Image forming device

Abstract

To prevent air sent from a blower from flowing into a fixing unit.SOLUTION: An image forming apparatus comprises: a fixing unit that applies heat and pressure to a sheet to fix a toner image to the sheet; a conveying unit 17 that conveys the sheet to which the toner image is fixed by the fixing unit; a blower that sends air into the conveying unit; a conveying roller 18a that is rotatably arranged in the conveying unit and conveys the sheet; and a rib 120 that is arranged in the conveying unit to block the flow of air sent by the blower and extends over the entire length of the conveying roller near the conveying roller in a direction along a rotation axis RA of the conveying roller. The conveying roller has different outer diameters between both ends and a center part in the direction along the rotation axis, and a leading end of the rib opposite to the conveying roller is bent along an outer peripheral part of the conveying roller.SELECTED DRAWING: Figure 11

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus having a fixing unit. [Background technology]

[0002] An image forming apparatus that forms an image on a recording medium with toner using an electrophotographic method has a fixing section that melts the toner and fixes it to the recording medium. The fixing section has a fixing member (heating member) and a pressure member. The recording medium onto which the toner image has been transferred is heated and pressurized through a nip between the fixing member and the pressure member, and the toner melts and the toner image is fixed to the recording medium. Since the melted toner has adhesiveness, the recording medium may stick to the fixing member or pressure member. Therefore, paraffin wax is contained in the toner as a release agent to improve the separability of the recording medium from the fixing member or pressure member.

[0003] However, the paraffin wax contained in the toner volatilizes in the nip portion and adheres to the induction heating portion that heats the fixing member. When the amount of paraffin wax adhered increases to a certain amount or more, the adhered paraffin wax adheres from the induction heating portion to the fixing member, and further adheres from the fixing member to the recording medium, resulting in a decrease in image quality. Therefore, Patent Document 1 discloses a cleaning means that provides a heating element in the induction heating portion, applies electricity to the heating element to liquefy the paraffin wax adhered to the induction heating portion, and discharges the paraffin wax from the induction heating portion. Patent Document 2 discloses a cleaning device that cleans the fixing member by contacting the fixing member with a cleaning web that contains a capturing material that captures volatile organic compounds and fine particles. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2010-217580 A [Patent Document 2] JP 2011-112708 A [Patent Document 3] Japanese Patent Application Publication No. 4-44075 Summary of the Invention [Problem to be solved by the invention]

[0005] The release wax vaporized from the toner melted by the fixing unit of the image forming apparatus is cooled in the air and condensed to become dust (dust) of about several tens of nm. It is desirable to keep the dust inside the fixing unit so that it does not leak out of the fixing unit. However, when a blower for cooling the sheet discharged from the fixing unit is provided, the air blown from the blower may flow into the fixing unit. The air flowing into the fixing unit causes the dust to diffuse outside the fixing unit and adhere to the sheet conveying member outside the fixing unit, and may accumulate on the sheet conveying member as dirt made of release wax. When the amount of dust accumulation increases, the dust moves from the sheet conveying member to the sheet, degrading the quality of the image formed on the sheet. In addition, when the image forming apparatus has an exhaust filter, the dust may adhere to the exhaust filter and cause clogging of the exhaust filter.

[0006] SUMMARY OF THE PRESENTLY PREFERRED EMBODIMENTS In view of the above, the present invention provides an image forming apparatus capable of preventing air blown from a blower from flowing into a fixing unit. [Means for solving the problem]

[0007] An image forming apparatus according to an embodiment of the present invention comprises: an image forming unit that forms a toner image on a sheet; a fixing section that applies heat and pressure to the sheet to fix the toner image to the sheet; a conveying roller disposed downstream of the fixing unit in a conveying direction of the sheet, the conveying roller conveying the sheet on which the toner image has been fixed by the fixing unit; a blower that blows air onto the sheet conveyed by the conveying rollers; a housing that supports the conveying roller and has a guide surface that guides a sheet conveyed by the conveying roller, the housing forming a duct through which air blown by the blower passes, the housing having an opening that is provided on the guide surface and through which the air that has passed through the duct passes; a protrusion that is disposed in the duct so as to block the flow of the air blown by the blower, and that protrudes in the vicinity of the conveying roller in a direction along the rotation axis of the conveying roller so as to extend over the entire length of the conveying roller; Equipped with the conveying roller has different outer diameters at both ends and a center portion in the direction along the rotation axis, The tip of the protrusion facing the transport roller is curved along the outer periphery of the outer diameter of the transport roller. A gap is formed between the conveying roller and the It is characterized by the fact that Effect of the Invention

[0008] According to the present invention, it is possible to prevent air blown from the blower from flowing into the fixing unit. [Brief description of the drawings]

[0009] [Figure 1] FIG. [Diagram 2] FIG. 2 is a block diagram of a control system for controlling the image forming apparatus. [Diagram 3] FIG. [Figure 4] FIG. [Diagram 5] FIG. [Figure 6] FIG. [Figure 7] FIG. [Figure 8] FIG. 4 is a perspective view showing a sheet cooling fan, a discharge unit, and a fixing device. [Figure 9] An explanatory diagram of the properties of dust vaporized from release wax. [Figure 10]FIG. 11 is a cross-sectional view of a discharge unit as a reference example. [Figure 11] 10 is a cross-sectional view of the discharge unit taken along line XI-XI shown in FIG. 3 . [Figure 12] FIG. 12 is a cross-sectional view of a discharge unit of a reference example taken along the same cross section as in FIG. 11 . DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] <Image forming device> FIG. 1 is a cross-sectional view of an image forming apparatus 1. The image forming apparatus 1 is a four-color full-color laser beam printer (color electrophotographic image forming apparatus) using an electrophotographic process. However, the image forming apparatus 1 is not limited to a color laser beam printer. The image forming apparatus 1 may be, for example, an electrophotographic copying machine (e.g., a digital copying machine), a color LED printer, an MFP (multifunction machine), a facsimile machine, or the like. In the direction perpendicular to the drawing of FIG. 1, the near side is the front side of the image forming apparatus 1, and the far side is the rear side of the image forming apparatus 1. The left and right sides of the image forming apparatus 1 are the left and right sides when viewed from the front side.

