Heating device and image processing apparatus

According to an embodiment, a heating device includes a cylindrical belt, a heater, a press roller, and a holder. The heater is disposed on an inner surface of the belt. The press roller rotates and is configured to contact an outer surface of the belt at a position opposite the heater to form a nip. The end portions of the press roller project by a first amount in a radial direction more than the central portion of the press roller. A holder has a first surface supporting the heater to face the cylindrical belt. The holder has a second surface that is opposite of the first surface with a center portion projecting in a direction orthogonal to the first surface more than the end portions of the second surface by a second projection amount that is greater than the first projection amount.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2020-131824, filed on Aug. 3, 2020 the entire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a heating device and an image processing apparatus.

BACKGROUND

An image forming apparatus that prints images on sheets with toner is known. An image forming apparatus of such type includes a fixing device incorporating a heating device. The fixing device heats and presses a toner image on formed on a sheet and thus fixes the toner image to the sheet. When the sheet passes through the fixing device, wrinkles, creases, or the like on the sheet sometimes occur. There is a demand for a fixing device that can avoid or reduce wrinkling and creasing of a printed sheet.

DETAILED DESCRIPTION

Certain embodiments provide a heating device and an image processing apparatus that can reduce creasing and the like of sheets being printed.

In general, according to an embodiment, a heater includes a cylindrical belt having an axial direction, a heater disposed on an inner surface of the cylindrical belt and having a longitudinal direction parallel to the axial direction, and a press roller. The press roller is configured to contact an outer surface of the cylindrical belt at a position opposite the heater and form a nip. The press roller is configured to rotate to convey a sheet through the nip. Both longitudinal end portions of the press roller project in a radial direction by a first projection amount that is more than a central portion of the press roller between the longitudinal end portions of the press roller in the longitudinal direction. A holder has a first surface supporting the heater and facing the cylindrical belt. The holder has a second surface that is opposite the first surface and has a center portion between the longitudinal end portions of the second surface that projects in a direction orthogonal to the first surface by a second projection amount that is greater than the first projection amount.

Certain examples of a heating device and an image processing apparatus according to an embodiment are explained with reference to the drawings.

FIG.1depicting aspects of an image processing apparatus1according to an embodiment. The image forming apparatus1performs processing for forming an image on a sheet. The sheet may be paper or the like.

In following description, a Z direction, an X direction, and a Y direction are arbitrarily set for explanatory convenience. The Z direction is the vertical direction inFIG.1and the +Z direction is an upward direction. The X direction and the Y direction are the horizontal directions. The X direction is set to be the page width (left-right) direction of the image forming apparatus1. The +X direction is towards right-hand direction ofFIG.1. The Y direction is the direction into the page ofFIG.1of the image forming apparatus1. The +Y direction is a direction outward from the page ofFIG.1.

The image forming apparatus1includes a reading unit R, an image forming unit P, and a paper feeding cassette unit C.

The reading unit R reads image information for copying a target object as brightness and darkness of light and generates an image signal accordingly.

The image forming unit P prints an image based on an image signal received from the reading unit R or, alternatively, from an external device. The printed image is an image formed by a toner or another recording material. The image forming unit P in this examples transfers a toner image onto the surface of a sheet. The image forming unit P includes a fixing device30. The fixing device30heats and presses the toner image that has been transferred onto the sheet and thus fixes or fuses the toner image to the sheet.

The image forming unit P in this example includes a laser scanning unit200and photoconductive drums201Y,201M,201C, and201K. The laser scanning unit200includes a polygon mirror208and an optical system241. The laser scanning unit200selectively irradiates, based on image signals corresponding to the respective colors, the surfaces of the photoconductive drums201Y,201M,201C, and201K with a laser beam. The laser scanning unit200forms electrostatic latent images on the surfaces of the photoconductive drums201Y,201M,201C, and201K.

The electrostatic latent images on the photoconductive drums201Y to201K are developed using respective color toners supplied from a developing device (e.g., toner cartridge). The toner selective adheres to the electrostatic latent images and toner images are thus formed on the photoconductive drums201Y,201M,201C, and201K. The photoconductive drums201Y,201M,201C, and201K hold the toner images until the toner images are transferred onto a transfer belt207. The transfer belt207is an endless belt and conveys the transferred toner images to a secondary transfer position T.

A conveyance path101connects the paper feeding cassette unit C, the secondary transfer position T, the fixing device30, and a discharge tray211. A sheet stocked in the paper feeding cassette unit C can be conveyed to the transfer position T along the conveyance path101. At the secondary transfer position T, the toner images are transferred from the transfer belt207onto the sheet.

