EP3660595A1

EP3660595A1 - Fixing device and image forming apparatus incorporating the same

Info

Publication number: EP3660595A1
Application number: EP19209564.4A
Authority: EP
Inventors: Yutaka Naitoh
Original assignee: Ricoh Co Ltd
Current assignee: Ricoh Co Ltd
Priority date: 2018-11-27
Filing date: 2019-11-15
Publication date: 2020-06-03
Anticipated expiration: 2039-11-15
Also published as: EP3660595B1

Abstract

A fixing device (7) includes an endless fixing belt (22), a heater (31), a pressing member (23), a nip formation pad (32) disposed inside a loop of the fixing belt (22) and configured to contact the pressing member (23) to form a fixing nip, and a reflector (34) disposed inside the loop of the fixing belt (22) and configured to reflect heat from the heater (31). The nip formation pad (32) has a wider width at a portion corresponding to a central region in a longitudinal direction than an end portion of the nip formation pad (32) in the longitudinal direction. The reflector (34) extends in a longitudinal direction of the nip formation pad (32) and has an opening (34k) opened to the nip formation pad (32). The opening (34k) has a wider width at a portion corresponding to the central region in the longitudinal direction than a width at an end portion of the reflector (34) in the longitudinal direction; and corresponding image forming apparatus.

Description

BACKGROUND

Technical Field

Embodiments of the present disclosure generally relate to a fixing device and an image forming apparatus incorporating the fixing device.

Description of the Related Art

An image forming apparatus such as a copier, a printer, a facsimile machine, and a multi-functional apparatus including at least two functions of the copier, printer, facsimile machine includes a fixing device to convey a recording medium such as a sheet on which an unfixed image is formed to a fixing nip formed between a fixing belt and a pressure rotator in the fixing device, heat the recording medium, and fix the unfixed image on the recording medium.
One type of the fixing devices includes a nip formation pad disposed inside of a loop of the fixing belt and a pressing member such as a pressure roller that presses against the nip formation pad via the fixing belt to form the fixing nip between the fixing belt and the nip formation pad. The fixing device also includes an urging member that urges one of the nip formation pad and the pressing member toward the other one to form the fixing nip in which the fixing belt and the pressing member contact each other with a predetermined pressure.
For example, with reference to FIG. 16, JP-2017-68056-A discloses members, such as a nip formation pad 101, a halogen lamp 102, a reflection plate 103 and a stay 104, which are disposed inside of the loop of the fixing belt 110. A center portion of the nip formation pad 101 in the longitudinal direction is wider than each end portion of the nip formation pad 101. In contrast, each of the reflection plate 103 and the stay 104 has an opening facing the nip formation pad 101 and having a uniform width in the longitudinal direction. Additionally, as illustrated in FIG. 17, springs 106 urge side guides 105 that support both ends of the stay 104 to urge the nip formation pad 101 toward a pressure roller. As a result, the nip formation pad 101 contacts the pressure roller via the fixing belt 110 to form a fixing nip.
In the above configuration, the urging forces of the springs 106 press both ends of the nip formation pad 101 in the longitudinal direction and bend and deform the nip formation pad 101 in the direction in which the center portion of the nip formation pad 101 in the longitudinal direction is further away from the pressure roller. This causes a difference in the width of the fixing nip between the center portion and each end portion in the longitudinal direction, resulting in a problem of uneven temperature in the fixing nip. However, in the configuration disclosed in JP-2017-68056-A , the relatively wider center portion of the nip formation pad 101 in the longitudinal direction than each end portion enlarges the width at the center of the fixing nip in the longitudinal direction and reduces the difference in the width of the fixing nip between the center portion and the each end portion.
In the configuration including the wider center portion of the nip formation pad 101 in the longitudinal direction than each end portion as described in JP-2017-68056-A , the temperature at the center portion in the longitudinal direction of the nip formation pad 101 rises more slowly than the temperature at each end portion, which causes problems such as the temperature difference of the fixing nip in the longitudinal direction and extra heating required to heat the center portion of the nip formation pad 101 to a target fixing temperature.
An object of the present disclosure is to provide a fixing device capable of reducing the temperature difference in the longitudinal direction of the fixing nip.

SUMMARY

It is a general object of the present disclosure to provide an improved and useful fixing device in which the above-mentioned problems are eliminated. In order to achieve the above-mentioned object, there is provided a fixing device according to claim 1. Advantageous embodiments are defined by the dependent claims.
Advantageously, the fixing device includes an endless fixing belt, a heater configured to heat the fixing belt, a pressing member configured to press an outer surface of the fixing belt, a nip formation pad disposed inside a loop of the fixing belt and configured to contact the pressing member to form a fixing nip, and a reflector disposed inside the loop of the fixing belt and configured to reflect heat from the heater. The nip formation pad has a wider width at a portion corresponding to a central region in a longitudinal direction than an end portion of the nip formation pad in the longitudinal direction. The reflector extends in a longitudinal direction of the nip formation pad and has an opening opened to the nip formation pad. The opening has a wider width at a portion corresponding to the central region in the longitudinal direction than a width at an end portion of the reflector in the longitudinal direction.
According to the present disclosure, the opening of the reflector has a wider width at the portion corresponding to the central region in the longitudinal direction having wider width than a width of the opening at each end portion of the reflector in the longitudinal direction. Such a configuration can reduce the temperature difference in the longitudinal direction of the fixing nip.

