shown in Figure 9, it is transported without deflection between the switchback roller pair 21 and the driven roller 17.

[0038] As shown in Figure 9, the printing surface side of the transfer material S is in contact with the driven roller 17, and the driven roller 17 rotates in conjunction with the transport of the transfer material S. The driven roller 17 has an outer surface that protrudes beyond the transport guide 20a of the transport path 20 and is pulled by the switchback roller pair 21, so the printing surface side of the transfer material S does not come into contact with the transport guide 20a. Therefore, it is possible to prevent image defects caused by friction between the printing surface of the transfer material S and the transport guide 20a.

[0039] (Operation during second-side printing of transfer material) Next, the operation of the transfer material S and driven rollers 16 and 17 during the printing of the second side of double-sided printing will be explained. Figure 10 is a cross-sectional view showing how the transfer material S is transported from the fuser 14 to the discharge roller pair 19 during the printing of the second side of double-sided printing, and shows that the leading edge of the transfer material S in the transport direction has not yet reached the discharge roller pair 19. Figure 11 is a cross-sectional view showing how the transfer material S is transported from the fuser 14 to the discharge roller pair 19 during the printing of the second side of double-sided printing, and shows that the transfer material S is being held between the discharge roller pair 19. In addition, the double-sided flapper 15 is set so that the opening to the transport path 20 is closed, and the transported transfer material S is transported towards the transport path 18 side.

[0040] As shown in Figure 10, when the transfer material S is transported from the fixing nip section of the fixing device 14 toward the discharge roller pair 19, it is transported along the transport guide 18b opposite the transport guide 18a of the transport path 18. At this time, the first printed surface side of the transfer material S (the side that has passed the pressure roller 14b of the fixing nip section) comes into contact with the driven roller 17, and the driven roller 17 rotates in conjunction with the transport of the transfer material S. Since the outer surface of the driven roller 17 protrudes beyond the transport guide 18b of the transport path 18, the first printed surface side of the transfer material S does not come into contact with the transport guide 18b. Therefore, it is possible to prevent image defects caused by the first printed surface of the transfer material S rubbing against the transport guide 18b.

[0041] Subsequently, the leading edge of the transfer material S transported from the fixing device 14 moves toward the discharge roller pair 19 and is held between them. The transfer material S, held between the fixing nip of the fixing device 14 and the discharge roller pair 19, is pulled by the discharge roller pair 19 and transported without deflection between the discharge roller pair 19 and the driven roller 16, as shown in Figure 11. As shown in Figure 11, the second printing surface of the transfer material S is in contact with the driven roller 16, and the driven roller 16 rotates in conjunction with the transport of the transfer material S. The driven roller 16's outer surface protrudes beyond the transport guide 18a of the transport path 18 and is pulled by the discharge roller pair 19, so the second printing surface of the transfer material S does not come into contact with the transport guide 18a. Similarly, the first printing surface of the transfer material S does not come into contact with the transport guide 18b. As a result, it is possible to prevent image defects caused by friction between the first and second printing surfaces of the transfer material S and the transport guides 18a and 18b of the transport path 18.

[0042] [Operation of the driven roller when double-sided printing of the transfer material is performed continuously] Next, the operation of the transfer material S and driven rollers 16 and 17 when double-sided printing is performed continuously will be explained. Figure 12 is a cross-sectional view showing how the transfer material S2, which has been printed on the first side, is transported to the transport path 20, reversed by the switchback roller pair 21 and transported to the double-sided transport path 23, and how the transfer material S1, which has been printed on the second side, is transported from the fixing nip section of the fixing device 14 to the discharge roller pair 19. In Figure 12, the transfer material S2 is transported to the transport path 20 first, and then the transfer material S1 is transported to the transport path 18. Therefore, in Figure 12, the double-sided flapper 15 is set to a state where the opening to the transport path 20 is closed, and the transported transfer material S1 is set to be transported to the transport path 18 side.

[0043] As shown in Figure 12, the first printing surface of the transfer material S2, which is reverse-conveyed by the switchback roller pair 21, is conveyed to the double-sided conveying path 23 while in contact with the driven roller 17. Meanwhile, the transfer material S1 being conveyed in the conveying path 18 is conveyed toward the discharge roller pair 19 while the conveying surface facing the conveying guide 18a of the conveying path 18 is in contact with the driven roller 17. At this time, both the transfer materials S1 and S2 are in contact with the driven roller 17. The transfer material S2 being conveyed toward the double-sided conveying path 23 is in contact with the driven roller 17, causing the driven roller 17 to rotate clockwise in the figure. On the other hand, the transfer material S1 being conveyed toward the discharge roller pair 19 is in contact with the driven roller 17, causing the driven roller 17 to rotate clockwise in the figure. In this way, the driven roller 17 is driven to rotate clockwise in the figure by the transfer materials S1 and S2, so the transfer materials S1 and S2 do not rub against the driven roller 17.

