JP2004029060A - Image forming apparatus having bent conveyance path - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent a toner image from being damaged, when a sheet after the fixing of a toner image is passed through a bent sheet-guide path. <P>SOLUTION: A conveyance distance a sheet is conveyed from a fixing unit to a pair of paper ejection vertical-path rollers along the outermost portion of the bending section of an upward bending conveyance path which bends and guides a sheet along the shape of an arc is assumed as Lmax. A conveyance distance the sheet is conveyed from the fixing unit to the pair of paper ejection vertical-path rollers along the imaginary innermost portion of the bending section of the upward bending conveyance path is assumed to be Lmin. The maximum length of a sheet to be conveyed is assumed to be S, the speed at which a fixing unit conveys the sheet is assumed to be V1, and the speed at which the sheet is conveyed by a sheet conveying means is assumed to be V2. In this case, the relation expressed by the formula 0<(V2-V1)(S-Lmax)/V2<(Lmax-Lmin) is established. The inner guide face of the upward bending transport path is disposed inside the imaginary innermost arcuate section which has a length that satisfies the value of Lmin of the formula. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an image forming apparatus that forms a toner image on a sheet, and more particularly, to an image forming apparatus that includes a sheet guide path that guides a sheet in an arc shape immediately downstream of a heat fixing unit that fixes a toner image to a sheet. About.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, an image forming apparatus for forming a toner image on a sheet includes a copying machine, a printer, a facsimile, and a multifunction peripheral thereof. The conventional image forming apparatus mainly has a structure based on a copying machine. However, due to the rapid spread of personal computers, recent image forming apparatuses have been mainly structured based on printers. Further, many of recent image forming apparatuses have a double-sided printing function for forming a toner image on both sides of a sheet from the viewpoint of environmental measures and resource saving.
[0003]
A four-color printer, which is an example of a conventional image forming apparatus, will be described with reference to FIG. The sheet P fed from the sheet feeding unit 21 to the image forming unit 10 is transferred with the toner image by the image forming unit 10. Then, the fixing device 40 fixes the toner image on the sheet. The sheet that has passed through the fixing device 40 is discharged to a face-down tray (hereinafter, referred to as an “FD tray”) 80 with the toner image forming surface facing downward by switching of the conveyance path switching gate 51. If toner images are to be formed on both sides of the sheet, the sheet is guided by a reversing conveyance path 90 and conveyed in reverse by a pair of reversing rollers 92. Thereafter, the sheet passes through the two-sided conveyance path 100, and the toner image is transferred to the opposite surface of the sheet by the image forming unit 10. Finally, the sheet is discharged to the FD tray 80.
[0004]
[Problems to be solved by the invention]
By the way, in recent years, high-density images have been used as backgrounds in the preparation of presentation materials, and color photographic images have been output due to the spread of digital cameras. The opportunity to output a color image of a density by a printer is increasing.
[0005]
A fixing device in a color printer has to melt more toner and fix it to a sheet as compared with a fixing device in a conventional black-and-white printer. In the case of (1), it is necessary to pass the nip portion of the fixing roller pair over a long time. For this reason, the sheet comes out of the fixing device with the temperature of the sheet surface being higher than that of the conventional monochrome printer. Specifically, a fixing device of a color printer is supplied with about twice to three times as much heat as a fixing device of a black and white printer. Further, in the color printer, due to the miniaturization of the printer, it is necessary to arrange a bent conveyance path that changes the direction of the conveyance path by approximately 90 degrees by a conveyance path switching gate immediately after the fixing device.
[0006]
For this reason, in particular, a small-sized color printer strongly presses the high-density color toner image forming surface as described above against the guide surface of the transfer path switching gate or the transfer guide surface of the bent transfer path. This causes the sheet to be bent and conveyed, causing the color toner image to be damaged, and the color toner image forming surface to be temporarily stuck to the conveying surface, resulting in sheet conveyance failure. I was holding it.