BRIEF DESCRIPTION OF THE DRAWINGS

The aforementioned and other aspects, features, and advantages of the present disclosure would be better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic diagram illustrating a configuration of an image forming apparatus;
FIG. 2 is a vertical cross-sectional view of a fixing device viewed from a lateral side of the fixing device;
FIG. 3 is a perspective view of the fixing device with the vertical cross-sectional view of the fixing device;
FIG. 4 is a vertical cross-sectional view of the fixing device viewed from a front side of the fixing device;
FIG. 5 is a perspective view of a belt holder;
FIG. 6 is a perspective view of a variation of the belt holder;
FIG. 7 is an exploded perspective view illustrating parts disposed inside a loop of a fixing belt;
FIG. 8 is a perspective view illustrating a state after the parts illustrated in FIG. 7 are assembled;
FIG. 9A is an explanatory diagram illustrating a range of a fixing nip in a comparative example of a nip formation pad having a different shape from an embodiment of the present disclosure;
FIG. 9B is an explanatory diagram illustrating a range of a fixing nip in a nip formation pad in the embodiment the present disclosure;
FIG. 10 is a perspective view illustrating a reflector and other parts in the fixing device of another embodiment;
FIG. 11 is a cross-sectional view illustrating the reflector and other parts along a line passing through a bent portion illustrated in FIG. 10;
FIG. 12 is a perspective view illustrating the reflector and other parts in the fixing device according to yet another embodiment;
FIG. 13 is a cross-sectional view illustrating the reflector and other parts along a line passing through a bulge portion illustrated in FIG. 12;
FIG. 14 is a perspective view illustrating a stay having different shape;
FIG. 15 is a perspective view illustrating a stay having another different shape;
FIG. 16 is an exploded perspective view of a comparative fixing device.
FIG. 17 is a perspective view illustrating the comparative fixing device; and
FIG. 18 is a perspective view illustrating the fixing device according to yet another embodiment.

The accompanying drawings are intended to depict embodiments of the present disclosure and should not be interpreted to limit the scope thereof. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted.

