Patent Description:
A fusing process accompanies a process of applying heat and pressure to the toner. The fuser includes a heating roller and a pressing roller that are engaged with each other and that form a fusing nip. The heating roller is heated by a heater such as a halogen lamp. The recording medium to which the toner image is transferred receives heat and pressure while passing the fusing nip, and the toner image is fused to the recording medium.

In response to demands for high speed printing and low energy fusing, a fusing belt with smaller heat capacity than the heating roller may be used. As a heater, a plate heater for locally heating the fusing belt in the fusing nip may be adopted. <CIT> discloses an image heating apparatus including heat generating resistors; <CIT> discloses a heating device having heating resistors.

<FIG> is a schematic block diagram of an example of a printer to which a heater and a fuser according to the disclosure are applied. Referring to <FIG>, the printer may include a printing unit <NUM> for forming visible toner images on a recording medium P, for example, paper and a fuser <NUM> for fusing the toner images to the recording medium P. The printing unit <NUM> according to the example forms color toner images on the recording medium P by an electro-photographic method.

The printing unit <NUM> may include a plurality of photosensitive drums <NUM>, a plurality of developing devices <NUM>, and a paper feed belt <NUM>. The photosensitive drum <NUM>, as an example of photoconductor on a surface of which an electrostatic latent image is formed, may include a conductive metal pipe and a photosensitive layer formed on an outer circumference of the conductive metal pipe. The plurality of developing devices <NUM> respectively correspond to the plurality of photosensitive drums <NUM>. The plurality of developing devices <NUM> forms toner images on the surfaces of the plurality of photosensitive drums <NUM> by supplying toners to the electrostatic latent images formed on the plurality of photosensitive drums <NUM> and performing development. The plurality of developing devices <NUM> may be respectively replaced independent of the plurality of photosensitive drums <NUM>. In addition, the plurality of developing devices <NUM> may be in the forms of cartridges including the photosensitive drums <NUM>.

For color printing, the plurality of developing devices <NUM> may include a plurality of developing devices 10Y, <NUM>, 10C, and <NUM> that accommodate yellow Y, magenta M, cyan C, and black K toners, respectively. Other than the above-described colors, developing devices that accommodate toners of various colors such as light magenta and white may be further adopted. Hereinafter, the printer including the plurality of developing devices 10Y, <NUM>, 10C, and <NUM> will be described. Unless otherwise described, when Y, M, C, and K are added to reference numerals, the reference numerals denote components for printing an image by using the yellow Y, magenta M, cyan C, and black K toners.

The developing device <NUM> supplies the toner accommodated therein to the electrostatic latent image formed on the photosensitive drum <NUM> and develops the electrostatic latent image as the visible toner image. The developing device <NUM> may include developing roller <NUM>. The developing roller <NUM> supplies the toner in the developing device <NUM> to the photosensitive drum <NUM>. A developing bias voltage may be applied to the developing roller <NUM>. A regulating member that is not shown regulates amounts of the toners supplied to a developing region in which the photosensitive drum <NUM> and the developing roller <NUM> face each other by the developing roller <NUM>.

When a two-component developing method is adopted, magnetic carrier and the toner may be accommodated in the developing device <NUM>. The developing roller <NUM> may be separate from the photosensitive drum <NUM> by tens or hundreds of microns. Although not shown in the drawing, the developing roller <NUM> may have a shape in which a magnetic roller is arranged in hollow cylindrical sleeve. The toner are attached to a surface of the magnetic carrier. The magnetic carrier is attached to a surface of the developing roller <NUM> and is carried to the developing region in which the photosensitive drum <NUM> and the developing roller <NUM> face each other. Due to the developing bias voltage applied between the developing roller <NUM> and the photosensitive drum <NUM>, the toner is supplied to the photosensitive drum <NUM> and develops the electrostatic latent image formed on the surface of the photosensitive drum <NUM> as the visible toner image. The developing device <NUM> may include an agitator (not shown) for mixing the toner and the magnetic carrier with each other, agitating the mixture, and carrying the agitated mixture to the developing roller <NUM>. The agitator may be, for example, an auger and a plurality of agitators may be provided in the developing device <NUM>.

When a one-component developing method in which the magnetic carrier is not used is adopted, the developing roller <NUM> may rotate while contacting the photosensitive drum <NUM>. The developing roller <NUM> may rotate while being separate from the photosensitive drum <NUM> by tens or hundreds of microns. The developing device <NUM> may further include supply roller (not shown) for attaching the toner to the surface of the developing roller <NUM>. A supply bias voltage may be applied to the supply roller. The developing device <NUM> may further include an agitator (not shown). The agitator may agitate the toner so that the toner is triboelectrically charged. The agitator may be, for example, an auger.

Charging roller <NUM> is an example of a charging device for charging the photosensitive drum <NUM> so that the photosensitive drum <NUM> has a uniform surface electric potential. Instead of the charging roller <NUM>, a charging brush or corona charging device may be adopted.

Cleaning blade <NUM> is an example of a cleaning member for removing the toner and alien substances that reside on the surface of the photosensitive drum <NUM> after a transferring process. Instead of the cleaning blade <NUM>, a cleaning apparatus of another type such as a rotating brush may be adopted.

An example of a developing method of a printer as an example was described in detail. However, other various developing methods may be adopted by the printer.

