Image forming device

Abstract

To provide a product with high image quality with high productivity regardless of a thickness of a sheet.SOLUTION: An image forming apparatus has: a transfer unit that forms a transfer nip part; a fixing unit that forms a fixing nip part; first belt adsorption and conveyance means that conveys a sheet while adsorbing the sheet on a first belt; and second belt adsorption and conveyance means that conveys the sheet while adsorbing the sheet on a second belt. The fixing nip part is arranged at a position above the transfer nip part. In a sheet conveyance direction, an upstream end of the first belt is provided below the transfer nip part so that the sheet conveyed from the transfer unit can be received and a downstream end of the first belt is provided above an upstream end of the second belt, and a downstream end of the second belt is provided below the fixing nip part. The first belt and the second belt are inclined from a lower part to an upper part from the upstream side toward the downstream side in the sheet conveyance direction.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus that forms an image on a sheet. [Background technology]

[0002] Conventionally, as disclosed in Patent Document 1, an electrophotographic image forming apparatus includes a transfer unit that transfers an image onto a sheet, a fixing unit that fixes the image transferred onto the sheet, and a belt adsorption conveying means that adsorbs the sheet onto a belt using a fan and conveys it.

[0003] Regarding the relative height positions of the transfer nip portion, which sandwiches and transports the sheet in the transfer portion, the fixing nip portion, which sandwiches and transports the sheet in the fixing portion, and the belt surface, which adsorbs the sheet with the belt adsorption conveying means, the transfer nip portion and the fixing nip portion are located at approximately the same height, and the belt adsorption surface is located below the transfer nip portion and the fixing nip portion. Furthermore, the upstream and downstream ends of the belt adsorption surface in the sheet transport direction are located at approximately the same height. This allows the sheet to be transported horizontally from the transfer portion to the fixing portion. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-83416 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in recent years, there has been a demand for image forming apparatuses that can achieve both high image quality and high productivity. In particular, there is a demand for sheet conveyance with high productivity regardless of the type of sheet (thick or thin). Generally, the thicker the sheet being conveyed, the more heat is required in the fixing section to fix the image on the sheet, and therefore the sheet conveyance speed must be reduced.

[0006] To fix an image on a sheet at a constant high speed regardless of the type of sheet, a stable supply of an appropriate amount of heat is required, which increases the size of the fixing unit. For example, not only does the fixing unit's image side (upper side) become larger due to the fixing web, but a cooling device is also required to cool the fixing roller on the non-image side (lower side) of the fixing unit to prevent it from overheating, which increases the size of the non-image side (lower side). Without a cooling device, there is a risk of poor image quality when using thin coated paper, or poor image quality due to tension or slack in the sheet at the fixing unit caused by fluctuations in rotation speed due to thermal expansion of the fixing roller.

[0007] In order to arrange the devices of an image forming apparatus, including a large fixing unit, while preventing the image forming apparatus from becoming larger, it is conceivable to arrange the fixing nip of the sheet in the fixing unit above the transfer nip. However, the prior art does not take into consideration stable sheet transport by a belt suction transport means from the transfer unit to the fixing unit when the fixing nip is arranged above the transfer nip.

[0008] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an image forming apparatus that can produce high-quality finished products with high productivity and can stably transport a sheet onto which a toner image has been transferred to a fixing unit. [Means for solving the problem]

