Image forming apparatus and fixing device

An image forming apparatus includes an image forming device for forming a toner image on a recording medium and a fixing device for fixing the toner image formed on the recording medium by applying heat and pressure to the recording medium. In the fixing device, an endless belt, having flexibility, moves to apply heat to the recording medium. A metal thermal conductor, having a pipe shape and provided inside a loop formed by the endless belt, guides the moving endless belt. A heat source heats the metal thermal conductor. A pressing member presses the metal thermal conductor via the endless belt to form a nip between the endless belt and the pressing member. At the nip, the endless belt and the pressing member nip the recording medium bearing the toner image to apply heat and pressure to the recording medium.

PRIORITY STATEMENT

The present patent application claims priority of Japanese Patent Application No. 2006-168628 filed on Jun. 19, 2006 in the Japan Patent Office, the entire contents of which are incorporated by reference herein.

BACKGROUND

1. Technical Field

Some example embodiments generally relate to an image forming apparatus and/or a fixing device, for example, for fixing a toner image on a recording medium.

2. Description of Background Art

A background image forming apparatus, for example, a copying machine, a facsimile machine, a printer, or a multifunction printer having copying, printing, scanning, and facsimile functions, forms a toner image on a recording medium (e.g., a sheet) according to image data by an electrophotographic method. For example, a charger charges a surface of an image carrier (e.g., a photoconductor). An optical writer emits a light beam on the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to image data. The electrostatic latent image is developed with a developer (e.g., toner) to form a toner image on the photoconductor. A transfer device transfers the toner image formed on the photoconductor onto a sheet. A fixing device applies heat and pressure to the sheet bearing the toner image to fix the toner image on the sheet. The sheet bearing the fixed toner image is output onto the outside of the image forming apparatus.

The fixing device generally includes a pressing roller and/or a fixing roller. The pressing roller and the fixing roller oppose each other to form a nip between the pressing roller and the fixing roller. While the pressing roller and the fixing roller nip a sheet bearing a toner image, the pressing roller and the fixing roller apply pressure and heat to the sheet bearing the toner image to fix the toner image on the sheet. Alternatively, the fixing device may include a fixing belt instead of the fixing roller.

FIG. 1illustrates an example background fixing device41RA using a belt method. In the fixing device41RA, a fixing belt103is looped over a fixing roller102and a heating roller104. The fixing roller102opposes a pressing roller101via the fixing belt103. A tension applier107includes a spring108and/or a roller109and applies tension to the fixing belt103at a position between the fixing roller102and the heating roller104. The heating roller104includes a heater105and heats the fixing belt103. The pressing roller101presses the fixing roller102via the fixing belt103to form a nip between the pressing roller101and the fixing belt103. While the pressing roller101and the fixing belt103nip a sheet P bearing a toner image, the pressing roller101and the fixing belt103apply pressure and heat to the sheet P to fix the toner image on the sheet P. A separator106separates the sheet P bearing the fixed toner image and fed by the pressing roller101and the fixing belt103in a direction C from the fixing belt103. A thermistor110detects a temperature of the fixing belt103.

FIG. 2illustrates another example background fixing device41RB using a SURF method (e.g., a film method). In the fixing device41RB, a ceramic heater113opposes a pressing roller111via a fixing belt112having an endless belt shape. A holder114holds the ceramic heater113. A support115supports the holder114. The pressing roller111presses the ceramic heater113via the fixing belt112to form a nip between the pressing roller111and the fixing belt112. The ceramic heater113heats the fixing belt112at the nip.

FIG. 3illustrates yet another example background fixing device41RC using a roller method. In the fixing device41RC, a pressing roller121presses a fixing roller122to form a nip between the pressing roller121and the fixing roller122. A heater123is provided inside the fixing roller122having a thin thickness.

Image forming apparatuses may need to shorten a warm-up time period needed to increase the temperature of the image forming apparatus up to a reference temperature at which a print operation is properly performed after the image forming apparatus is powered on. Image forming apparatuses may also need to shorten a first print time period needed for the image forming apparatus to finish outputting a sheet bearing a fixed toner image onto the outside of the image forming apparatus after the image forming apparatus receives a print request. Image forming apparatuses may also need to form a toner image on a sheet at a higher speed.

When the fixing device41RA (depicted inFIG. 1) is provided in a high-speed image forming apparatus, the fixing belt103may rotate at a high speed. Therefore, an increased amount of heat may be radiated at a portion of the fixing belt103other than the nip, resulting in faulty fixing.

The fixing device41RB (depicted inFIG. 2) has a decreased heat capacity compared to the fixing device41RA (depicted inFIG. 1) and thereby is quickly heated with a compact structure. However, the ceramic heater113heats the fixing belt112at the nip only. Heat is easily drawn from the fixing belt112to a sheet bearing a toner image and having a decreased temperature at an entrance to the nip, resulting in faulty fixing. In the fixing device41RB, the holder114and the support115are provided inside a loop formed by the fixing belt112. The holder114and the support115having an increased heat capacity absorb heat generated by the ceramic heater113, resulting in a decreased thermal conversion efficiency. When the rotating fixing belt112moves away from the ceramic heater113, forced convection cools the fixing belt112, resulting in a decreased thermal conversion efficiency.

In the fixing device41RA or41RB, the rotating fixing belt103or112may move in a thrust direction to collide with a stopper, and may be damaged. When a user removes a jammed sheet from the fixing device41RA or41RB, a force is applied to the fixing belt103or112. When the applied force bends the fixing belt103or112, a small or large kink is formed on the fixing belt103or112. The small kink may break the fixing belt103or112. The large kink may appear as a faulty toner image on a sheet when a fixing operation is performed.

The fixing device41RC (depicted inFIG. 3) having a simple structure has a decreased heat capacity. However, a center of curvature of the nip faces a toner image on a sheet nipped by the pressing roller121and the fixing roller122. Therefore, the sheet is adhered around the fixing roller122via the toner image. The fixing device41RC may include a separator (e.g., a nail, a plate, and/or the like) for preventing the sheet from adhering around the fixing roller122. However, the separator needs to apply an increased force to the sheet and the fixing roller122to separate the sheet from the fixing roller122. The separator may scrape the toner image on the sheet, resulting in a faulty toner image on the sheet.

SUMMARY

At least one embodiment may provide an image forming apparatus that includes an image forming device and a fixing device. The image forming device forms a toner image on a recording medium. The fixing device fixes the toner image formed on the recording medium by applying heat and pressure to the recording medium. The fixing device includes an endless belt, a metal thermal conductor, a heat source, and a pressing member. The endless belt, having flexibility, moves to apply heat to the recording medium. The metal thermal conductor has a pipe shape and is provided inside a loop formed by the endless belt. The metal thermal conductor guides the moving endless belt. The heat source heats the metal thermal conductor. The pressing member presses the metal thermal conductor via the endless belt to form a nip between the endless belt and the pressing member. At the nip, the endless belt and the pressing member nip the recording medium bearing the toner image to apply heat and pressure to the recording medium.