DETAILED DESCRIPTION OF EMBODIMENTS

In describing embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that have a similar function, operate in a similar manner, and achieve a similar result.
Although the embodiments are described with technical limitations with reference to the attached drawings, such description is not intended to limit the scope of the disclosure and all of the components or elements described in the embodiments of this disclosure are not necessarily indispensable.
Referring now to the drawings, embodiments of the present disclosure are described below. In the drawings illustrating the following embodiments, the same reference codes are allocated to elements (members or components) having the same function or shape and redundant descriptions thereof are omitted below.
As illustrated in FIG. 1, the image forming apparatus 1 includes an image forming device 2 disposed in a center portion of the image forming apparatus 1. The image forming device 2 includes four process units 9Y, 9M, 9C, and 9K removably installed in the image forming apparatus 1. The process units 9Y, 9M, 9C, and 9K have an identical structure except that the process units 9Y, 9M, 9C, and 9K contain developers (e.g., yellow, magenta, cyan, and black toners) in different colors, that is, yellow, magenta, cyan, and black corresponding to color separation components of a color image.
Each process unit 9 includes a photoconductor drum 10 serving as a rotatable image bearer to bear toner as the developer on the surface of the photoconductor drum 10, a charging roller 11 to uniformly charge the surface of the photoconductor drum 10, and a developing device 12 that includes a developing roller to supply toner to the surface of the photoconductor drum 10.
Below the process units 9Y, 9C, 9M, and 9K, an exposure device 3 is disposed. The exposure device 3 emits laser light beams based on image data.
Above the image forming device 2, a transfer device 4 is disposed. The transfer device 4 includes, e.g., a drive roller 14, driven rollers 15 and 27, an intermediate transfer belt 16, a belt cleaning unit 280, and four primary transfer rollers 17. The drive roller 14 rotates the intermediate transfer belt 16. The intermediate transfer belt 16 is an endless belt rotatably stretched around the drive roller 14, the driven rollers 15 and 27, and the like. Each of the four primary transfer rollers 17 is disposed opposite the corresponding photoconductor drum 10 in each of the process units 9Y, 9C, 9M, and 9K via the intermediate transfer belt 16. At different positions, the primary transfer rollers 17 press against an inner circumferential surface of the intermediate transfer belt 16. Thus, primary transfer nips are formed at positions at which the photoconductor drums 10 contact portions of the intermediate transfer belt 16 pressed by the primary transfer rollers 17.
In addition, the transfer device 4 includes a secondary-transfer roller 18 disposed opposite the drive roller 14 via the intermediate transfer belt 16. The secondary-transfer roller 18 is pressed against an outer circumferential surface of the intermediate transfer belt 16, and thus a secondary-transfer nip is formed between the secondary-transfer roller 18 and the intermediate transfer belt 16.
A sheet feeder 5 is disposed in a lower portion of the image forming apparatus 1. The sheet feeder 5 includes a sheet tray 19, which contain sheets P as recording media, and a sheet feeding roller 20 to feed the sheets P from the sheet tray 19.
The sheets P are conveyed along a conveyance passage 6 from the sheet feeder 5 toward a sheet ejector 8. Conveyance roller pairs including a registration roller pair 21 are disposed along the conveyance passage 6.
A fixing device 7 is disposed downstream from the secondary-transfer nip in a sheet conveyance direction. The fixing device 7 includes a fixing belt 22 heated by a heater and a pressure roller 23 as a pressing member to press the fixing belt 22.
The sheet ejector 8 is disposed at an extreme downstream side of the conveyance passage 6 in the image forming apparatus 1. The sheet ejector 8 includes an ejection roller pair 24 and an output tray 25. The ejection roller pair 24 ejects the sheets P onto the output tray 25 disposed atop a housing of the image forming apparatus 1. Thus, the sheets P lie stacked on the output tray 25.
In an upper portion of the image forming apparatus 1, removable toner bottles 29Y, 29C, 29M, and 29K are disposed. The toner bottles 29Y, 29C, 29M, and 29K are filled with fresh toner of yellow, cyan, magenta, and black, respectively. A toner supply tube is interposed between each of the toner bottles 29Y, 29C, 29M, and 29K and the corresponding developing device 12. The fresh toner is supplied from each of the toner bottles 29Y, 29C, 29M, and 29K to the corresponding developing device 12 through the toner supply tube.
Next, a description is given of a basic operation of the image forming apparatus 1 with reference to FIG. 1.
As the image forming apparatus 1 receives a print job and starts an image forming operation, the exposure device 3 emits laser light beams onto the outer circumferential surfaces of the photoconductor drums 10 of the process units 9Y, 9M, 9C, and 9K according to image data, thus forming electrostatic latent images on the photoconductor drums 10. The image data used to expose the respective photoconductor drums 10 by the exposure device 3 is monochrome image data produced by decomposing a desired full color image into yellow, magenta, cyan, and black image data. After the exposure device 3 forms the electrostatic latent images on the photoconductor drums 10, the drum-shaped developing rollers 13 of the developing devices 12 supply yellow, magenta, cyan, and black toners stored in the developing devices 12 to the electrostatic latent images, rendering visible the electrostatic latent images as developed visible images, that is, yellow, magenta, cyan, and black toner images, respectively.
In the transfer device 4, the intermediate transfer belt 16 moves along with rotation of the drive roller 14 in a direction indicated by arrow A in FIG. 1. A power supply applies a constant voltage or a constant current control voltage having a polarity opposite a polarity of the toner to each primary transfer roller 17. As a result, a transfer electric field is formed at the primary transfer nip. The yellow, magenta, cyan, and black toner images are primarily transferred from the photoconductor drums 10 onto the intermediate transfer belt 16 successively at the primary transfer nips such that the yellow, magenta, cyan, and black toner images are superimposed on the same position on the intermediate transfer belt 16.
As the image forming operation starts, the sheet feeding roller 20 of the sheet feeder 5 disposed in the lower portion of the image forming apparatus 1 is driven and rotated to feed a sheet P from the sheet tray 19 toward the registration roller pair 21 through the conveyance passage 6. The registration roller pair 21 conveys the sheet P fed to the conveyance passage 6 by the sheet feeding roller 20 to the secondary-transfer nip formed between the secondary-transfer roller 18 and the intermediate transfer belt 16 supported by the drive roller 14, timed to coincide with the superimposed toner image on the intermediate transfer belt 16. At this time, a transfer voltage having a polarity opposite the toner charge polarity of the toner image formed on the surface of the intermediate transfer belt 16 is applied to the sheet P and the transfer electric field is generated in the secondary-transfer nip. Due to the transfer electric field generated in the secondary-transfer nip, the toner images formed on the intermediate transfer belt 16 is collectively transferred onto the sheet P.
The sheet P bearing the full color toner image is conveyed to the fixing device 7 where the fixing belt 22 and the pressure roller 23 fix the full color toner image on the sheet P under heat and pressure. The sheet P bearing the full color toner image is separated from the fixing belt 22 and conveyed by the conveyance roller pair to the sheet ejector 8. The ejection roller pair 24 of the sheet ejector 8 ejects the sheet P onto the output tray 25.
