Patent Description:
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, and multifunction peripherals (MFP) having two or more of copying, printing, scanning, facsimile, plotter, and other functions, typically form an image on a recording medium according to image data.

Such image forming apparatuses include a fixing device that includes a fixing rotator and a plurality of heaters that heats the fixing rotator. The heaters have different heat generation properties, respectively. The fixing device fixes the image on the recording medium.

<CIT> (<CIT>) discloses a fixing device that includes a main heater and a sub heater. The main heater includes a center portion and lateral end portions in a longitudinal direction of the main heater. The main heater has a heat generation property in which a heat generation amount of the center portion is greater than a heat generation amount of each of the lateral end portions. The sub heater includes a center portion and lateral end portions in a longitudinal direction of the sub heater. The sub heater has a heat generation property in which a heat generation amount of each of the lateral end portions is greater than a heat generation amount of the center portion. The fixing device further includes a first temperature detecting sensor and a second temperature detecting sensor. The first temperature detecting sensor detects a temperature of a center span of a fixing rotator in an axial direction thereof. The second temperature detecting sensor detects a temperature of a lateral end span of the fixing rotator in the axial direction thereof. A controller controls the main heater based on the temperature of the center span of the fixing rotator, that is detected by the first temperature detecting sensor. The controller controls the sub heater based on the temperature of the lateral end span of the fixing rotator, that is detected by the second temperature detecting sensor. The heat generation amount of each of the lateral end portions of the sub heater is greater than the heat generation amount of the center portion of the main heater.

However, the fixing device may be constructed of parts in an increased number, increasing manufacturing costs.

<CIT> discloses a fixing device and image forming apparatus to reduce a load of lighting control and control the temperature of a fixing roller to be kept uniform. When a warm-up operation is started by power-on of an image forming apparatus (S10), a control unit set a series lighting mode (S11). After lapse of a predetermined period of time, the control unit switches the lighting mode to a parallel lighting mode (S12). After that, when the measured temperature of the end portions of a heating roller is equal to or lower than a reference temperature, the control unit sets the lighting mode to an end heat up mode (S16), and subjects an end heater to full-lighting control and subjects a flat heater to lighting control in a duty pattern at a duty ratio between <NUM>% and <NUM>% (S17). Meanwhile, when the measured temperature of the end portions of the heating roller exceeds the reference temperature, the control unit sets the lighting mode to a flat mode (S18), and subjects the flat heater to full-lighting control and subjects the end heater to lighting control in a duty pattern at a duty ration between <NUM>% and <NUM>% (S19).

It is a general object of the present disclosure to provide an improved and useful fixing device in which the above-mentioned problems are eliminated.

It is another object of the present disclosure to provide an improved and useful image forming apparatus in which the above-mentioned problems are eliminated.

Advantageously, the image forming apparatus includes the fixing device described above.

Accordingly, the fixing device and the image forming apparatus reduce manufacturing costs.

A more complete appreciation of the embodiments and many of the attendant advantages and features thereof can be readily obtained and understood from the following detailed description with reference to the accompanying drawings, wherein:.

A description is provided of a construction of a printer <NUM> according to an embodiment of the present disclosure, that is, a color printer employing an electrophotographic method.

The printer <NUM> serves as an image forming apparatus incorporating a fixing device according to an embodiment of the present disclosure.

<FIG> is a schematic cross-sectional view of the printer <NUM> according to the embodiment of the present disclosure.

The printer <NUM> depicted in <FIG> is a color printer employing a tandem system in which a plurality of image forming devices that forms images in a plurality of colors, respectively, is arranged in a stretch direction of a transfer belt <NUM> serving as an intermediate transferor. However, the image forming apparatus employing the fixing device according to the embodiment of the present disclosure is not limited to the printer <NUM> employing the tandem system. The image forming apparatus employing the fixing device according to the embodiment of the present disclosure may be a copier, a facsimile machine, or the like instead of a printer.

The printer <NUM> employs the tandem system in which photoconductive drums 20Y, 20C, <NUM>, and 20Bk are arranged. The photoconductive drums 20Y, 20C, <NUM>, and 20Bk serve as image bearers that bear images in yellow, cyan, magenta, and black as color separation components, respectively.

In the printer <NUM>, visible images, that is, toner images, formed on the photoconductive drums 20Y, 20C, <NUM>, and 20Bk, respectively, are primarily transferred onto the transfer belt <NUM> in a primary transfer process. The transfer belt <NUM> is an endless belt that rotates in a rotation direction A1 while the transfer belt <NUM> is disposed opposite the photoconductive drums 20Y, 20C, <NUM>, and 20Bk. In the primary transfer process, the visible images, that is, yellow, cyan, magenta, and black toner images, are transferred onto the transfer belt <NUM> such that the yellow, cyan, magenta, and black toner images are superimposed on the transfer belt <NUM>. Thereafter, the visible images formed on the transfer belt <NUM> are transferred collectively onto a sheet P serving as a recording medium in a secondary transfer process.

Each of the photoconductive drums 20Y, 20C, <NUM>, and 20Bk is surrounded by image forming units that form the visible image as each of the photoconductive drums 20Y, 20C, <NUM>, and 20Bk rotates. Taking the photoconductive drum 20Bk that forms the black toner image as an example, a charger 30Bk, a developing device 40Bk, a primary transfer roller 12Bk, and a cleaner 50Bk which form the black toner image are arranged in a rotation direction of the photoconductive drum 20Bk. Similarly, chargers 30Y, 30C, and <NUM>, developing devices 40Y, 40C, and <NUM>, primary transfer rollers 12Y, 12C, and <NUM>, and cleaners 50Y, 50C, and <NUM> are arranged in a rotation direction of the photoconductive drums 20Y, 20C, and <NUM>, respectively. An optical writing device <NUM> is used for optical writing with a light beam Lb after the charger 30Bk charges the photoconductive drum 20Bk uniformly.

While the transfer belt <NUM> rotates in the rotation direction A1, the toner images formed on the photoconductive drums 20Y, 20C, <NUM>, and 20Bk, respectively, are transferred onto the transfer belt <NUM> such that the toner images are superimposed on a same position on the transfer belt <NUM>. The primary transfer rollers 12Y, 12C, <NUM>, and 12Bk disposed opposite the photoconductive drums 20Y, 20C, <NUM>, and 20Bk, respectively, via the transfer belt <NUM> apply a voltage to primarily transfer the toner images formed on the photoconductive drums 20Y, 20C, <NUM>, and 20Bk at different times from the upstream photoconductive drum 20Y to the downstream photoconductive drum 20Bk in the rotation direction A1 of the transfer belt <NUM>.

The photoconductive drums 20Y, 20C, <NUM>, and 20Bk are arranged in this order from the upstream photoconductive drum 20Y to the downstream photoconductive drum 20Bk in the rotation direction A1 of the transfer belt <NUM>. Imaging stations that form the yellow, cyan, magenta, and black toner images include the photoconductive drums 20Y, 20C, <NUM>, and 20Bk, respectively.

The printer <NUM> includes four imaging stations and a transfer belt unit <NUM>. The four imaging stations form the yellow, cyan, magenta, and black toner images, respectively. The transfer belt unit <NUM> is disposed opposite and above the photoconductive drums 20Y, 20C, <NUM>, and 20Bk in <FIG>. The transfer belt unit <NUM> includes the transfer belt <NUM> and the primary transfer rollers 12Y, 12C, <NUM>, and 12Bk. The printer <NUM> further includes a secondary transfer roller <NUM> and a belt cleaner <NUM>. The secondary transfer roller <NUM> is disposed opposite the transfer belt <NUM> and rotates in accordance with rotation of the transfer belt <NUM>. The belt cleaner <NUM> is disposed opposite the transfer belt <NUM> and cleans the transfer belt <NUM>. The optical writing device <NUM> is disposed opposite and below the four imaging stations in <FIG>.

The optical writing device <NUM> includes a semiconductor laser serving as a light source that writes an electrostatic latent image, a coupling lens, an f-θ lens, a toroidal lens, a reflection mirror, and a polygon mirror serving as a deflector. The optical writing device <NUM> emits light beams Lb that correspond to yellow, cyan, magenta, and black image data onto the photoconductive drums 20Y, 20C, <NUM>, and 20Bk, forming electrostatic latent images on the photoconductive drums 20Y, 20C, <NUM>, and 20Bk, respectively. Although <FIG> illustrates the light beam Lb directed to the imaging station that forms the black toner image, the light beams Lb are also directed to the imaging stations that form the yellow, cyan, and magenta toner images, respectively.