At least one embodiment may provide a fixing device for fixing a toner image on a recording medium by applying heat and pressure to the recording medium. The fixing device includes an endless belt, a metal thermal conductor, a heat source, and a pressing member. The endless belt, having flexibility, moves to apply heat to the recording medium. The metal thermal conductor has a pipe shape and is provided inside a loop formed by the endless belt. The metal thermal conductor guides the moving endless belt. The heat source heats the metal thermal conductor. The pressing member presses the metal thermal conductor via the endless belt to form a nip between the endless belt and the pressing member. At the nip, the endless belt and the pressing member nip the recording medium bearing the toner image to apply heat and pressure to the recording medium.

Additional features and advantages of example embodiments will be more fully apparent from the following detailed description, the accompanying drawings, and the associated claims.

The accompanying drawings are intended to depict example embodiments 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 EXAMPLE EMBODIMENTS

Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, particularly toFIG. 4, an image forming apparatus100according to an example embodiment is explained.

As illustrated inFIG. 4, the image forming apparatus100includes an optical unit35, an image forming device30, an intermediate transfer belt36, a paper tray38, a feeding roller39, a registration roller pair40, a transferor37, a fixing device41, an output roller pair42, and/or an output tray43. The image forming device30includes photoconductors31,32,33, and/or34.

The image forming apparatus100may be a copying machine, a facsimile machine, a printer, a multifunction printer having copying, printing, scanning, and facsimile functions, or the like. According to example embodiments, the image forming apparatus100functions as a color printer for forming a color image on a recording medium by an electrophotographic method.

The optical unit35emits laser beams corresponding to yellow, cyan, magenta, and black image data sent from an external device (e.g., a personal computer) toward the image forming device30. In the image forming device30, the photoconductors34,33,32, and31receive the laser beams to form electrostatic latent images corresponding to the yellow, cyan, magenta, and black image data, respectively. Developing devices (not shown) visualize the electrostatic latent images with yellow, cyan, magenta, and black toners to form yellow, cyan, magenta, and black toner images, respectively. Transferors (not shown) transfer the yellow, cyan, magenta, and black toner images formed on the photoconductors34,33,32, and31respectively onto the intermediate transfer belt36, so that the yellow, cyan, magenta, and black toner images are superimposed on the intermediate transfer belt36to form a color toner image.

The paper tray38loads a recording medium (e.g., sheets). The feeding roller39feeds the sheets one by one toward the registration roller pair40. The registration roller pair40feeds the sheet at a proper time to a transfer nip formed between the intermediate transfer belt36and the transferor37opposing each other. At the transfer nip, the transferor37transfers the color toner image formed on the intermediate transfer belt36onto the sheet fed by the registration roller pair40. The intermediate transfer belt36and the transferor37feed the sheet bearing the color toner image toward the fixing device41. In the fixing device41, heat and pressure is applied to the sheet bearing the color toner image to fix the color toner image on the sheet. The fixing device41feeds the sheet bearing the fixed color toner image toward the output roller pair42. The output roller pair42feeds the sheet bearing the fixed color toner image onto the output tray43.

Referring toFIGS. 5 and 6, the following describes the fixing device41.FIG. 5is a sectional front view of the fixing device41.FIG. 6is a sectional side view of the fixing device41. As illustrated inFIG. 5, the fixing device41includes an endless belt1, a metal thermal conductor2, a heat source3, and/or a pressing roller4. The pressing roller4includes a metal roller5and/or an elastic layer6.

The endless belt1has flexibility. The metal thermal conductor2has a hollow pipe shape and is provided inside a loop formed by the endless belt1. The heat source3includes a heater disposed in the hollow of the metal thermal conductor2. The pressing roller4serves as a pressing member. In the pressing roller4, the elastic layer6is formed on an outer circumferential surface of the metal roller5having a hollow shape. As illustrated inFIG. 6, a length of the pressing roller4in a longitudinal direction of the pressing roller4is shorter than a length of the endless belt1in a longitudinal direction of the endless belt1. The elastic layer6(depicted inFIG. 5) of the pressing roller4pressingly contacts the endless belt1.

As illustrated inFIG. 5, the metal thermal conductor2presses the pressing roller4via the endless belt1to form a nip N between the endless belt1and the pressing roller4contacting each other. A portion in which the endless belt1contacts the pressing roller4(e.g., the nip N) has a flat shape. The hollow pipe shape of the metal thermal conductor2includes a cylindrical shape as illustrated inFIG. 5. However, the metal thermal conductor2may have a hollow polygonal shape as illustrated inFIG. 7. Alternatively, the metal thermal conductor2may have a shape formed by rolling a metal plate. For example, the metal thermal conductor2may be a cylinder having a slit in a longitudinal direction of the cylinder.

As illustrated inFIG. 5, in the pressing roller4, the elastic layer6(e.g., a silicon rubber layer) is formed on the outer circumferential surface of the metal roller5. A surface layer (not shown) for providing a releasing property is formed on the elastic layer6. The surface layer includes a fluoroplastic resin such as a PFA (perfluoroalkoxy) resin and/or a PTFE (polytetrafluoroethylene) resin. A driving source (e.g., a motor) provided in the image forming apparatus100(depicted inFIG. 4), in which the fixing device41is provided, generates a driving force. The driving force is transmitted via a gear, for example, to the pressing roller4to rotate the pressing roller4.

As illustrated inFIG. 6, a pressing member (e.g., a spring) applies a pressing force to the pressing roller4in a direction F, so that the pressing roller4pressingly contacts the endless belt1. As illustrated inFIG. 5, the pressing force deforms the elastic layer6to cause the nip N to have a reference length in a sheet conveyance direction.

The pressing roller4may be a solid roller. However, the pressing roller4may be a hollow roller because the hollow roller has a small heat capacity. The pressing roller4may include a heat source (not shown) such as a halogen heater.

The endless belt1includes a metal belt including nickel and/or stainless steel (SUS) and/or an endless loop belt including a resin (e.g., a polyimide and/or the like). The endless belt1includes a releasing layer (not shown) serving as a surface layer for providing a releasing property to prevent a toner particle forming a toner image on a sheet from adhering to the endless belt1. The releasing layer includes a PFA resin and/or a PTFE resin.

The endless belt1may further include an elastic layer (not shown) formed between a base (not shown) and the releasing layer and including a silicon rubber. When the elastic layer is not provided, the endless belt1has a small heat capacity and thereby provides an increased fixing property. However, when the pressing roller4presses a sheet bearing a toner image toward the endless belt1, surface asperities of the endless belt1are transferred onto the toner image and appear on the toner image as orange peel. To prevent this, the elastic layer needs to have a layer thickness not smaller than about 100 μm. When the elastic layer is deformed, the elastic layer absorbs the surface asperities of the endless belt1and thereby the orange peel does not appear on the toner image on the sheet. However, the nip N has a decreased thermal conductivity and the endless belt1provides a decreased fixing property.

The metal thermal conductor2having a hollow pipe shape includes a metal (e.g., aluminum, iron, stainless steel, and/or the like). The cross section of the metal thermal conductor2illustrated inFIG. 5has a circular shape. However, the cross section of the metal thermal conductor2may have a rectangular shape, a square shape, or other shape. A nip portion N1of the metal thermal conductor2, which contacts an inner circumferential surface of the endless belt1to form the nip N between an outer circumferential surface of the endless belt1and the pressing roller4, has a flat or concave shape to improve a releasing property for releasing a sheet from the endless belt1. The nip portion N1may be shaped by a cutting or press work or by extruding a metal material to have a reference cross section.