An exposure device <NUM> irradiates lights modulated in accordance with image information onto the photosensitive drums 1Y, <NUM>, 1C, and <NUM> and forms the electrostatic latent images corresponding to yellow (Y), magenta (M), cyan (C), and black (K) images. As an example of the exposure device <NUM>, a laser scanning unit (LSU) that uses a laser diode as a light source and a light emitting diode (LED) exposure device using an LED as a light source may be adopted.

The paper feed belt <NUM> supports and feeds the recording medium P. The paper feed belt <NUM>, for example, may be supported by supporting rollers <NUM> and <NUM> and may be circulation driven. The recording medium P is picked up by a pickup roller <NUM> from a loading table <NUM> one by one, is fed by feed rollers <NUM>, and may be attached to the paper feed belt <NUM> by, for example, electrostatic force. A plurality of transfer rollers <NUM> may be arranged in positions in which the plurality of transfer rollers <NUM> face the plurality of photosensitive drums 1Y, <NUM>, 1C, and <NUM> with the paper feed belt <NUM> interposed therebetween. The plurality of transfer rollers <NUM> are an example of transfer units for transferring the toner images from the plurality of photosensitive drums 1Y, <NUM>, 1C, and <NUM> to the recording medium P supported by the paper feed belt <NUM>. The transfer bias voltage for transferring the toner images to the recording medium P is applied to the plurality of transfer rollers <NUM>. Instead of the transfer rollers <NUM>, corona transfer units or transfer units of a pin scorotron method may be adopted.

The fuser <NUM> applies heat and/or pressure to the image transferred to the recording medium P and may fuse the image to the recording medium P. The recording medium P that passes the fuser <NUM> is discharged by discharge roller <NUM>.

By the above-described configuration, the exposure device <NUM> irradiates lights modulated in response to image information of the respective colors onto the plurality of photosensitive drums 1Y, <NUM>, 1C, and <NUM> and forms the electrostatic latent images. The plurality of developing devices 10Y, <NUM>, 10C, and <NUM> respectively supply the Y, M, C, and K colored toners to the electrostatic latent images formed on the plurality of photosensitive drums 1Y, <NUM>, 1C, and <NUM> and respectively form the Y, M, C, and K colored visible toner images on the surfaces of the plurality of photosensitive drums 1Y, <NUM>, 1C, and <NUM>. The recording medium P loaded on the loading table <NUM> is supplied to the paper feed belt <NUM> by the pickup roller <NUM> and the feed rollers <NUM> and is maintained on the paper feed belt <NUM>, for example, by electrostatic force. The Y, M, C, and K colored toner images are sequentially transferred onto the recording medium P fed by the paper feed belt <NUM> by the transfer bias voltage applied to the plurality of transfer rollers <NUM>. When the recording medium P passes the fuser <NUM>, the toner images are fused to the recording medium P by heat and pressure. The recording medium P to which the toner images are completely fused is discharged by the discharge roller <NUM>.

The printer illustrated in <FIG> adopts a method of directly transferring the toner images developed on the plurality of photosensitive drums 1Y, <NUM>, 1C, and <NUM> to the recording medium P supported by the paper feed belt <NUM>. However, another transfer method is available. For example, a method of intermediate transferring the toner images developed on the plurality of photosensitive drums 1Y, <NUM>, 1C, and <NUM> to an intermediate transfer belt (not shown) and transferring the toner images to the recording medium P may be adopted.

When a single-colored image, for example, a black image is printed, the printer may include the developing device <NUM> among the plurality of developing devices 10Y, <NUM>, 10C, and <NUM>. The paper feed belt <NUM> may not be provided. The recording medium P is fed between the photosensitive drum <NUM> and the transfer roller <NUM> and the toner image formed on the photosensitive drum <NUM> may be transferred to the recording medium P by the transfer bias voltage applied to the transfer roller <NUM>.

The fuser <NUM> applies heat and pressure to the toner image and fuses the toner image to the recording medium P. In order to increase a printing speed and to reduce energy consumption, a heated portion with small heat capacity may be adopted to the fuser <NUM>. For example, a thin film-shaped endless belt may be adopted as the heated portion. Therefore, a temperature of the fuser <NUM> may be rapidly increased to a temperature at which the toner image may be fused and printing may be performed within a short time after the printer is turned on.

<FIG> is a schematic block diagram of an example of a fuser. Referring to <FIG>, the fuser <NUM> includes a flexible endless belt <NUM>, a heater <NUM>, and a backup member <NUM>. The heater <NUM> is positioned inside the flexible endless belt <NUM> and heats the flexible endless belt <NUM>. The backup member <NUM> is positioned outside the flexible endless belt <NUM> to face the heater <NUM>. A pressing member <NUM> may provide pressing force to at least one of the heater <NUM> and the backup member <NUM>. The heater <NUM> and the backup member <NUM> are pressed to each other by the pressing force of the pressing member <NUM> so that a fusing nip <NUM> is formed. The heater <NUM> heats the flexible endless belt <NUM> in the fusing nip <NUM>. When the recording medium P on a surface of which the toner images T are formed passes the fusing nip <NUM>, the toner images T are fused to the recording medium P by heat and pressure. Although not shown in the drawing, the fuser <NUM> may further include a temperature sensor for detecting a temperature of the heater <NUM> in order to control the temperature and a thermostat as a safety device.