[0011] FIG. 2 is a block diagram of a control system 30 that controls the image forming apparatus 1. The control system 30 has a control circuit section A provided in the image forming apparatus 1, an external host device B, and an operation section E provided in the image forming apparatus 1. The image forming apparatus 1 forms an image on a recording medium (hereinafter referred to as a sheet) P (FIG. 3) based on an electrical image signal input from an external host device B such as a personal computer or an image reader to the control circuit section A as a control means. The control circuit section A has a central processing unit (CPU). The sheet P is, for example, plain paper, recycled paper, thick paper, second original paper, OHP sheet, colored paper, pre-punched paper, pre-printed paper, letterhead, or label paper. The control circuit section A exchanges various electrical information with the external host device B and the operation section E, and comprehensively controls the image forming operation of the image forming apparatus 1 according to a control program and a reference table. The control circuit section A is electrically connected to a ROM 31 and a RAM 32. The ROM 31 stores a control program. The RAM 32 stores data required for controlling the image forming apparatus 1.

[0012] The image forming apparatus 1 includes four image forming stations (hereinafter referred to as image forming units) 5Y, 5M, 5C, and 5K. The image forming units 5Y, 5M, 5C, and 5K are arranged in order from left to right in a horizontal line in the approximate center of the main body 1A of the image forming apparatus 1. The image forming units 5Y, 5M, 5C, and 5K each include a rotating drum-type electrophotographic photoreceptor (hereinafter referred to as drum) 6Y, 6M, 6C, and 6K as an image carrier on which a toner image is formed. Around the drums 6Y, 6M, 6C, and 6K, charging rollers (charging means) 7Y, 7M, 7C, and 7K, developing units (developing means) 9Y, 9M, 9C, and 9K, and cleaning members (cleaning means) 41Y, 41M, 41C, and 41K are arranged. The charging rollers 7Y, 7M, 7C and 7K, the developing units 9Y, 9M, 9C and 9K, and the cleaning members (cleaning means) 41Y, 41M, 41C and 41K are process means acting on the drums 6Y, 6M, 6C and 6K.

[0013] The toner storage chamber 9aY of the developing unit 9Y of the image forming section 5Y stores yellow developer (hereinafter, referred to as toner). The image forming section 5Y forms a yellow toner image using yellow toner. The toner storage chamber 9aM of the developing unit 9M of the image forming section 5M stores magenta toner. The image forming section 5M forms a magenta toner image using magenta toner. The toner storage chamber 9aC of the developing unit 9C of the image forming section 5C stores cyan toner. The image forming section 5C forms a cyan toner image using cyan toner. The toner storage chamber 9aK of the developing unit 9K of the image forming section 5K stores black toner. The image forming section 5K forms a black toner image using black toner. The suffixes Y, M, C, and K of the reference symbols indicate yellow, magenta, cyan, and black, respectively. In the following description, the suffixes Y, M, C, and K of the reference symbols may be omitted unless otherwise necessary. The four image forming units 5 have the same structure except for the color of the toner. In this embodiment, the toner contains a release agent.

[0014] Below the image forming section 5, a laser scanner unit (optical scanning device) 8 is disposed as an exposure means for exposing the surface of the drum 6. Above the image forming section 5, an endless intermediate transfer belt (intermediate transfer body) 10 is disposed. The intermediate transfer belt 10 is stretched over a drive roller 10a, a tension roller 10b, and primary transfer rollers 11Y, 11M, 11C, and 11K. The primary transfer rollers 11Y, 11M, 11C, and 11K are disposed facing the drums 6Y, 6M, 6C, and 6K, respectively, with the intermediate transfer belt 10 sandwiched therebetween. The drive roller 10a abuts against the secondary transfer roller 12 with the intermediate transfer belt 10 sandwiched therebetween, forming a secondary transfer section ST between the intermediate transfer belt 10 and the secondary transfer roller 12. A belt cleaning device 10c is disposed facing the tension roller 10b, with the intermediate transfer belt 10 sandwiched therebetween.

[0015] A feeding cassette 2 is disposed below the laser scanner unit 8. The feeding cassette 2 is attached to the device body 1A so as to be able to be pulled out from the device body 1A. A vertical sheet transport path D is disposed inside the right side of the device body 1A, which transports the sheet P fed from the feeding cassette 2 upward. In the sheet transport path D, a roller pair of a feeding roller 2a and a retard roller 2b, a registration roller pair 4, a secondary transfer roller 12, a fixing device 103, a double-sided flapper 15a, and a discharge roller pair 14 are disposed in this order from bottom to top. A discharge tray (discharged sheet stacking section) 16 is provided on the top surface of the device body 1A.

[0016] <Image formation operation> Next, an electrophotographic image forming process (hereinafter referred to as image forming operation) in which the image forming apparatus 1 forms a full-color image on the sheet P will be described. The control circuit section A starts the image forming operation of the image forming apparatus 1 based on a print start signal sent from an external host device B or an operation section E. The control circuit section A rotates the drum 6 of the image forming section 5 in a clockwise direction at a predetermined speed in accordance with the image formation timing. The intermediate transfer belt 10 is rotated in a counterclockwise direction at a rotation speed corresponding to the rotation speed of the drum 6. The control circuit section A starts driving the laser scanner unit 8. In synchronization with the start of driving the laser scanner unit 8, in each image forming section 5, the surface of the drum 6 is uniformly charged to a predetermined polarity and a predetermined potential by the charging roller 7 to which a predetermined charging bias is applied.