The heat source3heats the metal thermal conductor2and the endless belt1to increase the temperature of the metal thermal conductor2and the endless belt1. The heat source3includes a halogen heater as illustrated inFIGS. 5 and 6.

As illustrated inFIG. 8, the fixing device41further includes a resistant heat generator7. The resistant heat generator7is disposed on an inner circumferential surface of the metal thermal conductor2and may serve as a heat source instead of the halogen heater serving as the heat source3illustrated inFIGS. 5 and 6.

As illustrated inFIG. 9, the fixing device41may further include an induction heater8. The induction heater8faces the outer circumferential surface of the endless belt1and may serve as a heat source instead of the halogen heater serving as the heat source3illustrated inFIGS. 5 and 6. The induction heater8heats the metal thermal conductor2via the endless belt1to increase the temperature of the metal thermal conductor2.

An external roller drives the endless belt1to move around its circumferential direction. For example, as illustrated inFIG. 5, a driver (not shown) generates a driving force to rotate the pressing roller4. The driving force is transmitted from the pressing roller4to the endless belt1at the nip N to rotate the endless belt1. At the nip N, the endless belt1moves in a state that the endless belt1is sandwiched between the pressing roller4and the metal thermal conductor2. At a position other than the nip N, the metal thermal conductor2guides the endless belt1so that the endless belt1separates from the metal thermal conductor2with a reference distance or smaller provided between the endless belt1and the metal thermal conductor2.

The metal thermal conductor2having a polygonal pipe shape may be provided inside the loop formed by the endless belt1, for example. However, the metal thermal conductor2having a cylindrical shape similar to the endless belt1may be disposed inside the loop formed by the endless belt1with a clearance of from about 0 mm to about 2 mm provided between the endless belt1and the metal thermal conductor2, so as to reduce variation of the temperature of the endless belt1

FIG. 10illustrates a fixing device41B according to another example embodiment. The fixing device41B includes elements common to the fixing device41depicted inFIG. 5. However, the nip portion N1of the metal thermal conductor2does not have a flat shape (depicted inFIG. 5) but has a concave shape. Namely, the nip portion N1of the metal thermal conductor2has the concave shape to cause the nip N formed between the endless belt1and the pressing roller4to have concave shape.

The metal thermal conductor2and the endless belt1have a similar cylindrical shape in cross section taken on line perpendicular to an axial direction of the metal thermal conductor2and the endless belt1. The metal thermal conductor2and the endless belt1are disposed close to each other. Alternatively, as illustrated inFIG. 11, the metal thermal conductor2may contact an entire inner circumferential surface of the endless belt1with a clearance of about 0 mm provided between the endless belt1and the metal thermal conductor2. Actually, a looseness allowing the endless belt1to rotate or a looseness allowing the heated metal thermal conductor2to thermally expand is provided between the endless belt1and the metal thermal conductor2.

Only the metal thermal conductor2and the heat source3are provided inside a loop formed by the endless belt1. Namely, a guide including a resin and an elastic member including a rubber are not provided. Thus, the fixing device41(depicted inFIG. 5) or41B may have a small heat capacity. However, when a thermistor, a thermostat, and/or grease having a small heat capacity and a heat resistance is provided inside the loop formed by the endless belt1, the fixing device41or41B may have a small heat capacity.

In the fixing device41or41B, the endless belt1and the metal thermal conductor2are heated. Namely, the fixing device41or41B includes a decreased number of elements and does not include a plurality of rollers provided inside the loop formed by the endless belt1, a tension roller contacting the endless belt1, and a resin guide and a metal support stay provided inside the loop formed by the endless belt1. Thus, the fixing device41or41B has a small heat capacity and a compact size. The fixing device41or41B may be quickly heated, resulting in a shortened warm-up time period. When the image forming apparatus100(depicted inFIG. 4) is in a standby mode (e.g., when the image forming apparatus100is pre-heated), the entire endless belt1is heated. When a fixing operation is requested, the entire endless belt1is almost uniformly heated. Thus, the fixing operation may quickly start, resulting in a shortened first print time period (e.g., a time period needed until the image forming apparatus100outputs a sheet bearing a fixed toner image after the image forming apparatus100receives a print request).

In the fixing device41or41B, the metal thermal conductor2having an increased thermal conductivity forms the nip N. Even when the heat source3supplies a decreased amount of heat for a fixing operation, heat stored in the metal thermal conductor2is transmitted to the endless belt1to compensate for the shortage of heat, preventing a decreased fixing temperature. The metal thermal conductor2for supplying heat to the nip N has a pipe shape and includes the nip portion N1forming the nip N and another portion not forming the nip N. When the metal thermal conductor2supplies heat to the endless belt1at the nip N, heat stored in the portion not forming the nip N flows to the nip portion N1forming the nip N, because the entire metal thermal conductor2has an increased thermal conductivity.

The metal thermal conductor2is provided inside the loop formed by the endless belt1. Therefore, airflow may not cool the metal thermal conductor2, unlike a rotating heating roller. Thus, the metal thermal conductor2may effectively keep heat without a temperature detector such as a thermistor, preventing a decreased temperature of the endless belt1caused by time lag in temperature detection and delay in control.

The heat source3directly or indirectly heats the metal thermal conductor2. Convection in an air layer formed between the endless belt1and the metal thermal conductor2, radiant heat generated by the metal thermal conductor2, or heat conduction from the metal thermal conductor2to the endless belt1heats the entire endless belt1. Thus, the fixing device41or41B provides a smaller temperature variation in a circumferential direction of the endless belt1than a fixing device using a SURF method or a belt method. As a result, the nip N may provide a decreased temperature variation (e.g., a decreased temperature ripple) and thereby may provide a stable fixing property.

When the fixing device41or41B is provided in a high-speed image forming apparatus, a sheet is conveyed at an increased speed and thereby the endless belt1moves at an increased speed. In a fixing device using the SURF method, an endless belt is heated mainly at a nip formed between the endless belt and a pressing roller. After a heated portion on the endless belt moves out of the nip, the heated portion on the endless belt is not heated until the heated portion reaches the nip again. Therefore, the heated portion has a decreased temperature when the heated portion enters the nip. When the endless belt moves at an increased speed, the endless belt has a decreased temperature at an entrance to the nip, resulting in faulty fixing. However, in the fixing device41or41B, the entire endless belt1is heated simultaneously. Namely, the endless belt1is properly heated while the endless belt1moves, reducing faulty fixing.

The fixing device using the SURF method may include a guide for guiding the endless belt so that the endless belt properly moves. When an increased friction generates between the guide and the endless belt contacting each other, the friction may apply an increased load to the endless belt, preventing proper moving of the endless belt.

