Image forming apparatus

Abstract

 An image forming apparatus (10) employing a fixing device (4) having a heating roller (41) with a plurality of heaters (h1, h2) embedded therein and a pressing roller (42) opposed to the heating roller (41). During a printing operation, only a first heater (h1), which generates a greater amount of heat from its center portion than from its end portions, is controlled to be turned on and off. In a standby state, a second heater (h2), which generates a greater amount of heat from its end portions than the amount of heat generated from the end portions of the first heater (h1), is controlled to be turned on and off.

Description

TECHNICAL FIELD

[0001] The present invention relates to an image forming apparatus, such as a copying machine for forming a toner image on a recording sheet, a printer, a facsimile or a multi-function apparatus having functions as two or more of these machines, and more particularly to a fixing device for fixing a toner image onto a recording sheet.

BACKGROUND ART

[0002] In an image forming apparatus, such as a copying machine for forming a toner image on a recording sheet, a printer, a facsimile or a multi-function apparatus having functions as two or more of these machines, the following processes are performed: an electrostatic latent image is formed on an image carrier by an electrostatic latent image forming process (for example, an electrostatic latent image is formed on an image carrier such as a photoreceptor by an exposure process); the electrostatic latent image is developed into a toner image by a developing process; the toner image is transferred to a recording sheet by a transfer process, or the toner image is transferred to an intermediate transfer belt (primary transfer) and then transferred from the intermediate transfer belt to a recording sheet (secondary transfer); and the toner image transferred to the recording sheet is fixed thereon by a fixing process.

[0003] In the fixing process for fixing a toner image, which has been formed but has not been fixed on a recording sheet, onto the recording sheet, a fixing device is used. Various types of fixing devices are known.

[0004] One of the known types is a fixing device that comprises a rotatable heating member (for example, a heating roller) with an embedded heater and a rotatable pressing member (for example, a pressing roller), the rotatable heating member and the pressing member opposed to each other to form a fixing nip portion (a nip portion which as a recording sheet with an unfixed toner image thereon is passing through, the toner image is fixed on the sheet).

[0005] In this type of fixing device, as a recording sheet with a toner image thereon is passing through the nip portion, the toner image is heated and pressed and thereby can be fixed on the recording sheet.

[0006] Also, it has been suggested that a plurality of heaters (for example, two heaters), of which heat-generation distributions in the direction of rotation axis of the heating member are different from each other, are embedded in the heating member so that the temperature of the heating member can be controlled with the heaters based on the size, especially the width (the dimension in the direction of rotation axis of the heating member) of the recording sheet to be passed through the fixing nip portion.

[0007] Japanese Patent Laid-Open Publication No. 2011-118261 teaches that a sub-heater and a main-heater are switched depending on whether the temperature of the heating member detected by a temperature detector is lower than a reference temperature for switchover of heaters and that the reference temperature for switchover of heaters is set to different values for a state wherein the heating member is rotating and for a state wherein the heating member stops.

[0008] When the conventional type of fixing device comprising a rotatable heating member (for example, a heating roller) and a rotatable pressing member (for example, a pressing roller) opposed to each other, the rotatable heating member having one heater embedded therein, is used, heat is emitted from end portions of the heating member with respect to the direction of rotation axis of the heating member during a standby period when no recording sheet is passing through the fixing device, and there may be cases to set the amount of heat generation from the end portions of the heater higher so as to prevent a drop in temperature of the end portions of the heating member.

[0009] In such a fixing device, as a plurality of recording sheets are passing through the fixing nip portion continuously, in the range corresponding to the width of the recording sheets, heat transfers to the recording sheets, which causes a drop in temperature of the heating member. In order to prevent the drop in temperature, the heater is energized for a longer time.

[0010] In this moment, out of the range corresponding to the width of the recording sheet, that is, around the ends of the heating roller, however, heat transfer to the recording sheets does not occur, while the heater is energized for a long time. Therefore, the temperature around the ends of the heating member is apt to become high.

[0011] Not only the fixing device having one heater but also fixing devices having two or more heaters to comply with various sizes of recording sheets have this problem.

[0012] Incidentally, in recent years, UFP (ultrafine particle equal to or less than 100nm) radiated from image forming apparatuses receives attention.

[0013] Concerning fixing devices, a type of fixing devices using silicon rubber (for example, the type of fixing devices of which heating members (heating rollers) have silicone rubber surface layers) generate much UFP under high temperature. In order to suppress the generation of UFP, it is demanded to prevent the ends of heating members (heating rollers) from reaching a high temperature.