[0017] The laser scanner unit 8 exposes the uniformly charged surfaces of the drums 6Y, 6M, 6C, and 6K in the main scanning direction with laser beams modulated according to image information signals for yellow, magenta, cyan, and black. As a result, an electrostatic latent image corresponding to the image information signal for the corresponding color is formed on the surface of each drum 6. The electrostatic latent image formed on the surface of the drum 6 is developed into a toner image (developer image) by a developing roller (developing member) of the developing unit 9. A predetermined developing bias is applied to the developing roller.

[0018] A yellow toner image corresponding to the yellow component of the full-color image is formed on the surface of the drum 6Y of the image forming unit 5Y. The yellow toner image on the surface of the drum 6Y is transferred to the intermediate transfer belt 10 by the primary transfer roller 11Y. A magenta toner image corresponding to the magenta component of the full-color image is formed on the surface of the drum 6M of the image forming unit 5M. The magenta toner image on the surface of the drum 6M is transferred by the primary transfer roller 11M so as to be superimposed on the yellow toner image already transferred to the intermediate transfer belt 10. A cyan toner image corresponding to the cyan component of the full-color image is formed on the surface of the drum 6C of the image forming unit 5C. The cyan toner image on the surface of the drum 6C is transferred by the primary transfer roller 11C so as to be superimposed on the magenta toner image already transferred to the intermediate transfer belt 10. A black toner image corresponding to the black component of the full-color image is formed on the surface of the drum 6K of the image forming unit 5K. The black toner image on the surface of the drum 6K is transferred by the primary transfer roller 11K so as to be superimposed on the cyan toner image already transferred to the intermediate transfer belt 10.

[0019] A primary transfer bias of a predetermined potential, which is opposite in polarity to the charge polarity of the toner, is applied to the primary transfer rollers 11Y, 11M, 11C, and 11K at a predetermined control timing. In this manner, unfixed yellow toner image, magenta toner image, cyan toner image, and black toner image are formed by superimposing them on the moving intermediate transfer belt 10. By the rotation of the intermediate transfer belt 10, the unfixed toner images of the four colors are transported to the secondary transfer section ST. In each image forming section 5, the residual toner remaining on the surface of the drum 6 after the toner image is transferred to the intermediate transfer belt 10 is wiped off by a cleaning member (cleaning blade) 41.

[0020] Meanwhile, the sheets P in the feeding cassette 2 are fed one by one by the feeding roller 2a and the retard roller 2b at a predetermined control timing and conveyed to the registration roller pair 4. In the manual feeding mode, the sheet P on the manual feed tray 3 is fed by the feeding roller 3a and conveyed to the registration roller pair 4 by the conveying roller pair 3b. The sheet P is conveyed to the secondary transfer portion ST by the registration roller pair 4 at a predetermined control timing. A secondary transfer bias of a predetermined potential, which is the polarity opposite to the charge polarity of the toner, is applied to the secondary transfer roller 12 at a predetermined control timing. As a result, the four-color toner images on the intermediate transfer belt 10 are transferred collectively to the surface of the sheet P at the secondary transfer portion ST. The transfer residual toner remaining on the surface of the intermediate transfer belt 10 after the toner image is transferred to the sheet P is removed from the surface of the intermediate transfer belt 10 by the belt cleaning device 10c.

[0021] The sheet P that has left the secondary transfer portion ST is separated from the intermediate transfer belt 10 and conveyed to a fixing device 103 as a fixing portion. In the fixing device 103, the sheet P is heated and pressed, the toner image is fixed to the sheet P, and a full-color image is formed on the sheet P. In this embodiment, the sheet P is conveyed upward in the direction of gravity along a conveying path (sheet conveying path) D that leads from the secondary transfer portion (first position) ST to a fixing nip portion (second position) 101b (FIG. 3) of the fixing device 103, and is introduced into the fixing device 103. That is, in this embodiment, the fixing device 103 is located above the intermediate transfer belt 10. Therefore, the fixing nip portion 101b is located above the secondary transfer portion ST.

[0022] In the conveying direction (sheet conveying direction) X (FIG. 3) of the sheet P, a discharge unit 17 is disposed downstream of the fixing device 103 as a conveying section. A double-sided flapper 15a is disposed downstream of the discharge unit 17. The double-sided flapper 15a can be swung between an upper position where it is swung upward to discharge the sheet P to the discharge tray 16 and a lower position where it is swung downward to invert the sheet P. In the case of a single-sided printing mode in which an image is formed only on one side of the sheet P, the double-sided flapper 15a is swung to the upper position. The sheet P, which has an image formed on one side (first side) and is conveyed from bottom to top through the fixing device 103, passes under the double-sided flapper 15a in the upper position and is discharged to the discharge tray 16 by the discharge roller pair 14.

[0023] In the case of a double-sided printing mode in which an image is formed on both sides of the sheet P, the double-sided flapper 15a is swung to a lower position. The sheet P, on which an image is formed on one side (first side) and discharged from the fixing device 103, passes above the double-sided flapper 15a in the lower position, and is conveyed to the switchback roller pair (reverse conveying section) 15 without being discharged to the discharge tray 16. The sheet P is conveyed above the discharge tray 16, but when the rear end of the sheet P reaches above the double-sided flapper 15a, the double-sided flapper 15a is swung to an upper position and the switchback roller pair 15 is reversed. As a result, the sheet P is conveyed in the reverse direction, conveyed downward in the double-sided conveying path (recirculating conveying path) 15b, and conveyed again to the registration roller pair 4 by the conveying roller pair 15c. In the secondary transfer section ST, a toner image is transferred to the back side (second side) of the sheet P, and the toner image is fixed to the back side of the sheet P by the fixing device 103, forming an image on the back side of the sheet P. The sheet P with images formed on both sides is discharged onto a discharge tray 16 by a pair of discharge rollers 14 .