The sheet to which the toner images have been transferred is conveyed to the fixing device30along the conveyance path101. The fixing device30heats and presses the sheet to fix the toner images to the sheet. After the toner image has been fixed to the sheet, the sheet can be discharged to the discharge tray211via the conveyance path101.

A control unit801is a controller that controls various components and mechanisms of the image forming apparatus1. The control unit801includes a central arithmetic unit, such as a CPU (Central Processing Unit), and a volatile and/or nonvolatile storage device(s). The central arithmetic unit executes an arithmetic operations according to a program stored in a storage device, whereby the control unit801controls the components and the mechanisms of the image forming apparatus1. In some examples, some or all of functions of the control unit801may be implemented as a dedicated hardware circuit or the like.

FIG.2is a cross-sectional view of fixing device30. The fixing device30is a fixing unit of a so-called “direct heat” type in this example. The fixing device30includes a belt38, a press roller32, a heating member40, and a frame50.

The belt38is formed in a tubular or cylindrical shape of a flexible material. The belt38can be referred to as endless belt, a fixing belt, a film unit, or the like. The belt38includes a base layer, an elastic layer, and a surface release layer. The base layer is made of a thin-film material having high heat resistance. The base layer can be made of a metal material such as nickel or stainless steel, a resin material such as polyimide (PI), or the like. Surface coating or lubricant may be applied to the inner surface of the base layer in order to improve sliding (reduce friction) of the belt38against the heating member40. The elastic layer is made of an elastic material such as silicone rubber. The surface release layer is made of a tetrafluoroethylene/perfluoroalkylvinyl ether copolymer (PFA), polytetrafluoroethylene (PTFE), or the like. The belt38is supported by supporting mechanism on its axial ends (Y direction ends in this description) and is capable of rotating about a central axis parallel to the Y direction.

The press roller32is disposed adjacent to the belt38. The press roller32includes a core member33and an elastic layer34. The core member33is formed in a columnar shape by metal or the like. Both Y-direction ends of the core member33can be supported by a housing or the like of image forming apparatus1via a bearing or the like. The core member33is capable of rotating around its central axis (Y-direction). The elastic layer34is provided on the outer circumferential surface of the core member33. The elastic layer34is formed of a silicone rubber foam, silicone rubber, fluorocarbon rubber, or the like. A release layer (not separately illustrated) may be formed on the outer circumferential surface of the elastic layer34. PFA, PTFE, or the like can be used in the release layer.

The press roller32is pressed against the belt38by pressing means to contact with the outer circumferential surface of the belt38the opposite a heater42that is positioned within the interior region formed by the belt. Where the press roller32and the belt38are in contact, the elastic layer34is elastically compressed, whereby a nip N is formed. A sheet S can be held in the nip N and conveyed onward. The nip N has predetermined width in the conveying direction of the sheet S.

The press roller32is driven to rotate by a driving source such as a motor. If the press roller32rotates, a driving force is transmitted to the belt38in the nip N. The belt38is rotated in the direction of an arrow Dl. The press roller32conveys the sheet S by rotating in a state in which the sheet S is placed in the nip N. In the example illustrated inFIG.2, the conveying direction of the sheet S slightly tilts toward the +X direction away from the Z direction axis.

The heating member40is disposed on the inner side of the belt38. The heating member40includes a heater42and a holder45. The heater42is formed in a long plate shape.

In the present description, an x direction, a y direction, and a z direction (note lower case usage in this context) are used as directions in a local coordinate system for describing aspects of the fixing device30. The y direction is the longitudinal direction of the heater42and is parallel to the Y direction of the global coordinate system. The x direction is the width (short planar dimension) direction of the heater42and the +x direction is the conveying direction of the sheet S (a direction toward the downstream side of the sheet conveyance path). The z direction is the thickness direction of the heater42and the +z direction is a direction going towards the press roller32from the heater42and the −z direction is a direction going away from the press roller32towards the heater42.

The heater42is disposed with its longitudinal direction set in parallel to the y direction, the width direction set in parallel to the x direction, and the thickness direction set in parallel to the z direction. The heater42includes a resistance film, a substrate, and a protective layer (each not separately illustrated).

The substrate is made of ceramic, stainless steel, or the like.

The resistance film is formed on the +z direction surface of the substrate. The resistance film can be energized to generate heat. The resistance film may be referred to as a resistive heater, a resistive heating element, or the like. The resistance film may be divided into a plurality of resistance elements spaced along the y direction. It may be preferable that the different resistance elements can be energized independently from one another. With independently controllable resistance elements, the temperature of each element can be set independently. Therefore, it can be possible to heat only a particular region through which passes a sheet S of less than full width of the fixing device30.