[0014] In regard to this point, in the device disclosed by Japanese Patent Laid-Open Publication No. 2011-118261, as mentioned above, the sub-heater and the main-heater are switched depending on whether the temperature of the heating member detected by the temperature detector is lower than a reference temperature for switching of heaters, and therefore, it appears that this device can prevent the heating member (such as a heating roller) from reaching a high temperature.

[0015] In the fixing device disclosed by Japanese Patent Laid-Open Publication No. 2011-118261, however, the temperature distribution of the heating member after switching of heaters is different from the temperature distribution of the heating member before the switching, which may result in a waste of heat around the ends of the heating member generated before the switching. Also, when the reference temperature for switching of heaters is set to different values for a case wherein the heating member is rotating and for a case wherein the heating member stops, the temperature control may not achieve a desired temperature in time for passing of recording sheets through the fixing nip portion.

SUMMARY OF INVENTION

[0016] A first object of the present invention is to provide an image forming apparatus that is configured to form a toner image on an image carrier, to transfer the toner image to a recording sheet and to fix the toner image onto the recording sheet with a fixing device comprising a rotatable heating member having a plurality of heaters embedded therein and a rotatable pressing member, the rotatable heating member and the rotatable pressing member opposed to each other to form a nip portion, which a fixing process is performed while the recording sheet is passing through so that the toner image is heated and pressed to be fixed on the recording sheet, wherein during a printing operation, even if the printing operation is to form a plurality of printed sheets continuously, ends of the heating member in a direction of rotation axis are prevented from reaching an excessively high temperature, thereby suppressing generation of UFP from the ends of the heating member in the direction of rotation axis.

[0017] A second object of the present invention is to provide an image forming apparatus that can attain the first object, wherein when the apparatus changes from a standby state where the apparatus does not operate for printing to a printing state, the temperature of the heating member can be stabilized to a temperature appropriate for the fixing process quickly without energy waste.

[0018] In order to attain the first object, the present invention provides an image forming apparatus that is configured to form a toner image on an image carrier, to transfer the toner image to a recording sheet and to fix the toner image on the recording sheet with a fixing device comprising a rotatable heating member having a plurality of heaters embedded therein and a rotatable pressing member, the heating member and the pressing member opposed to each other to form a nip portion, which a fixing process is performed while the recording sheet is passing through so that the toner image is heated and pressed to be fixed on the recording sheet, the image forming apparatus comprising:

a power source for electrifying the plurality of heaters;

a temperature detector for detecting a temperature of the heating member; and

a controller for controlling the power source,

wherein the plurality of heaters includes a first heater that generates a larger amount of heat from its central portion with respect to a direction of a rotation axis of the heating member than from its end portions with respect to the direction of the rotation axis of the heating member;

wherein the controller controls the power source such that at least one of the plurality of heaters is controlled to be turned on and off based on a difference between the temperature detected by the temperature detector and a target temperature of the heating member so that the temperature of the heating member can become the target temperature; and

wherein the controller controls the power source such that only the first heater is controlled to be turned on and off during a printing operation.

[0019] In the image forming apparatus according to the present invention, a toner image is formed on the image carrier, the toner image is transferred to a recording sheet, and the toner image is fixed on the recording sheet with a fixing device. In the fixing device, while the recording sheet with the toner image thereon is passing through the nip portion formed by the rotatable heating member with a plurality of heaters embedded therein and the rotatable pressing member opposed to the heating member, the toner image is heated and pressed to be fixed onto the recording sheet.

[0020] In the image forming apparatus according to the present invention, the power source of the fixing device is controlled by the controller. Specifically, the controller controls the power source such that at least one of the plurality of heaters is controlled to be turned on and off based on a difference between the temperature detected by the temperature detector and a target temperature of the heating member so that the temperature of the heating member can become the target temperature.

[0021] The "target temperature" is one of some different target temperatures predetermined for different states of the fixing device. The target temperature during a warm-up of the fixing device is a temperature which permits the fixing device to complete its warm-up, that is, a "standby temperature" which permits the fixing device to transit from a standby state to a printing operation immediately. The target temperature during a printing operation is a "fixing temperature" appropriate for the fixing process.