[0009] In order to solve the above-mentioned problems, one aspect of the present invention is a sheet conveying device including: a transfer unit that forms a transfer nip that sandwiches and conveys a sheet and transfers an image onto the sheet; a fixing unit that is provided downstream of the transfer unit in a sheet conveying direction and forms a fixing nip that sandwiches and conveys a sheet and fixes the image transferred by the transfer unit onto the sheet; a first belt suction and conveying means that is provided between the transfer unit and the fixing unit in the sheet conveying direction and conveys the sheet while adsorbing it to a first belt; and a second belt suction and conveying means that is provided between the first belt suction and conveying means and the fixing unit in the sheet conveying direction and conveys the sheet while adsorbing it to a second belt, In the sheet conveying direction, an upstream end of the first belt of the first belt attracting and conveying means is provided below the transfer nip portion so as to receive the sheet conveyed from the transfer portion, and a downstream end of the first belt of the first belt attracting and conveying means is provided above an upstream end of the second belt of the second belt attracting and conveying means, and a downstream end of the second belt of the second belt attracting and conveying means is provided below the fixing nip portion, and the first belt of the first belt attracting and conveying means and the second belt of the second belt attracting and conveying means are inclined from the bottom up as they move from the upstream to the downstream in the sheet conveying direction. a nip line at the transfer nip portion extends in a direction inclined downward toward downstream in the sheet conveying direction and intersects with the first belt of the first belt attracting and conveying means, and a tangent line tangent to a belt surface of the second belt of the second belt attracting and conveying means intersects with the nip line at the fixing nip portion downstream of the fixing nip portion; It is characterized by: [Effects of the Invention]

[0010] According to the present invention, it is possible to provide an image forming apparatus that can produce high-quality finished products with high productivity and can stably transport a sheet onto which a toner image has been transferred to a fixing unit. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a schematic diagram illustrating the configuration of an image forming apparatus according to a first embodiment. [Figure 2] FIG. 3 is a cross-sectional view showing a secondary transfer unit, a belt attracting and conveying unit, and a fixing unit in the first embodiment. [Figure 3]FIG. 3 is a cross-sectional view showing the positional relationship between the secondary transfer unit, the belt attracting and conveying unit, and the fixing unit in the first embodiment. [Figure 4] FIG. 3 is a perspective view of a belt suction conveying means in the first embodiment. [Figure 5] FIG. 2 is a control block diagram of the image forming apparatus according to the first embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, exemplary embodiments of the present invention will be described with reference to the drawings. [Example]

[0013] <Image forming device> FIG. 1 is a schematic diagram of an image forming apparatus 100 according to a first embodiment. First, the configuration of the image forming apparatus 100 will be described with reference to FIG. 1. The image forming apparatus 100 can be divided into a sheet conveying unit that mainly conveys sheets and an image forming unit that forms an image on the conveyed sheet. The sheet conveying unit mainly includes a feeding unit 110 that feeds sheets, a registration unit that controls the start of sheet conveyance, and a post-conveying unit 903 that conveys sheets on which a toner image has been fixed. Meanwhile, the image forming unit mainly includes an image forming unit 920 that forms a toner image, a belt attracting and conveying unit 904 that attracts the sheet on which the toner image has been transferred onto a belt and conveys it, and a fixing unit 50 that fixes the transferred toner image onto the sheet.

[0014] First, we will explain the image forming means that forms an image on a sheet. Image forming unit 920, which is part of the image forming means, is a so-called tandem type in which electrophotographic image forming stations 200Y, 200M, 200C, and 200K that form toner images of Y (yellow), M (magenta), C (cyan), and K (black) are arranged in series.

[0015] The image forming stations 200Y, 200M, 200C, and 200K have the same configuration except for the toner color. Therefore, the configuration of image forming station 200Y will be described here as an example, and descriptions of the configurations of image forming stations 200M, 200C, and 200K will be omitted. Note that in FIG. 1, the components of image forming station 200Y are designated with a "Y" at the end of their reference numerals, the components of image forming station 200M with an "M" at the end of their reference numerals, the components of image forming station 200C with a "C" at the end of their reference numerals, and the components of image forming station 200K with a "K" at the end of their reference numerals. Image forming station 200Y has a photosensitive drum 120Y, a primary charging device 121Y, an exposure device 122Y, and a developing device 123Y.

[0016] Furthermore, the image forming unit 920 includes an intermediate transfer belt 125 as an image carrier that carries the toner images visualized by the image forming stations 200Y, 200M, 200C, and 200K. The intermediate transfer belt 125 is supported in a state in which it is stretched across a drive roller 126, a tension roller 127, and an inner transfer roller 128, and is rotated in the direction of arrow R2 in FIG. 1 by the drive of the drive roller 126.