[0024] <Fixing device> FIG. 3 is a cross-sectional view of the fixing device 103 and the discharge unit 17. The fixing device 103 is a belt (film) heating type-pressure member driving type heating device that uses a planar (thin plate) heater 101a such as a ceramic heater as a heat source. This type of heating device is known from Patent Document 3. The fixing device 103 has a horizontally elongated shape extending in a longitudinal direction LD (FIG. 4) parallel to a direction perpendicular to the conveying direction X of the sheet P in the sheet conveying path surface. The fixing device 103 has a heating unit 101, a pressure roller 102 as an opposing member (pressure member) facing the heating unit 101, and a housing (fixing housing) 100 that houses the heating unit 101 and the pressure roller 102. The housing 100 is provided with an entrance opening 100a and an exit opening 100b that allow the sheet P to pass through.

[0025] Fig. 4 is an exploded perspective view of the heating unit 101. Note that Fig. 4 also illustrates the pressure roller 102. The heating unit 101 is an assembly having a holder (holding member) 104, a heater 101a, a stay 104a, an endless belt (film)-like sleeve 105 as a heating member, a one end flange 106L, and an other end flange 106R.

[0026] The holder 104 is a gutter-shaped horizontally elongated member having a substantially semicircular cross section. The holder 104 is made of heat-resistant resin such as liquid crystal polymer. The heater 101a is a horizontally elongated plate-shaped heating element having a low heat capacity such as a ceramic heater whose temperature rises sharply when electricity is applied. The heater 101a is arranged along the longitudinal direction LD at the circumferential center on the outer surface side of the holder 104 and is held by the holder 104. The stay 104a is a horizontally elongated rigid member having a U-shaped cross section. The stay 104a is made of a metal such as iron. The stay 104a is arranged inside the holder 104. The sleeve 105 is loosely fitted (externally inserted) around the assembly of the holder 104, the heater 101a, and the stay 104a with a certain amount of slack.

[0027] The one-end flange 106L and the other-end flange 106R are molded products each made of heat-resistant resin and having a symmetrical shape. The one-end flange 106L and the other-end flange 106R are symmetrically attached to one end and the other end of the holder 104, respectively. The one-end flange 106L and the other-end flange 106R are holding members that rotatably hold the sleeve 105. The one-end flange 106L and the other-end flange 106R regulate the positions of both ends of the sleeve 105 and maintain the shapes of both ends of the sleeve 105 (shape retention).

[0028] As shown in FIG. 4, each of the one end flange 106L and the other end flange 106R has a flange portion 106a, a shelf portion 106b, and a pressed portion 106c. The flange portion 106a is a brim portion that receives the end of the sleeve 105 and restricts the movement of the sleeve 105 in the thrust direction. The flange portion 106a has an outer shape that is a predetermined amount larger than the outer shape of the sleeve 105. The shelf portion 106b is provided on the inner surface of the flange portion 106a and has an arc shape. The shelf portion 106b holds the inner surface of the end of the sleeve 105 to maintain the cylindrical shape of the sleeve 105. The pressed portion 106c is provided on the outer surface of the flange portion 106a. The pressed portion 106c receives a predetermined pressing force T from a biasing means (not shown).

[0029] FIG. 5 is a cross-sectional view of the heating unit 101 and the pressure roller 102. FIG. 5(a) is an enlarged cross-sectional view of the fixing nip portion 101b between the heating unit 101 and the pressure roller 102. FIG. 5(b) is an explanatory diagram of the layer structure of the sleeve 105. The sleeve 105 is a composite layer member in which an endless (cylindrical) base layer 105a, a primer layer 105b, an elastic layer 105c, and a release layer 105d are laminated from the inside to the outside. The sleeve 105 is a thin-walled member with low heat capacity that is flexible overall. The sleeve 105 has a shape retention property that allows it to maintain an approximately cylindrical shape in a free state.

[0030] The base layer 105a is made of a metal such as SUS (stainless steel). The base layer 105a has a thickness of about 30 μm to withstand thermal stress and mechanical stress. The primer layer 105b is formed by applying a conductive primer with an appropriate amount of conductive particles such as carbon dispersed therein to a thickness of about 5 μm on the base layer 105a. The elastic layer 105c functions to make the release layer 105d adhere to the toner image by deforming when pressed against the sheet P on which the toner image is formed. The release layer 105d is made of PFA (tetrafluoroethylene-perfluoroalkylvinylether copolymer) resin, which has excellent releasability and heat resistance, in order to ensure adhesion suppression performance of toner and paper powder. The release layer 105d has a thickness of about 20 μm from the viewpoint of ensuring heat conductivity.

[0031] 5(c) is a cross-sectional view showing the layer structure of the pressure roller 102. The pressure roller 102 is an elastic roller having a metal (aluminum or iron) core 102a, an elastic layer 102b made of silicone rubber or the like, and a release layer 102c that covers the elastic layer 102b. The tubular elastic layer 102b is covered with the release layer 102c. The release layer 102c is made of a fluorine-based resin such as PFA resin. The circumferential length of the sleeve 105 and the circumferential length of the pressure roller 102 are approximately the same.