[0007]
The most problematic condition is when high-density toner images are formed on both sides of a sheet. The situation where this problem occurs will be described.
[0008]
As shown in FIG. 6, the sheet conveying speed of the conveying roller pair 91 is set slightly higher than the sheet conveying speed of the fixing roller pairs 41 and 42. For this reason, when the color toner image is transferred and fixed to the first surface of the sheet, and is guided to the reversing conveyance path 90 by the guide of the conveyance path switching gate 51, the toner image non-forming surface on which the toner image is not formed is formed. Is conveyed while being pressed against the conveyance guide 53. In this case, since the toner image is not damaged and the toner image forming surface does not stick to the lower surface 51b of the transport path switching gate 51 or the upper guide surface 53a of the transport guide 53 in the bent transport path, the sheet is It is transported smoothly.
[0009]
Thereafter, the sheet is conveyed through a double-sided conveyance path 100 shown in FIG. 5, a high-density toner image is transferred to the second side, and then passed through a fixing device 40 to fix the toner image. At this time, the toner image on the first surface as well as the toner image on the second surface is melted again by the heat of the fixing device. Therefore, the conveyance path of the sheet is switched upward by the conveyance path switching gate 51 before the toner images on both sides are completely cooled and hardened.
[0010]
At this time, until the leading edge of the sheet reaches the discharge vertical path roller pair 71, the sheet is transferred to the conveyance path switching gate 51 by the resistance of the sheet conveyed upward, the weight of the sheet carrying the high density toner image, and the like. (1st side) is pressed. With such a pressing force, by providing a slanted rib or the like on the upper surface 51a of the transport path switching gate 51, it is possible to prevent the sheet from sticking to the transport path switching gate or to prevent a defective transport.
[0011]
However, after the leading edge of the sheet reaches the discharge vertical path roller pair 71, if there is a speed difference between the sheet conveyance speed of the fixing roller pairs 41 and 42 and the sheet conveyance speed of the discharge vertical path roller pair 71, the following occurs. Causes malfunction.
[0012]
For example, as shown in FIG. 7, when the sheet conveying speed of the paper discharge vertical pass roller 71 is slower than the sheet conveying speed of the fixing roller pair 41, 42, the back surface (the first surface) of the sheet is moved by the conveying path switching gate 51 or It is gradually and strongly pressed against the transport guide 54. The pressing force becomes maximum immediately before the trailing edge of the sheet passes through the fixing roller pairs 41 and 42. For this reason, the sheet may adhere to the conveyance path switching gate 51 or the conveyance guide 54, or the toner image may be rubbed.
[0013]
As shown in FIG. 8, when the sheet conveyance speed of the paper discharge vertical pass roller 71 is faster than the sheet conveyance speed of the fixing roller pair 41, 42, the surface of the sheet (second surface) gradually moves to the inner guide 52. Pressed strongly. The pressing force becomes maximum immediately before the trailing edge of the sheet passes through the fixing roller pairs 41 and 42. For this reason, the toner image may be damaged, or the sheet surface (second surface) may be stuck to the inner guide surface 52a of the inner guide 52, resulting in a defective conveyance of the sheet.
[0014]
As a countermeasure, it is conceivable that the toner image forming surface of the sheet is cooled by a cooling means such as a fan immediately after the downstream side of the fixing device 40, but the cost is reduced by electric parts such as a fan and cooling air guide parts such as a duct. It is conceivable that the size of the printer increases and the entire printer becomes large.
[0015]
The above-mentioned problems are remarkable in the above color printers, and the same problems exist not only in the above color printers but also in black and white printers, copiers, facsimile machines, and the like. A similar problem occurs not only when a toner image is formed on both sides of a sheet, but also when a toner image is formed on one side of a sheet and the sheet is conveyed as shown in FIG.