The above describes the image forming operation of the image forming apparatus 1 to form the full color toner image on the sheet P. Alternatively, the image forming apparatus 1 may form a monochrome toner image by using any one of the four process units 9Y, 9M, 9C, and 9K or may form a bicolor toner image or a tricolor toner image by using two or three of the process units 9Y, 9M, 9C, and 9K.
As illustrated in FIGS. 2 and 3, the fixing device 7 includes the fixing belt 22, the pressure roller 23, a halogen heater 31 as a heater, a nip formation pad 32, a stay 33 as a supporter, a reflector 34, and a fixing temperature sensor 28.
As illustrated in FIG. 2, the fixing belt 22 is a cylindrical fixing member to fix an unfixed image T to the sheet P and is disposed on the side of the sheet P on which the unfixed image T is held. The fixing belt 22 in the present embodiment is an endless belt or film including a base layer formed at an inner side of the fixing belt 22 and made of metal such as nickel and stainless steel (SUS) or resin such as polyimide and a release layer formed at an outer side of the fixing belt 22 and made of tetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA), polytetrafluoroethylene (PTFE), or the like. The fixing belt 22 extends in a direction perpendicular to the surface of the paper on which FIG. 2 is drawn, that is, a longitudinal direction. The longitudinal direction of the fixing belt 22 is also the longitudinal direction of the stay 33 and the reflector 34 and is hereinafter simply referred to as the longitudinal direction.
Optionally, an elastic layer made of rubber such as silicone rubber, silicone rubber foam, and fluoro rubber may be interposed between the base layer and the release layer. While the fixing belt 22 and the pressure roller 23 press the unfixed toner image T onto the sheet P to fix the toner image on the sheet P, the elastic layer having a thickness of about 100 micrometers elastically deforms to absorb slight surface asperities of the fixing belt 22, preventing variation in gloss of the toner image on the sheet P. In the present embodiment, the fixing belt 22 is thin and has a small loop diameter to decrease the thermal capacity of the fixing belt 22. For example, the base layer of the fixing belt 22 has a thickness of from 20 µm to 50 µm and the release layer has a thickness of from 10 µm to 50 µm. Thus, the fixing belt 22 has a total thickness not greater than 1 mm. When the fixing belt 22 includes the elastic layer, the thickness of the elastic layer may be set to 100 to 300 µm. In order to further decrease the thermal capacity of the fixing belt 22, the fixing belt 22 may have a total thickness not greater than 0.20 mm and preferably not greater than 0.16 mm. In the present embodiment, the fixing belt 22 may have a loop diameter from 20 to 40 mm and preferably 30 mm or less.
The pressure roller 23 is an opposed member disposed opposite an outer circumferential surface of the fixing belt 22. The pressure roller 23 includes a core bar; an elastic layer coating the core bar and being made of silicone rubber foam, fluoro rubber, or the like; and a release layer coating the elastic layer and being made of PFA, PTFE, or the like. According to the present embodiment, the pressure roller 23 is a solid roller. Alternatively, the pressure roller 23 may be a hollow roller. When the pressure roller 23 is a hollow roller, a heater such as a halogen heater may be disposed inside the pressure roller 23. The elastic layer of the pressure roller 23 may be made of solid rubber. Alternatively, if no heater is disposed inside the pressure roller 23, the elastic layer of the pressure roller 23 is preferably made of sponge rubber to enhance thermal insulation of the pressure roller 23. Such a configuration reduces heat conduction from the fixing belt 22 to the pressure roller 23 and improves heating efficiency of the fixing belt 22.
A driver inside the image forming apparatus 1 drives and rotates the pressure roller 23 in the direction indicated by arrow A in FIG. 2. The rotation of the pressure roller 23 drives the fixing belt 22 to rotate in a direction indicated by arrow B in FIG. 2 (hereinafter, belt rotation direction B) due to frictional force therebetween. After the toner image is transferred onto the sheet P, the sheet P bearing the unfixed toner image T is conveyed in a direction indicated by arrow J in FIG. 2 to a fixing nip N between the fixing belt 22 and the pressure roller 23. The rotating fixing belt 22 and the rotating pressure roller 23 convey the sheet P, and the sheet P passes through the fixing nip N. When the sheet P passes through the fixing nip N, heat and pressure applied to the sheet P fix the unfixed toner image T to the sheet P.
The halogen heater 31 is a heater disposed inside the loop of the fixing belt 22 to emit infrared light, and radiant heat from the halogen heater 31 heats the fixing belt 22 from the inside. Alternatively, instead of the halogen heater 31, a carbon heater, a ceramic heater or the like may be employed as the heater. In the present embodiment, one halogen heater 31 is disposed in the loop of the fixing belt 22, but a plurality of halogen heaters 31 having different heat-generating areas may be used according to the width of the sheet.
The nip formation pad 32 sandwiches the fixing belt 22 together with the pressure roller 23, to form the fixing nip N. Specifically, the nip formation pad 32 is disposed inside the loop of the fixing belt 22 and extends in the longitudinal direction. The nip formation pad 32 includes a nip formation portion 32a that is a plate to contact the inner circumferential surface of the fixing belt 22 and a pair of bent portions 32b that are bent from both end portions of the nip formation portion 32a in a belt rotation direction B to the opposite side to the pressure roller 23. A pressing member such as a spring presses the pressure roller 23 against the nip formation pad 32, which causes the pressure roller 23 to contact the fixing belt 22 and form the fixing nip N between the pressure roller 23 and the fixing belt 22.
A nip formation surface 32c on the nip formation portion 32a facing the fixing belt 22 directly contacts the inner circumferential surface of the fixing belt 22. Therefore, when the fixing belt 22 rotates, the fixing belt 22 slides along the nip formation surface 32c. In order to improve the abrasion resistance and the slidability of the nip formation surface 32c, preferably the nip formation surface 32c is treated with an alumite or a fluororesin material coating. Additionally, a lubricant such as a fluorine-based grease may be applied to the nip formation surface 32c in order to ensure slidability over time. In the present embodiment, the nip formation surface 32c is planar. Alternatively, the nip formation surface 32c may define a recess or other shape. For example, the nip formation surface 32c having a concave shape recessed to the side opposite to the pressure roller 23 leads the outlet of the sheet in the fixing nip N to be closer to the pressure roller 23, which improves separation of the sheet from the fixing belt 22.
The nip formation pad 32 is made of a material having a thermal conductivity greater than a thermal conductivity of the stay 33. For example, the material of the nip formation pad 32 is preferably copper (thermal conductivity: 398 W / mk) or aluminum (thermal conductivity: 236 W / mk). The nip formation pad 32 made of the material having such a large thermal conductivity absorbs the radiant heat from the halogen heater 31 and effectively transmits heat to the fixing belt 22. For example, setting the thickness of the nip formation pad 32 to 1 mm or less can shorten a heat transfer time in which the heat transfers from the nip formation pad 32 to the fixing belt 22, which is advantageous in shortening a warm-up time of the fixing device 7. In contrast, setting the thickness of the nip formation pad 32 larger than 1 mm and 5 mm or less can improve a heat storage capacity of the nip formation pad 32.