As an example, the flexible endless belt <NUM> may include a film-shaped base layer (not shown). The base layer may be a metal thin film such as stainless steel, nickel (Ni), or Ni-copper (Cu) or a polymer film having thermal resistance and abrasion resistance, which may withstand a fusing temperature, such as a polyimide film, a polyamide film, or a polyimideamide film. A thickness of the base layer may be set so that the flexible endless belt <NUM> may have flexibility and elasticity in which the flexible endless belt <NUM> is flexibly transformed in the fusing nip <NUM> and, after deviating from the fusing nip <NUM>, may be recovered to an original state. For example, the thickness of the base layer may be about <NUM> to <NUM>. The thickness of the base layer may be, for example, about <NUM> to <NUM>.

A release layer (not shown) may be provided on a surface of the base layer toward the backup member <NUM> or both surfaces of the base layer. The release layer may be a resin layer with high separability. The release layer may include, for example, one or more among perfluoroalkoxy (PFA), polytetrafluoroethylenes (PTFE), and fluorinated ethylene prophylene (FEP). A thickness of the release layer may be, for example, about <NUM> to <NUM>.

In order to form the fusing nip <NUM> to be wide and flat, an elastic layer (not shown) may be interposed between the base layer and the release layer. The elastic layer may be formed of a material having thermal resistance, which may withstand a fusing temperature. For example, the elastic layer may be formed of a rubber material such as fluoro rubber or silicon rubber. A thickness of the elastic layer may be, for example, about <NUM> to <NUM>.

As an example, the backup member <NUM> may be a backup roller that rotates while being pressed to the heater <NUM> with the flexible endless belt <NUM> interposed therebetween and drives the flexible endless belt <NUM>. The backup member <NUM> may include an elastic layer (not shown). As an example, a material of the elastic layer may be a rubber material such as fluoro rubber, silicon rubber, natural rubber, isoprene rubber, butadiene rubber, nitrile rubber, chloroprene rubber, butyl rubber, acryl rubber, hydrin rubber, or urethane rubber or one of various thermoplastic elastomers such as styrene based thermoplastic elastomer, polyolefin based thermoplastic elastomer, polyvinyl chloride based thermoplastic elastomer, polyurethane based thermoplastic elastomer, polyester based thermoplastic elastomer, polyamide based thermoplastic elastomer, polybutadiene based thermoplastic elastomer, trans polyisoprene based thermoplastic elastomer, and chlorinated polyethylene based thermoplastic elastomer. The release layer may include one or more among perfluoroalkoxy (PFA), polytetrafluoroethylenes (PTFE), and fluorinated ethylene prophylene (FEP).

The pressing member <NUM> may provide a pressing force toward the backup roller <NUM>, for example, to the heater <NUM>. The pressing force may be directly or indirectly provided to the heater <NUM>. As an example, referring to <FIG>, the pressing member <NUM> may provide a pressing force to a supporting member <NUM> by which the heater <NUM> is supported or a pressing bracket <NUM> connected to the supporting member <NUM>. A structure in which a pressing force is provided to the heater <NUM> is not limited to the structure illustrated in <FIG>.

<FIG> is a schematic side view of an example of the heater <NUM>. Referring to <FIG>, the heater <NUM> includes a substrate <NUM>, heating elements <NUM> provided on a surface 211a of the substrate <NUM> that faces the backup member <NUM>, and an electrode (not shown) for supplying a current to the heating elements <NUM>. The heating elements <NUM> receive electric energy and emit heat. A structure of the heating elements <NUM> will be described later. An insulating layer <NUM> covers the heating elements <NUM> and the electrode. The insulating layer <NUM> may function as a sliding layer that contacts the flexible endless belt <NUM>. The insulating layer <NUM> may be, for example, a glass layer.

<FIG> is a schematic side view of an example of the heater <NUM>. Referring to <FIG>, the heater <NUM> includes the substrate <NUM>, the heating elements <NUM> provided on a surface 211b of the substrate <NUM> opposite to the surface 211a that faces the backup member <NUM>, and an electrode (not shown) for supplying a current to the heating elements <NUM>. The heating elements <NUM> receive electric energy and emit heat. The structures of the heating elements <NUM> will be described later. The insulating layer <NUM> covers the heating elements <NUM> and the electrode. A sliding layer <NUM> may be provided on the surface 211a of the substrate <NUM> that faces the backup member <NUM>. The sliding layer <NUM> may be, for example, a glass layer or a polyimide layer.

In the examples of the heater <NUM> illustrated in <FIG> and <FIG>, the substrate <NUM> may be, for example, a ceramic substrate. For example, alumina (Al2O3) or nitride aluminum (AIN) may be used as a ceramic material. The heating elements <NUM> may be, for example, metal heating elements such as a silver-palladium (Ag-Pd) alloy. The electrode may be, for example, a silver-platinum (Ag-Pt) electrode or an Ag electrode.

A shape of the heating elements <NUM> may be determined considering thermal efficiency, fusibility, and prevention of overheating of a region in which the recording medium P does not pass.

The heating elements <NUM> may include a pair of heating elements symmetrically arranged in a width direction of the substrate <NUM>. The pair of heating elements extend in a length direction of the substrate <NUM>. The width direction of the substrate <NUM> is a direction in which the recording medium P is fed through the fusing nip <NUM>. The length direction of the substrate <NUM> is a width direction of the recording medium P and is orthogonal to a direction in which the recording medium P is fed.