In the fixing device41or41B, the metal thermal conductor2serves as a guide for guiding the endless belt1. Namely, the guide has an increased temperature.FIG. 12is a graph showing a relationship between a temperature of the metal thermal conductor2and the endless belt1and a friction coefficient caused between the metal thermal conductor2including a metal (e.g., aluminum) and the endless belt1including a resin. As illustrated inFIG. 12, the friction coefficient decreases as the temperature increases. In the fixing device41or41B, an action for decreasing a friction resistance works between the metal thermal conductor2and a resin member forming a surface layer of the endless belt1. Thus, a proper slipping property may be provided between the endless belt1and the metal thermal conductor2contacting each other, resulting in proper movement of the endless belt1.

In the fixing device41or41B, the metal thermal conductor2contacts or is disposed close to the endless belt1, reducing temperature variation in the circumferential direction of the endless belt1and maintaining a constant temperature of the endless belt1. Further, the metal thermal conductor2and the endless belt1have a similar shape, providing a substantially constant clearance between the metal thermal conductor2and the endless belt1. Thus, an amount of heat conducted to the endless belt1may be uniform in the circumferential direction of the endless belt1. As a result, a uniform surface temperature of the endless belt1may prevent temperature variation of the endless belt1.

The metal thermal conductor2contacts the endless belt1to conduct heat from the metal thermal conductor2to the endless belt1so as to increase the temperature of the endless belt1. The entire endless belt1has a uniform temperature. Namely, the temperature of the endless belt1does not fluctuate at the nip N, reducing a temperature ripple of the endless belt1. Even in a standby mode when the endless belt1does not move, the entire endless belt1is already heated. Thus, a fixing operation may quickly start upon a fixing request.

When the metal thermal conductor2is not disposed close to the endless belt1but contacts the entire inner circumferential surface of the endless belt1as illustrated inFIG. 7, a looseness is not provided between the metal thermal conductor2and the endless belt1. The endless belt1is disposed parallel to the nip N and does not have a serpentine shape. Even when an external force is applied to the endless belt1, the endless belt1may not bend or break because the metal thermal conductor2contacts and supports the inner circumferential surface of the endless belt1. In a fixing device using the SURF method or the belt method, a part of an inner circumferential surface of an endless belt is not supported. When the endless belt moves, the inner and outer circumferential surfaces of the part of the endless belt not supported are cooled down, providing a decreased thermal conversion efficiency. In the fixing device41or41B, at least inner circumferential surface of the endless belt1contacts the metal thermal conductor2. Airflow may not cool the endless belt1, resulting in an increased thermal conversion efficiency.

When the metal thermal conductor2is disposed close to the endless belt1with a clearance provided between the metal thermal conductor2and the endless belt1, a decreased torque may be needed to move the endless belt1, resulting in smooth movement of the endless belt1.

The fixing device41or41B may have a decreased heat capacity inside the loop formed by the endless belt1compared to a fixing device using the SURF method or the belt method, because the fixing device41or41B includes no elements to be heated inside the loop formed by the endless belt1. Thus, the fixing device41or41B provides an increased thermal conversion efficiency. The fixing device41or41B includes only the heat source3having a heat resistant property and the metal thermal conductor2having a high melting point inside the loop formed by the endless belt1. For example, the fixing device41or41B does not include a resin member which is included in a fixing device using the SURF method and a silicon rubber which is included in a fixing device using the belt method. Namely, the fixing device41or41B does not include the resin member and the silicon rubber which may be deformed and damaged respectively, when the heat source3is accidentally out of control and continuously performs heating.

FIG. 13Aillustrates a tester fixing device41TA in which the heat source3is provided between the metal thermal conductor2and the endless belt1. In the tester fixing device41TA, when the endless belt1moves closer to the heat source3, the endless belt1may be excessively heated. Even when the endless belt1is stably positioned with respect to the heat source3, the endless belt1may have various temperatures because the endless belt1having a small heat capacity is quickly heated.

FIG. 13Billustrates a tester fixing device41TB in which the heat source3is provided outside a loop formed by the endless belt1. In the tester fixing device41TB, the heat source3radiates heat in a direction in which the endless belt1is not disposed as well as in a direction in which the endless belt1is disposed, providing a decreased thermal conversion efficiency. Therefore, the heat source3may be disposed inside a hollow formed by the metal thermal conductor2.

In the fixing device41or41B (depicted inFIG. 5or10respectively), the metal thermal conductor2is provided inside the loop formed by the endless belt1. The nip N has a flat or concave shape. As illustrated inFIG. 14, when a sheet bearing a toner image contacts the endless belt1via the toner image having a viscosity, a conveyance direction A of the sheet separates from a moving direction B of the endless belt1at an exit of the nip N (e.g., a downstream portion of the nip N in a sheet conveyance direction). The metal thermal conductor2guiding the endless belt1regulates the moving direction B of the endless belt1toward an outer circumferential direction of the endless belt1. A separating force generates in the conveyance direction A equivalent to a direction tangent to a curve of the nip N at the exit of the nip N. The separating force separates the sheet bearing the toner image from the endless belt1against the viscosity of the toner image.

The fixing device41or41B (depicted inFIG. 5orFIG. 10respectively) includes a halogen heater serving as the heat source3and disposed at a substantially center of the hollow formed by the metal thermal conductor2. The entire inner circumferential surface of the metal thermal conductor2is coated in black to form a blackbody surface. Thus, the entire metal thermal conductor2is heated. At the nip N at which the endless belt1opposes the heat source3via the metal thermal conductor2, heat is conducted from the metal thermal conductor2to the endless belt1. A portion of the endless belt1which does not form the nip N is heated by radiant heat generated by the metal thermal conductor2and heat transmitted via the air layer formed between the endless belt1and the metal thermal conductor2. The above-described configuration may provide a proper fixing property. However, the endless belt1may be locally heated at the nip N to provide an increased fixing property.

When the endless belt1is locally heated at the nip N, an increased amount of heat is supplied to the endless belt1to compensate for heat drawn to a sheet. Thus, even when the fixing device41or41B performs a fixing operation at a high speed (e.g., even when the image forming apparatus100depicted inFIG. 4performs an image forming operation at a high speed), the fixing device41or41B may provide a proper fixing property. At the entrance to the nip N (e.g., an upstream portion of the nip N in the sheet conveyance direction), a temperature difference between the sheet and the endless belt1is substantially large. Therefore, an increased amount of heat needs to be supplied to the entrance to the nip N. For example, the upstream portion of the nip N in the sheet conveyance direction may be locally heated.

The nip portion N1of the metal thermal conductor2forming the nip N as well as the other portion of the metal thermal conductor2include a common material. Thus, even when the endless belt1is locally heated at the nip N, heat is transmitted to the other portion of the metal thermal conductor2to heat the endless belt1when heat is excessively supplied to the sheet. The endless belt1may have a constant temperature, preventing variation in fixing property and gloss of a fixed toner image.

FIGS. 15A and 15Billustrate fixing devices41CA and41CB, respectively, according to example embodiments. Each of the fixing devices41CA and41CB includes elements common to the fixing device41(depicted inFIG. 5). Each of the fixing devices41CA and41CB includes the heat source3(e.g., a halogen heater) located near the nip portion N1to locally heat the nip portion N1. For example, as illustrated inFIG. 15A, the heat source3is disposed near a center of the nip portion N1in a sheet conveyance direction. As illustrated inFIG. 15B, the heat source3is disposed near an upstream portion N2of the nip portion N1in a sheet conveyance direction.