The stay 33 is a support member to support the nip formation pad 32 against pressure from the pressure roller 23. Similar to the nip formation pad 32, the stay 33 extends in the longitudinal direction inside the loop of the fixing belt 22. In the present embodiment, the stay 33 has a U-shaped cross-section including a pair of vertical wall portions 33a and a bottom wall portion 33b that connects the pair of vertical wall portions 33a. The stay 33 has an opening 33k facing the nip formation pad 32. The opening 33k extends along the longitudinal direction of the nip formation pad 32.
The pair of vertical wall portions 33a of the stay 33 supports both ends of the nip formation pad 32 in the belt rotation direction B. The vertical wall portions 33a extending in a direction in which the pressure roller 23 presses against the nip formation pad 32 that is a vertical direction in FIG. 2 strengthens the rigidity of the stay 33 in the direction in which the pressure roller 23 presses against the nip formation pad 32 and reduces the bend of the nip formation portion 32a caused by the pressure force of the pressure roller 23. Such a configuration results in a uniform width of the nip in the longitudinal direction. The stay 33 is preferably made of an iron-based metal such as stainless steel (SUS) or Steel Electrolytic Cold Commercial (SECC) that is electrogalvanized sheet steel to ensure rigidity.
The reflector 34 is disposed opposite the halogen heater 31 inside the loop of the fixing belt 22 to reflect radiant heat that is infrared light emitted from the halogen heater 31 to the nip formation pad 32. In the present embodiment, the reflector 34 includes a reflector portion 34a formed as an ellipse in the cross-section of the reflector 34, vertical wall portions 34b extending from both ends of the reflector portion 34a to the nip formation pad 32 in the vertical direction, and a pair of bent portions 34c bent from both ends of the vertical wall portions 34b in a direction in which the bent portions separate from each other. Between the vertical wall portions 34b, the reflector 34 has an opening 34k facing the nip formation pad 32. The opening 34k extends along the longitudinal direction of the nip formation pad 32. Each bent portion 34c is sandwiched between each vertical wall portion 33a of the stay 33 and the nip formation portion 32a of the nip formation pad 32 to hold the reflector 34.
An opening of an ellipse concave surface of the reflector portion 34a that opens toward the nip formation pad 32 reflects the radiant heat from the halogen heater 31 toward the nip formation pad 32. That is, the halogen heater 31 directly irradiates the nip formation pad 32 with the infrared light, and, additionally, the nip formation pad 32 is also irradiated with the infrared light reflected by the reflector portion 34a. Therefore, the nip formation pad 32 is effectively heated.
Since the reflector portion 34a is interposed between the halogen heater 31 and the stay 33, the reflector portion 34a functions to block the radiant heat from the halogen heater 31 to the stay 33 and eliminates wasted energy. In other words, the radiant heat from the halogen heater 31 is applied to the space surrounded by the reflector 34 and the nip formation portion 32a and is blocked from parts disposed outside the space, such as the stay 33. Additionally, in the present embodiment, thermal insulation of the layer of air in a gap between the stay 33 and the reflector portion 34a blocks heat transfer to the stay 33.
As described above, the reflector 34 is provided to cover the halogen heater 31, and the radiant heat from the halogen heater 31 and the radiant heat reflected by the reflector 34 are efficiently collected to the opening 34k of the reflector 34, that is, to the nip formation pad 32. The heated nip formation pad 32 can efficiently heat the fixing belt 22 in the fixing nip N. The stay 33 is disposed to cover the outer circumferential surface of the reflector 34. In other words, the reflector 34 and the halogen heater 31 are disposed in the opening 33k of the stay 33. The vertical wall portions 33a of the stay 33 are disposed outside the vertical wall portions 34b of the reflector 34 and extend along the longitudinal direction of the vertical wall portions 34b. The fixing belt 22 is disposed outside the stay 33. That is, in the circumferential direction of the fixing belt 22, the fixing belt 22 faces the halogen heater 31 through the nip formation pad 32 in the fixing nip N and, in the portion other than the fixing nip N, faces the halogen heater 31 through the reflector 34 and the stay 33.
The surface of the reflector portion 34a of the reflector 34 facing the halogen heater 31 is treated with a mirror finish or the like to increase reflectance. In the present embodiment, reflectance is measured using the spectrophotometer that is the ultraviolet visible infrared spectrophotometer UH4150 manufactured by Hitachi High-Technologies Corporation in which the incident angle is set 5°. In general, the color temperature of the halogen heater varies depending on the application. The color temperature of the heater for the fixing device is about 2500 K. The reflectance of the reflector 34 used in the present embodiment is preferably 70% or more with wavelengths of high emission intensity in the halogen heater 31, that is, specifically the wavelengths of 900 to 1600 nm, and more preferably 70% or more with wavelengths of 1000 to 1300 nm.
Alternatively, the stay 33 may have the function of reflection and thermal insulation of the reflector 34. For example, performing the thermal insulation treatment or the mirror finishing on the inner surface of the stay 33 facing the halogen heater 31 enables the stay 33 to function as the reflector 34. In this case, the reflector 34 that is a separate part from the stay 33 can be removed. The reflectance of the stay 33 subjected to the mirror finishing is preferably similar to the reflectance of the reflector 34.
As illustrated in FIG. 4, urging members such as springs urge both ends in the longitudinal direction of the pressure roller 23 toward the fixing belt 22 in a direction indicated by arrows H in FIG. 4 and press the pressure roller 23 against the fixing belt 22. As a result, the elastic layer of the pressure roller 23 is pressed and deforms, and the fixing nip N is formed.
A pair of belt holders 35 is inserted in both lateral ends of the fixing belt 22 in the axial direction of the fixing belt 22 to rotatably support the fixing belt 22. As described above, the belt holders 35 inserted in the inner periphery of the fixing belt 22 support the fixing belt 22 in a state in which basically no tension in the circumferential direction is applied when the fixing belt 22 does not rotate, that is, the so-called free belt method.
As illustrated in FIGS. 3 to 5, the belt holders 35 include a C-shaped supporter 35a inserted in the inner periphery of the fixing belt 22 to support the fixing belt 22 and a flange 35b that contacts an end face of the fixing belt 22 to stop a movement of the fixing belt 22 in the width direction, that is, walking of the fixing belt 22 in the width direction. As illustrated in FIG. 6, the supporter 35a may have a cylindrical shape which is continuous over its entire circumference. As illustrated in FIG. 4, each of the belt holders 35 is fixed on a pair of side plates 36 that are frames of the fixing device 7. The belt holders 35 define openings 35c as illustrated in FIG. 5, and both ends of the halogen heater 31 and the stay 33 are fixed to the side plates 36 through the openings 35c. The halogen heater 31 and the stay 33 may be fixed to the belt holders 35.