[0017] The secondary transfer roller 131 is pressed against the intermediate transfer belt 125, which is supported from the inside of the intermediate transfer belt 125 by the inner transfer roller 128, to form a transfer nip N2 between the secondary transfer roller 131 and the intermediate transfer belt 125. The secondary transfer roller 131, the intermediate transfer belt 125, and the inner transfer roller 128 constitute a secondary transfer unit 130, which serves as the transfer unit of this embodiment. The cleaning device 128' rubs a cleaning web against the intermediate transfer belt 125 to remove residual toner, paper dust, and the like remaining on the surface of the intermediate transfer belt 125 after it has passed through the transfer nip N2.

[0018] Having explained the configuration of the image forming unit 920 above, we will now explain the sequence of steps in the image formation process for forming an image on a sheet. Based on an image formation job input to the image forming apparatus 100, first, the exposure device 122Y exposes the photosensitive drum 120Y to light to form an electrostatic latent image on the surface of the photosensitive drum 120Y. The electrostatic latent image on the photosensitive drum 120Y is developed by the development device 123Y and visualized as a toner image. The toner image carried on the surface of the photosensitive drum 120Y is primarily transferred onto the intermediate transfer belt 125 by the primary transfer device 124Y. The toner images (multiple) carried on the surfaces of the photosensitive drums 120Y, 120M, 120C, and 120K are transferred onto the intermediate transfer belt 125 so as to overlap each other, forming a full-color toner image (added to restore the toner image to a single image). The toner image (primary transfer) transferred onto the intermediate transfer belt 125 is then (secondary transfer) transferred to the sheet S fed from the feed unit 110 at the transfer nip N2 in this embodiment. The intermediate transfer belt 125 is driven to rotate by a drive roller 126 that rotates at a constant speed, and its circumferential velocity (rotational speed) is maintained at a constant transfer speed. Therefore, the sheet transport speed at the transfer nip N2 is the circumferential velocity of the intermediate transfer belt 125. Hereinafter, the sheet transport speed at the secondary transfer unit 130 will be referred to as the "transfer speed VT." The transfer speed VT is the sheet transport speed when the toner image is transferred at the secondary transfer unit 130. In this manner, the toner image formed by the image forming unit 920 is transferred to the sheet.

[0019] The sheet onto which the toner image has been transferred is then transported toward the fixing unit to fix the toner image thereon. In this embodiment, a belt adsorption / conveyance unit 904 is disposed between the secondary transfer unit 130 and the fixing unit 50 in the sheet transport direction FD. The belt adsorption / conveyance unit 904 is composed of a first belt adsorption / conveyance unit 10 disposed upstream in the sheet transport direction FD and a second belt adsorption / conveyance unit 20 disposed downstream. A fixing unit 50 is also disposed downstream of the secondary transfer unit 130 in the sheet transport direction FD. The fixing unit 50 is a fixing unit that fixes the toner image to the sheet by heat and pressure. The fixing unit 50 includes a heating roller 52 with an internal heater and a pressure roller 53 that is disposed in contact with the heating roller 52 and forms a fixing nip N together with the heating roller 52. The toner image on the transferred sheet is fixed to the sheet by melting it in the fixing unit. The detailed configurations of the belt adsorption / conveyance unit 904 and the fixing unit 50 will be described later.

[0020] Next, the sheet conveying means will be described. The feeding section 110 includes a sheet cassette 111 that stores sheets, a pickup roller 112 that picks up sheets from the sheet cassette 111, and a separation device 113 that separates and feeds the sheets picked up by the pickup roller 112. The feeding section 110 also includes a delivery roller 114 that conveys the sheets in a feeding conveying path 901 along which the sheets separated and fed by the separation device 113 are conveyed, and a registration roller 115.