[0022] During a printing operation, the controller controls the power source such that only the first heater, which generates a greater amount of heat from its center portion than from its end portions with respect to the direction of rotation axis of the heating member, is controlled to be turned on and off so that the temperature of the heating member can become the target temperature. Thereby, the center portion of the heating member with respect to the direction of rotation axis is heated, and a drop in temperature in the center portion due to heat transfer to a recording sheet passing through the fixing device can be compensated. Also, the end portions of the heating member are heated by the heat generated from the center portion. In this way, the temperature of the heating member reaches the target temperature (fixing temperature).

[0023] The end portions of the heating member is heated due to heat migration from the center portion, and therefore, even if the first heater is turned on for a long time during a printing operation to form a plurality of printed sheets continuously, the end portions of the heating member will not become too high a temperature. Accordingly, generation of UFP from the end portions of the heating member with respect to the direction of rotation axis of the heating member can be suppressed. Even if the surface layer of the heating member is made of a material that readily generates UFP under high temperature, such as silicon rubber, generation of UFP can be suppressed.

[0024] In the image forming apparatus according to the present invention, the plurality of heaters embedded in the heating member may further comprise a second heater that generates a greater amount of heat from its end portions with respect to the direction of rotation axis of the heating member than the amount of heat generated from the end portions of the first heater with respect to the direction of rotation axis of the heating member. In this case, the controller may control the power source such that only the second heater is controlled to be turned on and off during a standby state of not performing printing.

[0025] The second heater is controlled to be turned on and off in a standby state of not performing printing, and accordingly, at the time of a transition from the standby state to a printing operation, the end portions of the heating member have been heated already. Then, by changing the object of control from the second heater to the first heater, the temperature of the heating member can be stabilized to the temperature appropriate for the fixing process quickly without wasting the energy generated by the second heater.

[0026] The controller may calculate a time when the recording sheet with the toner image thereon passes through the nip portion to be subjected to the fixing process and may determine a time to stop controlling the second heater and to start controlling the first heater in a transition from the standby state to the printing operation based on the calculated time when the recording sheet passes through the nip portion in such a manner to ensure the fixing process be conducted on the recording sheet.

[0027] The "time to stop controlling the second heater and to start controlling the first heater" is, for example, the time when the leading edge of the recording sheet comes into the nip portion, a predetermined time before the time when the leading edge of the recording sheet comes into the nip portion or a time immediately after the time when the leading edge of the recording sheet comes into the nip portion, as long as it ensures the fixing process be conducted on the recording sheet.

[0028] The first heater may have a heat-generating portion having an effective length in the direction of rotation axis of the heating member smaller than a length of a maximum sheet-passing range corresponding to a maximum dimension in the direction of rotation axis of the heating member of a recording sheet passing through the nip portion among recording sheets of regular sizes that can be subjected to the fixing process. Thereby, heat radiation from the heating member can be suppressed.

[0029] The first heater may have a greater power distribution from the power source than the second heater has. The power distribution to the second heater can be small because it is not necessary to consider heat transfer to the recording sheet. Accordingly, a small power distribution to the second heater can be sufficient to prevent heat radiation from end portions of the heating member, thereby preventing a drop in temperature of the heating member so that a transition from a standby state to a printing operation can be done smoothly.

[0030] As the temperature detector, a plurality of detectors or a single detector may be provided. When a single detector is provided, the detector may be located within the maximum sheet-passing range in the direction of rotation of the heating member. In this case, the controller can use information detected by the single detector as temperature information of the heating member.

ADVANTAGEOUS EFFECTS OF INVENTION

[0031] As described above, the present invention can provide an image forming apparatus that forms a toner image on an image carrier, that transfers the toner image to a recording sheet and that fixes the toner image onto the recording sheet with a fixing device comprising a rotatable heating member having a plurality of heaters embedded therein and a rotatable pressing member, the rotatable heating member and the rotatable pressing member opposed to each other to form a nip portion, which a fixing process is performed while the recording sheet is passing through so that the toner image is heated and pressed to be fixed on the recording sheet, wherein during a printing operation, even if the printing operation is to form a plurality of printed sheets continuously, ends of the heating member in a direction of rotation axis are prevented from reaching an excessively high temperature, thereby suppressing generation of UFP from the ends of the heating member in the direction of rotation axis.

[0032] The present invention also can provide an image forming apparatus having the advantages above, wherein when the apparatus changes from a standby state of not performing printing to a printing state, the temperature of the heating member can be stabilized to a temperature appropriate for the fixing process quickly without energy waste.