The heat source3may include a halogen heater including a glass tube having a mirror-finished half surface or a ceramic heater disposed close to the nip portion N1.FIG. 16illustrates a fixing device41D according to yet another example embodiment. The fixing device41D includes an induction heater12instead of the heat source3(depicted inFIG. 5). The induction heater12includes a coil10and/or a core11. The coil10is coiled around the core11. The induction heater12locally heats the nip N. The other elements of the fixing device41D are common to the fixing device41(depicted inFIG. 5).

In the fixing device41or41B (depicted inFIG. 5or10respectively), the entire endless belt1is simultaneously heated. For example, a portion of the endless belt1forming the nip N and the other portion of the endless belt1are heated. Namely, heat may be supplied to the endless belt1more stably than in a fixing device using the SURF method. In the fixing device41CA (depicted inFIG. 15A),41CB (depicted inFIG. 15B), or41D (depicted inFIG. 16), the nip N is locally heated. However, after the nip N is heated, heat is transmitted from the nip N to a portion of the endless belt1not forming the nip N.

In the fixing device41CA,41CB, or41D, heat is quickly transmitted to the nip N at which heat is quickly drawn to a sheet. Even when heat is excessively generated, the heat is diffused to an entire circumferential surface of the endless belt1because a temperature gradient generates between the nip portion N1of the metal thermal conductor2forming the nip N and the other portion of the metal thermal conductor2not forming the nip N. Namely, a sheet may not quickly cool the nip N. Even when the nip N is excessively heated, heat is transmitted from a portion of the endless belt1forming the nip N to the other portion of the endless belt1not forming the nip N, resulting in a constant temperature of the nip N and a stable fixing property.

As illustrated inFIG. 16, in the fixing device41D, a portion of the induction heater12which generates heat may be limited by changing a coiling of the coil10so as to locally heat the endless belt1. When heat is drawn from the endless belt1to a sheet at the nip N, the induction heater12may quickly supply heat to the endless belt1, improving a fixing property.

As illustrated inFIGS. 15A and 15B, when a halogen heater is used as the heat source3, the fixing device41CA or41CB includes a decreased number of elements compared to the fixing device41D including the coil10and the core11(depicted inFIG. 16). Thus, the heat source3may have a decreased heat capacity and may reduce cost. The halogen heater may have a simpler structure for adjusting a temperature of an end portion of the halogen heater compared to the ceramic heater, resulting in decreased manufacturing costs.

The halogen heater (e.g., the heat source3) or the induction heater12may not be disposed near the nip N due to a layout of elements of the fixing device41CA,41CB, or41D. Further, a mirror-finished halogen heater or the induction heater12may increase manufacturing costs.

FIG. 17illustrates a fixing device41E according to yet another example embodiment. The fixing device41E includes a blackbody surface13, a mirror surface14, and/or a thin portion2A. The other elements of the fixing device41E are common to the fixing device41(depicted inFIG. 5). A halogen heater is used as the heat source3. The blackbody surface13is provided on an inner circumferential surface of the nip portion N1of the metal thermal conductor2forming the nip N. Thus, the halogen heater may locally heat the nip N even when the halogen heater is disposed in a center of a loop formed by the endless belt1. The mirror surface14is mirror-finished and is provided on an inner circumferential surface of a portion other than the nip portion N1of the metal thermal conductor2, the portion not forming the nip N. The mirror surface14reflects light or heat generated by the halogen heater. Thus, the halogen heater may locally heat the blackbody surface13provided on the nip portion N1.

The thin portion2A is provided in the nip portion N1forming the nip N, and has a thickness smaller than a thickness of a portion of the metal thermal conductor2other than the nip portion N1. Thus, the fixing device41E may provide an increased thermal conductivity to a sheet and may thereby provide a proper fixing property.

When the blackbody surface13is provided on the inner circumferential surface of the nip portion N1of the metal thermal conductor2, the nip portion N1absorbs an increased amount of radiant heat generated by the halogen heater. Namely, the halogen heater may locally heat the nip N. When the mirror surface14is provided on the inner circumferential surface of the portion other than the nip portion N1of the metal thermal conductor2, the mirror surface14reflects radiant heat generated by the halogen heater, even when the radiant heat is emitted toward the inner circumferential surface of the portion other than the nip portion N1of the metal thermal conductor2. Thus, the halogen heater may locally heat the nip N. The thin portion2A is provided in the nip portion N1of the metal thermal conductor2. Thus, a part of the metal thermal conductor2may have a small heat capacity. The thin portion2A of the metal thermal conductor2may be quickly heated and may quickly conduct heat to the endless belt1. When a sheet draws heat from the endless belt1at the nip N, the metal thermal conductor2may quickly conduct heat to the endless belt1at the nip N, increasing a fixing property.

The endless belt1moves on the metal thermal conductor2provided inside the loop formed by the endless belt1. For example, the endless belt1slides on the metal thermal conductor2at the nip N. When an increased friction generates at an interface between the endless belt1and the metal thermal conductor2, the endless belt1may be scraped and damaged by friction.

As illustrated inFIG. 18A, the fixing device41E may further include a lubricant15. The lubricant15includes grease and/or oil and is applied to the interface between the endless belt1and the metal thermal conductor2to reduce friction between the endless belt1and the metal thermal conductor2. Thus, the endless belt1may not be scraped and damaged by friction.

The rotating endless belt1rotates the lubricant15. As a result, the lubricant15provided at the nip N may be reduced or may become empty. As illustrated inFIG. 18B, the fixing device41E may include a lubricant sheet16instead of the lubricant15(depicted inFIG. 18A). The lubricant sheet16is impregnated with a lubricant and is provided between the metal thermal conductor2and the endless belt1to continuously supply the lubricant to the nip N. For example, the lubricant sheet16is sandwiched between the metal thermal conductor2and the endless belt1at the interface between the metal thermal conductor2and the endless belt1. The lubricant sheet16is fixed to the metal thermal conductor2so that the lubricant sheet16is constantly placed at the interface between the metal thermal conductor2and the endless belt1at the nip N. The lubricant sheet16impregnated with a lubricant may maintain its lubricating property.

The lubricant sheet16sandwiched between the metal thermal conductor2and the endless belt1may provide an increased thermal resistance and thereby the fixing device41E may provide a decreased fixing property.

As illustrated inFIG. 19, the fixing device41E may include a resin-coated layer17instead of the lubricant sheet16(depicted inFIG. 18B). The resin-coated layer17is formed by coating a portion of the metal thermal conductor2, which contacts the endless belt1(depicted inFIG. 18B), with fluoroplastic (e.g., a PFA resin, a PTFE resin, and/or the like). The resin-coated layer17may have a layer thickness of several tens of μm. The resin-coated layer17has a small heat resistance and a small surface friction coefficient. Thus, the resin-coated layer17may provide a lubricating property for a longer time period than the lubricant sheet16while the fixing device41E provides a proper fixing property.

FIG. 20illustrates a fixing device41F according to example embodiments. The metal thermal conductor2of the fixing device41F includes a convex portion18. The other elements of the fixing device41F are common to the fixing device41E (depicted inFIG. 17). A part of the nip portion N1of the metal thermal conductor2protrudes to form the convex portion18. The convex portion18presses a sheet bearing a toner image with an increased pressure. Thus, the fixing device41F provides an increased fixing property.