As illustrated in FIG. 7, the nip formation portion 32a of the nip formation pad 32 has an arc shape in which a portion corresponding to a "central region in the longitudinal direction" bulges outward, and the portion corresponding to the "central region in the longitudinal direction" is wider in the sheet conveyance direction than each of end portions in the longitudinal direction of the nip formation pad 32 itself. Specifically, a width of the nip formation pad 32 in the portion corresponding to the "central region in the longitudinal direction", in particular, a width Da in the sheet conveyance direction at the central position in the longitudinal direction of the portion corresponding to the "central region in the longitudinal direction" is larger than a width Db in the sheet conveyance direction at the end portion in the longitudinal direction of the nip formation pad 32 itself. In the present embodiment, the "central region in the longitudinal direction" means a central region of the fixing belt 22 divided into three in the longitudinal direction and does not necessary coincide with a central region of the nip formation pad 32 itself divided into three in the longitudinal direction. However, this "central region in the longitudinal direction" may not be the central region of the fixing belt 22 divided into three in the longitudinal direction and may be a central region of the pressure roller 23 divided into three in the longitudinal direction or a central region of an image formation region divided into three in the longitudinal direction. The image formation region is a region of the sheet having the maximum width and used in the fixing device. In addition, in the present embodiment, the widths of the nip formation pad 32 at both ends in the longitudinal direction are the same but are not limited to be the same, the widths at the both ends may be different.
In the configuration in which the pressure roller 23 is urged and pressed against the fixing belt 22 in the direction indicated by arrows H in FIG. 4 to form the fixing nip, the pressure force from the pressure roller 23 bends the nip formation pad 32 so that the portion corresponding to the central region in the longitudinal direction in the nip formation pad 32 becomes convex in a direction of the pressure force. When a nip formation portion 32a' of the nip formation pad 32' that is different from the nip formation pad 32 of the present embodiment has a rectangular shape as illustrated in FIG. 9A, that is, has the same width in the sheet conveyance direction that is a vertical direction in FIG. 9A across the longitudinal direction that is a horizontal direction in FIG. 9A, the bend in the nip formation pad 32' as described above causes the central portion of the fixing nip N to be narrower than each of the end portions of the fixing nip N as illustrated in the hatched part in FIG. 9A. That is, the bend described above causes a large difference in the nip width between the portion corresponding to the central region in the longitudinal direction and the end portion in the longitudinal direction of the nip formation pad 32, causing temperature unevenness in the fixing nip N.
In contrast, in the present embodiment, as illustrated in FIG. 9B, the width Db in the sheet conveyance direction at the end portion in the longitudinal direction of the nip formation portion 32a is smaller than the width Da in the sheet conveyance direction at the portion corresponding to the central region in the longitudinal direction, which causes the nip width at the end portion in the longitudinal direction of the nip formation portion 32a to be small as illustrated in the hatching part of FIG. 9B. Such a configuration reduces the difference in the width of the fixing nip N in the sheet conveyance direction between the portions in the longitudinal direction and the temperature unevenness of the fixing belt 22. In the present embodiment, the pressure roller 23 is pressed against the nip formation pad 32, but conversely, the nip formation pad 32 may be pressed against the pressure roller 23. Even in this case, the shape of the nip formation pad 32 in the present embodiment can reduce the difference in the width of the fixing nip N in the sheet conveyance direction between the portions in the longitudinal direction.
As a configuration different from the present embodiment, using the reflector and the stay each having openings of uniform width in the longitudinal direction and the nip formation pad having a relatively large width at the portion corresponding to the central region in the longitudinal direction prevents the heater from efficiently heating the nip formation pad. For example, as the configuration different from the present disclosure, the fixing device illustrated in FIG. 17 includes a reflection plate 103 and a stay 104 each having the opening with uniform width in the longitudinal direction and a nip formation pad 101 having a wide central portion. The above-described configuration generates a region C in the central portion in which the nip formation pad 101 is out of the reflection plate 103 and the stay 104. Since the halogen lamp 102 is disposed inside the reflection plate 103 and the stay 104, heat from the halogen lamp 102 is not directly transmitted to the region C of the nip formation pad 101.
On the other hand, in the present embodiment, as illustrated in FIG. 7, the stay 33 and the reflector 34 have the openings 33k and 34k that are formed corresponding to the nip formation portion 32a. That is, in edges 33a1 that are edges of the opening 33k and edges of the vertical wall portions 33a of the stay 33, a portion configured by the edges 33a1 corresponding to the central region in the longitudinal direction is wider than each of end portions configured by end portions of the edges 33a1 in the longitudinal direction. In other words, a width of the opening 33k in the portion corresponding to the central region in the longitudinal direction, in particular, a width Ea of the opening 33k in the sheet conveyance direction at the central position in the longitudinal direction of the portion corresponding to the central region in the longitudinal direction is set larger than a width Eb in the sheet conveyance direction at end portion in the longitudinal direction of the opening 33k. In addition, in edges 34b1 that are edges of the opening 34k and edges of the vertical wall portions 34b of the reflector 34, a portion configured by the edges 34b1 corresponding to the central region in the longitudinal direction is wider than each of end portions configured by end portions of the edges 34b1 in the longitudinal direction. Specifically, a width of the opening 34k in the portion corresponding to the central region in the longitudinal direction, in particular, a width Fa in the sheet conveyance direction at the central position in the longitudinal direction of the portion corresponding to the central region in the longitudinal direction is larger than a width Fb in the sheet conveyance direction at end portion in the longitudinal direction of the opening 34k. In the present embodiment, both end portions in each of the openings 33k and 34k in the longitudinal direction have the same width, but the present disclosure is not limited to this.
In the reflector 34 configured as described above, the opening 34k can cover the nip formation portion 32a even in the portion corresponding to the central region in the longitudinal direction, which has the large width in the sheet conveyance direction as illustrated in FIG. 2, though the opening 34k does not cover the bent portions 32b. As illustrated in FIG. 8, the opening 34k illustrated in FIG. 7 can cover an entire region of the nip formation portion 32a in the longitudinal direction. That is, a region in which the radiant heat from the halogen heater 31 can directly heat the nip formation portion 32a becomes widest, and the halogen heater 31 can efficiently heat the nip formation portion 32a and the central region of the fixing belt 22 in the longitudinal direction, which eliminates the temperature unevenness in the fixing nip N in the longitudinal direction. In addition, the stay 33 configured as described above can have the vertical wall portions 34b of the reflector 34 inside the vertical wall portions 33a of the stay 33 in an entire region of the stay 33 as illustrated in FIG. 2. Therefore, the stay 33 does not need a clearance to avoid interference caused by a wide portion of the reflector 34 corresponding to the central region in the longitudinal direction, which is described below.