The protective layer is provided on the surfaces of the resistance film and the substrate. For example, the protective layer is made of SiO2.

The holder45can be made of an elastic material such as silicone rubber or fluorocarbon rubber, heat resistant resin such as polyimide resin, polyphenylene sulfide (PPS), polyether sulphone (PES), or liquid crystal polymer (LCP), or the like. The holder45is formed generally in a long plate shape. The heater42is disposed on a first surface46on the +z direction side of the holder45. Specifically, a recess47is formed in the first surface46and the heater42is attached to the bottom surface of the recess47. The longitudinal direction of the first surface46is parallel to the y direction. The width direction of the first surface46is parallel to the z direction. The direction normal to the first surface46is parallel to the +x direction. In the example illustrated inFIG.2, the thickness of the holder45increases from the −z direction side to the +z direction side. The longitudinal direction of a second surface48on the −x direction side of the holder45is parallel to the Y direction in this example. The width direction of the second surface48is parallel to the Z direction of the global coordinate system. The direction normal to the second surface48is parallel to the −X direction. The second surface48may be parallel to the first surface46in some examples.

A heat conductive member with high thermal conductance may be disposed between the bottom surface of the recess47and the heater42. The heat conductive member can be formed of a material having a thermal conductivity higher than the thermal conductivity of the substrate of the heater42and the holder45. For example, the heat conductive member is made of a metal material such as copper or aluminum. A graphite sheet may be adopted as the heat conductive member in some examples. The heat conductive member functions to reduce a possible temperature gradient along the y direction of the belt38and the heater42and prevent the temperature of the holder45from locally exceeding a thermal resistant temperature.

FIG.3depicts a view of heating member40and the press roller32taken along the III-III line inFIG.2. The press roller32has a concave shape in which both the y-direction end portions (the axial ends) of the press roller32project further in the radial direction than the central portion of the press roller32. That is, a diameter DC of the outer circumference in the central portion of the press roller32is less than a diameter DE of the outer circumference of the axial end portions of the press roller32. A first projection amount CP, which reflects the amount of concavity, for the press roller32is represented by the equation: CP=(DE−DC)/2. InFIG.3, the outer circumferential portion of the press roller32has a concaved shape. The concaved shape may be a circular arc shape or may be another curved line shape such as an elliptical arc shape, a parabolic shape, or a hyperbolic shape.

In the heating member40, the heater42is attached on the first surface46side of the holder45. The first surface46has a flat planar portion. The second surface48of the holder45has a convex shape in which the central portion of the second surface48along the y direction (between the y-direction ends) projects further in the −X direction than do the y-direction end portions of the second surface48.

InFIG.3, thickness TC along the x direction of the holder45in the central portion is greater than thickness TE along the x direction of the holder45at the y-direction end portions. A second projection amount CH, which reflects the amount convexity, for the second surface48of the holder45is represented by the equation CH=TC−TE. InFIG.3, the second surface48of the holder45has a convexed shape. The convexed shape may be a circular arc shape or may be another curved line shape such as an elliptical arc shape, a parabolic shape, or a hyperbolic shape.

FIG.4depicts a fixing device30when the press roller32is not being pressed against the heating member40. InFIG.4, the belt38is omitted from the illustrated.

The frame50is disposed on the −X direction side of the heating member40. The frame50is long in the Y direction. The frame50is supported by a housing or the like of the image forming apparatus1at both the Y-direction ends. As illustrated inFIG.2, when viewed from the Y direction, the frame50has a U shape opening towards the +X direction. In other examples, the frame50may have an H shape. The frame50includes a coupling section53and a pair of supporting sections52.

The supporting sections52are formed in a long plate shape. The supporting sections52are disposed with the longitudinal dimension set in parallel to the Y direction, the width dimension set in parallel to the Z direction, and the thickness direction set in parallel to the X direction. The pair of supporting sections52is disposed at an interval in the Z direction. The pair of supporting sections52is disposed at both the Z direction end portions of the holder45(which corresponds to both the x direction end portions in the local coordinate system). The pair of supporting sections52supports the holder45. The coupling section53connects the −X direction end portions in the of the pair of supporting sections52to each other. The pair of supporting sections52and the coupling section53may be integrally formed by bending a steel plate material or the like.