When the convex portion18is provided at an exit of the nip N (e.g., a downstream portion of the nip portion N1in a sheet conveyance direction), a curve formed by the metal thermal conductor2has a small curvature at the convex portion18. As a result, a sheet bearing a toner image may easily separate from the endless belt1, preventing a sheet from being wound around the endless belt1and being jammed.

The convex portion18causes the endless belt1and the pressing roller4to pressingly contact each other with an increased pressure. When the pressing roller4rotates the endless belt1, the pressing roller4drives the endless belt1with an increased friction, preventing the endless belt1from slipping on the pressing roller4.

Generally, a safety device is provided near the heat source3to cope with a situation in which temperature control does not properly work. The safety device includes a thermal fuse and/or a thermostat.

According to the above-described example embodiments, the nip N is heated more quickly than any other elements. When the safety device is provided outside a loop formed by the endless belt1, the safety device may activate at a delayed time when temperature control does not properly work, because the outside of the loop formed by the endless belt1is slowly heated. As a result, the heat source3may emit smoke or may catch fire depending on an output of the heat source3or a heat capacity of elements forming the heat source3.

As illustrated inFIG. 21, the fixing device41F may further include a thermal fuse19, a thermistor20, and/or a thermopile21. The thermal fuse19serves as a safety device. The thermal fuse19is provided between the endless belt1and the metal thermal conductor2to quickly detect the temperature of the endless belt1and/or the metal thermal conductor2in a case that temperature control does not properly work. A thermostat instead of the thermal fuse19may be used as a safety device.

The thermistor20serves as a temperature detector to control the temperature of the nip N. A thermocouple instead of the thermistor20may be used as a temperature detector. At a portion other than the nip N, the metal thermal conductor2is heated up to a reference temperature more quickly than the endless belt1. Therefore, the thermistor20may directly detect the temperature of the metal thermal conductor2instead of the endless belt1to detect a response to heat generated by the heat source3. For example, the thermistor20is provided between the endless belt1and the metal thermal conductor2to detect the temperature of the metal thermal conductor2. The heat source3is controlled based on a detection result so that the heat source3heats the metal thermal conductor2up to a reference target temperature.

The amount of heat drawn to a sheet bearing a toner image at the nip N varies depending on type and temperature of the sheet. Therefore, the temperature of the endless belt1needs to be detected to determine how much the temperature of the endless belt1is decreased after the endless belt1passes the nip N.

A non-contact type temperature detector may be used to detect the temperature of the endless belt1. When a contact type temperature detector is used, fine particles (e.g., toner particles) adhered to the endless belt1move from the endless belt1onto the contact type temperature detector. When the toner particles are accumulated on the contact type temperature detector, the accumulated toner particles may deteriorate detection accuracy or may damage the endless belt1, forming a faulty line image on a sheet. In the fixing device41F, the thermopile21is used as a non-contact type temperature detector.

In the fixing device41F including two temperature detectors (e.g., the thermistor20and the thermopile21), a basic temperature control is performed based on a detection result provided by the thermistor20to adjust the temperature of the endless belt1to a reference target temperature. The thermopile21detects a temperature difference caused by type and temperature of a sheet and/or environmental conditions. The thermistor20and the thermopile21send detection results to a controller (not shown) for controlling the temperature of the endless belt1.

When the endless belt1is disposed close to the metal thermal conductor2or contacts the metal thermal conductor2, heat is quickly transmitted from the metal thermal conductor2to the endless belt1. Therefore, the safety device (e.g., the thermal fuse19) and the temperature detector (e.g., the thermistor20and/or the thermopile21) may be disposed outside the loop formed by the endless belt1.

As illustrated inFIG. 22, the fixing device41F further includes a driving roller22and/or a cleaning roller23.FIG. 22illustrates an example structure of a driver for driving the endless belt1, which may be provided in the fixing device41,41B,41CA,41CB,41D,41E, or41F depicted inFIG. 5,10,15A,15B,16,17, or20respectively. The driving roller22serves as a belt driver for driving the endless belt1, and is provided outside the loop formed by the endless belt1. The driving roller22includes a gear (not shown) and/or a silicon rubber layer (not shown). The gear is provided on an end of a shaft of the driving roller22and transmits a driving force generated by a driver not shown (e.g., a motor). The silicon rubber layer forms a surface layer contacting the endless belt1and having an increased surface friction coefficient. The driving roller22transmits a driving force generated by the driver to the endless belt1contacting the driving roller22.

The driving roller22may further include a releasing layer (not shown) forming a surface layer of the driving roller22. The releasing layer includes a PFA resin and/or a PTFE resin. The cleaning roller23may contact the surface of the driving roller22. The cleaning roller23includes a surface layer (not shown) including metal, a silicon rubber, and/or felt to collect a substance (e.g., toner particles) adhered to the driving roller22. When the cleaning roller23contacts the driving roller22, the cleaning roller23causes the driving roller22to generate a smaller driving force compared to the driving roller22including the silicon rubber layer and not being contacted by the cleaning roller23. However, the cleaning roller23may prevent a substance adhered to the driving roller22from falling onto the endless belt1.

A tension T is applied to the endless belt1between the exit (e.g., a downstream portion) of the nip N in a sheet conveyance direction and the driving roller22, so that the rotating endless belt1does not slip.

As illustrated inFIG. 23, the fixing device41F further includes a heat resistant elastic layer26. The endless belt1includes a base24and/or an elastic layer25.

The endless belt1serves as a multilayered belt including the base24and the elastic layer25. The base24includes a polyimide resin and/or nickel. The elastic layer25includes a silicon rubber.

When the fixing device41F performs a fixing operation on a sheet bearing a toner image, the elastic layer25may include a releasing layer (not shown) which forms a surface layer (e.g., a layer contacting a toner image on a sheet) of the elastic layer25. The releasing layer includes a PFA resin and/or a PTFE resin. For example, a silicon rubber layer having a layer thickness of from about 100 μm to about 500 μm is formed on the base24including a polyimide resin. A releasing layer including a PFA resin and having a layer thickness of from about 10 μm to about 50 μm is formed on the silicon rubber layer.

When the endless belt1contacts a sheet bearing a toner image, the elastic layer25absorbs asperities of the toner image on the sheet. Thus, the endless belt1may uniformly apply heat to the sheet, reducing a faulty toner image appearing as orange peel on the sheet and thereby improving quality of a fixed toner image.

The heat resistant elastic layer26is provided between the nip portion N1of the metal thermal conductor2and the endless belt1. The heat resistant elastic layer26includes a silicon rubber and/or a heat resistant felt pad. When the heat resistant elastic layer26includes a silicon rubber, the silicon rubber is formed in a sponge shape to provide an increased insulation property. Thus, a decreased amount of heat may be drawn to a sheet at the nip N, preventing faulty fixing.

The silicon rubber or the felt is impregnated with a lubricant (e.g., a silicon oil and/or the like) to provide an improved sliding property of the endless belt1at the nip N.