The printer <NUM> further includes a sheet feeder <NUM> (e.g., a sheet tray) that loads sheets P to be conveyed to a secondary transfer nip formed between the secondary transfer roller <NUM> and the transfer belt <NUM>. The printer <NUM> further includes a registration roller pair <NUM> that feeds a sheet P conveyed from the sheet feeder <NUM> to the secondary transfer nip formed between the secondary transfer roller <NUM> and the transfer belt <NUM> at a predetermined time when the yellow, cyan, magenta, and black toner images formed on the transfer belt <NUM> by the imaging stations, respectively, reach the secondary transfer nip. The printer <NUM> further includes a sensor that detects that a leading edge of the sheet P reaches the registration roller pair <NUM>.

The printer <NUM> further includes a fixing device <NUM>, a sheet ejecting roller pair <NUM>, an output tray <NUM>, and toner bottles 9Y, 9C, <NUM>, and 9Bk. The fixing device <NUM> is a fuser unit that fixes a color toner image on the sheet P while the fixing device <NUM> contacts and heats the sheet P. The color toner image is formed by transferring the yellow, cyan, magenta, and black toner images formed on the transfer belt <NUM> onto the sheet P. The sheet ejecting roller pair <NUM> ejects the sheet P bearing the fixed color toner image onto an outside of a body of the printer <NUM>. The output tray <NUM> is disposed atop the body of the printer <NUM>. The output tray <NUM> stacks the sheets P ejected onto the outside of the body of the printer <NUM> by the sheet ejecting roller pair <NUM>. The toner bottles 9Y, 9C, <NUM>, and 9Bk are disposed below the output tray <NUM> in <FIG> and disposed inside the body of the printer <NUM>. The toner bottles 9Y, 9C, <NUM>, and 9Bk are replenished with yellow, cyan, magenta, and black toners, respectively.

In addition to the transfer belt <NUM> and the primary transfer rollers 12Y, 12C, <NUM>, and 12Bk, the transfer belt unit <NUM> includes a driving roller <NUM> and a driven roller <NUM> over which the transfer belt <NUM> is looped.

The driven roller <NUM> also serves as a tension applicator that applies tension to the transfer belt <NUM>. A biasing member such as a spring biases the driven roller <NUM> against the transfer belt <NUM>. The transfer belt unit <NUM>, the primary transfer rollers 12Y, 12C, <NUM>, and 12Bk, the secondary transfer roller <NUM>, and the belt cleaner <NUM> construct a transfer device <NUM>.

The sheet feeder <NUM> is disposed in a lower portion of the body of the printer <NUM>. The sheet feeder <NUM> includes a sheet feeding roller <NUM> that comes into contact with an upper surface of an uppermost sheet P. As the sheet feeding roller <NUM> is driven and rotated counterclockwise in <FIG>, the sheet feeding roller <NUM> feeds the uppermost sheet P to the registration roller pair <NUM>.

The belt cleaner <NUM> installed in the transfer device <NUM> includes a cleaning brush and a cleaning blade that are disposed opposite and brought into contact with the transfer belt <NUM>. The cleaning brush and the cleaning blade of the belt cleaner <NUM> scrape and remove a foreign substance such as residual toner from the transfer belt <NUM>, cleaning the transfer belt <NUM>.

The belt cleaner <NUM> further includes a discharging device that conveys the residual toner removed from the transfer belt <NUM> for disposal.

A description is provided of a construction of the fixing device <NUM> incorporated in the printer <NUM>.

<FIG> is a schematic cross-sectional view of the fixing device <NUM>.

The fixing device <NUM> includes a fixing belt <NUM> and a pressure roller <NUM>. The fixing belt <NUM> serves as a fixing rotator that is rotatable in a rotation direction indicated with an arrow in <FIG>. The pressure roller <NUM> serves as a pressure rotator that is disposed opposite the fixing belt <NUM> and rotatable in a rotation direction indicated with an arrow in <FIG>. Within a loop formed by the fixing belt <NUM> are a main heater 102a serving as a first heater, a sub heater 102b serving as a second heater, a pad <NUM> serving as a nip formation pad, a support <NUM>, a slide aid <NUM>, a reflector <NUM>, and the like. Each of the main heater 102a, the sub heater 102b, the pad <NUM>, the support <NUM>, the slide aid <NUM>, and the reflector <NUM> that are disposed within the loop formed by the fixing belt <NUM> has a length that is greater than a length of the fixing belt <NUM> in an axial direction thereof.

The fixing belt <NUM> is an endless belt or film made of metal such as nickel and stainless used steel (SUS) or a resin material such as polyimide. The fixing belt <NUM> includes a base layer and a release layer. The release layer serves as a surface layer made of perfluoroalkoxy alkane (PFA), polytetrafluoroethylene (PTFE), or the like, facilitating separation of toner of the toner image on the sheet P from the fixing belt <NUM> and preventing the toner from adhering to the fixing belt <NUM>. Optionally, an elastic layer made of silicone rubber or the like may be interposed between the base layer and the release layer. If the fixing belt <NUM> does not incorporate the elastic layer, the fixing belt <NUM> attains a decreased thermal capacity that improves a fixing property of being heated quickly. However, when the pressure roller <NUM> presses and deforms an unfixed toner image to fix the toner image on the sheet P, slight surface asperities of the fixing belt <NUM> may be transferred onto the toner image, causing a disadvantage that an orange peel mark remains on a solid part of the toner image as uneven gloss of the toner image or an orange peel image. To address this circumstance, the elastic layer has a thickness of <NUM> or more. As the elastic layer deforms, the elastic layer absorbs the slight surface asperities, preventing the orange peel mark on the toner image.

The pressure roller <NUM> includes a core metal <NUM>, an elastic rubber layer <NUM>, and a release layer. The elastic rubber layer <NUM> is disposed on the core metal <NUM>. The release layer serves as a surface layer that facilitates separation of the sheet P from the pressure roller <NUM>. The release layer is made of PFA, PTFE, or the like. A driving force is transmitted to the pressure roller <NUM> from a driver such as a motor disposed in the printer <NUM> through a gear, thus rotating the pressure roller <NUM>. A spring or the like presses the pressure roller <NUM> against the fixing belt <NUM>. As the spring presses and deforms the elastic rubber layer <NUM>, the pressure roller <NUM> forms a fixing nip N having a predetermined length in a sheet conveyance direction DP. Alternatively, the pressure roller <NUM> may be a hollow roller. A heater such as a halogen heater may be disposed inside the pressure roller <NUM> as the hollow roller. The elastic rubber layer <NUM> may be made of solid rubber. Alternatively, if no heater is disposed inside the pressure roller <NUM>, sponge rubber may be used. The sponge rubber enhances thermal insulation of the pressure roller <NUM>, preferably causing the pressure roller <NUM> to draw less heat from the fixing belt <NUM>.

The pad <NUM> serving as a nip formation pad is disposed within the loop formed by the fixing belt <NUM>. The pad <NUM> is disposed opposite the pressure roller <NUM> via the fixing belt <NUM> to form the fixing nip N between the fixing belt <NUM> and the pressure roller <NUM>. The pad <NUM> mounts the slide aid <NUM> over which an inner circumferential surface of the fixing belt <NUM> slides. The support <NUM> supports the pad <NUM>.

The pad <NUM> depicted in <FIG> has an opposed face that is disposed opposite the pressure roller <NUM> and is planar. Alternatively, the opposed face of the pad <NUM> may be curved or recessed or may have other shapes. If the opposed face of the pad <NUM> is recessed, the opposed face of the pad <NUM> causes the fixing nip N to be recessed toward the fixing belt <NUM>. Accordingly, the fixing nip N directs the leading edge of the sheet P toward the pressure roller <NUM> when the sheet P is ejected from the fixing nip N, facilitating separation of the sheet P from the fixing belt <NUM> and thereby preventing the sheet P from being jammed.

The support <NUM> prevents the pad <NUM> from being bent by pressure received from the pressure roller <NUM>, attaining a uniform length of the fixing nip N in the sheet conveyance direction DP throughout an entire span of the fixing belt <NUM> in the axial direction thereof.

Each of the main heater 102a and the sub heater 102b is a halogen heater. The main heater 102a and the sub heater 102b disposed opposite the inner circumferential surface of the fixing belt <NUM> heat the fixing belt <NUM> directly with radiant heat. Alternatively, each of the main heater 102a and the sub heater 102b may be an induction heater (IH), a resistive heat generator, a carbon heater, or the like as long as the main heater 102a and the sub heater 102b heat the fixing belt <NUM>.

According to this embodiment, the reflector <NUM> (e.g., a reflecting plate) is interposed between the main heater 102a and the support <NUM> and between the sub heater 102b and the support <NUM>. The reflector <NUM> reflects radiant heat and the like from the main heater 102a and the sub heater 102b, preventing the radiant heat and the like from heating the support <NUM> and suppressing resultant waste of energy. Alternatively, instead of the reflector <NUM>, a surface of the support <NUM> may be treated with thermal insulation or specular surface finish to attain similar advantages.