The heating elements <NUM> according to the disclosure may include a plurality of pairs of heating elements that symmetrically make pairs based on the width direction of the substrate <NUM>. The plurality of pairs of heating elements extends in the length direction of the substrate <NUM>. One end of each of the plurality of pairs of heating elements may be connected to a common electrode. The other ends of the plurality of pairs of heating elements may be connected to a plurality of driving electrodes. The plurality of pairs of heating elements may be individually driven or may be driven together. The plurality of pairs of heating elements refers to at least two pairs of heating elements.

At least one of the first heating element or the corresponding third heating element has a heating value per a unit length that is different in a center of the at least one of the first heating element or the corresponding third heating element, in the length direction (L), compared to an end of the first heating element or the corresponding third heating element, in the length direction (L). In the above-described configuration, the pair of inner heating elements and the pair of outer heating elements are simultaneously or individually driven so that, when a fusing process is performed on the recording medium P of one of various sizes, it is possible to prevent a region in which the recording medium P does not pass from being overheated.

A length of at least a pair of heating elements among the plurality of pairs of heating elements may be different from lengths of the other pairs of heating elements. For example, the pair of outer heating elements may correspond to the recording medium P of a largest size. A length of the pair of inner heating elements may be smaller than that of the pair of outer heating elements. In the above-described configuration, the pair of inner heating elements and the pair of outer heating elements are individually or simultaneously driven so that, when the fusing process is performed on the recording medium P of one of various sizes, it is possible to prevent the region in which the recording medium P does not pass from being overheated.

An inner distance between the pair of heating elements may affect thermal efficiency and fusibility in the fusing nip <NUM>. A pair of heating elements each having a width of <NUM> are formed on an alumina substrate having a width of <NUM> and, while changing the inner distance between the pair of heating elements to <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>, temperatures in a center and at both ends in a width direction of the fusing nip <NUM> and an average temperature between the temperatures in the center and at the both ends are measured. As a result, as the inner distance is larger, a temperature in the fusing nip <NUM> is high at the both ends in the width direction and is low in the center. To the contrary, as the inner distance is small, the temperature in the fusing nip <NUM> is low at the both ends in the width direction and is high in the center. An average temperature in the fusing nip <NUM> is higher as the inner distance is larger, which means that, as the inner distance is larger, an average quantity of heat that the toner image on the recording medium P receives is larger. That is, as the inner distance is larger, thermal efficiency of the heater <NUM> is higher.

Based on the width direction, a difference in temperature between the center and the both ends of the fusing nip <NUM> is larger as the inner distance is smaller. When the difference in temperature between the center and the both ends of the fusing nip <NUM> is large, during a rise in temperature, thermal stress applied to the heater <NUM> is concentrated on the center so that possibility of the heater <NUM> being damaged increases. According to the above, the inner distance is no less than a thickness of the center of each of the pair of heating elements so that the difference in temperature between the center and the both ends of the fusing nip <NUM> is reduced and the possibility of the heater <NUM> being damaged by concentration of thermal stress may be reduced. In addition, when the inner distance is no less than the thickness of the center of each of the pair of heating elements, since an influence of thermal stress is small, the temperature of the heater <NUM> may rapidly rise.

As the inner distance is larger, fusibility may increase. Fusibility may be measured by, for example, a taping method. In the taping method, optical density of the image printed to the recording medium P is measured, an adhesive tape is attached to the recording medium P and is detached from the recording medium P, optical density of the image recorded in the recording medium P is measured again, and fusibility is determined as a ratio between optical densities before and after taping. A pair of heating elements each having a width of <NUM> are formed on an alumina substrate having a width of <NUM> and, while changing the inner distance between the pair of heating elements to <NUM>, <NUM>, and <NUM>, fusibility is measured. According to the above, in the respective cases, fusibilities are <NUM>%, <NUM>%, and <NUM>%. Therefore, it is noted that fusibility of no less than <NUM>% may be secured by having the inner distance be no less than the thickness of the center of each of the pair of heating elements.

The above-described conditions about the inner distance between a pair of heating elements may be applied to an inner distance between the pair of heating elements positioned at the innermost side among the plurality of pairs of the heating elements as follows. The inner distance between the pair of the innermost heating elements among the plurality of pairs of heating elements is no less than a value obtained by adding a width of a center in a length direction of a substrate of the pair of the innermost heating elements and a width of a center in a length direction of a substrate of a pair of outer heating elements adjacent to the pair of the innermost heating elements. According to the above-described configuration, as described above, high thermal efficiency and high fusibility may be implemented, the possibility of the heater <NUM> being damaged may be reduced, and rapid rise in temperature may be performed.

Hereinafter, various examples of the heating elements <NUM> will be described.