As explained above, the press roller32comes into contact with the belt38on the side opposite of the holder45and the heater42. The press roller32forms the nip N with the belt38. The frame50supports the holder45at both the z direction end portions. The +z direction is the conveying direction of the sheet S in the nip N. A pressing force acting on the sheet S in the nip N will tend to be larger at the y-direction end portions than in the center portion between the y-direction end portions. Force applied from the center portion towards both the end portions in the y direction acts on the sheet S. The sheet S will be pulled toward both the y direction end portions. Consequently, creases extending in the y direction will less easily occur on the sheet S.

As illustrated inFIG.4, a positioning mechanism55connects the heating member40and the frame50to one another. The positioning mechanism55incorporates a positioning member56and a locking claw58. The positioning member56has a locking hole57into which the locking claw58can be inserted. The positioning member56is attached to the pair of supporting sections52of the frame50near the +X direction ends. The positioning member56is at or near the Y direction center of these +X direction ends of the frame50. The locking claw58is formed on the −X direction end side of the holder40to be near the Y direction center of the holder45. The locking claw58which is formed on the heating member40can be inserted into the locking hole57of the positioning member56. The positioning member56is attached to the frame50. Consequently, the heating member40can be positioned with respect to the frame50by the engagement of the locking claw58with the locking hole57.

As explained above, the second surface48of the holder45has a convex shape. As such, at both the Y-direction end portions of the holder45, there will be a gap left between the frame50and the second surface48of the holder45.FIG.2also depicts positioning member56being on both the Z direction sides of the frame50and the holder45.

A fixing operation of the fixing device30is explained.

The press roller32illustrated inFIG.3is pressed or urged toward the belt38by a spring or the like. The press roller32comes into contact with the outer circumferential surface of the belt38and forms the nip N between the press roller32and the belt38at a position opposite the heater42. The sheet S is fed to the nip N. The sheet S in the nip N is conveyed toward the +z direction by rotation of the press roller32. Toner images on the sheet S are pressed and heated while the sheet S passes through the nip N. Consequently, the toner images are fixed (fused) to the sheet S.

The press roller32has a concave shape in which both the y-direction end portions of the press roller32project more in the radial direction than the central portion of the press roller32. A pressing force of the press roller32against the sheet S is larger at the end portions than at central portion. Force from the central portion toward both the end portions acts on the sheet S held in the nip N. The sheet S is pulled toward both the end portions. Consequently, creasing along the x direction occur less easily on the sheet S.

The holder45has a convex in which the central portion of the holder45projects further in the x direction (radial direction) than the y direction end portions. If the nip N is formed between the holder45and the press roller32(which has a concave shape), the width of the nip N can be kept uniform along the y direction. Consequently, fixing performance of the fixing device30will be homogeneous along the y direction. The holder45has a convex shape on its second surface48. The first surface46is a flat plane. Compared with when the holder45has the convex shape on the first surface46(on which the heater42is disposed), bending deformation of the heater42involved in press contact and separation of the press roller32is reduced. Since the first surface46is a flat plane, bending of the sheet S is avoided. Consequently, if the sheet S is an envelope or the like, creasing can be suppressed.

The holder45is supported by the frame50in the center of the second surface48. At both the y-direction end portions in the of the second surface48, there will be a gap left between the second surface48and the frame50. When the press roller32having a concave shape is pressed toward the holder45, both the y-direction end portions of the press roller32will come into contact with the belt38. By this contacting of these end portions with the belt38, both the end portions of the holder45bend (flex) in the −x direction. If the press roller32is further pressed, the entire y direction length of the press roller32will come into contact with the belt38. The nip N is thus formed over the entire y direction length.

The second projection amount CH, which corresponds to the amount convexity, of the holder45is greater than the first projection amount CP, which corresponds to the amount of concavity, of the press roller32(that is, amount CH>amount CP). Accordingly, both the y-direction end portions of the holder45continue to bend toward the −x direction until the y direction center portion of the press roller32comes into contact with the belt38. Since the nip N is formed in along entire y direction length by a small pressing force, an excessive pressing force does not act on the nip N. Consequently, creases of the sheet S can be avoided.

As explained above, since the press roller32has a concave shape, creasing of the sheet S is suppressed. However, if the concave amount of the press roller32increases, a force pulling the sheet S to both the y direction end portions of the sheet S increases and curls may occur in the sheet S. The convexity of the holder45is also considered to contribute to creases and curls of the sheet S. In an experiment, the first projection amount CP of a press roller32and the second projection amount CH, of a holder45were changed and the occurrence of creases and curls was checked. A result of the experiment is illustrated inFIG.5.