Outside the loop formed by the fixing belt <NUM> is a temperature detecting sensor <NUM> that detects the temperature of a surface of the fixing belt <NUM>. The temperature detecting sensor <NUM> is a temperature sensor, such as a thermopile, that has an enhanced temperature responsiveness. The temperature detecting sensor <NUM> is disposed opposite a center span CS of the fixing belt <NUM> in the axial direction thereof and detects the temperature of the center span CS of the fixing belt <NUM> as described below with reference to <FIG>.

The fixing belt <NUM> rotates in accordance with rotation of the pressure roller <NUM>. With the construction of the fixing device <NUM> depicted in <FIG>, as the driver drives and rotates the pressure roller <NUM>, the driving force is transmitted from the pressure roller <NUM> to the fixing belt <NUM> at the fixing nip N, rotating the fixing belt <NUM> in accordance with rotation of the pressure roller <NUM>. As a sheet P bearing a toner image is conveyed through the fixing nip N, the fixing belt <NUM> and the pressure roller <NUM> fix the toner image on the sheet P under heat and pressure.

With the construction described above, the fixing device <NUM> improves productivity and fixing performance at reduced costs.

<FIG> is a perspective view of a guide <NUM> incorporated in the fixing device <NUM>. <FIG> is a front view of the guide <NUM>.

The guides <NUM> having an identical shape are disposed opposite both lateral ends of the fixing belt <NUM> in the axial direction thereof, respectively. As illustrated in <FIG>, the guide <NUM> includes an attachment portion 451b and a guide portion 451a. The attachment portion 451b is attached to a side plate of the fixing device <NUM>. The guide portion 451a is disposed opposite the inner circumferential surface of the fixing belt <NUM> at a lateral end of the fixing belt <NUM> in the axial direction thereof.

The guide portion 451a is substantially tubular and has a slit disposed opposite the pressure roller <NUM>. An outer diameter of the guide portion 451a is equivalent to an inner diameter of the fixing belt <NUM>. The guide portion 451a has a length in the axial direction of the fixing belt <NUM>, that is defined inward from a lateral edge of the fixing belt <NUM> in the axial direction thereof, when the guide portion 451a is inserted into the fixing belt <NUM> for a predetermined amount. As the guide portion 451a is inserted into the fixing belt <NUM> at the lateral end of the fixing belt <NUM> in the axial direction thereof such that the fixing belt <NUM> slides over the guide portion 451a, the guide portion 451a retains a circular shape of the fixing belt <NUM> in cross section.

As illustrated in <FIG>, the attachment portion 451b includes a through hole 451c disposed opposite an interior of the guide portion 451a. The support <NUM>, the main heater 102a, and the sub heater 102b are attached to the side plate of the fixing device <NUM> through the through hole 451c.

<FIG> is a block diagram of the printer <NUM>, illustrating a controller <NUM> that controls turning on of each of the main heater 102a and the sub heater 102b of the fixing device <NUM>.

The controller <NUM> includes a central processing unit (CPU), a read only memory (ROM), a random access memory (RAM), and a nonvolatile flash memory. The ROM is a memory that is read-only and stores a control program. The RAM is a memory that is readable and writable and stores data temporarily. The controller <NUM> is connected to the main heater 102a, the sub heater 102b, the temperature detecting sensor <NUM>, and a control panel <NUM>. The control panel <NUM> includes a display and a control portion and receives an instruction input by a user.

The nonvolatile flash memory stores data relating to a size of a sheet P placed in the sheet feeder <NUM>, that is input by the user using the control panel <NUM>. The controller <NUM> controls turning on of each of the main heater 102a and the sub heater 102b based on the data relating to the size of the sheet P, that is stored in the nonvolatile flash memory, and a temperature of the fixing belt <NUM>, that is detected by the temperature detecting sensor <NUM>.

<FIG> is a diagram of a comparative fixing device 100C, illustrating a configuration of heaters incorporated therein.

As illustrated in <FIG>, the comparative fixing device 100C includes a center heater 202a and a lateral end heater 202b. The center heater 202a has a heat generation property in which a center portion of the center heater 202a in a longitudinal direction thereof generates heat solely. The lateral end heater 202b has a heat generation property in which lateral end portions of the lateral end heater 202b in a longitudinal direction thereof generate heat solely. A heat generation span LC produced when the center heater 202a and the lateral end heater 202b are turned on is not smaller than a maximum conveyance span in an axial direction of a fixing belt where a sheet having a maximum width is conveyed over the fixing belt.

The comparative fixing device 100C further includes a lateral end temperature detecting sensor 210b and a center temperature detecting sensor 210a. The lateral end temperature detecting sensor 210b detects a temperature of a lateral end span of the fixing belt in the axial direction thereof. The center temperature detecting sensor 210a detects a temperature of a center span of the fixing belt in the axial direction thereof. When a large sheet is conveyed over the fixing belt, a controller controls turning on of the lateral end heater 202b based on the temperature of the lateral end span of the fixing belt, that is detected by the lateral end temperature detecting sensor 210b. The controller controls turning on of the center heater 202a based on the temperature of the center span of the fixing belt, that is detected by the center temperature detecting sensor 210a. Accordingly, the center heater 202a and the lateral end heater 202b retain the fixing belt at a predetermined fixing temperature substantially throughout an entire span of the fixing belt in the axial direction thereof.

The comparative fixing device 100C includes the center heater 202a and the lateral end heater 202b that have the heat generation properties described above, respectively. Hence, when a small sheet is conveyed over the fixing belt, the controller turns off the lateral end heater 202b, thus allowing the fixing belt to fix a toner image on the small sheet without causing the lateral end heater 202b to heat the lateral end spans of the fixing belt in the axial direction thereof. Accordingly, when printing is performed continuously on a great number of small sheets with a short interval between successive small sheets, the comparative fixing device 100C suppresses overheating of the lateral end spans of the fixing belt in the axial direction thereof. However, image forming apparatuses located in offices barely print a great number of sheets continuously and are barely requested to improve productivity in continuous printing. The image forming apparatuses located in the offices are requested to shorten a first print out time at reduced costs.

To address this circumstance of the comparative fixing device 100C, as illustrated in <FIG>, the fixing device <NUM> according to the embodiment of the present disclosure includes the main heater 102a and the sub heater 102b. The main heater 102a has a heat generation property in which the main heater 102a generates heat evenly in a longitudinal direction thereof. The sub heater 102b includes a center portion 102b1 and lateral end portions 102b2 arranged with the center portion 102b1 in a longitudinal direction of the sub heater 102b. The sub heater 102b has a heat generation property in which a heat generation amount of each of the lateral end portions 102b2 is greater than a heat generation amount of the center portion 102b1. Accordingly, the fixing device <NUM> reduces the number of temperature detecting sensors and manufacturing costs compared to the comparative fixing device 100C depicted in <FIG>. The controller <NUM> turns on both the main heater 102a and the sub heater 102b, heating the fixing belt <NUM> quickly to the fixing temperature substantially evenly in the axial direction of the fixing belt <NUM> and thus suppressing degradation in the first print out time for a large sheet.

Referring to drawings, a description is provided of a construction of the fixing device <NUM> specifically.

<FIG> is a diagram of the fixing device <NUM> according to the embodiment of the present disclosure, illustrating a configuration of the main heater 102a and the sub heater 102b.

As illustrated in <FIG>, the fixing device <NUM> includes the main heater 102a and the sub heater 102b. The main heater 102a has the heat generation property in which the main heater 102a generates heat evenly in the longitudinal direction thereof. The sub heater 102b has the heat generation property in which the heat generation amount of each of the lateral end portions 102b2 is greater than the heat generation amount of the center portion 102b1.

A heat generation span L produced by the main heater 102a and the sub heater 102b is not smaller than a maximum conveyance span in the axial direction of the fixing belt <NUM> where a sheet having a maximum width available in the printer <NUM> is conveyed over the fixing belt <NUM>. The heat generation amount of the center portion 102b1 of the sub heater 102b is smaller than a heat generation amount of the main heater 102a in the center span CS. According to this embodiment, a single temperature detecting sensor, that is, the temperature detecting sensor <NUM>, is disposed opposite the center span CS of the fixing belt <NUM>. The controller <NUM> controls the main heater 102a and the sub heater 102b by using the single, temperature detecting sensor <NUM> so that the fixing belt <NUM> retains a predetermined temperature (e.g., a standby temperature or a fixing temperature). According to this embodiment, the temperature detecting sensor <NUM> is disposed opposite the center span CS of the fixing belt <NUM>. Alternatively, the temperature detecting sensor <NUM> may be disposed opposite other span of the fixing belt <NUM> in the axial direction thereof where a sheet having a minimum width available in the printer <NUM> is conveyed over the fixing belt <NUM>. According to this embodiment, the minimum width is a width of <NUM> of an A6 size sheet in portrait orientation.