[0016]
The present invention provides an image forming apparatus that does not damage a toner image of a sheet that passes through a bent sheet guide path after fixing a toner image, or a sheet conveyance failure does not occur due to the toner image forming surface sticking to the sheet guide path. It is intended to provide.
[0017]
[Means for Solving the Problems]
In order to achieve the above object, an image forming apparatus according to the present invention includes: a heat fixing unit configured to heat and convey an unfixed toner image transferred to a sheet to the sheet by heating and conveying the sheet; and a heat fixing unit. A sheet guide path for bending and guiding the passed sheet in an arc shape, and sheet conveying means for conveying the sheet downstream of the arcuate bent portion of the sheet guide path; and a bent portion of the sheet guide path. The transport distance of the sheet traveling along the outermost portion from the heat fixing unit to the sheet transporting unit is Lmax, from the thermal fixing unit to the sheet transporting unit of the sheet traveling along the virtual innermost portion of the bent portion. Is the transport distance of Lmin, the length of the longest sheet to be transported is S, the sheet transport speed of the heat fixing unit is V1, and the sheet transport speed of the sheet transport unit is V2.
0 <(V2-V1) (S-Lmax) / V2 <(Lmax-Lmin)
The inside guide surface of the sheet guide path is arranged inside the arc-shaped virtual innermost portion having a length satisfying the value of Lmin in the formula.
[0018]
The image forming apparatus according to the present invention may further include a sheet thickness detecting unit disposed upstream of the heat fixing unit to detect a thickness of the sheet, and a sheet thickness detecting unit configured to detect the sheet thickness based on the sheet thickness detected by the sheet thickness detecting unit. Control means for varying the sheet conveying speed of the sheet conveying means.
[0019]
In the image forming apparatus of the present invention, the sheet conveying speed of the sheet conveying unit is set to increase as the thickness of the sheet increases.
[0020]
BEST MODE FOR CARRYING OUT THE INVENTION
A printer as an example of an image forming apparatus according to an embodiment of the present invention will be described with reference to the drawings. Note that the image forming apparatus includes a copying machine, a printer, a facsimile, and a multifunction machine thereof, but the present invention is not limited to the printer and can be applied to other devices. The sheet is a sheet on which a toner image can be transferred. Examples of the sheet include plain paper, a thin resin sheet which is a substitute for the plain paper, a postcard, a letter, and the like.
[0021]
(Description of the schematic configuration and operation of the entire printer)
First, the configuration and operation of the printer will be outlined with reference to FIG. The printer 2 of the present embodiment is a four-color printer. Note that the same reference numerals are used for the same or substantially the same parts as those of the conventional color printer.
[0022]
In the main body 1 of the printer 2, sheets on which images are to be formed are stacked and stored, and a sheet supply unit 21 that feeds the sheets one by one to the image forming unit 10, an image forming unit 10 that forms a toner image on the sheets, and conveyance. For example, a fixing device 40, a conveyance direction switching unit 50 for switching the conveyance direction of the sheet, and a sheet with the toner image forming surface on the lower side, which is a heat fixing unit for fixing the toner image of the formed sheet to the sheet almost permanently. A sheet is guided from the face-down tray (hereinafter, referred to as an “FD tray”) 80 on which the sheet is stacked and the conveyance direction switching unit 50 to the image forming unit 10 again, and a toner image is formed on the other surface of the sheet. The double-sided conveyance path 100 that enables the sheet thickness to be detected, the sheet thickness detecting unit 60 serving as a sheet thickness detecting unit, and the sheet thickness detected by the sheet thickness detecting unit 60 depending on the sheet thickness. A sheet conveying means, and a sheet conveying speed control section 65 (see FIG. 3) for controlling the sheet conveying speed of Haishitate pass roller pair 71.