BRIEF DESCRIPTION OF DRAWINGS

[0033] 

Fig. 1 is a perspective view of an image forming apparatus according to an embodiment of the present invention.

Fig. 2 is a block diagram of a control circuit of the image forming apparatus shown by Fig. 1.

Fig. 3 is a schematic sectional view of a fixing device employed in the image forming apparatus shown by Fig. 1.

Fig. 4 is a schematic view of a heating roller of the fixing device shown by Fig. 3, showing a structure raging from an end to the other end in a direction of rotation axis of the heating roller.

Fig. 5 shows another example of the heating roller.

Fig. 6 shows a still another example of the heating roller.

Fig. 7 is a chart showing switching control of a first heater and a second heater in each of the heating rollers shown by Figs. 4 to 6.

Fig. 8 is a chart showing a temperature distribution in the direction of rotation axis of each of the heating rollers shown by Figs. 4 to 6 during a continuous printing operation.

Fig. 9 is a chart showing a temperature distribution in the direction of rotation axis of a conventional heating roller having a long heater with its heat-generating portion long in the direction of rotation axis during continuous printing operation.

DESCRIPTION OF EMBODIMENTS

[0034] An image forming apparatus according to the present invention is hereinafter described with reference to the drawings.

[0035] Fig. 1 is an image forming apparatus according to an embodiment of the present invention. Fig. 2 schematically shows a control circuit of the image forming apparatus.

[0036] The image forming apparatus 10 shown by Fig. 1 is a printer for forming a monochromatic image by electrophotography and printing the image on a recording sheet.

[0037] The printer 10 comprises an image forming section 1 for forming a toner image by electrophotography, a sheet feeding section 2 for feeding a recording sheet S to the image forming section 1, a transfer section 3 for transferring the toner image from the image forming section 1 to the recording sheet S fed from the sheet feeding section 2, a fixing device 4 for fixing the toner image transferred to the recording sheet S thereon, a sheet ejecting device 5 for ejecting the recording sheet S with the toner image fixed thereon to a printed-sheet tray 50, etc.

[0038] The printer 10 further comprises a controller C for controlling the operation of the printer 10. The action of every element of the printer 10 is controlled by the controller C. An operation panel PA to be operated by a user is connected to the controller C. The operation panel PA includes a ten-key for an input of the number of printed sheets to be made, a sheet-size key for designating a size of recording sheets to be used, a start key for commanding a start of printing.

[0039] The image forming section 1 comprises a drum-shaped photoreceptor 11, which is to be driven to rotate, as an electrostatic latent image carrier. Around the photoreceptor 11, there are arranged a charger 12, a developing device 14, a cleaner 15, etc. in this order. Further, an exposure device 13 is arranged in such a way to expose the photoreceptor 11 from between the charger 12 and the developing device 14.

[0040] The exposure device 13 exposes the photoreceptor 11 with a laser in accordance with image information sent from a scanner, a computer or the like (not shown).

[0041] The transfer section 3 comprises a transfer roller 31 to be impressed with a transfer bias from a transfer power source (not shown). The transfer roller 31 faces to the photoreceptor 11 at a position downstream of the developing device 14 and upstream of the cleaner 15 with respect to the rotation direction (counterclockwise in Fig. 1) of the photoreceptor 11.

[0042] The transfer roller 31 is pressed by a presser (not shown) so that a nip portion is formed between the transfer roller 31 and the photoreceptor 11. The transfer roller 31 rotates following the rotation of the photoreceptor 11 or a travel of a recording sheet fed to the nip portion as will be described later, or the transfer roller 31 is driven to rotate.

[0043] The sheet feeding section 2 comprises a cassette 200 for storing recording sheets S and a sheet feeder 20 for feeding the sheets S from the cassette 200 one by one.

[0044] The fixing device 4 will be described later.

[0045] The sheet ejecting device 5 comprises a pair of ejection rollers 51 for ejecting a recording sheet S fed from the fixing device 4 to the printed-sheet tray 50. Further, a sheet reversing/circulating path (not shown), which is known, may be provided so as to permit printing on both sides of a sheet.

[0046] In the printer 10, when the start key on the operation panel PA connected to the controller C is pressed, feeding of a recording sheet S from the sheet feeding section 2 is started.

[0047] When a sensor (not shown) detects the recording sheet S fed from the sheet feeding section 2, the image forming section 1 starts an image forming process, and the recording sheet S is once stopped by a pair of the timing rollers 6.