As illustrated in <FIG>, a total heat generation amount obtained by adding a heat generation amount of the lateral end portion 102b2 of the sub heater 102b to a heat generation amount of a lateral end span LS of the main heater 102a in the longitudinal direction thereof when both the sub heater 102b and the main heater 102a are turned on is greater than a total heat generation amount obtained by adding a heat generation amount of the center portion 102b1 of the sub heater 102b to a heat generation amount of the center span CS of the main heater 102a. Conversely, when the sub heater 102b is turned off and the main heater 102a is turned on, as illustrated in <FIG>, the main heater 102a attains a heat generation amount that is substantially even in the longitudinal direction of the main heater 102a, heating the fixing belt <NUM> substantially evenly in the axial direction thereof.

According to this embodiment, the center portion 102b1 of the sub heater 102b attains a predetermined heat generation amount. Alternatively, the center portion 102b1 of the sub heater 102b may provide a heat generation amount of <NUM> [W].

<FIG> is a timing chart of a control for turning on each of the main heater 102a and the sub heater 102b as one example.

The controller <NUM> turns on both the sub heater 102b and the main heater 102a when the printer <NUM> is powered on for warming up.

When the printer <NUM> is warmed up to heat the fixing belt <NUM> to the predetermined temperature (e.g., the fixing temperature or the standby temperature), the guides <NUM> serving as lateral end contact members draw heat from the lateral end spans LS of the fixing belt <NUM> because the guides <NUM> include the guide portions 451a that contact both lateral ends of the fixing belt <NUM> in the axial direction thereof, respectively. Both lateral ends of the fixing belt <NUM> in the axial direction thereof slide over the guide portions 451a, respectively. Hence, the lateral end spans LS of the fixing belt <NUM> are subject to temperature decrease in which the temperature of each of the lateral end spans LS of the fixing belt <NUM> decreases compared to the temperature of the center span CS of the fixing belt <NUM>.

To address this circumstance, the controller <NUM> for the fixing device <NUM> according to this embodiment turns on both the sub heater 102b and the main heater 102a when the fixing device <NUM> is warmed up, thus increasing the heat generation amount of the sub heater 102b and the main heater 102a in each of the lateral end spans LS of the fixing belt <NUM> compared to the center span CS of the fixing belt <NUM>. Accordingly, even if the guides <NUM> draw heat slightly from the lateral end spans LS of the fixing belt <NUM>, respectively, the fixing device <NUM> suppresses temperature decrease in the lateral end spans LS of the fixing belt <NUM>. Accordingly, the fixing device <NUM> causes the sub heater 102b and the main heater 102a to heat each of the lateral end spans LS of the fixing belt <NUM> to the predetermined temperature (e.g., the fixing temperature or the standby temperature) quickly like the center span CS of the fixing belt <NUM>. Consequently, the fixing device <NUM> shortens a warm up time taken after the printer <NUM> is powered on until the fixing device <NUM> is heated to the predetermine temperature and a first print out time taken after the printer <NUM> receives an instruction to start printing until a trailing edge of a first sheet P is ejected onto the output tray <NUM>. Additionally, the fixing device <NUM> attains proper fixing performance for fixing a toner image on a sheet P conveyed over the fixing belt <NUM> first after warming up of the fixing belt <NUM> is finished even in the lateral end spans LS of the fixing belt <NUM>.

According to this embodiment, when a large sheet P is conveyed through the fixing nip N, the controller <NUM> turns on both the sub heater 102b and the main heater 102a. As the controller <NUM> turns on both the sub heater 102b and the main heater 102a, the fixing device <NUM> prevents the fixing belt <NUM> from fixing a toner image on a sheet P while the lateral end spans LS of the fixing belt <NUM> suffer from temperature decrease, thus suppressing faulty fixing of the toner image in the lateral end spans LS of the sheet P. According to this embodiment, a large sheet P has an increased width in a width direction thereof parallel to the axial direction of the fixing belt <NUM>. The increased width is not smaller than a width of <NUM> of a B4 size sheet in portrait orientation. A small sheet P has a decreased width in a width direction thereof parallel to the axial direction of the fixing belt <NUM>. The decreased width is smaller than the width of the B4 size sheet in portrait orientation. Alternatively, the increased width and the decreased width may be defined properly according to a configuration of an image forming apparatus (e.g., the printer <NUM>).

When a predetermined time period elapses after conveyance of a sheet P starts (e.g., after fixing starts), the controller <NUM> turns off the sub heater 102b. The controller <NUM> turns on the main heater 102a based on a detection result sent from the temperature detecting sensor <NUM>, retaining the fixing belt <NUM> at the fixing temperature.

Immediately after fixing starts, the guides <NUM> have a temperature not higher than the fixing temperature. Hence, heat is conducted from the lateral end spans LS of the fixing belt <NUM> to the guides <NUM>. Conversely, after the predetermined time period elapses, the guides <NUM> are heated to a temperature close to the fixing temperature, decreasing conduction of heat from the lateral end spans LS of the fixing belt <NUM> to the guides <NUM>. As described above, the length of the fixing belt <NUM> in the axial direction thereof is not smaller than the maximum width of the sheet P available in the printer <NUM>. Hence, both lateral ends of the fixing belt <NUM> in the axial direction thereof do not directly contact the sheet P having the maximum width. Accordingly, the lateral end spans LS of the fixing belt <NUM> are less susceptible to drawing of heat by the sheet P than the center span CS of the fixing belt <NUM>. As conduction of heat from the lateral end spans LS of the fixing belt <NUM> to the guides <NUM> decreases, an amount of heat drawn from each of the lateral end spans LS of the fixing belt <NUM> by a sheet P conveyed over the lateral end spans LS of the fixing belt <NUM> and the guides <NUM> is equivalent to an amount of heat drawn from the center span CS of the fixing belt <NUM> by the sheet P conveyed over the center span CS of the fixing belt <NUM>. As a result, even if the heat generation amount of each of the sub heater 102b and the main heater 102a in each of the lateral end spans LS is not greater than the heat generation amount of each of the sub heater 102b and the main heater 102a in the center span CS, the lateral end spans LS of the fixing belt <NUM> do not suffer from temperature decrease. Thus, the fixing belt <NUM> retains the fixing temperature substantially throughout the entire span of the fixing belt <NUM> in the axial direction thereof.

When the controller <NUM> turns on the main heater 102a without turning on the sub heater 102b, as illustrated in <FIG>, the main heater 102a attains the heat generation amount that is substantially even in the longitudinal direction of the main heater 102a, heating the fixing belt <NUM> substantially evenly in the axial direction thereof. Accordingly, after the fixing device <NUM> attains a condition in which the lateral end spans LS of the fixing belt <NUM> are immune from temperature decrease, the controller <NUM> controls turning on of the main heater 102a based on a temperature of the fixing belt <NUM>, that is detected by the temperature detecting sensor <NUM> disposed opposite the center span CS of the fixing belt <NUM>. Consequently, the fixing belt <NUM> retains the fixing temperature substantially throughout the entire span of the fixing belt <NUM> in the axial direction thereof, thus suppressing faulty fixing of a toner image in the lateral end spans LS of a sheet P.

According to this embodiment, the main heater 102a has the heat generation property in which the main heater 102a generates heat substantially evenly in the longitudinal direction thereof. After the fixing device <NUM> attains the condition in which the lateral end spans LS of the fixing belt <NUM> are immune from temperature decrease, the controller <NUM> performs a control described below to retain the fixing belt <NUM> at the fixing temperature substantially throughout the entire span of the fixing belt <NUM> in the axial direction thereof. For example, the controller <NUM> turns off the sub heater 102b and turns on the main heater 102a based on the detection result sent from the temperature detecting sensor <NUM>. Accordingly, the fixing device <NUM> eliminates the lateral end temperature detecting sensor 210b of the comparative fixing device 100C depicted in <FIG>, that is used to control turning on of the sub heater 202b to retain both lateral end spans of the fixing belt in the axial direction thereof at the fixing temperature. Thus, the fixing device <NUM> reduces the number of parts and manufacturing costs compared to the comparative fixing device 100C depicted in <FIG>.

When a small sheet P having the decreased width is conveyed through the fixing nip N, the controller <NUM> turns on the main heater 102a without turning on the sub heater 102b based on the detection result sent from the temperature detecting sensor <NUM>, retaining the fixing belt <NUM> at the fixing temperature.

When the small sheet P is conveyed over the fixing belt <NUM>, a toner image on the small sheet P passes over an inboard span that is inboard from both lateral ends of the fixing belt <NUM> in the axial direction thereof. Both lateral ends of the fixing belt <NUM> may suffer from temperature decrease. Hence, the toner image on the small sheet P is immune from an adverse effect caused by temperature decrease of the fixing belt <NUM>. Accordingly, when the small sheet P or a sheet P bearing a toner image having a decreased width in the width direction of the sheet P is conveyed over the fixing belt <NUM>, the controller <NUM> does not turn on the sub heater 102b and turns on the main heater 102a. Thus, the fixing device <NUM> reduces power consumption compared to a configuration in which the controller <NUM> turns on both the sub heater 102b and the main heater 102a to fix a toner image on a sheet P. Additionally, the fixing device <NUM> suppresses overheating of the lateral end spans LS of the fixing belt <NUM> compared to the configuration in which the controller <NUM> turns on both the sub heater 102b and the main heater 102a.