[0032] 6 is an exploded perspective view of the fixing device 103. The fixing device 103 has a housing 100. The housing 100 has a horizontally long inner frame 115 made of sheet metal, and a horizontally long outer frame 125 made of heat-resistant resin attached to the outside of the inner frame 115. The inner frame 115 is composed of a base plate 109, a stay 108, one end side plate 107L, and the other end side plate 107R. The outer frame 125 is composed of a rear cover 110, a first upper cover 111, a front lower cover 112, a second upper cover (not shown), one end side cover 117L, and the other end side cover 117R.

[0033] One end side and the other end side of the core metal 102a of the pressure roller 102 are supported by one end side plate 107L and the other end side plate 107R of the inner frame body 115 via bearings (not shown), respectively, so that the pressure roller 102 is rotatable. The elastic layer 102b and the release layer 102c of the pressure roller 102 are disposed between the one end side plate 107L and the other end side plate 107R. The heater 101a (FIG. 5(a)) of the heating unit 101 faces the pressure roller 102 between the one end side plate 107L and the other end side plate 107R of the inner frame body 115, and is disposed parallel to the pressure roller 102.

[0034] The flange 106L on one end side and the flange 106R on the other end side of the heating unit 101 are fitted into guide holes 107La and 107Ra formed in the plate 107L on one end side and the plate 107R on the other end of the inner frame 115, respectively, and are slidable in a direction toward the pressure roller 102. The flange 106L on one end and the flange 106R on the other end are pressed toward the pressure roller 102 with a predetermined pressing force T by a biasing means (not shown). The flange 106L on one end, the flange 106R on the other end, the stay 104a (FIG. 5(a)), the holder 104 (FIG. 5(a)), and the heater 101a (FIG. 5(a)) move toward the pressure roller 102 due to the predetermined pressing force T. The heater 101a is pressed against the pressure roller 102 with a predetermined pressing force T via the sleeve 105 against the elastic force of the elastic layer 102b of the pressure roller 102. As a result, a fixing nip portion 101b (FIG. 5(a)) having a predetermined width in the conveying direction X is formed between the sleeve 105 and the pressure roller .

[0035] Next, the fixing operation of the fixing device 103 will be described. The control circuit section A rotates the pressure roller 102 at a predetermined rotation speed in the clockwise direction indicated by the arrow R102 in FIG. 5(a) at a predetermined control timing. A driving gear G (FIG. 4) is integrally attached to the other end of the core metal 102a of the pressure roller 102. The driving force of the driving source 33 is transmitted to the driving gear G, thereby rotating the pressure roller 102. When the pressure roller 102 rotates, a rotation torque acts on the sleeve 105 due to the frictional force with the pressure roller 102 in the fixing nip section 101b. As a result, the sleeve 105 rotates counterclockwise as indicated by the arrow R105 around the holder 104 and the stay 104a at approximately the same rotation speed as the rotation speed of the pressure roller 102, while the inner surface of the sleeve 105 slides in close contact with the heater 101a.

[0036] As shown in FIG. 4, one end side and the other end side of the heater 101a are respectively equipped with a one end side energizing connector 101dL and an other end side energizing connector 101dR. The control circuit section A starts energizing the heater 101a from the power supply section 34 via the one end side energizing connector 101dL and the other end side energizing connector 101dR. This energization causes the heater 101a to rapidly increase in temperature over the entire effective length of the heater 101a. A thermistor TH is provided as a temperature detection means on the back surface of the heater 101a (the surface opposite to the fixing nip section 101b). The thermistor TH detects the temperature of the heater 101a. The control circuit section A controls the power supplied to the heater 101a based on the temperature detected by the thermistor TH so that the temperature of the heater 101a becomes the target set temperature. In this embodiment, the target set temperature is about 170°C.

[0037] With the temperature of the heater 101a controlled to the target set temperature, the sheet P carrying the unfixed toner image S is transported from the secondary transfer section ST to the fixing device 103. As shown in FIG. 3 and FIG. 5(a), the sheet P is guided by the transport guide 130b and the guide surface 110a of the rear cover 110 to be introduced into the nip section entrance 101c, and is sandwiched and transported by the fixing nip section 101b. In the process in which the sheet P is sandwiched and transported by the fixing nip section 101b, the heat of the heater 101a is applied to the sheet P via the sleeve 105. The unfixed toner image S is melted by the heat of the heater 101a, pressurized by the fixing nip section 101b, and heat-fixed (heat-pressure fixed) to the sheet P as a fixed image.

[0038] <Discharge unit> Next, the discharge unit (conveying section) 17 will be described. As shown in FIG. 3, the discharge unit 17 is disposed downstream of the fixing device 103 in the conveying direction X of the sheet P. The discharge unit 17 conveys the sheet P conveyed from the fixing device 103 to the discharge tray 16. The discharge unit 17 has a discharge entrance roller pair 18 consisting of conveying rollers 18a and 18b. The discharge entrance roller pair 18 (18a and 18b) is disposed in the discharge unit 17 and conveys the sheet P. The discharge entrance roller pair 18 is rotatably supported by a housing 17A of the discharge unit 17. The discharge entrance roller pair 18 is disposed in the transfer section of the discharge unit 17 near the exit opening 100b of the fixing device 103 so that the discharge entrance roller pair 18 can convey even a sheet P such as an envelope that is short in the conveying direction X.

[0039] When the sheet P is sandwiched between the pair of conveying rollers and conveyed immediately after the toner image is fixed to the sheet P by the fixing device 103, gloss unevenness may occur in the image formed on the sheet P. When the sheet P is sandwiched between the pair of conveying rollers immediately after the toner image is fixed, the temperature of the toner image drops rapidly due to heat transfer from the heated toner image to the pair of conveying rollers. Therefore, the gloss of the toner image changes by the width of the nip portion of the pair of conveying rollers, and this is visually recognized as gloss unevenness. Therefore, in order that the nip portion of the pair of discharge entrance rollers 18 in this embodiment can cover the entire width of the sheet P, the width of the nip portion of the pair of discharge entrance rollers 18 in the longitudinal direction LD is set to be larger than the width of the sheet P being conveyed.