[0021] The registration rollers 115 are kept waiting with the leading edge of the sheet S abutting against them while the rollers 115 are stopped from rotating, and convey the sheet S toward the transfer nip N2 in synchronization with the toner image formed on the intermediate transfer belt 125. The sheet S carrying the toner image transferred at the transfer nip N2 is conveyed from the transfer nip N2 to the fixing unit 50 by a belt attraction conveying means 904. In the fixing unit 50, the sheet S is sandwiched at the fixing nip N and heat and pressure are applied to the unfixed toner image to fix the toner image to the sheet. The sheet S sent out from the fixing unit 50 is discharged outside the image forming apparatus 100 by discharge rollers 911, which are part of the post-conveying unit 903. The post-conveying unit 903 also includes a reversing roller 912 that reverses and conveys the sheet, and a double-sided conveying path 913 that conveys the sheet reversed by the reversing roller 912 and merges with the feeding conveying path 901. When forming images on both sides of a sheet, the sheet sent out from the fixing unit 50 is conveyed to the reversing rollers 912, and after being reversed by the reversing rollers 912, is conveyed toward the double-sided conveying path 913. Then, the sheet is conveyed again to the feeding conveying path 901 via the double-sided conveying path 913, and a toner image is formed on the second side (back side) of the sheet in the same manner as on the first side (front side).

[0022] As described above, the image forming apparatus in this embodiment is capable of executing the image forming process by the image forming means and the sheet conveying process by the sheet conveying means in a mutually interlocking manner, thereby producing a sheet on which an image has been formed as a deliverable.

[0023] <Detailed configuration from the transfer unit to the fixing unit> 2 and 3 are cross-sectional views showing the secondary transfer unit 130, belt attracting and conveying means 904, and fixing unit 50. Fig. 4 is a perspective view of belt attracting and conveying means 904. Fig. 5 is a control block diagram of the image forming apparatus.

[0024] 2, the belt attraction / conveyance means 904 serving as the belt attraction / conveyance means of this embodiment includes a first belt attraction / conveyance means 10 and a second belt attraction / conveyance means 20. The first belt attraction / conveyance means 10 is disposed downstream of the transfer nip portion N2 in the sheet conveyance direction FD, and the second belt attraction / conveyance means 20 is disposed downstream of the first belt attraction / conveyance means 10 and upstream of the fixing nip portion N. In the sheet conveyance direction FD, a post-transfer guide 951 is provided between the first belt attraction / conveyance means 10 and the transfer nip portion N2 to guide the sheet conveyed from the transfer nip portion N2 toward the first belt attraction / conveyance means 10. In addition, a pre-fixing guide 952 is provided between the second belt attraction / conveyance means 20 and the fixing nip portion N in the sheet conveyance direction FD to guide the sheet conveyed by the second belt attraction / conveyance means 20 to the fixing nip portion N.

[0025] The first belt suction conveying means 10 includes a first conveyor belt 11 serving as the first belt in this embodiment, a first drive roller 12 that rotatably stretches the first conveyor belt 11, and driven rollers 12a, 12b, and 12c. The first tensioning member in this embodiment is the first drive roller 12 and the driven rollers 12a, 12b, and 12c. The first belt suction conveying means 10 also includes a first drive motor 14 that rotates the first drive roller 12 to rotate the first conveyor belt 11. The first conveyor belt 11 is an endless belt formed with a number of holes 13, and is a breathable member that allows air to pass between the inside and outside of the circumferential surface of the first conveyor belt 11 through the holes 13. A first suction fan 15 is disposed on the inside of the circumferential surface of the first conveyor belt 11 to attract sheets to the circumferential surface of the first conveyor belt 11. The first suction fan 15 is capable of sucking air from the outside to the inside of the circumferential surface of the first conveyor belt 11 through a large number of holes 13 formed in the first conveyor belt 11, thereby applying suction force for conveying sheets to the circumferential surface of the first conveyor belt 11. The first suction fan 15 serves as the first air suction unit in this embodiment.