In a standby mode in which the fixing device <NUM> waits for a fixing job, for example, the controller <NUM> turns on both the main heater 102a and the sub heater 102b based on a detection result sent from the temperature detecting sensor <NUM>, retaining the fixing belt <NUM> at the standby temperature.

In the standby mode, heat is not drawn from the fixing belt <NUM> by a sheet P. Conversely, the guides <NUM> draw heat from the lateral end spans LS of the fixing belt <NUM>, respectively, even in the standby mode. As a result, if the controller <NUM> is configured to turn on the main heater 102a, without turning on the sub heater 102b, based on a detection result sent from the temperature detecting sensor <NUM> that detects the temperature of the center span CS of the fixing belt <NUM> so as to retain the fixing belt <NUM> at the standby temperature, a disadvantage below may occur. For example, a temperature difference between each of the lateral end spans LS and the center span CS of the fixing belt <NUM> increases gradually, causing a temperature of each of the lateral end spans LS of the fixing belt <NUM> to be lower than a temperature of the center span CS of the fixing belt <NUM> disadvantageously.

To address this circumstance, according to this embodiment, in the standby mode, the controller <NUM> turns on both the sub heater 102b and the main heater 102a based on the detection result sent from the temperature detecting sensor <NUM>, retaining the fixing belt <NUM> at the standby temperature. As described above, the controller <NUM> turns on both the sub heater 102b and the main heater 102a, causing the combined heat generation amount that combines the heat generation amount of the sub heater 102b and the heat generation amount of the main heater 102a in each of the lateral end spans LS to be greater than the combined heat generation amount of the heat generation amount of the sub heater 102b and the heat generation amount of the main heater 102a in the center span CS. Thus, in the standby mode, the fixing device <NUM> prevents a temperature of each of the lateral end spans LS of the fixing belt <NUM> from being lower than a temperature of the center span CS of the fixing belt <NUM>. Additionally, in the standby mode, heat is not drawn from the fixing belt <NUM> by the sheet P. Hence, the fixing device <NUM> decreases a lighting amount of the main heater 102a and the sub heater 102b per unit time to retain the fixing belt <NUM> at the standby temperature. Accordingly, even if a difference between the combined heat generation amount in each of the lateral end spans LS and the combined heat generation amount in the center span CS increases slightly, the lateral end spans LS of the fixing belt <NUM> are immune from overheating.

If the center portion 102b1 of the sub heater 102b also generates heat in a predetermined heat generation amount that is greater than <NUM> [W], the controller <NUM> may turn on the sub heater 102b, without turning on the main heater 102a, based on a detection result sent from the temperature detecting sensor <NUM>, retaining the fixing belt <NUM> at the standby temperature as illustrated in <FIG>. If the controller <NUM> turns on the sub heater 102b and does not turn on the main heater 102a in the standby mode, a heat generation amount of the sub heater 102b is smaller than a combined heat generation amount combining a heat generation amount of the sub heater 102b and a heat generation amount of the main heater 102a when the controller <NUM> turns on both the sub heater 102b and the main heater 102a. As a result, the fixing device <NUM> increases the lighting amount of the main heater 102a and the sub heater 102b per unit time to retain the fixing belt <NUM> at the standby temperature. Accordingly, if a difference between a heat generation amount of each of the lateral end portions 102b2 of the sub heater 102b and a heat generation amount of the center portion 102b1 of the sub heater 102b increases, the lateral end spans LS of the fixing belt <NUM> may overheat. Hence, if the controller <NUM> turns on the sub heater 102b and does not turn on the main heater 102a in the standby mode, the controller <NUM> sets the difference between the heat generation amount of each of the lateral end portions 102b2 of the sub heater 102b and the heat generation amount of the center portion 102b1 of the sub heater 102b to be smaller than that when the controller <NUM> turns on both the sub heater 102b and the main heater 102a in the standby mode.

If each of the guides <NUM> has an increased thermal capacity and is barely subject to temperature decrease, the controller <NUM> may turn on the main heater 102a without turning on the sub heater 102b in the standby mode also, retaining the fixing belt <NUM> at the standby temperature.

As described above, when a large sheet P is conveyed through the fixing nip N, the controller <NUM> turns on the main heater 102a and the sub heater 102b for the predetermined time period. However, even when the large sheet P is conveyed through the fixing nip N, if a toner image on the large sheet P is not conveyed over both lateral ends of the fixing belt <NUM> in the axial direction thereof, that suffer from temperature decrease, faulty fixing does not occur on the toner image on the large sheet P. Accordingly, if the large sheet P is conveyed through the fixing nip N and the toner image on the large sheet P is not situated in reference spans extended inboard from both lateral edges of the large sheet P in the width direction thereof, respectively, that is, if an image area rate in each of the reference spans on the large sheet P is zero, the controller <NUM> may turn on the main heater 102a without turning on the sub heater 102b like in a configuration in which a small sheet P is conveyed through the fixing nip N. Thus, the fixing device <NUM> reduces power consumption compared to the configuration in which the controller <NUM> turns on both the sub heater 102b and the main heater 102a.

<FIG> is a flowchart illustrating processes of a control for turning on each of the main heater 102a and the sub heater 102b to fix a toner image on a sheet P.

When the controller <NUM> receives a print instruction from an external device such as a personal computer, the controller <NUM> reads data relating to a size (e.g., a width) of a sheet P placed in the sheet feeder <NUM> from the nonvolatile flash memory. In step S1, the controller <NUM> determines whether or not the width of the sheet P, that is read from the nonvolatile flash memory, is the increased width. For example, according to this embodiment, the increased width is not smaller than the width of the B4 size sheet in portrait orientation.

If the controller <NUM> determines that the width of the sheet P is the decreased width that is smaller than the width of the B4 size sheet in portrait orientation (NO in step S1), as described above, as the sheet P is conveyed over the fixing belt <NUM>, the sheet P passes over the inboard span that is inboard from both lateral ends of the fixing belt <NUM> in the axial direction thereof, that may suffer from temperature decrease. Hence, the toner image on the sheet P having the decreased width is immune from an adverse effect caused by temperature decrease of the fixing belt <NUM>. Accordingly, if the controller <NUM> determines that the sheet P has the decreased width, the controller <NUM> does not turn on the sub heater 102b and turns on the main heater 102a in step S6.

Conversely, if the controller <NUM> determines that the sheet P has the increased width that is not smaller than the width of the B4 size sheet in portrait orientation (YES in step S1), the controller <NUM> determines whether or not the toner image is within at least one of the reference spans extended inboard from the lateral edges of the sheet P in the width direction thereof based on image data according to which the toner image is formed on the sheet P in step S2. Even if the controller <NUM> determines that the sheet P has the increased width, if the controller <NUM> determines that the toner image is not within the reference spans on the sheet P, that are disposed opposite the lateral ends of the fixing belt <NUM> in the axial direction thereof, respectively, that may suffer from temperature decrease (NO in step S2), the controller <NUM> turns on the main heater 102a in step S6 and does not turn on the sub heater 102b.

Each of the reference spans extended inboard from the lateral edges of the sheet P in the width direction thereof, respectively, where faulty fixing may occur due to temperature decrease of the fixing belt <NUM>, varies depending on the size (e.g., the width) of the sheet P. To address this circumstance, the controller <NUM> changes the reference span according to the size of the sheet P as indicated in table <NUM> below.

Conversely, if the controller <NUM> determines that the toner image is within at least one of the reference spans extended inboard from the lateral edges of the sheet P in the width direction thereof, respectively (YES in step S2), the controller <NUM> turns on both the sub heater 102b and the main heater 102a in step S3. In step S4, the controller <NUM> determines whether or not a predetermined time period elapses after a fixing job starts, that is, after the controller <NUM> turns on both the main heater 102a and the sub heater 102b. If the controller <NUM> determines that the predetermined time period elapses after the fixing job starts (YES in step S4) and the guides <NUM> are heated to the temperature close to the fixing temperature, thus decreasing conduction of heat from both lateral ends of the fixing belt <NUM> in the axial direction thereof to the guides <NUM>, respectively, the controller <NUM> turns off the sub heater 102b in step S5.

The predetermined time period that elapses after the controller <NUM> turns on the sub heater 102b until the controller <NUM> turns off the sub heater 102b is preferably changed according to a width of the sheet P conveyed through the fixing nip N.

<FIG> is a graph illustrating temperature change of the lateral end span LS of the fixing belt <NUM>.