[0040] Since gloss unevenness is sensitive to the nip pressure of the pair of discharge inlet rollers 18, the material of the outer periphery of the conveying roller 18a of the pair of discharge inlet rollers 18 is a sponge made of foamed urethane. The outer periphery of the conveying roller 18a is locally deformed, so that the nip pressure of the pair of discharge inlet rollers 18 is uniformed. In addition, the conveying roller 18a has a crown shape so that the outer diameter (diameter) of the center of the conveying roller 18a is larger than the outer diameter (diameter) of both ends of the conveying roller 18a in the longitudinal direction LD. FIG. 7 is a cross-sectional view of the conveying roller 18a. The outer diameter D1 and D3 of both ends of the conveying roller 18a are different from the outer diameter D2 of the center of the conveying roller 18a in the direction along the rotation axis RA of the conveying roller 18a. In this embodiment, in FIG. 7, the outer diameter D2 of the center of the conveying roller 18a is set to 14.7 mm, the outer diameter D1 of one end is set to 14.0 mm, and the outer diameter D3 of the other end is set to 14.0 mm.

[0041] The outer diameter D1 of one end of the conveying roller 18a, the outer diameter D2 of the center, and the outer diameter D3 of the other end may be different in the direction along the rotation axis RA. The difference (outer diameter difference) between the outer diameter D1 of one end of the conveying roller 18a or the outer diameter D3 of the other end and the outer diameter D2 of the center may be 1.5 mm to 0.3 mm. The outer diameter D1 of one end of the conveying roller 18a or the outer diameter D3 of the other end in the direction along the rotation axis RA may be shorter than the outer diameter D2 of the center of the conveying roller 18a by 1.5 mm to 0.3 mm. The conveying roller 18a is biased toward the other conveying roller 18b (FIG. 3) by springs (not shown) arranged at both ends. The biasing force of the springs (not shown) causes the conveying roller 18a to bend. The amount of bending of the conveying roller 18a is greatest at the center of the conveying roller 18a where no support is provided. As a result, the nip pressure at the center of the conveying roller 18a decreases. Therefore, by making the outer diameter D2 of the center of the conveying roller 18a larger than the outer diameter D1 at one end and the outer diameter D3 at the other end, the amount of squeezing of the sponge on the outer periphery of the conveying roller 18a increases at the center of the conveying roller 18a. This prevents a decrease in the nip pressure at the center due to bending of the conveying roller 18a, and makes the nip pressure of the discharge entrance roller pair 18 uniform.

[0042] The discharge unit 17 has a sheet cooling function for lowering the sheet temperature of the toner image surface of the sheet P on which the toner image is fixed. When the toner has a thermoplastic property and exceeds a certain temperature, it exhibits adhesiveness even after fixing. Therefore, if the sheet P is stacked on the discharge tray 16 while the sheet temperature is still high, poor sheet alignment on the discharge tray 16 and adhesion between sheets occur due to the adhesiveness of the toner.

[0043] In this embodiment, in order to reduce the sheet temperature of the toner image surface of the sheet P on which the toner image is fixed, the image forming apparatus 1 has a sheet cooling fan (air blower) 122 (FIG. 8) that blows air into the discharge unit 17. FIG. 8 is a perspective view showing the sheet cooling fan 122, the discharge unit 17, and the fixing device 103. The sheet cooling fan 122 blows cooling air 121 to a duct 123 provided in the discharge unit 17. The duct 123 forms an air flow path in the discharge unit 17. The duct 123 is provided on the opposite side of the conveying roller 18a to the conveying path D of the sheet P. The duct 123 is provided with a plurality of openings 124 that lead to the conveying path D. Downstream of the conveying roller 18a in the conveying direction X of the sheet P, the air (cooling air 121) flows from the duct 123 through the plurality of openings 124 into the conveying path D as indicated by an arrow 126 to cool the sheet P. The sheet cooling fan 122 blows cooling air 121 through the discharge unit 17 onto the sheet P being transported by the discharge unit 17, thereby cooling the sheet P.

[0044] <Release wax contained in toner> Next, the release wax (releasing agent) contained in the toner will be described. In the image forming apparatus 1 that forms an image using toner, a phenomenon called offset, in which the toner adheres to the sleeve 105, may occur. The toner adhering to the sleeve 105 causes image defects and temperature fluctuations in the fixing nip portion 101b. Therefore, in this embodiment, the toner encapsulates (contains) a release wax as a releasing agent. When the toner is heated by the fixing device 103, the release wax seeps out from the molten toner. The release wax has the function of being present at the interface between the sleeve 105 and the toner image on the sheet P and preventing the toner from adhering to the sleeve 105.

[0045] The melting point Tm of the release wax is about 75° C. The melting point Tm of the release wax is set so that the release wax exuded from the toner instantly melts and spreads to the interface between the toner image and the sleeve 105 when the target temperature of the fixing nip portion 101b is set to 170° C. When the release wax melts, a part of the release wax, such as low molecular weight components in the release wax, vaporizes. The release wax is composed of long chain molecular components, but the length is not uniform and has a distribution within a predetermined range. In other words, the release wax contains low molecular components with short chains and low boiling points and polymer components with long chains and high boiling points, so that when the release wax melts, the low molecular components that are part of the release wax vaporize.