[0048] At the start of image forming process in the image forming section 1, the photoreceptor 11 is driven to rotate counterclockwise in Fig. 1. Thereby, the surface of the photoreceptor 11 is charged to a predetermined potential by the charger 12, and the charged portion of the photoreceptor 11 is exposed to light from the exposure device 13 in accordance with an image to be formed. In this way, an electrostatic latent image is formed on the photoreceptor 11, and in this embodiment, the electrostatic latent image is subjected to reversal development (developed with negatively-charged toner) by the developing device 14 to be developed into a visible toner image.

[0049] The recording sheet S, which has been staying at the pair of timing rollers 6, is fed therefrom to the transfer section 3, and the transfer bias is applied from the transfer power source (not shown) to the transfer roller 31, whereby the toner image is transferred from the photoreceptor 11 to the recording sheet S. The recording sheet S, after receiving the toner image from the photoreceptor 11, is continuously fed to the fixing device 4, where the recording sheet S is heated and pressed, so that the toner image is fixed onto the recording sheet S. Thereafter, the recording sheet S is fed to the pair of ejection rollers 51 and is ejected to the printed-sheet tray 50 through the pair of ejection rollers 51.

[0050] After the toner image transfer, the cleaner 15 cleans the photoreceptor 11 so as to remove residual toner.

[0051] Next, the fixing device is described.

1. Fixing Device 4

[0052] Fig. 3 is a schematic sectional view of the fixing device 4 employed in the printer 10. Fig. 4 schematically shows the structure of a heating roller 41a of the fixing device 4 entirely in the direction of rotation axis thereof.

[0053] The heating roller 41a has a roller body having a silicon rubber elastic layer 412 on a cored bar 411, and a first heater ah1 and a second heater ah2 are embedded in the roller body and are arranged in parallel to each other. The heating roller 41a is to come into contact with a surface of the recording sheet S supporting an unfixed toner image t. The elastic layer 412 may be covered with a lubricating layer, for example, made of PFA (perfluoroalkoxy).

[0054] A pressing roller 42 of the fixing device 4 has a spongy layer of foamed silicon rubber on a cored bar.

[0055] The rollers 41a and 42 are supported by bearings (not shown) in such a way to be rotatable. Based on commands sent from the controller C, the heating roller 41a is driven by a motor (not shown) to rotate counterclockwise in Fig. 3 at the right time for a toner-image fixing process.

[0056] The pressing roller 42 is pressed by a presser (not shown) against the heating roller 41a elastically, so that the pressing roller 42 rotates counterclockwise in Fig. 3 following the heating roller 41a.

[0057] The heating roller 41a is heated to a predetermined toner-image fixing temperature, and as a recording sheet with a toner image thereon is passing through a nip portion N between the heating roller 41a and the pressing roller 42, the toner image is fixed on the recording sheet.

[0058] The heating roller 41a is heated by the two embedded heaters ah1 and ah2. Switching on/off of a power source PW (see Fig. 2) for the heaters ah1 and ah2 is controlled by the controller C, based on the temperature detected by a thermistor SM located to face to the heating roller 41a.

[0059] As the recording sheet is moving, the heat partly migrates together with the recording sheet. However, the other part of the heat remains inside the printer 10. The heat remaining inside the printer 10 partly raises the temperature of the both ends of the heating roller 41a. The other part of the remaining heat raises the elements and the air around the fixing device 4, and thereafter is sucked by an exhaust fan to be exhausted from the printer 10.

[0060] Fig. 4 schematically shows the structure of the heating roller 41a in the direction of rotation axis of the heating roller (which will be occasionally referred to as "axial direction"). Based on commands sent from the controller C, during a printing operation (image formation), the first heater ah1 generates heat from a filament, which is shorter than a sheet-passing range of the heating roller 41a, to heat the heating roller 41a. During a standby state, the second heater ah2 generates heat from a filament, which is longer than the sheet-passing range of the heating roller 41a, to prevent the temperature around the both ends of the sheet-passing range from dropping even with emission of heat through spaces made at the both ends of the heating roller 41a. For these operations, the first heater ah1 consumes an electric power of 800W, and the second heater ah2 consumes an electric power of 400W.

[0061] Since the power distribution to the first heater ah1 is larger, it is possible to supply sufficient heat to the toner for toner fixation on the recording sheet S by bringing only the first heater ah1 under switching control.