[0023]
The transport direction switching unit 50, the FD tray 80, the two-sided transport path 100, the sheet thickness detection unit 60, and the sheet transport speed control unit 65 are not necessarily required. When the conveyance direction switching unit 50 is not provided, the sheet that has passed through the fixing device 40 is guided by a conveyance path that is curved upward from the fixing device 40 and is conveyed by the paper discharge vertical path roller pair 71 and the face-down discharge roller pair 72. , And FD tray 80.
[0024]
When a toner image is formed on one side of the sheet, the toner image is transferred to the sheet sent from the sheet supply unit 21 by the image forming unit 10, and the toner image is fixed to the sheet by the fixing unit 40. Then, the sheet P is discharged and stacked on the FD tray 80 with the toner image forming surface facing down under the guidance of the transport direction switching unit 50.
[0025]
When toner images are formed on both sides of the sheet, the sheet fed from the sheet supply unit 21 is transferred with the toner image in the image forming unit 10, the toner image is fixed in the fixing unit 40, and the conveyance direction is switched. The sheet is turned upside down and fed into the double-sided conveyance path 100 by switchback conveyance by the guide of the section 50 to the reverse conveyance path 90 and the normal rotation / reversal rotation of the pair of reversing rollers 92. Thereafter, the toner image is transferred to the other surface of the sheet by the image forming unit 10. The toner image is fixed on the sheet by the fixing device 40. The sheets are discharged to the FD tray 80 and stacked by the guide of the conveyance direction switching unit 50.
[0026]
(Sheet feeding unit)
Sheets P are stored in a sheet cassette 25 arranged below the printer main body 1. The sheet P is sent out of the sheet cassette 25 by the rotation of the pickup roller 22 that has come into contact. The sheet P sent out by the pickup roller 22 is separated into one sheet by a separation unit (for example, a separation claw, a separation pad, and a reverse rotation roller) (not shown), and a pair of registration rollers 24 provided near the entrance of the image forming unit 10. Transported to
[0027]
The registration roller pair 24 once receives the leading edge of the sheet and stops it. If the direction of the conveyed sheet P is oblique, the sheet P is straightened. Then, the sheet P is conveyed to the image forming unit 10 by the rotation of the pair of registration rollers 24 synchronized with the image forming timing with the image forming unit 10. The sheet P can be supplied from the manual tray 27 as well.
[0028]
(Image forming unit)
The image forming unit 10 using an electrophotographic method includes a laser scanner 13 that irradiates a laser beam based on image data input to the printer 2, a photosensitive drum 11 as an image carrier, and uniformly charges the surface of the photosensitive drum 11. A primary charger 12, a developing unit 14 for visualizing the electrostatic latent image formed on the photosensitive drum 11 by adsorbing toner, and a transfer blade 35 for transferring the toner image formed on the photosensitive drum 11 to the sheet P For the next image forming operation, a cleaning device 15 for removing the toner remaining on the photosensitive drum 11 and cleaning the drum 11 is provided. The cleaning device 15 is not always necessary.
[0029]
The lowercase letters a, b, c, and d attached to the reference numerals 11, 12, 13, 14, 35, and 15 are codes for corresponding to the colors yellow, magenta, cyan, and black, respectively. In the description, the lowercase letters will be omitted.
[0030]
When the image forming operation is started, the surface of the photosensitive drum 11 is uniformly charged by the primary charger 12. Image data is written on the charged photosensitive drum 11 by a laser beam irradiated by a laser scanner 13. As a result, an electrostatic latent image is formed on the photosensitive drum 11.
[0031]
The developing device 14 attracts toner to the photosensitive drum 11 on which the electrostatic latent image is formed, so that a toner image to be transferred to a sheet is formed on the surface of the photosensitive drum 11. This process is performed on the photosensitive drums of yellow, magenta, cyan, and black in synchronization with the sheet conveyance timing.
[0032]
The sheet P conveyed from the pair of registration rollers 24 is conveyed by suction onto the transfer belt 31 and the toner image formed on the photosensitive drum 11 is synchronized with the above-described image forming operation. The transfer is performed by the transfer blade 35 in an overlapping manner for each color. Excess toner and the like remaining on the surface of each photosensitive drum 11 are removed by the cleaning device 15, and the photosensitive drum 11 is in a clean state in preparation for the next image forming operation.