<FIG> is a plan view of an example of the heater <NUM>, which illustrates a distribution of heating values. Referring to <FIG>, the heating elements <NUM> include a first pair of heating elements <NUM>(first heating element and second heating element) arranged outside based on the width direction W of the substrate <NUM> and a second pair of heating elements <NUM>(third heating element and fourth heating element) positioned at an inner side of the first pair of heating elements <NUM>. An inner distance d between the second pair of heating elements <NUM> has a value of no less than a value obtained by adding widths of centers in a length direction L of the first pair of heating elements <NUM> and the second pair of heating elements <NUM>. In other words, the first heating element and the second heating element are provided on the substrate <NUM> and extend in the length direction of the substrate <NUM>, and are respectively provided at a first side and a second side of the substrate <NUM> in the width direction of the substrate <NUM>. The second side is opposite of the first side. The third heating element and the fourth heating element are provided on the substrate <NUM>, extend in the length direction, and are provided between the first heating element and the second heating element in the width direction. A distance between the third heating element and the fourth heating element is greater than or equal to a sum of widths of each of the first heating element, second heating element, third heating element, and fourth heating element at a center in the length direction of each of the first heating element, second heating element, third heating element, and fourth heating element. Each the first pair of heating elements <NUM> and the second pair of heating elements <NUM> may be symmetrical with each other in the width direction W. A distance between the third heating element and the fourth heating element is greater than or equal to twice a sum of widths of the first heating element and the third heating element at a center in the length direction of each of the first heating element and third heating element. That is, d ≥ <NUM>×(d1+d2).

An end of each of the first, second, third, and fourth heating elements is connected to a common electrode <NUM>, another end of at least one of the first heating element or the second heating element is connected to a first driving electrode <NUM>, and another end of at least one of the third heating element or the fourth heating element is connected to a second driving electrode <NUM>. For example, one end of each of the first pair of heating elements <NUM> and the second pair of heating elements <NUM> is connected to a common electrode <NUM>. The other ends of the first pair of heating elements <NUM> are connected to a first driving electrode <NUM>. The other ends of the second pair of heating elements <NUM> are connected to a second driving electrode <NUM>.

The first pair of heating elements <NUM> and the second pair of heating elements <NUM> have lengths that may correspond to a largest recording medium P1. The lengths of the first pair of heating elements <NUM> and the second pair of heating elements <NUM> may be the same. At least one of the first heating element or the third heating element has a heating value per a unit length that is different in a center of the at least one of the first heating element or the third heating element in the length direction compared to an end of the at least one of the first heating element or the third heating element in the length direction. A heating value per a unit length of the first pair of heating elements <NUM> is larger at both ends in the length direction L than in the center. A heating value per a unit length of the second pair of heating elements <NUM> is larger in the center in the length direction L than at the both ends. Distributions of heating values in the length direction L of the first pair of heating elements <NUM> and the second pair of heating elements <NUM> are denoted by reference numerals <NUM> and <NUM>. The distributions of the heating values may be implemented, for example, by having the width of the first pair of heating elements <NUM> smaller at the both ends in the length direction L than in the center and having the width of the second pair of heating elements <NUM> smaller in the center in the length direction L than at the both ends. That is, a width of the first heating element at an end of the first heating element is less than a width of the first heating element at the center in the length direction of the first heating element, and the width of the third heating element at the center in the length direction of the third heating element is less than a width of the third heating element at an end of the third heating element. Length of the first heating element and the third heating element are equal. The first pair of heating elements <NUM> and the second pair of heating elements <NUM> may be complementary to each other in shape.

For the largest recording medium P1, the first pair of heating elements <NUM> and the second pair of heating elements <NUM> may be simultaneously driven so that a heating value in a length direction L of the heater <NUM> is uniform. For a small recording medium P2, the second pair of heating elements <NUM> is driven so that a heating value in the center is large by having the heating value of the second pair of heating elements <NUM> large. The above-described driving may be implemented, for example, by having an amount of supply of a current to the second pair of heating elements <NUM> larger than an amount of supply of a current to the first pair of heating elements <NUM>. According to the above-described configuration, it is possible to implement high thermal efficiency and to prevent a region in which the recording medium P does not pass from being overheated in a process of fusing the small recording medium P2.

<FIG> is a plan view of an example of the heater <NUM>, which illustrates a distribution of heating values. In comparison with the example of the heater <NUM> illustrated in <FIG>, a difference between the heater <NUM> illustrated in <FIG> and the heater <NUM> illustrated in <FIG> will be described. At least one of the first heating element or the third heating element has a heating value per unit length that is uniform in the length direction. Widths d1 and d2 of the first pair of heating elements <NUM> and the second pair of heating elements <NUM> are uniform in the length direction L. Therefore, heating values per a unit length of the first pair of heating elements <NUM> and the second pair of heating elements <NUM> are uniform in the length direction L. A length of the first pair of heating elements <NUM> is larger than that of the second pair of heating elements <NUM>. That is, a width of each of the first and second heating elements is uniform, a width of each of the third and fourth heating elements is uniform, and a length of each of the first and second heating elements is greater than A length of each of the third and fourth heating elements. The first pair of heating elements <NUM> may have the length corresponding to the largest recording medium P1. The second pair of heating elements <NUM> are positioned in the center in the length direction L. Distributions of heating values in the length direction L of the first pair of heating elements <NUM> and the second pair of heating elements <NUM> are denoted by the reference numerals <NUM> and <NUM> in <FIG>. An inner distance d between the second pair of heating elements <NUM> has a value of no less than a value obtained by adding widths of centers in the length direction L of the first pair of heating elements <NUM> and the second pair of heating elements <NUM>. That is, d ≥ <NUM>×(d1+d2).

For the largest recording medium P1, the first pair of heating elements may be driven. For the small recording medium P2, the second pair of heating elements may be driven. According to the above-described configuration, it is possible to implement high thermal efficiency and to prevent a region in which the recording medium P does not pass from being overheated in a process of fusing the small recording medium P2.