[0062] Since the second heater ah2 is turned on only during a standby state, where no recording sheet is passing through the fixing device 4, the small electric power distributed to the second heater ah2 is sufficient to keep the heating roller 41a at a temperature which permits the fixing device 4 to proceed from a standby state to a printing operation promptly. The second heater ah2 has an effective heat-generating portion covering almost the entire range of the second heater ah2 in the axial direction, and the amount of heat generation is substantially uniform in the effective heat-generating portion.

[0063] Fig. 5 shows another exemplary heating roller 41b.

[0064] In the fixing device 4 employing the heating roller 41b instead of the heating roller 41b, during a standby state, a second heater bh2 of the heating roller 41b generates heat from a filament, which is longer than the sheet-passing range of the heating roller 41b, to prevent the temperature around the both ends of a sheet-passing range of the heating roller 41b from dropping even with emission of heat through spaces made at both ends of the heating roller 41b.

[0065] The second heater bh2 generates almost no heat from the center portion of the filament with respect to the axial direction. This is for the following reason. Heat is not emitted from the center portion of the heating roller 14b, and heat migration together with a recording sheet does not occur during a standby state. Therefore, heat generated from only both ends of the filament is sufficient to keep the temperature. The first heater bh1 consumes an electric power of 800W, and the second heater bh2 consumes an electric power of 300W.

[0066] Fig. 6 shows still another exemplary heating roller 41c.

[0067] In the fixing device 4 employing the heating roller 41c instead of the heating roller 41a, during a standby state, a second heater ch2 of the heating roller 41c generates heat from a filament, which is longer than the sheet-passing range of the heating roller 41c, to prevent the temperature around the both ends of a sheet-passing range of the heating roller 41b from dropping even with emission of heat through spaces made at both ends of the heating roller 41b. The amount of heat generation from the both ends of the filament in the axial direction, where the temperature is likely to drop due to heat emission, is greater than the amount of heat generation from the center portion of the filament so that the temperature distribution can be uniform entirely in the axial direction. The first heater ch1 consumes an electric power of 800W, and the second heater ch2 consumes an electric power of 400W.

[0068] Fig. 7 shows the switching control state of the first heaters ah1, bh1 and ch1 and the second heaters ah2, bh2 and bh3 of the heating rollers 41a, 41b and 41c, and transition of the surface temperature of the heating rollers 41a, 41b and 41c.

[0069] The heating rollers 41a, 41b and 41c will be hereinafter collectively referred to as "heating roller 41". The first heaters ah1, bh1 and ch1 will be hereinafter collectively referred to as "first heater h1", and the second heaters ah2, bh2 and ch2 will be hereinafter collectively referred to as "second heater h2".

[0070] In a sleep state before a warm-up, wherein heat from the fixing device 4 does not exist in the printer 10, like in a power-off state, the heaters are off, and the temperature detected by the thermistor SM is equal to the room temperature. When the printer 10 becomes a warm-up state in response to an input through the operation panel PA, both the first heater h1 and the second heater h2 come under switching control, and the temperature detected by the thermistor rises. Since both of the heaters are turned on, the temperature rises quickly, and therefore, the printer 10 can proceed to a printing-permissible state promptly.

[0071] The thermistor SM is not in contact with the surface of the heating roller 41, and the temperature detected by the thermistor SM and sent to the controller C is not equal to the actual surface temperature of the heating roller 41. Therefore, the controller C makes a correction to the temperature detected by the thermistor SM to estimate the surface temperature of the heating roller 41. After completion of the warm-up, if there is no printing command, the printer 10 comes to a standby state. In this moment, the switching control of the first heater h1 is stopped under control of the controller C, and only the second heater h2 keeps under switching control.

[0072] The second heater h2 is turned on and off repeatedly so that the surface temperature of the heating roller 41 can be kept at a target standby temperature of 150 degrees C. In Fig. 7, the "ON/OFF" indicates not whether the heater is turned on or off but whether the heater is under switching control or not. Upon receiving a printing command, the target surface temperature of the heating roller is changed from 150 degrees C to 170 degrees C. In synchronized timing with a recording sheet's passing through the nip portion N, the switching control of the second heater h2 is stopped, and the switching control of the first heater is started. The first heater is turned on and off repeatedly so that the surface temperature of the heating roller 41 can be kept at the target printing temperature of 170 degrees C.