[0033]
(Fuser)
The fixing device 40 conveys the sheet P on which the toner image has been transferred while nipping the sheet P between a fixing roller 41 heated and rotated by a heater (not shown) and a fixing roller 42 pressed against the fixing roller 41. The toner is melted by applying heat and pressure to the sheet, and the toner image is fixed almost permanently on the sheet surface.
[0034]
(Face-down discharge)
The sheet P that has passed through the fixing roller pairs 41 and 42 is switched upward in the transport direction by the transport path switching gate 51 of the transport direction switching unit 50, and is discharged from the face-down discharge roller pair 72 to the FD tray 80.
[0035]
(Toner image formation on both sides of sheet)
The sheet P that has passed through the fixing roller pairs 41 and 42 is switched downward in the transport direction by the transport path switching gate 51, and then transported by the transport roller pair 91 and forward rotation of the reverse roller pair 92 in the reverse transport path 90. Switchback conveyance is performed by the subsequent reverse rotation. As a result, the sheet is turned over and sent to the double-sided conveyance path 100. Then, the sheet joins the conveyance path from the sheet supply unit 21 and undergoes the same process as that of the first surface to form a toner image on the other surface (second surface). Thereafter, the sheet is conveyed upward by the conveyance path switching gate 51, and is discharged to the FD tray 80 by the face-down discharge roller pair 72.
[0036]
(Transport direction switching section)
Next, the shape of the inner guide surface of the sheet guide path, which is a feature of the present invention, will be described.
[0037]
2, the transport direction switching unit 50 selectively guides the sheet to the reverse transport path 90 and the FD tray 80. The transport direction switching unit 50 includes an inner guide 52, a transport guide 53, an outer guide 54, a transport path switching gate 51, and the like. The inner guide 52, the transport guide 53, and the outer guide 54 form a T-shaped transport path. The transport path switching gate 51 is rotatably provided at a branch point of the T-shaped transport path. It is configured to be rotated by a motor or a plunger (not shown).
[0038]
The T-shaped conveyance path is an arc-shaped sheet guide path that guides the sheet from the fixing device 40 to the FD tray 80, for example, an upward bent conveyance path 57 and a sheet from the fixing device 40 to the reverse conveyance path 90. It is formed by an arc-shaped downwardly bent conveyance path 58 for guiding.
[0039]
The upward bent transport path 57 is formed by the inner guide surface 52a of the inner guide 52, the upper surface 51a of the transport path switching gate 51, and the upper guide surface 54a of the outer guide 54. The inner guide surface 52a and the upper guide surface 54a form an arc-shaped bent portion. The downward bent transport path 58 is formed by an upper guide surface 53 a of the transport guide 53, a lower surface 51 b of the transport path switching gate 51, and a lower guide surface 54 b of the outer guide 54.
[0040]
The sheet P passing through the fixing device 40 and heading toward the FD tray 80 is transferred to the upper surface 51 a of the transfer path switching gate 51 and the upper guide surface 54 a of the outer guide 54 in the upwardly bent transfer path 57 toward the discharge vertical path roller pair 71. Go along.
[0041]
In this case, the transport distance between the fixing device 40 and the paper discharge vertical path roller pair 71 in the upward bent transport path 57 is the longest distance Lmax. The sheet rubs against the outer guide 54 only by its own weight. No rubbing of the toner image and no defective toner image such as toner image streaks occur in the rubbed portion.