<FIG> is a plan view of an example of the heater <NUM>, which illustrates a distribution of heating values. In comparison with the example of the heater <NUM> illustrated in <FIG>, a difference between the heater <NUM> illustrated in <FIG> and the heater <NUM> illustrated in <FIG> will be described. A width d1 of the first pair of heating elements <NUM> is uniform in the length direction L. Therefore, heating values per a unit length of the first pair of heating elements <NUM> are uniform in the length direction L. A heating value per a unit length of the second pair of heating elements <NUM> is larger in a center in the length direction L than at both ends. Distributions of heating values in the length direction L of the first pair of heating elements <NUM> and the second pair of heating elements <NUM> are denoted by the reference numerals <NUM> and <NUM> in <FIG>. The distributions of heating values may be implemented, for example, by having a width of the second pair of heating elements <NUM> smaller in the center in the length direction L than at the both ends. The first pair of heating elements <NUM> and the second pair of heating elements <NUM> may have lengths corresponding to the largest recording medium P1. That is, a width of each of the first and second heating elements is uniform, the width of the third heating element at the center in the length direction of the third heating element is less than the width of the third heating element at an end of the third heating element, and the width of the fourth heating element at the center in the length direction of the fourth heating element is less than a width of the fourth heating element at an end of the fourth heating element. A length of the first pair of heating elements <NUM> may be the same as a length of the second pair of heating elements <NUM>. An inner distance d between the second pair of heating elements <NUM> has a value of no less than a value obtained by adding widths of centers in the length direction L of the first pair of heating elements <NUM> and the second pair of heating elements <NUM>. That is, d ≥ <NUM>×(d1+d2).

<FIG> is a plan view of an example of the heater <NUM>, which illustrates a distribution of heating values. Referring to <FIG>, the heating elements <NUM> include the first pair of heating elements <NUM>(first heating element and second heating element) arranged at the outermost side, the second pair of heating elements <NUM>(first heating element and second heating element) positioned at an inner side of the first pair of heating elements <NUM>, and a third pair of heating elements <NUM>(fifth heating element and sixth heating element) positioned at the innermost side, based on the width direction W of the substrate <NUM>. The fifth heating element and the sixth heating element are provided on the substrate <NUM>, extend in the length direction, and provided between the third heating element and the fourth heating element in the width direction. Each pair of the first pair of heating elements <NUM>, the second pair of heating elements <NUM>, and the third pair of heating elements <NUM> may be symmetrical with each other in the width direction W. An inner distance d4 between the third pair of heating elements <NUM> has a value of no less than a value obtained by adding widths of centers in the length direction L of the third pair of heating elements <NUM> and the second pair of heating elements <NUM> adjacent to the third heating elements <NUM>. That is, d4 ≥ <NUM>×(d2+d3). That is, a distance between the fifth heating element and the sixth heating element is greater than or equal to a sum of widths of each of the third heating element, fourth heating element, fifth heating element, and sixth heating element at a center in the length direction of each of the third heating element, fourth heating element, fifth heating element, and sixth heating element.

One end of each of the first pair of heating elements <NUM>, the second pair of heating elements <NUM>, and the third pair of heating elements <NUM> is connected to the common electrode <NUM>. The other ends of the first pair of heating elements <NUM> are connected to the first driving electrode <NUM>. The other ends of the second pair of heating elements <NUM> are connected to the second driving electrode <NUM>. The other ends of the third pair of heating elements are connected to a third driving electrode <NUM>.

A length of at least one of the first heating element or the second heating element is different from a length of at least one of the third heating element or the fourth heating element. The first pair of heating elements <NUM> has a length that may correspond to the largest recording medium P1. A heating value of the first pair of heating elements <NUM> per a unit length is uniform in the length direction L. That is, a width of the first pair of heating elements <NUM> is uniform in the length direction L. Lengths of the second pair of heating elements <NUM> and the third pair of heating elements <NUM> are smaller than the length of the first pair of heating elements <NUM>. In other words, a width of each of the first and second heating elements is uniform, and lengths of each of the third, fourth, fifth, and sixth heating elements is less than a length of each of the first and second heating elements. As an example, the lengths of the second pair of heating elements <NUM> and the third pair of heating elements <NUM> may be the same. The second pair of heating elements <NUM> and the third pair of heating elements <NUM> may be positioned in centers in the length direction L.