Hereinafter, other embodiments of the fixing device according to the present disclosure are described that have different configurations from the above-described embodiment. The following description is chiefly limited to parts different from those of the embodiment of the fixing device described above, with a description of shared structures omitted where appropriate.
In the fixing device illustrated in FIG. 10, the configuration of the reflector 34 is different from that of the above-described embodiment. FIG. 10 is a perspective view illustrating the reflector 34 that is disposed on the nip formation pad 32 and covers the halogen heater 31, in which the stay 33 is omitted. FIG. 12 described below is a similar perspective view illustrating reflector 34.
As illustrated in FIG. 10, the reflector 34 has cut and bent portions 34d as widening portions bent to both sides in the sheet conveyance direction at the portion corresponding to the central region in the longitudinal direction. As illustrated in FIG. 11, the cut and bent portion 34d is inclined in the direction in which the opening 34k is widened toward the nip formation pad 32. The above-described configuration can enlarge the width in the sheet conveyance direction of the nip formation portion 32a that can receive the radiant heat from the halogen heater 31 and the radiant heat reflected by the reflector 34 at the portion corresponding to the central region in the longitudinal direction in which the width in the sheet conveyance direction of the nip formation portion 32a is large. Therefore, the halogen heater 31 can efficiently heat the nip formation portion 32a and, in particular, the central region of the fixing belt 22 in the longitudinal direction, which eliminates the temperature unevenness in the fixing nip N in the longitudinal direction.
In the embodiment illustrated in FIGS. 12 and 13, the reflector 34 has bulging portions 34e as the widening portions on both sides in the sheet conveyance direction at the portion corresponding to the central region in the longitudinal direction. The bulging portions 34e are formed by drawing and have a shape that bulges to both sides in the sheet conveyance direction. The bulging portions 34e can enlarge the width in the sheet conveyance direction of the nip formation portion 32a that can receive the radiant heat from the halogen heater 31 and the radiant heat reflected by the reflector 34 at the portion corresponding to the central region in the longitudinal direction of the nip formation portion 32a. Therefore, the halogen heater 31 can efficiently heat the nip formation portion 32a and, in particular, the central region of the fixing belt 22 in the longitudinal direction, which eliminates the temperature unevenness in the fixing nip N in the longitudinal direction.
In addition, the stay 33 may have a clearance disposed at the portion corresponding to the central region in the longitudinal direction to avoid contact between the stay 33 and the cut and bent portion 34d or the bulging portion 34e of the reflector 34 as described above.
For example, as illustrated in FIG. 14, the stay 33 may have a cutout portion 33c to provide the clearance at the portion corresponding to the central region in the longitudinal direction to avoid the contact between the stay 33 and the cut and bent portion 34d of the reflector 34. The cutout portion 33c is a substantially rectangular cutout portion provided in a range corresponding to the cut and bent portion 34d. Alternatively, as illustrated in FIG. 15, the stay 33 may have an arc-shaped cutout portion 33d to provide the clearance corresponding to the bulging portion 34e. The above-described configuration enables the opening of the stay 33 to hold the reflector 34. In addition, providing the cutout portion 33c or 33d can reduce the thermal capacity of the stay 33 to improve the heating efficiency of the nip formation pad 32 and the fixing belt 22.
The present disclosure is not limited to the details of the embodiments described above and various modifications and improvements are possible.
The image forming apparatus 1 according to the present embodiment of the present disclosure is applicable not only to the color image forming apparatus illustrated in FIG. 1 but also to a monochrome image forming apparatus, a copier, a printer, a facsimile machine, or a multifunction peripheral including at least two functions of the copier, printer, and facsimile machine.
The sheets P serving as recording media may be thick paper, postcards, envelopes, plain paper, thin paper, coated paper, art paper, tracing paper, overhead projector (OHP) transparencies, plastic film, prepreg, copper foil, and the like.
In the above-described embodiment, although the portion corresponding to the central region in the longitudinal direction of the nip formation portion 32a has the arc shape that bulges outward on both sides in the sheet conveyance direction, the portion corresponding to the central region in the longitudinal direction of the nip formation portion 32a may have a shape that bulges outward on one side in the sheet conveyance direction. In this case, the portions corresponding to the central region in the longitudinal direction of the reflector 34 and the stay 33 may have bulge outward on one side in the sheet conveyance direction corresponding to the nip formation portion 32a as illustrated in FIG. 7. In addition, the reflector 34 may have the cut and bent portion 34d or the bulging portion 34e on one side in the sheet conveyance direction corresponding to the nip formation portion 32a, and similarly, the stay 33 may also have the cutout portion 33c or 33d on one side in the sheet conveyance direction corresponding to the cut and bent portion 34d or the bulging portion 34e, respectively.
As illustrated in FIG. 18, there may be a substantially uniform gap in the longitudinal direction between the vertical wall portion 33a of the stay 33 and the vertical wall portion 34b of the reflector 34. In this case, the portion of the stay 33 corresponding to the central region in the longitudinal direction bulges outward as illustrated in FIG. 7, the portion of the reflector 34 similarly bulges outward, and the openings of the stay 33 and the reflector 34 become large in the portions. The above-described configuration generates an air layer between the vertical wall portion 33a and the vertical wall portion 34b, which prevents the heat of the reflector 34 from transmitting to the stay 33, prevents the stay 33 from being overheated, and efficiently heats the fixing belt 22. Providing the vertical wall portion 33a and the vertical wall portion 34b along the longitudinal direction does not need the clearance described above and enables the stay 33 to support the entire nip formation pad 32 uniformly in the longitudinal direction.
In the present disclosure, the center position of the nip formation pad 32, the stay 33, and the reflector 34 in the longitudinal direction does not necessarily coincide with the center position of the fixing belt 22 in the longitudinal direction.
In the above description, although the width of the portion of the nip formation pad 32 (or the opening 33k of the stay 33 or the opening 34k of the reflector 34) corresponding to the central region in the longitudinal direction is larger than the width of the longitudinal end portion of the nip formation pad 32 (or the opening 33k of the stay 33 or the opening 34k of the reflector 34), the present disclosure is not limited to the case in which the width of the entire portion corresponding to the central region in the longitudinal direction is larger than the width of the end portion in the longitudinal direction. A part of the portion corresponding to the central region in the longitudinal direction may be larger than the width of the end portion in the longitudinal direction.
The present disclosure is not limited to the above-described embodiments, and the configuration of the present embodiment can be appropriately modified other than suggested in each of the above embodiments within the scope of the technological concept of the present disclosure. In addition, the positions, the shapes, and the number of components are not limited to the disclosed embodiments, and they may be modified suitably in implementing the present disclosure.