[0026] The second belt suction conveying means 20 includes a second conveyor belt 21 serving as the second belt in this embodiment, a second drive roller 22 that rotatably stretches the second conveyor belt 21, and driven rollers 22a, 22b, and 22c. The second tensioning members in this embodiment are the second drive roller 22 and the driven rollers 22a, 22b, and 22c. The second belt suction conveying means 20 also includes a second drive motor 24 that rotates the second drive roller 22 to rotate the second conveyor belt 21. The second conveyor belt 21 is an endless belt formed with a number of holes 23, and is a breathable member that allows air to pass between the inside and outside of the circumferential surface of the second conveyor belt 21 through the holes. A second suction fan 25 is disposed on the inside of the circumferential surface of the second conveyor belt 21 to attract sheets to the circumferential surface of the second conveyor belt 21. The center position of the second suction fan 25 may be located downstream of the center of the second conveyor belt 21 in the sheet conveying direction FD. In this way, the sheet can be conveyed to the fixing nip N while being brought close to the second belt surface 21a. The second suction fan 25 can apply suction force to the circumferential surface of the second conveyor belt 21 by sucking air from the outside to the inside of the circumferential surface of the second conveyor belt 21 through a large number of holes formed in the second conveyor belt 21, thereby conveying the sheet to the circumferential surface of the second conveyor belt 21. The second suction fan 25 serves as the second air suction unit in this embodiment.

[0027] Furthermore, a sheet detection sensor 116 that detects a sheet is provided between the registration roller 115 and the transfer nip portion N2 in the sheet conveyance direction FD. The sheet detection sensor 116 detects the presence or absence of a sheet at a detection position P1 between the registration roller 115 and the transfer nip portion N2 in the sheet conveyance direction FD. A signal output by the sheet detection sensor 116 is sent to the control unit 170 (see FIG. 5).

[0028] The fixing unit 50 also includes a heating roller temperature sensor 70 that detects the surface temperature of the heating roller 52 and a pressure roller temperature sensor 71 that detects the surface temperature of the pressure roller 53. The heating roller temperature sensor 70 and the pressure roller temperature sensor 71 are provided to maintain the surface temperatures of the heating roller 52 and the pressure roller 53 at appropriate temperatures, respectively. The airflow rate of the cooling fan 561 is adjusted according to the reading of the pressure roller temperature sensor 71, thereby controlling the surface temperature of the pressure roller 53 to be constant. This achieves high image quality and high productivity.

[0029] Below the fixing unit, there is a cooling unit 56 that cools the pressure roller 53. The cooling unit 56 has a cooling fan 561 and a cooling duct 562 that uniformly blows air from the cooling fan onto the pressure roller 53 in the longitudinal direction. To maintain a high-quality, highly productive image forming apparatus, the cooling fan 561 needs to have a strong airflow, and a larger fan is used than the first suction fan 15 and second suction fan 25 used in the belt suction transport unit. Furthermore, to avoid image defects in the main scanning direction, which is the width direction of the sheet, it is necessary to maintain a uniform temperature distribution in the longitudinal direction of the pressure roller 53. For this reason, the cooling duct 562 is also large.

[0030] <Arrangement of the transfer unit, belt attraction / conveyance means, and fixing unit> Next, the positional relationship between the transfer nip portion N2, the belt attracting and conveying means 904, and the fixing nip portion N will be described with reference to FIG.

[0031] First, the fixing nip N is located above the secondary transfer nip N2 (L2 in FIG. 3). This is because a large area is required below the fixing unit 50 and because space is required to clear paper jams in the duplex conveying path 913. As described above, a cooling unit 56 for cooling the pressure roller 53 constituting the fixing nip N must be located below the fixing unit 50. Furthermore, as shown in FIG. 1, the duplex conveying path 913 is located below the transfer unit, the belt suction conveying unit 904, and the fixing unit. Therefore, locating the fixing unit 50 and the cooling unit 56 above improves the ease with which a user can clear a jammed sheet in the duplex conveying path 913. In other words, workability is improved by increasing the space for clearing jammed sheets. On the other hand, to avoid a decrease in sheet processing efficiency by locating the duplex conveying path 913 below, it is possible to consider a method of moving only the portion of the duplex conveying path 913 directly below the fixing unit downward and moving the upstream and downstream portions upward. However, the radius of curvature of the bent portion of the sheet transport path is small, meaning that the curvature of the transport path is too large for sheet transport. In this case, it may be impossible to transport high-rigidity sheets such as thick sheets, or even if transport is possible, there is a risk of image defects. Therefore, it is possible to move both the fuser and the secondary transfer unit upward, but as shown in Figure 1, the primary transfer unit and toner cartridge are located above the secondary transfer unit. This would also raise the position of the toner cartridge, making it difficult for users to replace it. For this reason, as in this embodiment, the fixing nip N is located above the transfer nip N2. The cooling unit 56 is located below the fixing unit 50 and above the double-sided transport path 913.