[0042]
However, if the sheet conveying speed V2 of the pair of discharged vertical path rollers 71 is lower than the sheet conveying speed V1 of the pair of fixing rollers 41 and 42 of the fixing device 40, the leading end of the sheet is nipped by the pair of discharged vertical path rollers 71. Then, the sheet is loosened, the loop amount in the upwardly bent conveyance path 57 increases, and the sheet is pressed against the upper surface 51a of the conveyance path switching gate 51 and the outer guide surface 54a of the outer guide 54, and the toner image is damaged. May be attached.
[0043]
Therefore, it is necessary to set the sheet speed V2 of the paper discharge vertical path roller pair 71 to a speed higher than the sheet speed V1 of the fixing roller pairs 41 and 42.
When represented by mathematical expressions, 0 <(V2−V1)...
[0044]
When the speed is set to the formula (1), the loop amount of the sheet in the upwardly bent conveyance path 57 decreases, but at the moment when the rear end of the sheet passes through the nip of the pair of fixing rollers 41 and 42. The conveying distance between the sheet fixing roller pair 41 and the sheet discharging vertical path roller pair 71 in the upward bent conveying path 57 is the shortest distance Lmin. However, even if the length of the inner guide surface 52a is set to the length of the extreme end distance Lmin, the toner image forming surface of the sheet will rub the inner guide surface 52a. Therefore, the inner guide surface having the length of the extreme end distance Lmin is defined as a virtual inner guide surface, and the actual inner guide surface 52a is disposed inside the virtual inner guide surface. That is, the inner guide surface 52a is arranged in a direction away from the transport guide 53 and the outer guide 54.
[0045]
As a result, since the toner image forming surface of the sheet does not strongly contact the inner guide surface 52a of the inner guide 52, a toner image defect due to the rubbing of the toner image and the sticking of the sheet image forming surface to the inner guide surface 52a are prevented. It is possible to prevent the occurrence of sheet conveyance failure or the like due to sticking.
[0046]
Lmin is obtained by the following equation.
[0047]
(V2−V1) (S−Lmax) / V2 <Lmax−Lmin Equation (2)
S is the length of the longest sheet P to be conveyed.
[0048]
By equation (1) and equation (2),
0 <(V2−V1) (S−Lmax) / V2 <Lmax−Lmin Equation (3) is obtained.
[0049]
Further, the sheet immediately after fixing the toner image by the fixing device 40 may be greatly curled. For this reason, there is a possibility that the sheet is jammed without smoothly entering the introduction portion of the upward bending conveyance path 57. Further, a sheet having no rigidity may be jammed in the introduction portion. Therefore, the upstream end of the transport path switching guide 51 is tapered, and the introduction portion 56 of the upwardly bent transport path 57 is formed in a divergent shape (flapper shape) toward the upstream side. Therefore, the upwardly bent conveyance path 57 can smoothly receive the sheet in the divergent introduction portion 56, and can prevent the sheet from being jammed.
[0050]
The gaps (intervals) in the sheet thickness direction of the upwardly bent conveyance path 57 and the downwardly bent conveyance path 58 are such that the sheets do not buckle. The interval is, by the way, not less than the dimension exceeding the thickness of the sheet and not more than 10 mm.
[0051]
The printer described above forms a color toner image on both sides of a sheet. However, the present invention can be applied to a printer in which a black and white toner image is formed on both sides.
[0052]
In the case of a printer that forms a color toner image or a black-and-white toner image only on one side of a sheet, a mechanism necessary for the reverse conveyance of the sheet, such as the conveyance direction switching unit 50, the reverse conveyance path 90, and the two-sided conveyance path 100. Becomes unnecessary. In this case, the upwardly bent transport path from the fixing device 40 to the FD tray 80 is formed by the inner guide surface 52a of the inner guide 52, the upper surface 51a of the transport path switching gate 51, and the upper guide surface 54a of the outer guide 54. The shape is the same as that of the upwardly-bent conveyance path 57 to be formed. Also in this case, the inside guide surface of the upwardly bent conveyance path is disposed inside the virtual inside guide surface based on the equation (3). That is, in FIG. 2, it is disposed at substantially the same position as the inside guide surface 52a.