[0044] As described above, in this embodiment, the rotation center of the driven roller 17 is arranged coaxially with the rotation center of the double-sided flapper 15. Therefore, even in an image forming apparatus body where the distance over which the transfer material S is transported from the fixing device 14 to the switchback roller pair 21 is short, friction between the transfer material S and the transport guide of the transport path can be prevented. Furthermore, because the rotation center of the driven roller 17 is arranged coaxially with the rotation center of the double-sided flapper 15, the position in which the driven roller 17 contacts the transfer material S is stable regardless of the setting state of the double-sided flapper 15. Even as the image forming apparatus 1 is miniaturized and the space between the fixing device 14 and the discharge roller pair 19, and between the fixing device and the switchback roller pair 21 becomes narrower, friction between the transfer material and the transport ribs can be prevented, thus preventing image defects such as image streaks and gloss spots.

[0045] In the above-described embodiment, a full-color laser beam printer equipped with multiple photosensitive drums was used as an example of an image forming apparatus. However, the present invention can also be applied to monochrome copiers and printers equipped with a single photosensitive drum.

[0046] As described above, this embodiment makes it possible to prevent image defects caused by friction between the transfer material and the transport ribs of the transport path. [Explanation of symbols]

[0047] 6 Image forming unit 14 Fixing device 15 Double-sided flappa 17 Driven roller 18 Conveyor path 19 Discharge roller pair 20 Conveyor paths 21 Switchback Roller vs

Claims

1. An image forming unit that forms a toner image on the transfer material, A fixing unit having a heating member and a pressurizing member, which fixes the toner image formed on the transfer material in the image forming unit to the transfer material by sandwiching the transfer material in the nip portion formed by the heating member and the pressurizing member, A discharge unit for discharging the transfer material on which the toner image has been fixed in the fixing unit, A reversing unit reverses the transport direction of the transfer material in order to transport the transfer material, which has had the toner image fixed in the fixing unit, back to the double-sided transport path that leads to the image forming unit. A rotatably supported switching unit that contacts the leading edge of the transfer material that has passed through the fixing unit and switches the transport direction of the transfer material to the discharge unit or the reversing unit, A first transport path is provided between the switching section and the discharge section, and is formed by a first transport surface facing the side of the transfer material that contacts the heating member, and a second transport surface facing the side of the transfer material that contacts the pressurizing member. A second transport path is provided between the switching section and the reversing section, and is formed by a third transport surface facing the side of the transfer material that contacts the heating member, and a fourth transport surface facing the side of the transfer material that contacts the pressurizing member. A first rotating member is rotatably supported and positioned adjacent to the upstream ends of the second and third conveying surfaces in the conveying direction of the transfer material, having an outer peripheral surface that protrudes further toward the first conveying path than the second conveying surface and further toward the second conveying path than the third conveying surface, An image forming apparatus characterized by comprising:

2. The image forming apparatus according to claim 1, characterized in that the first rotating member is arranged adjacent to the switching portion on the downstream side in the transport direction of the transfer material of the switching portion.

3. The image forming apparatus according to claim 1, characterized in that the rotation center of the first rotating member is coaxial with the rotation center of the switching unit.

4. The image forming apparatus according to claim 2 or 3, characterized in that it comprises a second rotating member which is arranged downstream of the transfer material transport direction of the fixing section, adjacent to the upstream end of the first transport surface in the transfer material transport direction, has an outer peripheral surface that protrudes toward the first transport path side from the first transport surface, and is rotatably supported.

5. The image forming apparatus according to claim 4, characterized in that the side of the transfer material that is conveyed from the fixing section to the first transport path and is in contact with the pressurizing member is in contact with the first rotating member until the transfer material reaches the discharge section, and the first rotating member rotates in response to the contact of the transfer material.

6. The speed at which the discharge unit transports the transfer material to discharge it is set to be faster than the speed at which the nip unit transports the transfer material. The image forming apparatus according to claim 5, characterized in that the transfer material that reaches the discharge section is held in the discharge section and conveyed while in contact with the second rotating member, the second rotating member rotates as a result of contact with the transfer material, and the side of the conveyed transfer material that is in contact with the heating member does not come into contact with the first conveying surface.

7. The image forming apparatus according to claim 6, characterized in that the side of the transfer material that is transported from the fixing unit to the second transport path and is in contact with the heating member is in contact with the first rotating member until the transfer material reaches the reversing unit, and the first rotating member rotates in response to the contact of the transfer material.

8. The speed at which the reversing unit transports the transfer material until the timing at which it reverses the transport direction of the transfer material is set to be faster than the speed at which the nip unit transports the transfer material. The image forming apparatus according to claim 7, characterized in that the transfer material that reaches the reversal section is held in the reversal section and transported while in contact with the first rotating member until the timing, the first rotating member rotates as a result of contact with the transfer material, and the side of the transported transfer material that is in contact with the heating member does not come into contact with the third transport surface.

Images