[0032] 3, the nip line T2 at the secondary transfer nip N2 is formed to face slightly downward (in the direction D1 in FIG. 3). This is to prevent separation failure of thin sheets and the like onto the intermediate transfer belt 125. If the secondary transfer nip N2 is horizontal or upward, that is, if the contact angle with the intermediate transfer belt 125 is small, the leading edge of the thin sheet will stick to the intermediate transfer belt 125 and will not separate, resulting in it being discharged upward, resulting in separation failure. In particular, in devices that are compatible with thin sheets, the nip line at the secondary transfer nip N2 generally faces downward, in a direction away from the intermediate transfer belt 125.

[0033] In order to receive the sheet discharged downward from the transfer nip N2, the upstream end of the first belt surface 11a of the first belt attracting and conveying means 10 is positioned below the secondary transfer nip N2. Furthermore, in order to convey the sheet from the secondary transfer nip N2 to the fixing nip N above, the downstream end of the first belt surface 11a of the first belt attracting and conveying means 10 is positioned above the upstream end of the first belt surface. That is, the sheet conveyance angle of the first belt of the first belt attracting and conveying means 10 is upward (direction D2 in FIG. 3). In other words, the conveyance surface (upper surface of the first belt surface 11a) on which the first belt attracting and conveying means 10 attracts and conveys the sheet is inclined with respect to the horizontal. More specifically, the conveyance surface (upper surface of the first belt surface 11a) on which the first belt attracting and conveying means 10 attracts and conveys the sheet is inclined upward in the direction from the secondary transfer nip N2 toward the fixing nip N.

[0034] Similarly, the upstream end of the second belt surface 21a of the second belt attracting / conveying means 20 is positioned lower than the downstream end of the first belt surface 11a of the first belt attracting / conveying means 10 to receive the sheet from the first belt attracting / conveying means 10, and has a step L3. Then, to deliver the sheet to the upper fixing nip N, the downstream end of the second belt surface 21a of the second belt attracting / conveying means 20 is positioned higher than the upstream end of the second belt surface 21a. That is, the sheet conveyance angle of the second belt of the second belt attracting / conveying means 20 is also upward (direction D3 in FIG. 3), similar to that of the first belt attracting / conveying means 10. That is, the conveyance surface (upper surface of the second belt surface 21a) along which the second belt attracting / conveying means 20 attracts and conveys the sheet is inclined with respect to the horizontal. In detail, the conveying surface (the upper surface of the second belt surface 21a) on which the second belt adsorbing and conveying means 20 adsorbs and conveys the sheet is inclined upward in the direction from the secondary transfer nip N2 toward the fixing nip N.

[0035] An imaginary line S2 in the conveying direction on the second belt surface 21a of the second belt attracting / conveying means 20 intersects with the pre-fixing guide 952. As a result, the leading edge of the sheet conveyed from the second belt attracting / conveying means 20 to the fixing unit 50 always abuts against the pre-fixing guide 952, and the sheet conforms to the pre-fixing guide, so that the sheet is conveyed to the fixing nip N. In other words, it is possible to avoid image defects that occur when unfixed toner on the sheet comes into contact with the heating roller 52. Furthermore, the imaginary line S2 intersects with the fixing nip line T downstream of the fixing nip N in the sheet conveying direction. In this embodiment, the fixing nip line T is set to be approximately horizontal.

[0036] <Loop control between the transfer unit and the fixing unit> 2, the first belt suction conveying means 10 has a loop detection means 16. The loop detection means 16 has a loop detection flag 161 that swings depending on the height of the sheet S being conveyed. The loop detection flag 161 protrudes from the first belt surface 11a of the first conveying belt 11 and comes into contact with the sheet S to detect the height of the sheet S, that is, the amount of loop formed in the sheet between the secondary transfer nip N2 and the fixing nip N.