In the Table 1 ofFIG.5, the occurrence of creases and curls is rated as A, B, or C. A rating “A” for creases (in a “Crease” columns) indicates that creases did not occur. A rating “C” for creases (in a “Crease” column) indicates that creases were visually confirmed. A rating “B” for creases (in a “Crease” column) indicates that unevenness cannot be visually seen but can be confirmed by touch. A rating “A” for curls (in a “Curl” column) indicates that a lifting amount of curls is equal to or smaller than a first threshold amount. A rating “B” for curls (in a “Curl” column) indicates that the lifting amount of curls is larger than the first threshold amount but equal to or less than a second threshold amount. A rating “C” for curls (in a “Curl” column) indicates that the lifting amount of curls is larger than the second threshold amount.

As explained above, the second projection amount CH of the holder45and the first projection amount CP of the press roller32desirably satisfy the relationship CH>CP. InFIG.5, shading is applied to the evaluation results for which the relationship CH≤CP was satisfied.

The rating C for creases is absent the results for which CH>CP and CP≥150 μm are both satisfied. In the cases of CP≥150 μm, it is thought that an appropriate tensile force acts on the sheets S and creases are suppressed.

All experimental crease results in for which CH>CP, CP 150 μm, and 550 μm≥CH have the rating A. In the cases of CH≥600 μm, it is considered that the convexity amount of the holder45is too large and an appropriate tensile force less easily acts on the sheets S. However, for the cases of 550 μm≥CH, creases are effectively suppressed.

The rating C for curls is absent for experimental results for which CH>CP and 250 μm≥CP are both satisfied. In the cases of CP=300 μm, it is thought that a tensile force acting on the sheet S is too large and thus curls occur. In the cases of 250 μm≥CP, curls are suppressed.

All experimental results for curls for which CH>CP, 250 μm≥CP, and 550 μm. CH are met have the rating A. In this case, curls are effectively suppressed.

The rating C for creases and curls is absent for the results satisfying CH>CP and 250 μm≥CP≥150 μm. In these cases, both creases and curls are suppressed.

All experimental results for creases and curls meeting CH>CP, 250 μm≥CP≥150 μm, and 550 μm≥CH have the rating A. In these cases, both creases and curls are effectively suppressed.

As explained above, the fixing device30includes a tubular belt38, a heater42, a press roller32, and a holder45. The heater42is disposed on the inner side of the belt38and has the axial direction of the belt38as its longitudinal direction. The press roller32comes into contact with the belt38on the opposite side of the heater42to form the nip N. The press roller32rotates to convey the sheet S held in the nip N. In the press roller32, both the y direction end portions project in the radial direction by the first projection amount CP more than the y direction center portion in the radial direction. In the holder45, the first surface46(at which the heater42is disposed) is in a flat plane. In the holder45, the y direction center portion on the second surface48projects in the −x direction by the second projection amount CH more than both the y direction end portions. The second projection amount CH is greater than the first projection amount CP.

If the press roller32having a concave shape is pressed toward the holder45, both the y-direction end portions of the holder45(which has a convex shape) bend toward the −x direction. Since the first projection amount CP is less than the second projection amount CH, both the y-direction end portions of the holder45continue to bend in the −x direction until the y direction center portion of the press roller32eventually comes into contact with the belt38. Since the nip N is formed along the entire y direction length by a relatively small press force, an excessive pressing force does not act on the nip N. Consequently, creases of the sheet S can be avoided.

The first projection amount CP is desirably 150 μm or more so that a proper tensile force in the y direction acts on the sheet S and creases of the sheet S are suppressed.

The first projection amount CP is desirably 250 μm or less so that a tensile force in the y direction acting on the sheet S does not become excessive cause the sheet S to curl.

The second projection amount CH is desirably 550 μm or less so that a proper tensile force in the y direction acts on the sheet S and creasing and curling of the sheet S are suppressed.

The fixing device30further includes a frame50that is disposed on the −x direction side of the holder45and supports the holder45at both the z direction end portions in the nip N so that a tensile force in the z direction acts on the sheet S and creasing of the sheet S is suppressed.

In the fixing device30, creasing of the sheet S are suppressed. Therefore, the image forming apparatus1has improved image forming quality.

In an embodiment, the image forming apparatus1is an image processing apparatus, and fixing device30is a heating device. However, in other embodiments, a decoloring apparatus may be the image processing apparatus and a decoloring unit may be the heating device. A decoloring apparatus performs processing associated with a decoloring (erasing) operation on an image previously formed on a sheet using a decolorable toner. The decoloring unit of a decoloring apparatus heats a decolorable toner image on a sheet passing through a nip and thus decolors the toner image.

According to the at least one embodiment explained above, the second projection amount CH (convexity amount) of the holder45is greater than the first projection amount CP (concavity amount) of the press roller32. Consequently, creases in the sheet S can be avoided.