As illustrated in <FIG>, the controller <NUM> turns on both the sub heater 102b and the main heater 102a to heat the fixing belt <NUM> to a fixing temperature t. Thereafter, the controller <NUM> turns off the sub heater 102b. Thus, the temperature of the lateral end span LS of the fixing belt <NUM> changes when sheets <NUM> and <NUM> are conveyed through the fixing nip N. Each of the sheets <NUM> and <NUM> has a width not smaller than the width of <NUM> of the B4 size sheet in portrait orientation. The width of the sheet <NUM> is different from the width of the sheet <NUM>.

When the sheet <NUM> smaller than the sheet <NUM> in the width is conveyed through the fixing nip N, the sheet <NUM> draws heat less than the sheet <NUM> from both lateral end spans LS of the fixing belt <NUM>. Hence, the sheet <NUM> causes an amount of heat conducted from both lateral end spans LS of the fixing belt <NUM> to the guides <NUM> to be greater than that caused by the sheet <NUM>. Accordingly, the guides <NUM> are heated to the temperature close to the fixing temperature t in a shortened time period. Consequently, the lateral end spans LS of the fixing belt <NUM> recover the fixing temperature t in the shortened time period, eliminating temperature decrease of the fixing belt <NUM> in the lateral end spans LS. For example, when X<NUM> seconds elapse after conveyance of the sheet <NUM> through the fixing nip N starts, even if the controller <NUM> turns on the main heater 102a and does not turn on the sub heater 102b, temperature decrease in the lateral end spans LS of the fixing belt <NUM> does not occur.

Conversely, when the sheet <NUM> greater than the sheet <NUM> in the width is conveyed through the fixing nip N, the sheet <NUM> draws heat more than the sheet <NUM> from the lateral end spans LS of the fixing belt <NUM>. Hence, conduction of heat from the lateral end spans LS of the fixing belt <NUM> to the guides <NUM>, respectively, decreases. Accordingly, heat is conducted from the lateral end spans LS of the fixing belt <NUM> to the guides <NUM>, respectively, for an increased time period, taking time for the lateral end spans LS of the fixing belt <NUM> to recover the fixing temperature t. For example, when X<NUM> seconds that are longer than X<NUM> seconds elapse after conveyance of the sheet <NUM> through the fixing nip N starts, even if the controller <NUM> turns on the main heater 102a and does not turn on the sub heater 102b, temperature decrease in the lateral end spans LS of the fixing belt <NUM> does not occur.

As described above, even if the controller <NUM> turns on the main heater 102a and does not turn on the sub heater 102b, a time period taken to eliminate temperature decrease in the lateral end spans LS of the fixing belt <NUM> varies depending on the width of a sheet P conveyed through the fixing nip N. Hence, as illustrated in table <NUM> below, the predetermined time period that elapses after the controller <NUM> turns on the sub heater 102b until the controller <NUM> turns off the sub heater 102b is preferably changed according to the width of the sheet P.

The predetermined time period that elapses after the controller <NUM> turns on the sub heater 102b until the controller <NUM> turns off the sub heater 102b during fixing is preferably changed between a first image formation after the printer <NUM> is powered on and a later image formation after the printer <NUM> enters the standby mode. For example, when the printer <NUM> is powered on, the guides <NUM> including the guide portions 451a that contact both lateral ends of the fixing belt <NUM> in the axial contact thereof, respectively, have a substantially ambient temperature. Hence, it takes longer time for the guides <NUM> to be heated to the temperature close to the fixing temperature by conduction of heat from the lateral end spans LS of the fixing belt <NUM> to the guides <NUM> compared to the later image formation after the printer <NUM> enters the standby mode. Accordingly, during the first image formation after the printer <NUM> is powered on, the controller <NUM> increases the predetermined time period that elapses after the controller <NUM> turns on the sub heater 102b until the controller <NUM> turns off the sub heater 102b compared to the later image formation after the printer <NUM> enters the standby mode.

As described above, the controller <NUM> determines whether or not the toner image is within at least one of the reference spans extended inboard from the lateral edges of the sheet P in the width direction thereof, respectively, and determines whether the controller <NUM> turns on the main heater 102a without turning on the sub heater 102b or turns on both the sub heater 102b and the main heater 102a. Alternatively, the controller <NUM> may determine turning on of the sub heater 102b and the main heater 102a as described below. For example, based on a distance from the lateral edge of the fixing belt <NUM> to a lateral edge of a toner image on a sheet P in the axial direction of the fixing belt <NUM>, the controller <NUM> may determine whether the controller <NUM> turns on the main heater 102a without turning on the sub heater 102b or turns on both the sub heater 102b and the main heater 102a.

As described above, if the controller <NUM> determines that the toner image is within at least one of the reference spans extended inboard from the lateral edges of the sheet P in the width direction thereof, respectively, for example, if the image area rate in at least one of the reference spans on the sheet P is greater than zero, the controller <NUM> turns on both the sub heater 102b and the main heater 102a. However, the lateral end spans LS of the fixing belt <NUM> may barely suffer from temperature decrease depending on a configuration of the fixing device <NUM>. With the configuration of the fixing device <NUM>, that barely generates temperature decrease of the fixing belt <NUM>, for example, if the controller <NUM> determines that the image area rate in the reference span extended inboard from the lateral edge of the sheet P in the width direction thereof is not smaller than a predetermined value, the controller <NUM> may turn on both the sub heater 102b and the main heater 102a. Even if the controller <NUM> determines that the toner image is within the reference span extended inboard from the lateral edge of the sheet P in the width direction thereof, if the image area rate is small, the toner image draws slight heat from the fixing belt <NUM>. Accordingly, even if the lateral end spans LS of the fixing belt <NUM> suffer from temperature decrease, the fixing belt <NUM> fixes the toner image on the sheet P properly in the reference span extended inboard from the lateral edge of the sheet P in the width direction thereof.

The fixing device <NUM> may include a power interrupter that interrupts power supply to the sub heater 102b and the main heater 102a when the power interrupter detects an abnormal temperature of the surface of the fixing belt <NUM>.

The power interrupter is a thermopile, a thermal fuse, or the like. The power interrupter may include an abnormal temperature detecting sensor serving as an abnormal temperature detector such as a thermopile that is inferior to the temperature detecting sensor <NUM> in temperature responsiveness and is manufactured at reduced costs. The power interrupter interrupts power supply to the sub heater 102b and the main heater 102a based on a detection result sent from the abnormal temperature detecting sensor.

The thermopile, the thermal fuse, or the abnormal temperature detecting sensor is disposed opposite the fixing belt <NUM>. When the fixing belt <NUM> is heated to a predetermined temperature, the power interrupter is activated and interrupts power supply to the sub heater 102b and the main heater 102a.

<FIG> illustrates a power interrupter <NUM> (e.g., the thermopile, the thermal fuse, or the abnormal temperature detecting sensor) that is disposed opposite the lateral end span LS of the fixing belt <NUM>. The lateral end span LS of the fixing belt <NUM> receives heat in an increased amount when the main heater 102a and the sub heater 102b are turned on and therefore is subject to temperature increase. When a small sheet P having the decreased width in the width direction of the small sheet P is conveyed through the fixing nip N also, the lateral end span LS of the fixing belt <NUM> is subject to temperature increase. To address this circumstance, the power interrupter <NUM> is disposed opposite the lateral end span LS of the fixing belt <NUM> so that the power interrupter <NUM> detects an abnormal temperature of the fixing belt <NUM> early and interrupts power supply to each of the main heater 102a and the sub heater 102b.

According to this embodiment, when printing is performed continuously on a great number of small sheets P having the decreased width in the width direction thereof, a temperature of each of the lateral end spans LS of the fixing belt <NUM> tends to be higher than a temperature of the center span CS of the fixing belt <NUM>. Since the small sheets P that pass through the fixing nip N successively are conveyed over the center span CS of the fixing belt <NUM>, the small sheets P draw heat from the center span CS of the fixing belt <NUM>. Conversely, the small sheets P barely draw heat from the lateral end spans LS of the fixing belt <NUM>. Accordingly, after the guides <NUM> are heated to the temperature close to the fixing temperature, heat conducted from the main heater 102a to the lateral end spans LS of the fixing belt <NUM> is drawn to the small sheets P and other elements less than heat conducted to the center span CS of the fixing belt <NUM>. Consequently, when printing is performed continuously on the great number of small sheets P, the temperature of each of the lateral end spans LS of the fixing belt <NUM> tends to be higher than the temperature of the center span CS of the fixing belt <NUM>.