[0037] The loop detection sensor 162, such as a photointerrupter, switches between a light-blocking state and a light-transmitting state depending on the swing angle of the loop detection flag 161 and outputs an ON or OFF signal. When the loop detection flag 161 is at or above a predetermined height, i.e., when the loop amount is less than a predetermined amount, the speed of the fixing drive motor 54 is slowed down to increase the loop. Conversely, when the loop detection flag 161 is at or below the predetermined height, i.e., when the loop amount is greater than or equal to the predetermined amount, the speed of the fixing drive motor 54 is increased to increase the loop. The heating roller 52 and pressure roller 53 are driven to rotate by the fixing drive motor 54, such as a DC brushless motor. The sheet conveyance speed at the fixing nip N can be changed by changing the speed of the fixing drive motor 54. By performing this loop control, the sheet posture between the secondary transfer nip N2 and the fixing nip N can be maintained constant.

[0038] In this embodiment, the distance L1 between the transfer nip N2 and the fixing nip N is set to approximately 500 mm, greater than or equal to 19 inches (483 mm). When the length of a sheet in the conveying direction FD is 19 inches or less, the sheet is conveyed without being sandwiched between the transfer nip N2 and the fixing nip N. In other words, for a sheet length of 19 inches, the secondary transfer nip N2 and the fixing nip N do not nip simultaneously. This prevents image defects caused by the fixing nip N pulling the sheet against the transfer nip N2, thereby providing a high-quality, high-precision output. On the other hand, for sheets longer than 19 inches, commonly referred to as long paper, the secondary transfer nip N2 and the fixing nip N nip simultaneously nip the sheet. By conveying the sheet while performing the above-described loop control, image defects can be prevented, providing a high-quality, high-precision output.

[0039] As described above, in this embodiment, the fixing nip N is disposed above the transfer nip N2. The nip line T2 of the secondary transfer nip N2 faces downward. The upstream end of the first belt of the first belt attracting and conveying means 10, which receives the sheet S discharged from the transfer unit, is disposed below the transfer nip N. The upstream end of the second belt of the second belt attracting and conveying means 20 is disposed below the downstream end of the first belt of the first belt attracting and conveying means 10. The downstream end of the second belt of the second belt attracting and conveying means 20 is disposed below the fixing nip N. The first belt of the first belt attracting and conveying means 10 and the second belt of the second belt attracting and conveying means 20 are disposed upward from upstream to downstream in the sheet conveying direction.

[0040] The above configuration achieves the following functions and performance simultaneously. First, by positioning the nip line T2 of the secondary transfer nip N2 facing downward, separation performance of thin paper is ensured at the secondary transfer unit. Second, by positioning the first belt attraction / conveyance means below the secondary transfer nip and positioning the upstream end of the second belt below the downstream end of the first belt, stable sheet transport by the belt is achieved. Third, when a sheet is transported from the second belt attraction / conveyance means to the fixing unit, the leading edge of the sheet abuts against the pre-fixing guide 952, preventing image defects. Fourth, by positioning the fixing nip above the transfer nip N2, space below the fixing unit is secured, allowing stable cooling of the pressure roller 53 and providing high-quality finished products with high productivity. Fifth, by positioning the fixing nip above, space for handling sheet jams in the duplex conveyance path is secured. Sixth, a gently convex transport path can be formed from the secondary transfer unit to the fixing unit, enabling loop control. This means that high-quality output can be achieved even with long sheets of paper that are simultaneously nipped by the secondary transfer unit and the fixing unit.