[0053]
By the way, there are sheets of various thicknesses. For this reason, the sheet conveyance speed of the pair of fixing rollers 41 and 42 of the fixing device 40 changes depending on the thickness of the sheet. Thin paper (for example, basis weight 64 g / m ² ) To cardboard (eg, basis weight 160 g / m) ² In printers that can convey the sheet, the sheet conveying speed of the pair of fixing rollers 41 and 42 tends to increase as the sheet thickness increases.
[0054]
For this reason, as shown in FIG. 1, the printer 2 of the present embodiment includes a sheet thickness detection unit 60 on the upstream side of the registration roller pair 24, based on the sheet thickness detected by the sheet thickness detection unit 60. The sheet conveyance speed between the paper discharge vertical path roller pair 71 and the conveyance roller pair 91 can be changed.
[0055]
As shown in FIGS. 1 and 3, the sheet thickness detecting unit 60 includes a pair of rotatable sheet thickness detecting rollers 61 and 62 and a spring 63 that presses the vertically movable detecting roller 61 against a lower detecting and conveying roller 62. And an elasticity sensor 64 for detecting the elasticity of the spring 63. The sheet thickness detecting unit 60 detects, by an elasticity sensor 64, a change in the elasticity of the spring 63 depending on the thickness of the sheet that has entered the pair of conveying rollers 61, 62, and outputs the detection signal to a control unit, for example, The sheet is sent to the sheet conveying speed control section 65.
[0056]
The sheet conveyance speed control unit 65 stores in advance a table of the sheet conveyance speed of the pair of fixing rollers 41 and 42 with respect to the sheet thickness. The sheet conveyance speed controller 65 determines the thickness of the sheet based on the detection signal of the elasticity sensor 64, and determines the rotation speed of the motors 66 and 67 according to the sheet conveyance speed of the pair of fixing rollers 41 and 42 based on the table. Is changed to change the sheet conveyance speed of the discharge vertical path roller pair 71 and the conveyance roller pair 91. That is, when the sheet is thick, the sheet conveyance speed control unit 65 increases the sheet conveyance speed between the discharge vertical path roller pair 71 and the conveyance roller pair 91, and when the sheet is thin, the sheet conveyance speed control unit 65 The sheet conveyance speed between the conveyance roller 71 and the conveyance roller pair 91 is reduced. Note that the sheet thickness detection unit 60 may simply change the sheet conveyance speed of the pair of discharged vertical path rollers 71.
[0057]
Thus, the printer 2 can change the sheet conveying speed of the discharge vertical path roller pair 71 in accordance with the change even if the sheet conveying speed of the fixing roller pair 41 or 42 changes depending on the sheet thickness. Therefore, the toner image forming surface does not rub against the inner guide surface 52a, and the toner image is not damaged. Further, the sheet can be smoothly conveyed.
[0058]
Although the above-described sheet thickness detecting unit uses the elasticity of a spring, the sheet thickness detecting unit 68 using magnetism may be used as shown in FIG. The sheet thickness detecting section 68 includes a fixed magnetic field sensor 69 and a magnetic body 70 pressed against the magnetic field sensor 69 by a spring 73. The sheet thickness detecting section 68 changes the distance between the magnetic field sensor 69 and the magnetic body 70 according to the thickness of the sheet that enters and passes between the magnetic field sensor 69 and the magnetic body 70, and changes the magnetic force. Is detected by the magnetic field sensor 69, so that the thickness of the sheet can be detected.
[0059]
【The invention's effect】
Even if the image forming apparatus of the present invention is provided with a sheet guide path bent in an arc shape near the downstream side of the heat fixing unit, the sheet does not strongly contact the inner guide surface of the sheet guide path. Thus, the toner image on the sheet is not rubbed against the inner guide surface and the toner image is not damaged, so that it is possible to prevent the deterioration of the quality of the toner image. Further, the toner image forming surface does not stick to the sheet guide path, and the sheet can be smoothly conveyed. Further, since a sheet guide path bent in an arc shape can be provided near the downstream side of the heat fixing unit, the size of the image forming apparatus can be reduced.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view of a printer as an image forming apparatus according to an embodiment of the present invention, taken along a sheet conveying direction.