The heat resistant elastic layer26absorbs asperities of a toner image on a sheet to cause the sheet to properly contact the endless belt1, resulting in proper fixing and formation of a high quality image.

The fixing device (e.g., the fixing device41depicted inFIG. 5) includes an endless belt (e.g., the endless belt1depicted inFIG. 5) having flexibility, a metal thermal conductor (e.g., the metal thermal conductor2depicted inFIG. 5) provided inside a loop formed by the endless belt, a heat source (e.g., the heat source3depicted inFIG. 5), and/or a pressing member (e.g., the pressing roller4depicted inFIG. 5) for pressing the metal thermal conductor via the endless belt to form a nip between the endless belt and the pressing member. The fixing device applies heat and pressure to a recording medium (e.g., a sheet) conveyed through the nip to fix a toner image on a sheet. The metal thermal conductor has a pipe shape and is heated by the heat source. The metal thermal conductor is provided inside the loop formed by the endless belt. The endless belt is movable on the metal thermal conductor. While the metal thermal conductor pressingly opposes the pressing member via the endless belt to form the nip between the pressing member and the endless belt, the metal thermal conductor guides the endless belt moving on the metal thermal conductor. The endless belt and the metal thermal conductor having the pipe shape are heated. Therefore, the endless belt and the metal thermal conductor may be quickly heated, shortening a warm-up time period of the fixing device. The metal thermal conductor having an increased thermal conductivity forms the nip between the pressing member and the endless belt. Thus, even when the heat source supplies a decreased amount of heat to the metal thermal conductor during print operation, heat stored in the metal thermal conductor may be supplied to the endless belt to compensate for the shortage of heat, preventing temperature decrease of the endless belt. The metal thermal conductor for storing heat is provided inside the loop formed by the endless belt. Therefore, the metal thermal conductor may not be easily cooled by airflow, unlike a rotatable metal thermal conductor. Namely, the metal thermal conductor may effectively keep heat and thereby temperature decrease of the metal thermal conductor may be prevented.

The heat source directly or indirectly heats the metal thermal conductor. Convection in an air layer formed between the endless belt and the metal thermal conductor, radiant heat generated by the metal thermal conductor, and/or thermal conduction between the metal thermal conductor and the endless belt heat the entire endless belt. Temperature variation is reduced in the circumferential direction of the endless belt. Namely, the endless belt has a decreased temperature variation (e.g., a decreased temperature ripple) at the nip, providing a stable fixing property. The fixing device may be located in a high-speed image forming apparatus in which a sheet is conveyed at a high speed. The metal thermal conductor having an increased temperature guides the endless belt, reducing a friction resistance. Thus, the endless belt may properly contact and slide on the metal thermal conductor.

In the fixing device, the metal thermal conductor and the endless belt have a similar shape in cross section taken on line perpendicular to an axial direction of the metal thermal conductor and the endless belt. The metal thermal conductor having the pipe shape and the endless belt are disposed close to each other. Temperature variation is reduced in the circumferential direction of the endless belt, improving temperature stability. The metal thermal conductor and the endless belt have a similar shape and are disposed close to each other. Namely, a substantially common clearance is provided between the metal thermal conductor and the endless belt. A uniform amount of heat in the circumferential direction of the endless belt may be conducted to the endless belt. Thus, the endless belt has a uniform surface temperature, preventing temperature variation of the endless belt.

In the fixing device, the metal thermal conductor contacts an entire inner circumferential surface of the endless belt. No looseness is provided between the metal thermal conductor and the endless belt and thereby the endless belt moves in parallel with the nip without serpentining. At least inner circumferential surface of the endless belt contacts the metal thermal conductor and airflow does not cool the endless belt, providing an increased thermal conversion efficiency. Even when a force is applied to the endless belt, the endless belt may not bend or break because the metal thermal conductor supports the inner circumferential surface of the endless belt.

In the fixing device, the metal thermal conductor is provided inside the loop formed by the endless belt. Alternatively, the metal thermal conductor and the heat source are provided inside the loop formed by the endless belt. Thus, a heat capacity inside the loop of the endless belt may be reduced. An increased thermal conversion efficiency may be provided because the heat source needs to heat no extra element other than the metal thermal conductor and the endless belt.

In the fixing device, the heat source is provided in a hollow of the metal thermal conductor, providing an increased thermal conversion efficiency when the heat source heats the metal thermal conductor.

In the fixing device, the metal thermal conductor includes a nip portion (e.g., the nip portion N1depicted inFIG. 5) to form a nip between the endless belt and the pressing member. The nip portion has a flat shape or a concave shape. Therefore, a sheet bearing a toner image, which is adhered to the endless belt via the toner image having viscosity, may properly separate from the endless belt at an exit of the nip in a sheet conveyance direction and may move in a direction different from a direction in which the endless belt moves.

In the fixing device, the metal thermal conductor includes the nip portion to form the nip between the endless belt and the pressing member and the nip portion includes an upstream portion (e.g., the upstream portion N2depicted inFIG. 15B) in the sheet conveyance direction. The heat source locally heats the nip portion or the upstream portion, resulting in a stable temperature of the nip and a stable fixing property.

In the fixing device, an induction heater is used as the heat source for locally heating the nip portion or the upstream portion. A portion of the induction heater, which generates heat, may be limited by changing winding of a coil included in the induction heater. Even when heat is drawn from the endless belt to a sheet at the nip, the induction heater may quickly supply heat to the endless belt, providing an increased fixing property.

In the fixing device, a halogen heater is used as the heat source for locally heating the nip portion or the upstream portion. The halogen heater does not include a coil and/or a core included in the induction heater. Namely, a number of elements included in the heat source is decreased, resulting in a decreased heat capacity and a decreased manufacturing cost of the heat source.

In the fixing device, the fixing device further includes a blackbody surface (e.g., the blackbody surface13depicted inFIG. 17) for receiving radiant heat generated by the halogen heater, and provided on an inner circumferential surface of the nip portion or the upstream portion of the metal thermal conductor. Thus, radiant heat generated by the halogen heater may be substantially absorbed at the nip portion or the upstream portion. The metal thermal conductor may be locally heated and may quickly supply heat to the endless belt when heat is drawn from the endless belt to a sheet at the nip, providing an increased fixing property.

The fixing device further includes a mirror surface (e.g., the mirror surface14depicted inFIG. 17) to receive radiant heat generated by the halogen heater, and provided on an inner circumferential surface of a portion not forming the nip portion or the upstream portion of the metal thermal conductor. The mirror surface (e.g., a portion of the metal thermal conductor other than the nip portion) reflects radiant heat generated by the halogen heater. Namely, the halogen heater may intensively heat the nip portion. Thus, the metal thermal conductor may be locally heated and may quickly supply heat to the endless belt when heat is drawn from the endless belt to a sheet at the nip, providing an increased fixing property.

In the fixing device, the nip portion or the upstream portion of the metal thermal conductor has a thickness smaller than a thickness of a portion of the metal thermal conductor other than the nip portion or the upstream portion. Namely, a part (e.g., the nip portion or the upstream portion) of the metal thermal conductor has a small heat capacity and is easily heated. Thus, heat is quickly transmitted from the metal thermal conductor to the endless belt. The metal thermal conductor may quickly supply heat to the endless belt when heat is drawn from the endless belt to a sheet at the nip, providing an increased fixing property.