To address this circumstance, a thermal equalizer may be interposed between the pad <NUM> and the inner circumferential surface of the fixing belt <NUM>. The thermal equalizer facilitates conduction of heat in a longitudinal direction thereof and decreases unevenness of the temperature of the fixing belt <NUM> in a longitudinal direction, that is, the axial direction thereof. The thermal equalizer conducts heat from the lateral end spans LS to the center span CS of the fixing belt <NUM>. Accordingly, the thermal equalizer suppresses temperature decrease in the center span CS of the fixing belt <NUM> and suppresses temperature increase in the lateral end spans LS of the fixing belt <NUM>. Since the thermal equalizer suppresses temperature decrease in the center span CS of the fixing belt <NUM>, while the controller <NUM> performs a control to retain the fixing belt <NUM> at the fixing temperature based on a detection result sent from the temperature detecting sensor <NUM>, the controller <NUM> suppresses a lighting amount per unit time of the main heater 102a. Accordingly, the controller <NUM> suppresses a heating amount per unit time of heat supplied to the lateral end spans LS of the fixing belt <NUM>, thus, suppressing temperature increase in the lateral end spans LS of the fixing belt <NUM>.

The thermal equalizer eliminates temperature decrease in the lateral end spans LS of the fixing belt <NUM> quickly, shortening a lighting time period for which the controller <NUM> turns on both the main heater 102a and the sub heater 102b when a large sheet P having the increased width in the width direction of the large sheet P is conveyed through the fixing nip N. Thus, the thermal equalizer reduces power consumption of the fixing device <NUM>.

The above describes the embodiments of the present disclosure, that are applied to the fixing device <NUM> employing a belt fixing method using the fixing belt <NUM>. The embodiments of the present disclosure are also applied to a fixing device employing a roller fixing method using a fixing roller.

The above describes one example of the technology of the present disclosure. The technology of the present disclosure achieves advantages peculiar to aspects described below.

A description is provided of a first aspect of the technology of the present disclosure.

As illustrated in <FIG> and <FIG>, the fixing device <NUM> includes a fixing rotator (e.g., the fixing belt <NUM>) and a plurality of heaters that heats the fixing rotator and has different heat generation properties, respectively. While a recording medium (e.g., a sheet P) bearing an image (e.g., a toner image) is conveyed over the fixing rotator, the fixing device <NUM> fixes the image on the recording medium. The plurality of heaters includes a first heater (e.g., main heater 102a) and a second heater (e.g., the sub heater 102b). The first heater has a heat generation property in which the first heater generates heat evenly, that is, generates a heat generation amount that is even in an axial direction of the fixing rotator, in a maximum conveyance span (e.g., the heat generation span L) on the fixing rotator in the axial direction thereof. A recording medium having a maximum width, that is available for the fixing rotator, in a width direction of the recording medium, that is parallel to the axial direction of the fixing rotator, is conveyed over the maximum conveyance span on the fixing rotator. The second heater generates heat in the maximum conveyance span. The second heater includes a first portion (e.g., the center portion 102b <NUM>) and a second portion (e.g., the lateral end portions 102b2). The second heater has a heat generation property in which a heat generation amount of the second portion is greater than a heat generation amount of the first portion.

For example, the first portion of the second heater is disposed opposite a center span (e.g., the center span CS) of the fixing rotator in the axial direction thereof. The second portion of the second heater is disposed opposite a lateral end span (e.g., the lateral end span LS) of the fixing rotator in the axial direction thereof.

A comparative fixing device includes a main heater including a center portion and both lateral end portions in a longitudinal direction of the main heater. The main heater has a heat generation property in which a heat generation amount of the center portion is greater than a heat generation amount of each of the lateral end portions. The comparative fixing device further includes a sub heater including a center portion and both lateral end portions in a longitudinal direction of the sub heater. The sub heater has a heat generation property in which a heat generation amount of each of the lateral end portions is greater than a heat generation amount of the center portion and in which the heat generation amount of each of the lateral end portions of the sub heater is greater than the heat generation amount of the center portion of the main heater. With the heat generation properties described above, when a controller turns on the main heater and does not turn on the sub heater, both lateral end spans of a fixing rotator in an axial direction thereof suffer from temperature decrease. Conversely, when the controller turns on the sub heater and does not turn on the main heater, a center span of the fixing rotator in the axial direction thereof suffers from temperature decrease. To address this circumstance, the comparative fixing device includes a first temperature detecting sensor and a second temperature detecting sensor. The controller controls the main heater based on a temperature of the center span of the fixing rotator in the axial direction thereof, that is detected by the first temperature detecting sensor. The controller controls the sub heater based on a temperature of the lateral end span of the fixing rotator in the axial direction thereof, that is detected by the second temperature detecting sensor. Thus, the comparative fixing device retains the fixing rotator at a predetermined temperature (e.g., a standby temperature or a fixing temperature) substantially throughout an entire span of the fixing rotator in the axial direction thereof.

Conversely, according to the first aspect of the technology of the present disclosure, as illustrated in <FIG>, the first heater (e.g., the main heater 102a) has the heat generation property in which the first heater generates the heat generation amount that is even in the axial direction of the fixing rotator. Accordingly, when a controller (e.g., the controller <NUM>) turns on the first heater and does not turn on the second heater, the first heater heats the fixing rotator evenly in the axial direction thereof. When the controller turns on the first heater and the second heater, a combined heat generation amount combining the heat generation amount of the first heater and the heat generation amount of the second portion of the second heater in a second span (e.g., the lateral end span LS) in the axial direction of the fixing rotator is greater than a combined heat generation amount combining the heat generation amount of the first heater and the heat generation amount of the first portion of the second heater in a first span (e.g., the center span CS) in the axial direction of the fixing rotator.

For example, when a lateral end contact member (e.g., the guide <NUM>) that contacts the second span of the fixing rotator has a decreased temperature when the fixing device <NUM> is powered on, an amount of heat conducted from the second span of the fixing rotator to the lateral end contact member increases. To address this circumstance, the controller turns on the first heater and the second heater, causing the combined heat generation amount in the second span to be greater than the combined heat generation amount in the first span. Thus, the fixing rotator achieves an even temperature substantially throughout an entire span of the fixing rotator in the axial direction thereof. When the first heater and the second heater heat the fixing rotator for a predetermined time period, the lateral end contact member achieves a temperature equivalent to a temperature of the fixing rotator. Accordingly, an amount of heat conducted from the second span of the fixing rotator to the lateral end contact member decreases. Thus, even if the combined heat generation amount in the second span is not greater than the combined heat generation amount in the first span, the fixing rotator achieves the even temperature substantially throughout the entire span of the fixing rotator in the axial direction thereof. Hence, after the first heater and the second heater heat the fixing rotator for the predetermined time period, the controller turns off the second heater and turns on the first heater to retain the fixing rotator at the predetermined temperature based on a detection result sent from the temperature detecting sensor, retaining the fixing rotator at the predetermined temperature substantially throughout the entire span of the fixing rotator in the axial direction thereof. As described above, after the first heater and the second heater heat the fixing rotator for the predetermined time period, the first heater retains the fixing rotator at the predetermined temperature substantially throughout the entire span of the fixing rotator in the axial direction thereof, allowing the fixing device <NUM> to eliminate a temperature detecting sensor used for the second heater. Accordingly, the fixing device <NUM> according to the first aspect reduces the number of temperature sensors compared to the comparative fixing device, retaining the fixing rotator at the predetermined temperature substantially throughout the entire span of the fixing rotator in the axial direction thereof. Consequently, the fixing device <NUM> reduces manufacturing costs.

A description is provided of a second aspect of the technology of the present disclosure.

Based on the first aspect, when the fixing rotator fixes an image on a recording medium having a decreased width smaller than a reference width (e.g., the width of the B4 size sheet in portrait orientation according to the embodiments) in the width direction of the recording medium, the controller turns on the first heater to heat the fixing rotator without turning on the second heater. When the fixing rotator fixes an image on a recording medium having an increased width not smaller than the reference width in the width direction of the recording medium, the controller turns on the first heater without turning on the second heater or turns on both the first heater and the second heater to heat the fixing rotator.

Accordingly, as described above in the embodiments, if the recording medium has the decreased width smaller than the reference width in the width direction of the recording medium, the controller turns on the first heater to heat the fixing rotator without turning on the second heater. Thus, the fixing device <NUM> suppresses temperature increase in the second span (e.g., the lateral end span LS) of the fixing rotator compared to a configuration in which the controller turns on the first heater and the second heater to heat the fixing rotator.

If the recording medium has the increased width not smaller than the reference width in the width direction of the recording medium, based on a condition of temperature decrease in the second span of the fixing rotator and the image formed on the recording medium, the controller turns on the first heater without turning on the second heater or turns on both the first heater and the second heater to heat the fixing rotator that fixes the image on the recording medium.

A description is provided of a third aspect of the technology of the present disclosure.

Based on the second aspect, when the fixing rotator fixes the image on the recording medium having the increased width in the width direction of the recording medium, until the controller determines that a predetermined time period elapses after a fixing job starts, that is, after the controller turns on both the first heater and the second heater, the first heater and the second heater heat the fixing rotator. When the controller determines that the predetermined time period elapses, the controller turns on the first heater to heat the fixing rotator without turning on the second heater.