FIG. 2 is an enlarged view around a sheet guide path in FIG.
FIG. 3 is a diagram illustrating a sheet thickness detecting unit and a sheet conveyance speed control unit that varies a sheet conveyance speed.
FIG. 4 is a diagram illustrating a sheet thickness detection unit according to another embodiment and a sheet conveyance speed control unit that varies a sheet conveyance speed.
FIG. 5 is a cross-sectional view of the conventional image forming apparatus taken along a sheet conveying direction.
FIG. 6 is an enlarged view around a sheet guide path in FIG. 5;
7 is a diagram illustrating a sheet conveyance state in a case where the sheet conveyance speed of a discharge vertical path roller is lower than the sheet conveyance speed of a fixing roller pair in the image forming apparatus of FIG. 5;
8 is a diagram illustrating a sheet conveyance state in a case where the sheet conveyance speed of a discharge vertical path roller is faster than the sheet conveyance speed of a fixing roller pair in the image forming apparatus of FIG. 5;
[Explanation of symbols]
P sheet
V1 Sheet conveyance speed of fixing roller pair
V2 Sheet transport speed of the paper ejection vertical path roller pair
1 Printer body (image forming apparatus body)
2 Printer (image forming device)
11 Photosensitive drum
21 Sheet feeding unit
10 Image forming unit
40 Fixing device (heat fixing means)
41 Fixing roller
42 fixing roller
50 Transport direction switching unit
51 Transfer path switching gate
51a upper surface
51b bottom surface
52 Inside Guide
52a Inside guide surface (bent part)
53 Transport Guide
54 Outside Guide for Bent Conveying Path
54a Upper guide surface (bent part)
54b Lower guide surface
56 Introducing part of the bent conveyance path
57 Upward curved transport path (sheet guide path)
58 Downward curved transport path
60 Sheet thickness detector (sheet thickness detector)
65 Sheet transport speed control unit (control means)
68 Sheet thickness detector
71 Discharge Vertical Pass Roller Pair (Sheet Conveying Means)
90 Reverse transport path
100 Double-sided conveyance path

Claims (3)

Unfixed toner image transferred to the sheet, heat fixing means for fixing the sheet by heating and conveying the sheet,
A sheet guide path that guides the sheet that has passed through the heat fixing means in an arcuate shape,
Sheet conveying means for conveying the sheet on the downstream side of the arc-shaped bent portion of the sheet guide path,
The conveying distance of the sheet advancing along the outermost part of the bent portion of the sheet guide path from the heat fixing unit to the sheet conveying unit is Lmax, and the heat fixing of the sheet advancing along the virtual innermost part of the bent portion. When the conveying distance from the unit to the sheet conveying unit is Lmin, the length of the longest sheet conveyed is S, the sheet conveying speed of the thermal fixing unit is V1, and the sheet conveying speed of the sheet conveying unit is V2,
0 <(V2-V1) (S-Lmax) / V2 <(Lmax-Lmin)
Wherein the inner guide surface of the sheet guide path is disposed inside the arc-shaped virtual innermost portion having a length satisfying the value of Lmin in the following formula: A sheet thickness detection unit disposed upstream of the heat fixing unit to detect a thickness of the sheet, and a sheet conveyance speed of the sheet conveyance unit based on a sheet thickness detected by the sheet thickness detection unit. The image forming apparatus according to claim 1, further comprising a control unit configured to change the image forming apparatus. The image forming apparatus according to claim 1, wherein the sheet conveying speed of the sheet conveying unit increases as the thickness of the sheet increases.

Images