[0046] The vaporized low molecular components are cooled in the air and condense, becoming dust (dirt) of about tens to hundreds of nanometers. However, this dust is part of the release wax and is therefore sticky. The dust adheres to members inside the image forming apparatus 1. For example, the dust adheres to the pair of discharge inlet rollers 18 (FIG. 3) and the pair of discharge rollers 14 (FIG. 1) and accumulates as dirt. The dirt transfers to the sheet P and reduces the image quality. Furthermore, if an exhaust filter is attached to the image forming apparatus 1, the dust may adhere to the exhaust filter and cause clogging of the exhaust filter.

[0047] <Dust prevention measures> It is known that dust particles generally have two properties: they combine with each other to become larger, and they adhere to solid objects in the air. FIG. 9 is an explanatory diagram of the properties of dust particles vaporized from release wax. Using FIG. 9, the coalescence and adhesion phenomena of dust particles vaporized from a high-boiling-point substance 20 as release wax will be explained. As shown in FIG. 9(a), when a high-boiling-point substance 20 with a boiling point of 150 to 200° C. is placed on a heating source 20a and heated to about 200° C., volatile matter 21a is generated from the high-boiling-point substance 20. When the volatile matter 21a comes into contact with air at room temperature, it immediately becomes lower than the boiling point temperature, so it condenses in the air and changes into fine dust particles 21b of several nm to several tens of nm. This phenomenon is the same as the phenomenon in which water vapor becomes fine water droplets and generates fog when the temperature falls below the dew point temperature.

[0048] It is known that the minute dust particles 21b move through the air by Brownian motion, colliding with each other and merging to grow into larger dust particles 21c. The growth of the dust particles 21c gradually slows down and stops when the dust particles 21c reach a certain size or larger. This is presumably because when the dust particles 21c grow larger through merging, their movement through the air by Brownian motion becomes less active.

[0049] Next, referring to Fig. 9(b), consider a case where air α containing fine dust particles 21b and larger dust particles 21c is carried by air flow 22 toward wall 23. At this time, larger dust particles 21c are more likely to adhere to wall 23 than fine dust particles 21b. This is presumably because dust particles 21c have a large inertial force and collide with wall 23 with force. This phenomenon is the same when the air flow speed is 0.2 m / s or less, which is below the measurement limit of a general anemometer, that is, when the air flow is very weak.

[0050] Thus, dust has two properties: it coalesces and becomes larger, and when it becomes larger, it becomes easier to adhere to surrounding objects. The ease with which dust coalesces is related to the components, temperature, and concentration of the dust. For example, when sticky components become soft at high temperatures and the probability of dust particles colliding with each other increases under high concentrations, dust becomes more likely to coalesce. Considering the properties of dust, it is found that a measure to contain dust-containing air in the vicinity of the sleeve 105 is effective. The vicinity of the sleeve 105 is close to the source of dust, so the dust concentration is high, and the atmospheric temperature in the vicinity of the sleeve 105 is also high due to the surface heat of the sleeve 105, making it suitable for dust coalescence.

[0051] Since dust is a very light particle, when an airflow occurs around the sleeve 105, the dust is diffused by the airflow to the outside of the fixing device 103. Meanwhile, as described above, in order to prevent the sheet P from being misaligned on the discharge tray 16 immediately after fixing and to prevent adhesion between the toner surface and the sheet P, a duct 123 for passing cooling air 121 is provided in the discharge unit 17. The discharge unit 17 is configured so that the cooling air 121 passes through the inside of the discharge unit 17 and does not leak out to the fixing device 103 side. However, it is unavoidable to provide a certain amount of gap around a rotating body such as the discharge inlet roller pair 18.

[0052] 10 is a cross-sectional view of discharge unit 217 as a reference example. In discharge unit 217 as a reference example, as shown in FIG. 10, airflow 127 passes through gap 128 between conveyance roller 18a and housing 217A of discharge unit 217 from duct 123 and enters fixing device 103, generating an airflow around sleeve 105. Dust around sleeve 105 is diffused to the outside of fixing device 103 by airflow 127.

[0053] <Rib> Therefore, in the discharge unit 17 of this embodiment, in order to reduce the amount of air flowing out from the duct 123 into the gap 128 between the conveying roller 18a and the housing 17A, a rib (shielding rib) 120 is provided near the conveying roller 18a as shown in FIG. 3. The rib 120 is configured to block the air flow 129 that attempts to flow from the duct 123 into the gap 128 around the conveying roller 18a. The rib 120 is provided on the upstream side of the conveying roller 18a in the conveying direction X of the sheet P. The rib 120 extends from the housing 17A of the discharge unit 17 toward the conveying roller 18a. The rib 120 is configured to regulate the air blown by the sheet cooling fan 122 not to flow into the fixing device 103 arranged upstream of the conveying roller 18a in the conveying direction X.

[0054] FIG. 11 is a cross-sectional view of the discharge unit 17 taken along the line XI-XI shown in FIG. 3. The rib 120 extends over the entire length of the conveying roller 18a in the direction along the rotation axis RA of the conveying roller 18a in the vicinity of the conveying roller 18a. When the conveying roller 18a is viewed from the duct 123, the rib 120 is arranged so as to overlap with the outer diameter part of the conveying roller 18a. As described above, the conveying roller 18a has a crown shape so that the outer diameter of the central part of the conveying roller 18a in the longitudinal direction LD is larger than the outer diameter of both ends. Therefore, in this embodiment, as shown in FIG. 11, the rib 120 has a curved shape with respect to the rotation axis RA of the conveying roller 18a. The curved shape of the rib 120 is set based on the crown amount of the conveying roller 18a. The tip of the rib 120 is curved along the outer periphery of the conveying roller 18a. The distance between the outer periphery of the conveying roller 18a and the tip of the rib 120 is almost uniform over the entire length of the conveying roller 18a. Therefore, the gaps Ga1, Ga2, and Ga3 between the tips of the ribs 120 and the transport roller 18a at one end, the center, and the other end of the transport roller 18a are set to be the same. In order to make the gaps Ga1, Ga2, and Ga3 the same, both ends of the ribs 120 may protrude toward the transport roller 18a more than the center, and the tips of the ribs 120 may be formed in a dogleg shape.