[0073] The time to change the switching control from the second heater h2 to the first heater h1 may be synchronized with receipt of a printing command. Alternatively, the time of the change may be determined regardless of the time of receiving a printing command, and the controller C may calculate the time when a recording sheet passes through the nip portion N and may make the change at a predetermined time prior to the calculated time when the recording sheet passes through the nip portion N. Further, the time of making the change may be synchronized with or later than the arrival of the leading edge of a recording sheet S at the fixing nip portion N. The time of making the change may be determined depending on the power distributions to the heaters.

[0074] After completion of printing (image formation), the printer 10 stays in a standby state for a predetermined time period, and after the elapse of the predetermined time period, the printer 10 comes to a sleep state. The time period for a standby state can be arbitrarily set. When the printer 10 comes to a warm-up sate after a short-time sleep state, only the second heater h2 is turned on. Whether to turn on the first heater h1 may depend on the time length of the sleep state. For example, when the printer 10 comes to a warm-up state after a five-minute or longer sleep state, both of the first heater h1 and the second heater h2 are brought under switching control, and when the printer 10 comes to a warm-up state after a sleep state shorter than five minutes, only the second heater h2 is brought under switching control.

[0075] Further, the selection of the heater(s) to be brought under switching control may depend on the temperature detected by the thermistor SM. Also, two thermistors SM may be arranged around the center and around an end portion, respectively, in the axial direction, so that the thermistor SM arranged around the center may be used for the switching control of the first heater h1 and that the thermistor SM arranged around the end may be used for the switching control of the second heater h2. However, since the heater under switching control is changed depending on the state of the printer 10, only a single thermistor SM may be arranged around the center in the axial direction, and the single thermistor SM can be used both for the switching control of the first heater h1 and for the switching control of the second heater h2.

[0076] In the fixing device described above, the heating roller has two heaters embedded therein, but three or more heaters may be embedded in the heating roller. For example, in order to complete the warm-up more quickly, another heater to be used for the warm-up may be further provided.

[0077] Fig. 8 shows the surface temperature distribution of the heating roller 41 (41a, 41b and 41c) in the axial direction during continuous printing. Fig. 8 shows a case where the maximum sheet-passing range is 216mm.

[0078] The first heater h1 generates heat from its filament that is shorter than the maximum sheet-passing range, and the heat diffuses from the both ends of the filament toward the both ends of the heater h1. Thereby, as shown by Fig. 8, the temperature distribution in the maximum sheet-passing range becomes substantially uniform, and the temperature out of the maximum sheet-passing range does not rise above 170 degrees C.

[0079] Fig. 9 shows the surface temperature distribution of a conventional heating roller in the axial direction during continuous printing. The conventional heating roller employs a long heater of which heat-generating portion is long in the axial direction.

[0080] In the conventional example, the long heater is mainly used, and the long heater is turned on even during printing. Thereby, the temperature of portions out of the maximum sheet-passing range rises, and much UFP is generated from both end portions of the heating roller due to the rise in temperature.

[0081] The fixing device 4 described above has the following advantages.

[0082] During printing, only the first heater h1 having a greater power distribution is under switching control. Thereby, as heat transfers from the center portion of the heating roller 41 to the recording sheet S, the heat loss from the center portion of the heating roller 41 can be compensated.

[0083] Since the heat-generating portion of the first heater h1 is shorter than the sheet-passing range, excessive heat emission outward from the sheet-passing range can be prevented, thereby preventing the end portions of the heating roller 41 from rising in temperature during printing. Thus, by preventing a rise in temperature of the heating roller, generation of UFP can be suppressed.

[0084] Since both the first heater h1 and the second heater h2 are turned on for a warm-up, the rate of temperature rise is high, and the time period of a warm-up for printing can be shortened. For a warm-up shortly after turning-off of both the heaters h1 and h2, only the second heater h2 is turned on. In this case, this is sufficient to rise the temperature and to complete the warm-up for a short time.

[0085] During a standby state, only the second heater h2 is under switching control to heat the end portions of the heating roller 41, whereby a drop in temperature of the end portions of the heating roller 41 can be prevented even with heat emission from the end portions. In this state, heat transfer to a recording sheet does not occur, and the second heater h2 does not require a great electric power.

[0086] The control of the heaters h1 and h2 is changed depending on not the size of the recording sheet but the state of the printer 10 (a printing state, a standby state or a warm-up state), which results in efficient power consumption. This effect can be seen especially in an image forming apparatus that is applicable to recording sheets of around A4 size.