Accordingly, as described above in the embodiments, when the predetermined time period elapses after the fixing job starts, conduction of heat from the second span (e.g., the lateral end span LS) of the fixing rotator to the lateral end contact member decreases. Accordingly, even if the combined heat generation amount in the second span is not greater than the combined heat generation amount in the first span (e.g., the center span CS), the second span of the fixing rotator does not suffer from temperature decrease. Hence, until the controller determines that the predetermined time period elapses, the controller turns on both the first heater and the second heater to heat the fixing rotator. Thus, the fixing device <NUM> suppresses temperature decrease in the second span of the fixing rotator. When the controller determines that the predetermined time period elapses, the controller turns on the first heater to heat the fixing rotator without turning on the second heater. Thus, the fixing device <NUM> prevents a temperature of the second span of the fixing rotator from being higher than a temperature of the first span of the fixing rotator.

A description is provided of a fourth aspect of the technology of the present disclosure.

Based on the third aspect, the controller determines the predetermined time period based on a width of the recording medium in the width direction thereof.

Accordingly, as described above in the embodiments, if the recording medium has the decreased width, conduction of heat from the second span (e.g., the lateral end span LS) of the fixing rotator to the lateral end contact member decreases early. Accordingly, even if the combined heat generation amount in the second span is not greater than the combined heat generation amount in the first span (e.g., the center span CS), the second span of the fixing rotator does not suffer from temperature decrease. Hence, the controller determines the predetermined time period based on the width of the recording medium in the width direction thereof. Thus, the fixing device <NUM> suppresses temperature decrease in the second span of the fixing rotator properly. Additionally, the fixing device <NUM> prevents a temperature of the second span of the fixing rotator from being higher than a temperature of the first span of the fixing rotator.

A description is provided of a fifth aspect of the technology of the present disclosure.

Based on any one of the second to fourth aspects, if the controller determines that the recording medium has the increased width and that an image area rate in a reference span extended inboard from a lateral edge of the recording medium in the width direction thereof is not greater than a threshold, the controller turns on the first heater to heat the fixing rotator without turning on the second heater.

Accordingly, as described above in the embodiments, even if the recording medium conveyed through a fixing nip (e.g., the fixing nip N) has the increased width, if the image area rate in the reference span extended inboard from the lateral edge of the recording medium in the width direction thereof is not greater than the threshold, even if the second span (e.g., the lateral end span LS) of the fixing rotator suffers from temperature decrease, the image on the recording medium does not suffer from faulty fixing. Hence, if the controller determines that the recording medium has the increased width and that the image area rate in the reference span extended inboard from the lateral edge of the recording medium in the width direction thereof is not greater than the threshold, the controller turns on the first heater to heat the fixing rotator without turning on the second heater. Accordingly, the fixing device <NUM> suppresses faulty fixing and reduces power consumption compared to the configuration in which the controller turns on the first heater and the second heater to heat the fixing rotator.

A description is provided of a sixth aspect of the technology of the present disclosure.

Based on the fifth aspect, the controller determines the reference span based on the width of the recording medium in the width direction thereof.

Accordingly, as described above in the embodiments, the reference span extended inboard from the lateral edge of the recording medium in the width direction thereof, where faulty fixing may occur due to temperature decrease in the second span (e.g., the lateral end span LS) of the fixing rotator, varies depending on the width of the recording medium. To address this circumstance, the controller determines the reference span based on the width of the recording medium in the width direction thereof. Accordingly, the fixing device <NUM> suppresses faulty fixing effectively and reduces power consumption compared to the configuration in which the controller turns on the first heater and the second heater to heat the fixing rotator.

A description is provided of a seventh aspect of the technology of the present disclosure.

Based on any one of the first to sixth aspects, a temperature detecting sensor (e.g., the temperature detecting sensor <NUM>) is disposed opposite the fixing rotator in a minimum conveyance span (e.g., the center span CS) where the recording medium having a minimum width in the width direction thereof, that is available for the fixing rotator, is conveyed over the fixing rotator. The controller controls the first heater based on a detection result sent from the temperature detecting sensor.

Accordingly, the fixing device <NUM> retains the fixing rotator at the predetermined temperature (e.g., the fixing temperature).

A description is provided of an eighth aspect of the technology of the present disclosure.

Based on any one of the first to seventh aspects, the controller turns on the second heater without turning on the first heater or turns on both the first heater and the second heater to heat the fixing rotator in a standby mode in which the fixing device <NUM> waits for a fixing job.

Accordingly, as described above in the embodiments, the fixing device <NUM> suppresses temperature decrease in the second span (e.g., the lateral end span LS) of the fixing rotator in the standby mode.

A description is provided of a ninth aspect of the technology of the present disclosure.

Based on any one of the first to eighth aspects, if the controller determines that a temperature of the second span (e.g., the lateral end span LS) of the fixing rotator is not lower than a threshold, a power interrupter (e.g., the power interrupter <NUM>) depicted in <FIG>, that is disposed opposite the fixing rotator, interrupts power supply to each of the first heater and the second heater.

Accordingly, as described above in the embodiments, the power interrupter detects a temperature of the second span of the fixing rotator and interrupts power supply to the first heater and the second heater based on the detected temperature. Thus, the power interrupter detects an abnormal temperature of the fixing rotator early and interrupts power supply to the first heater and the second heater.

A description is provided of a tenth aspect of the technology of the present disclosure.

Based on any one of the first to ninth aspects, a heat generation amount of each of the first span (e.g., the center span CS) and the second span (e.g., the lateral end span LS) of the first heater in the axial direction of the fixing rotator is greater than a heat generation amount of the first portion (e.g., the center portion 102b1) of the second heater.

Accordingly, the controller turns on the first heater without turning on the second heater. Consequently, the fixing device <NUM> retains the fixing rotator at the predetermined temperature (e.g., the fixing temperature) properly.

A description is provided of an eleventh aspect of the technology of the present disclosure.

As illustrated in <FIG>, an image forming apparatus (e.g., the printer <NUM>) includes an image forming device that forms an image on a recording medium (e.g., a sheet P) and a fixing device (e.g., the fixing device <NUM>) that fixes the image on the recording medium. The image forming device includes an image bearer (e.g., the photoconductive drums 20Y, 20C, <NUM>, and 20Bk) that bears the image. The fixing device is configured based on any one of the first to tenth aspects.

Accordingly, the image forming apparatus reduces manufacturing costs and forms the image properly.

According to the embodiments described above, the fixing device <NUM> employs a center reference conveyance system in which a sheet P serving as a recording medium is centered on the fixing belt <NUM> while the sheet P is conveyed over the fixing belt <NUM>. Alternatively, the fixing device <NUM> may employ a lateral end reference conveyance system in which a sheet P is aligned along a lateral end of the fixing belt <NUM> in the axial direction thereof while the sheet P is conveyed over the fixing belt <NUM>.

According to the embodiments described above, the fixing belt <NUM> serves as a fixing rotator. Alternatively, a fixing roller, a fixing film, a fixing sleeve, or the like may be used as a fixing rotator. Further, the pressure roller <NUM> serves as a pressure rotator. Alternatively, a pressure belt or the like may be used as a pressure rotator.

Claim 1:
A fixing device (<NUM>) comprising:
a fixing rotator (<NUM>) over which a recording medium (P) bearing an image is conveyed;
a first heater (102a) configured to heat the fixing rotator (<NUM>), the first heater (102a) configured to generate heat evenly in a maximum conveyance span (L) where the recording medium (P) having a maximum width in a width direction of the recording medium (P) is conveyed, the maximum width being available for the fixing rotator (<NUM>);
a second heater (102b) configured to heat the fixing rotator (<NUM>) and generate heat in the maximum conveyance span (L),
the second heater (102b) including:
a first portion (102b1) configured to generate heat in a first heat generation amount; and
a second portion (102b2) configured to generate heat in a second heat generation amount that is greater than the first heat generation amount of the first portion (102b1);
a controller (<NUM>) configured to control the first heater (102a) and the second heater (102b),
wherein the controller (<NUM>) is configured to turn on the first heater (102a) without turning on the second heater (102b) if the controller (<NUM>) determines that the recording medium (P) has a decreased width that is smaller than a reference width in the width direction of the recording medium (P),
wherein the controller (<NUM>) is configured to turn on at least the first heater (102a) if the controller (<NUM>) determines that the recording medium (P) has an increased width that is not smaller than the reference width in the width direction of the recording medium (P),
wherein the controller (<NUM>) is configured to turn on the first heater (102a) and the second heater (102b) for a predetermined time period if the controller (<NUM>) determines that the recording medium (P) has the increased width in the width direction of the recording medium (P),
wherein the controller (<NUM>) is configured to turn off the second heater (102b) after the predetermined time period elapses, and
wherein the controller (<NUM>) is configured to determine the predetermined time period based on a width of the recording medium (P) in the width direction of the recording medium (P).