A heating value per a unit length of a pair of the second pair of heating elements <NUM> and the third pair of heating elements <NUM> is larger at both ends in the length direction L than in a center. A heating value per a unit length of the other pair of the second pair of heating elements <NUM> and the third pair of heating elements <NUM> is larger in a center in the length direction L than at both ends. The above-described distribution of heating values may be implemented, for example, by having a width of a pair of the second pair of heating elements <NUM> and the third pair of heating elements <NUM> smaller at the both ends in the length direction L than in the center and by having a width of the other pair of the second pair of heating elements <NUM> and the third pair of heating elements <NUM> smaller in the center in the length direction L than at the both ends. In other words, the width of the third heating element at the center in the length direction of the third heating element is less than a width of the third heating element at an end of the third heating element and the width of the fifth heating element at the center in the length direction of the fifth heating element is greater than a width of the fifth heating element at an end of the fifth heating element, or the width of the third heating element at the center in the length direction of the third heating element is greater than the width of the third heating element at the end of the third heating element and the width of the fifth heating element at the center in the length direction of the fifth heating element is less than a width of the fifth heating element at the end of the fifth heating element. According to the example, a heating value of the second pair of heating elements <NUM> per a unit length is larger at both ends in the length direction L than in a center. A heating value of the third pair of heating elements <NUM> per a unit length is larger in a center in the length direction L than at both ends. For example, a width of the second pair of heating elements <NUM> is smaller at both ends in the length direction L than in a center and a width of the third pair of heating elements <NUM> is smaller in a center in the length direction L than at both ends. The second pair of heating elements <NUM> and the third pair of heating elements <NUM> may be complementary to each other in shapes. Distributions of heating values in the length direction L of the first pair of heating elements <NUM>, the second pair of heating elements <NUM>, and the third pair of heating elements <NUM> are denoted by reference numerals <NUM>, <NUM>, and <NUM>, respectively in <FIG>.

For the largest recording medium P1, the first pair of heating elements <NUM> may be driven so that a heating value of the heater <NUM> in the length direction L is uniform. For the small recording medium P2, the second pair of heating elements <NUM> and the third pair of heating elements <NUM> may be simultaneously driven. For a smaller recording medium P3, the third pair of heating elements <NUM> may be driven. According to the above-described configuration, it is possible to implement high thermal efficiency and to prevent a region in which the small recording mediums P2 and P3 do not pass from being overheated in a process of fusing the small recording mediums P2 and P3.

<FIG> is a plan view of an example of the heater <NUM>, which illustrates a distribution of heating values. <FIG> is a modification of the example of the heater <NUM> illustrated in <FIG>, in which a heating value of the second pair of heating elements <NUM> per a unit length is larger in a center in the length direction L than at both ends and a heating value of the third pair of heating elements <NUM> per a unit length is larger at both ends in the length direction L than in a center. For example, a width of the second pair of heating elements <NUM> is smaller in the center in the length direction L than at the both ends and a width of the third pair of heating elements <NUM> is smaller at the both ends in the length direction L than in the center. The second pair of heating elements <NUM> and the third pair of heating elements <NUM> may be complementary to each other in shapes. Distributions of heating values in the length direction L of the first pair of heating elements <NUM>, the second pair of heating elements <NUM>, and the third pair of heating elements <NUM> are denoted by the reference numerals <NUM>, <NUM>, and <NUM>, respectively in <FIG>. An inner distance d4 between the third pair of heating elements <NUM> has a value of no less than a value obtained by adding widths of centers in the length direction L of the third pair of heating elements <NUM> and the second pair of heating elements <NUM> adjacent to the third heating elements <NUM>. That is, d4 ≥ <NUM>×(d2+d3).

For the largest recording medium P1, the first pair of heating elements <NUM> may be driven so that a heating value of the heater <NUM> in the length direction L is uniform. For the small recording medium P2, the second pair of heating elements <NUM> and the third pair of heating elements <NUM> may be simultaneously driven. For a smaller recording medium P3, the second pair of heating elements <NUM> may be driven. According to the above-described configuration, it is possible to implement high thermal efficiency and to prevent a region in which the small recording mediums P2 and P3 do not pass from being overheated in a process of fusing the small recording mediums P2 and P3.

<FIG> is a plan view of an example of the heater <NUM>, which illustrates a distribution of heating values. <FIG> is a modification of the example of the heater <NUM> illustrated in <FIG>, in which heating values of the first pair of heating elements <NUM>, the second pair of heating elements <NUM>, and the third pair of heating elements <NUM> per a unit length are uniform in the length direction L. That is, widths of the first pair of heating elements <NUM>, the second pair of heating elements <NUM>, and the third pair of heating elements <NUM> are uniform in the length direction L. The first pair of heating elements <NUM> has a length that may correspond to the largest recording medium P1. A length of the second pair of heating elements <NUM> is smaller than that of the first pair of heating elements <NUM>. A length of the third pair of heating elements <NUM> is smaller than that of the second pair of heating elements <NUM>. For example, the second pair of heating elements <NUM> and the third pair of heating elements <NUM> may have lengths respectively corresponding to the small recording mediums P2 and P3. In other words, widths of the first, second, third, fourth, fifth, and sixth heating elements are uniform, a length of each of the third and fourth heating elements is less than a length of each of the first and second heating elements, and a length of each of the fifth and sixth heating elements is less than the length of each of the third and fourth heating elements. The second pair of heating elements <NUM> and the third pair of heating elements <NUM> may be positioned in a center in the length direction L. Distributions of heating values in the length direction L of the first pair of heating elements <NUM>, the second pair of heating elements <NUM>, and the third pair of heating elements <NUM> are denoted by the reference numerals <NUM>, <NUM>, and <NUM>, respectively in <FIG>. An inner distance d4 between the third pair of heating elements <NUM> has a value of no less than a value obtained by adding widths of centers in the length direction L of the third pair of heating elements <NUM> and the second pair of heating elements <NUM> adjacent to the third heating elements <NUM>. That is, d4 ≥ <NUM>×(d2+d3).