The fixing device further includes a lubricant (e.g., the lubricant15depicted inFIG. 18A) provided between the endless belt and the metal thermal conductor. Thus, friction between the endless belt and the metal thermal conductor may be reduced, preventing wear of the endless belt.

The fixing device further includes a lubricant sheet (e.g., the lubricant sheet16depicted inFIG. 18B) impregnated with a lubricant and provided between the endless belt and the metal thermal conductor. The lubricant is constantly supplied to the inner circumferential surface of the endless belt. Thus, friction between the endless belt and the metal thermal conductor may be reduced for a long time period, preventing wear of the endless belt.

In the fixing device, a PFA resin or a PTFE resin is coated on a portion of the metal thermal conductor, which contacts the endless belt. Friction coefficient between the metal thermal conductor and the endless belt is decreased. Namely, friction between the metal thermal conductor and the endless belt may be reduced for a longer time period compared to a case in which grease is applied on the metal thermal conductor, preventing wear of the endless belt.

The metal thermal conductor further includes a convex portion (e.g., the convex portion18depicted inFIG. 20) provided on a part of the nip portion of the metal thermal conductor. The metal thermal conductor presses the pressing member via the endless belt with an increased pressure at the convex portion, providing an increased fixing property. The endless belt and the pressing member pressingly contact each other with an increased pressure. The pressing member may drive the endless belt with an increased friction, preventing the endless belt from slipping on the pressing member.

In the fixing device, the convex portion of the metal thermal conductor is provided on a downstream portion (e.g., an exit) of the nip portion of the metal thermal conductor in the sheet conveyance direction. The endless belt partially has an increased curvature. Thus, a sheet may easily separate from the endless belt, preventing the sheet from winding around the endless belt.

The fixing device further includes a safety device (e.g., the thermal fuse19depicted inFIG. 21) for detecting a temperature of the metal thermal conductor and provided between the metal thermal conductor and the endless belt. When the safety device detects an abnormal temperature, power supply to the heat source is stopped. Thus, the fixing device may detect an abnormal temperature more quickly than a fixing device in which a thermal fuse and/or a thermostat is provided outside a loop formed by an endless belt.

The fixing device further includes a temperature detector (e.g., the thermistor20depicted inFIG. 21) for detecting a temperature of the metal thermal conductor and provided between the metal thermal conductor and the endless belt. The fixing device may detect increase and decrease of the temperature of the metal thermal conductor caused by the heat source more quickly than a fixing device in which the temperature of a metal thermal conductor is detected via an endless belt. Thus, the fixing device may properly control the temperature of the endless belt and may reduce temperature ripple of the endless belt.

The fixing device further includes a non-contact type temperature detector (e.g., the thermopile21depicted inFIG. 21) for detecting the temperature of the endless belt and provided outside the loop formed by the endless belt. The non-contact type temperature detector may detect an amount of heat drawn from the endless belt to a sheet. A detection result is sent to a controller for controlling the temperature of the endless belt. Thus, a fixing temperature may be properly adjusted.

The fixing device further includes a belt driver (e.g., the driving roller22depicted inFIG. 22) for driving the endless belt. The belt driver has a roller shape and is provided outside the loop formed by the endless belt. The belt driver applies a tension to the endless belt while the endless belt moves from a downstream portion in the sheet conveyance direction of the nip formed between the endless belt and the metal thermal conductor to the belt driver. The belt driver collects a substance (e.g., toner particles) adhered to the endless belt, preventing the substance from adhering to the endless belt. The belt driver provided outside the loop of the endless belt drives the endless belt and applies a tension to the endless belt, preventing slipping of the endless belt.

In the fixing device, the endless belt has a multilayered structure in which the endless belt includes a base (e.g., the base24depicted inFIG. 23) and an elastic layer (e.g., the elastic layer25depicted inFIG. 23) formed on an outer circumferential surface of the base and having a layer thickness not smaller than about 100 μm. When the endless belt contacts a sheet bearing a toner image, the elastic layer absorbs asperities of the toner image on the sheet. Thus, the endless belt may uniformly apply heat to the sheet. Asperities may not appear as orange peel on the toner image on the sheet, resulting in proper fixing and formation of a high quality image.

The fixing device further includes a heat resistant elastic layer (e.g., the heat resistant elastic layer26depicted inFIG. 23) provided between the endless belt and the nip portion of the metal thermal conductor. The heat resistant elastic layer causes the endless belt to uniformly contact a sheet, resulting in proper fixing and formation of a high quality image.

An image forming apparatus (e.g., the image forming apparatus100depicted inFIG. 4) includes an image forming device (e.g., the image forming device30depicted inFIG. 4) for forming a toner image on a recording medium (e.g., a sheet) and a fixing device (e.g., the fixing device41depicted inFIG. 4) for fixing the toner image formed on the recording medium by applying heat and pressure to the recording medium. The image forming apparatus includes the fixing device according to the above-described example embodiments, providing a stable fixing property and formation of a high quality image.

In the fixing device according to the above-described example embodiments, the heat source heats the metal thermal conductor having a pipe shape and the endless belt. The metal thermal conductor and the endless belt may be quickly heated, shortening a warm-up time period. The metal thermal conductor having an increased thermal conductivity forms a nip between the endless belt and the pressing member. Even when the heat source supplies a decreased amount of heat for a fixing operation, heat stored in the metal thermal conductor is transmitted to the endless belt to compensate for the shortage of heat, preventing a decreased fixing temperature. The metal thermal conductor for storing heat is provided inside a loop formed by the endless belt. The metal thermal conductor may not be easily cooled by airflow, unlike a rotatable metal thermal conductor. Namely, the metal thermal conductor may effectively keep heat and thereby temperature decrease of the metal thermal conductor may be prevented.

The heat source directly or indirectly heats the metal thermal conductor. Convection in an air layer formed between the endless belt and the metal thermal conductor, radiant heat generated by the metal thermal conductor, or heat conduction from the metal thermal conductor to the endless belt heats the entire endless belt. Thus, the fixing device provides a decreased temperature variation in the circumferential direction of the endless belt. As a result, the nip may provide a decreased temperature variation (e.g., a decreased temperature ripple) and thereby may provide a stable fixing property. Therefore, the fixing device may be provided in a high-speed image forming apparatus in which a sheet is conveyed at a high speed. The metal thermal conductor having an increased temperature contacts and guides the endless belt. Therefore, an action for decreasing a friction resistance works between the metal thermal conductor and the endless belt. Thus, a proper slipping property may be provided between the endless belt and the metal thermal conductor contacting each other.

When an image forming apparatus includes the fixing device according to the above-described example embodiments, the image forming apparatus may provide a stable fixing property and formation of a high quality image.

The present invention has been described above with reference to specific example embodiments. Nonetheless, the present invention is not limited to the details of example embodiments described above, but various modifications and improvements are possible without departing from the spirit and scope of the present invention. It is therefore to be understood that within the scope of the associated claims, the present invention may be practiced otherwise than as specifically described herein. For example, elements and/or features of different illustrative example embodiments may be combined with each other and/or substituted for each other within the scope of the present invention.