[0041] Furthermore, in this embodiment, a configuration in which the first belt suction conveying means and the second belt suction conveying means are separated is used. The reason for this is the ease of handling sheet jams; using a single connected belt suction conveying means poses issues with sheet handling. If a sheet jam occurs in the belt suction conveying means, it is desirable to handle the sheet while the units around the belt conveying section are pulled out. However, when pulled out, the units are positioned in different locations, reducing the accuracy of sheet conveyance at the boundary between the pull-out and the belt conveying section. Because the sheet orientation at the secondary transfer section, where an image is transferred to the sheet, is extremely delicate, the secondary transfer section must be positioned with high precision relative to the upstream and downstream units. Furthermore, because the sheet orientation entering the fixing section is also very important for high image quality, the fixing section and the upstream conveying unit (second belt suction conveying means) must be positioned with high precision relative to each other. In other words, the unit needs to be pulled out to process the sheet, and the relative positions of the secondary transfer unit and the immediately downstream transport unit (first belt adsorption transport means), and the fixing unit and the upstream transport unit (second belt adsorption transport means) need to be highly accurate, so the first belt adsorption transport means and the second belt adsorption transport means need to be separate (a single long, large belt transport unit is not a good configuration). In this embodiment, two belt adsorption transport means are used, but they may be further divided into three or more belt adsorption transport means. [Explanation of symbols]

[0042] 10 First belt suction conveying means 11 First Belt 16 Height detection means 161 Height detection flag 20 Second belt suction conveying means 21 Second Belt 50 Fixing unit 52 Heating roller 53 Pressure roller 170 Control Means 56 Cooling means 904 Belt suction conveying means 952 Pre-fixation Guide N Fixing nip N2 Transfer nip T Fixing nip line T2 Transfer nip line

Claims

1. a transfer unit that forms a transfer nip that holds and conveys a sheet and transfers an image onto the sheet; a fixing unit that is provided downstream of the transfer unit in a sheet conveying direction, forms a fixing nip that sandwiches and conveys the sheet, and fixes the image transferred by the transfer unit onto the sheet; a first belt attracting / conveying unit provided between the transfer unit and the fixing unit in the sheet conveying direction, the first belt attracting / conveying unit conveying the sheet while attracting the sheet to the first belt; a second belt attracting / conveying means provided between the first belt attracting / conveying means and the fixing unit in the sheet conveying direction, for attracting the sheet to the second belt and conveying the sheet; the fixing nip portion is disposed at a position above the transfer nip portion, In the sheet conveying direction, an upstream end of the first belt of the first belt attracting and conveying means is provided below the transfer nip portion, and a downstream end of the first belt of the first belt attracting and conveying means is provided above an upstream end of the second belt of the second belt attracting and conveying means, and a downstream end of the second belt of the second belt attracting and conveying means is provided below the fixing nip portion, the first belt of the first belt attracting and conveying means and the second belt of the second belt attracting and conveying means are inclined from the bottom to the top as they move from the upstream to the downstream in the sheet conveying direction, a nip line in the transfer nip portion extends in a direction inclined downward toward the downstream side in the sheet conveying direction, and intersects with the first belt of the first belt attracting and conveying means; a tangent line of the second belt suction conveying means that contacts the belt surface of the second belt and a nip line of the fixing nip portion intersect downstream of the fixing nip portion; An image forming apparatus comprising:

2. 2. The image forming apparatus according to claim 1, wherein a downstream end of the second belt in the sheet transport direction is provided above a downstream end of the first belt in the sheet transport direction.

3. 3. The image forming apparatus according to claim 1, further comprising a cooling unit provided below the fixing unit for cooling the fixing unit.

4. the fixing unit has a heating roller and a pressure roller facing the heating roller, forming a fixing nip portion; cooling the pressure roller by the cooling means; 4. The image forming apparatus according to claim 3, wherein:

5. a detection unit that detects the distance of the sheet from the belt surface of the first belt of the first belt suction conveyance unit to the sheet being conveyed; a control unit for controlling a sheet conveying speed by the fixing unit, the control means, in a state in which a sheet is nipped and conveyed by both the transfer nip portion and the fixing nip portion, slows down the sheet conveying speed by the fixing portion when the distance of the sheet is equal to or greater than a predetermined distance based on the detection result by the detection means, and speeds up the sheet conveying speed by the fixing portion when the distance of the sheet is less than the predetermined distance; 5. The image forming apparatus according to claim 1, wherein:

6. the control means controls the sheet conveying speed by the fixing unit based on the detection result by the detection means while the sheet is being nipped and conveyed by both the transfer nip portion and the fixing nip portion, and performs loop control to maintain the posture of the sheet; 6. The image forming apparatus according to claim 5,

Images