For the largest recording medium P1, the first pair of heating elements <NUM> may be driven. For the small recording medium P2, the second pair of heating elements <NUM> may be driven. For a smaller recording medium P3, the third pair of heating elements <NUM> may be driven. According to the above-described configuration, it is possible to implement high thermal efficiency and to prevent a region in which the small recording mediums P2 and P3 do not pass from being overheated in a process of fusing the small recording mediums P2 and P3.

According to the above-described examples, a driving electrode is connected to the other ends of a pair of heating elements. However, a pair of driving electrodes corresponding to the pair of heating elements may be used. According to the above-described configuration, since the pair of heating elements may be simultaneously driven or one heating element of the pair of heating elements may be driven, it is possible to precisely control a temperature and to improve thermal efficiency.

According to the above-described examples, due to a shape in which a width slowly changes in the length direction L, a heating element in which heating values vary in a center and at both ends is implemented. The heating element in which the heating values vary in the center and at the both ends may be implemented by another shape. For example, the heating element in which the heating values vary in the center and at the both ends may be also implemented by a stepped shape in which the center has a first width and the both ends have a second width. In addition, a width of the heating element may change by stages from the center to the both ends.

A pressing force provided by the pressing member <NUM> to the heater <NUM> and the backup member <NUM> may vary. For example, while fusing is performed, sufficient pressing force is provided to the heater <NUM> and the backup member <NUM> in order to improve fusibility and, while fusing is not performed, in order to reduce stress applied to the flexible endless belt <NUM> and the backup member <NUM>, a pressing force may be reduced or removed. An envelope may be used as the recording medium P. When the pressing force is strong, the envelope may be wrinkled while the envelope passes the fusing nip <NUM>. The occurrence of wrinkling may be resolved by reducing pressing force. A pressing force at this time may be smaller than a pressing force while fusing is performed and may be larger than a pressing force while fusing is not performed.

<FIG>, and <FIG> are schematic block diagrams of an example of the fuser <NUM>. Referring to <FIG>, and <FIG>, a pressing force variable member <NUM> for varying pressing force is adopted. For example, the pressing force variable member <NUM> may include a pressing lever <NUM> rotatable about a hinge <NUM>-<NUM> and including a cam contacting portion <NUM>-<NUM>, and a rotating cam <NUM> that faces the cam contacting portion <NUM>-<NUM>. The pressing member <NUM> presses the pressing lever <NUM>. The pressing member <NUM> may be, for example, a compressive coil spring. The pressing lever <NUM> may press, for example, the pressing bracket <NUM>. The rotating cam <NUM> may include a first portion <NUM>-<NUM>, a second portion <NUM>-<NUM>, and a third portion <NUM>-<NUM> having different radiuses from a rotation center <NUM>-<NUM>. The radius from the rotation center <NUM>-<NUM> is the smallest in the first portion <NUM>-<NUM> and increases in the order of the second portion <NUM>-<NUM> and the third portion <NUM>-<NUM>. When the rotating cam <NUM> rotates, the first portion <NUM>-<NUM>, the second portion <NUM>-<NUM>, and the third portion <NUM>-<NUM> sequentially face the cam contacting portion <NUM>-<NUM>. The rotating cam <NUM> may be rotated by a motor that is not shown.

As illustrated in <FIG>, during fusing, the first portion <NUM>-<NUM> faces the cam contacting portion <NUM>-<NUM>. The first portion <NUM>-<NUM> may be separate from the cam contacting portion <NUM>-<NUM>. The largest pressing force is applied to the heater <NUM> and the backup member <NUM>.

When the envelope is used as the recording medium P, as illustrated in <FIG>, the second portion <NUM>-<NUM> contacts the cam contacting portion <NUM>-<NUM>. Then, the pressing lever <NUM> rotates about the hinge <NUM>-<NUM>, and pressing force is reduced. Therefore, since a small pressing force is applied, it is possible to prevent the envelope from being wrinkled in a fusing process.

When fusing is not performed, as illustrated in <FIG>, the third portion <NUM>-<NUM> contacts the cam contacting portion <NUM>-<NUM>. Then, the pressing lever <NUM> further rotates about the hinge <NUM>-<NUM> so that the pressing force may be reduced or removed.

Claim 1:
A heater (<NUM>) for a fuser (<NUM>), the heater (<NUM>) comprising:
a substrate (<NUM>);
a first heating element (<NUM>) and a second heating element (<NUM>), each provided on the substrate (<NUM>), extending in a length direction (L) of the substrate (<NUM>), and respectively provided at a first side and a second side of the substrate (<NUM>) in a width direction (W) of the substrate (<NUM>), the second side being opposite of the first side; and
a third heating element (<NUM>) and a fourth heating element (<NUM>), each provided on the substrate, extending in the length direction, to form a first inner area between the third and fourth heating elements,
the third heating element arranged in the width direction (W) to correspond to the first heating element and between the first heating element and fourth heating element, and the fourth heating element arranged in the width direction (W) to correspond to the second heating element and between the second heating element and the third heating element,
characterized in that an inner distance (d) at a center in the length direction (L) of the first inner area between the third heating element and the fourth heating element is greater than or equal to a sum of widths of each of the first heating element, second heating element, third heating element, and fourth heating element,
at least one of the first heating element or the corresponding third heating element has a heating value per a unit length that is different in a center of the at least one of the first heating element or the corresponding third heating element, in the length direction (L), compared to an end of the first heating element or the corresponding third heating element, in the length direction (L).