[0055] In comparison with the discharge unit 17 having the curved rib 120 according to the present embodiment, a discharge unit 317 of a reference example having a straight rib 320 will be described with reference to FIG. 12. FIG. 12 is a cross-sectional view of the discharge unit 317 of the reference example taken along the same cross section as FIG. 11. Since the conveying roller 18a is formed in a crown shape, when the rib 320 is formed in a straight line, the distance between the outer periphery of the conveying roller 18a and the tip of the rib 320 changes according to the position in the longitudinal direction LD. The gaps Gb1 and Gb3 between the tip of the rib 320 at one end and the other end of the conveying roller 18a and the conveying roller 18a are larger than the gap Gb2 between the tip of the rib 320 at the center of the conveying roller 18a and the conveying roller 18a. Even if the gap Ga2 is set to a minimum while preventing contact between the tip of the rib 320 and the conveying roller 18a, the gaps Gb1 and Gb3 are relatively larger than the gap Gb2. An air flow is generated from the duct 123 of the discharge unit 317 through the gaps Gb1 and Gb3 into the fixing device 103. Since dust is a very light particle, the air flow leaking from the gaps Gb1 and Gb3 causes the dust around the sleeve 105 to be diffused outside the fixing device 103.

[0056] In contrast, the rib 120 of this embodiment is curved so that the distance between the outer periphery of the conveying roller 18a and the tip of the rib 120 is almost uniform over the entire length of the conveying roller 18a. Therefore, the gap between the conveying roller 18a and the tip of the rib 120 can be made small over the entire length of the conveying roller 18a. This suppresses air leakage from the discharge unit 17 to the fixing device 103, and prevents dust in the fixing device 103 from leaking out of the fixing device 103 and soiling the members in the image forming apparatus 1. The dust generated in the fixing nip portion 101b is prevented from leaking out of the fixing device 103, and the dust generated in the fixing nip portion 101b is kept in the housing 100 of the fixing device 103. The dust in the housing 100 merges with each other to become larger, and adheres to the inner wall of the housing 100, the sleeve 105, and the pressure roller 102. The dust adhering to the sleeve 105 and the pressure roller 102 is transferred to the sheet P, but since it is a very fine particle, it does not affect the image.

[0057] According to this embodiment, it is possible to prevent the air blown from the sheet cooling fan 122 from flowing into the fixing device 103 . [Explanation of symbols]

[0058] 1...image forming apparatus, 5Y, 5M, 5C, 5K, 5...image forming section, 103...fixing device, 17...discharge unit, 18a...conveyor roller, 120...rib, 122...sheet cooling fan, RA...rotation axis, S...toner, P...sheet

Claims

1. an image forming unit that forms a toner image on a sheet; a fixing section that applies heat and pressure to the sheet to fix the toner image to the sheet; a conveying roller disposed downstream of the fixing unit in a conveying direction of the sheet, the conveying roller conveying the sheet on which the toner image has been fixed by the fixing unit; a blower that blows air onto the sheet conveyed by the conveying rollers; a housing that supports the conveying roller and has a guide surface that guides a sheet conveyed by the conveying roller, the housing forming a duct through which air blown by the blower passes, the housing having an opening that is provided on the guide surface and through which the air that has passed through the duct passes; a protrusion that is disposed in the duct so as to block the flow of the air blown by the blower, and that protrudes in the vicinity of the conveying roller in a direction along the rotation axis of the conveying roller so as to extend over the entire length of the conveying roller; Equipped with the conveying roller has different outer diameters at both ends and a center portion in the direction along the rotation axis, an end of the protrusion facing the transport roller is curved along the outer periphery of the outer diameter of the transport roller, and a gap is provided between the end of the protrusion and the transport roller;

2. 2. The image forming apparatus according to claim 1, wherein the outer diameter of the both end portions of the transport roller is shorter than the outer diameter of the central portion of the transport roller by 1.5 mm to 0.3 mm.

3. 3. The image forming apparatus according to claim 1, wherein the distance between the outer periphery of the transport roller and the tip of the protrusion is uniform over the entire length of the transport roller.

4. 4. The image forming apparatus according to claim 1, wherein the protrusion is provided on an upstream side of the transport roller in the transport direction of the sheet.

5. 5. The image forming apparatus according to claim 1, wherein the duct forms a flow path for the air blown by the blower on a side opposite to the sheet conveying path with respect to the conveying roller.

6. 6. The image forming apparatus according to claim 5, wherein the air flows into the transport path of the sheet downstream of the duct in the transport direction of the sheet to cool the sheet.

7. 7. The image forming apparatus according to claim 5, wherein the protrusion is disposed so as to overlap the outer periphery of the transport roller when the transport roller is viewed from the duct.

8. 8. The image forming apparatus according to claim 1, wherein the outer periphery of the transport roller is made of a sponge made of urethane foam.

9. the fixing unit includes a heating member that heats the sheet on which the toner image is formed, and a pressure roller that is disposed opposite the heating member and presses the sheet, 9. The image forming apparatus according to claim 1, wherein the toner of the toner image contains a release agent that enhances separation of the sheet from the heating member or the pressure roller.

10. An image forming device as described in any one of claims 1 to 9, characterized in that the protrusion is positioned inside the housing, inside the gap formed between the housing and the conveying roller.

Images