Claims

A fixing device (7) comprising:
an endless fixing belt (22);

a heater (31) configured to heat the fixing belt (22);

a pressing member (23) configured to press an outer surface of the fixing belt (22);

a nip formation pad (32) disposed inside a loop of the fixing belt (22) and configured to contact the pressing member (23) to form a fixing nip, the nip formation pad (32) having a wider width at a portion corresponding to a central region in a longitudinal direction than an end portion of the nip formation pad (32) in the longitudinal direction; and

a reflector (34) disposed inside the loop of the fixing belt (22) and configured to reflect heat from the heater (31), the reflector (34) extending in a longitudinal direction of the nip formation pad (32) and having an opening (34k) opened to the nip formation pad (32), the opening (34k) having a wider width at a portion corresponding to the central region in the longitudinal direction than a width at an end portion of the reflector (34) in the longitudinal direction.
The fixing device (7) according to claim 1,
wherein a width of the opening (34k) is largest at a position corresponding to a central position of the fixing belt (22) in the longitudinal direction.
The fixing device (7) according to claim 1 or 2,
wherein the reflector (34) has a pair of vertical wall portions (34b) extending in the longitudinal direction of the nip formation pad (32), and
wherein the opening (34k) of the reflector (34) is formed between the pair of vertical wall portions (34b),
wherein at least one of the pair of vertical wall portions(34b) has a widening portion (34d, 34e) in the central region in the longitudinal direction, and
wherein a width of the opening (34k) increases toward the nip formation pad (32) in the widening portion (34d, 34e).
The fixing device (7) according to claim 1 or 2, further comprising
a supporter (33) disposed inside the loop of the fixing belt (22) and configured to support the nip formation pad (32),
wherein the supporter has an opening (33k) extending in the longitudinal direction of the nip formation pad (32) and opening toward the nip formation pad (32),
wherein the opening (33k) has a larger width at a portion of the supporter (33) corresponding a central region of the fixing belt (22) in the longitudinal direction than a width at an end portion of the supporter (33) in the longitudinal direction.
The fixing device (7) according to claim 4,
wherein the reflector (34) has a pair of vertical wall portions (34b) extending in the longitudinal direction of the nip formation pad (32),
wherein the opening (34k) of the reflector (34) is formed between the pair of vertical wall portions (34b), and
wherein the supporter (33) has a pair of vertical wall portions (33a) extending in the longitudinal direction of the nip formation pad (32),
wherein the opening (33k) of the supporter (33) is formed between the pair of vertical wall portions (33a) of the supporter (33), and
wherein an end portion (34b1) of each of the pair of vertical wall portions (34b) of the reflector (34) at an open side of the reflector (34) is spaced from an end portion (33a1) of each of the pair of vertical wall portions (33a) of the supporter (33) at an open side of the supporter (33) and provided along the end portion (33a1) of each of the pair of vertical wall portions (33a) of the supporter (33) in the longitudinal direction.
The fixing device (7) according to any one of claims 1 to 3, further comprising a supporter (33) disposed inside the loop of the fixing belt (22) and configured to support the nip formation pad (32), the supporter (33) having an opening (33k) extending in the longitudinal direction of the nip formation pad (32) and opening toward the nip formation pad (32),
wherein the reflector (34) has a pair of vertical wall portions (34b) extending in the longitudinal direction of the nip formation pad (32),
wherein the opening (34k) of the reflector (34) is formed between the pair of vertical wall portions (34b), and
wherein the supporter (33) has a pair of vertical wall portions (33a) extending in the longitudinal direction of the nip formation pad (32), and the opening (33k) of the supporter (33) is formed between the pair of vertical wall portions (33a) of the supporter (33), and
wherein each of the pair of vertical wall portions (33a) of the supporter (33) has a clearance at a portion of the supporter (33) corresponding to a central portion of the fixing belt (22) in the longitudinal direction, to avoid contact with the pair of vertical wall portions (34b) of the reflector (34).
An image forming apparatus comprising the fixing device according to any one of claims 1 to 6.