[0087] Although a monochromatic image forming apparatus (printer) has been described as an embodiment of the present invention, the present invention is applicable also to color image forming apparatuses of a tandem type, a four-cycle type, etc. Further, the present invention is applicable not only to printers but also to copying machines, facsimiles and multi-function apparatuses having two or more functions of these machines.

INDUSTRIAL APPLICABILITY

[0088] The present invention is applicable to provide an image forming apparatus that forms a toner image on an image carrier, that transfers the toner image to a recording sheet and that fixes the toner image on the recording sheet with a fixing device comprising a rotatable heating member having a plurality of heaters embedded therein and a rotatable pressing member, the rotatable heating member and the rotatable pressing member opposed to each other to form a nip portion, wherein during a printing operation, even if the printing operation is to form a plurality of printed sheets continuously, ends of the heating member in a direction of rotation axis are prevented from reaching an excessively high temperature, thereby suppressing generation of UFP from the ends of the heating member in the direction of rotation axis, and wherein when the apparatus changes from a standby state of not performing printing to a printing state, the temperature of the heating member can be stabilized to a temperature appropriate for the fixing process quickly without energy waste.

Claims

1. An image forming apparatus (10) that is configured to form a toner image on an image carrier, to transfer the toner image (t) to a recording sheet (S) and to fix the toner image (t) on the recording sheet (S) with a fixing device (4) comprising a rotatable heating member (41) having a plurality of heaters (h1, h2) embedded therein and a rotatable pressing member (42), the heating member (41) and the pressing member (42) opposed to each other to form a nip portion (N), which a fixing process is performed while the recording sheet (S) is passing through so that the toner image (t) is heated and pressed to be fixed on the recording sheet (S), the image forming apparatus (10) comprising:

a power source (PW) for electrifying the plurality of heaters (h1, h2);

a temperature detector (SM) for detecting a temperature of the heating member (41); and

a controller (C) for controlling the power source (PW),

wherein the plurality of heaters (h1, h2) includes a first heater (h1) that generates a greater amount of heat from its central portion with respect to a direction of a rotation axis of the heating member (41) than from its end portions with respect to the direction of the rotation axis of the heating member (41);

wherein the controller (C) controls the power source (PW) such that at least one of the plurality of heaters (h1, h2) is controlled to be turned on and off based on a difference between the temperature detected by the temperature detector (SM) and a target temperature of the heating member (41) so that the temperature of the heating member (41) can become the target temperature; and

wherein the controller (C) controls the power source (PW) such that only the first heater (h1) is turned on and off during a printing operation.

2. An image forming apparatus (10) as claimed in claim 1,
wherein the plurality of heaters (h1, h2) includes a second heater (h2) that generates a greater amount of heat from its end portions with respect to the direction of rotation axis of the heating member (41) than the amount of heat generated from the end portions of the first heater (h1) with respect to the direction of rotation axis of the heating member (41); and
wherein the controller (C) controls the power source (SW) such that only the second heater (h2) is controlled to be turned on and off during a standby state of not performing printing.

3. An image forming apparatus (10) as claimed in claim 2, wherein the controller (C) calculates a time when the recording sheet (S) with the toner image (t) thereon passes through the nip portion (N) to be subjected to the fixing process and determines a time to stop controlling the second heater (h2) and to start controlling the first heater (h1) in a transition from the standby state to the printing operation based on the calculated time when the recording sheet (S) passes through the nip portion (N) in such a manner to ensure the fixing process be conducted on the recording sheet (S).

4. An image forming apparatus (10) as claimed in claim 1, 2 or 3, wherein the first heater (h1) has a heat-generating portion having an effective length in the direction of rotation axis of the heating member (41) smaller than a length of a maximum sheet-passing range corresponding to a maximum dimension in the direction of rotation axis of the heating member (41) of a sheet passing through the nip portion (N) among sheets of regular sizes that can be subjected to the fixing process.

5. An image forming apparatus (10) as claimed in claim 1, 2, 3 or 4, wherein the first heater (h1) has a greater power distribution from the power source than the second heater (h2) has.

6. An image forming apparatus (10) as claimed in claim 1, 2, 3, 4 or 5,
wherein as the temperature detector (SM), a single detector is located within the maximum sheet-passing range in the direction of rotation of the heating member (41); and
wherein the controller (C) uses information detected by the single detector as temperature information of the heating member (41).

Drawing