Fixing device and image forming apparatus

Abstract

To provide a configuration that, even with power restriction, can prevent the time from the start-up of a fixing device until the start of image formation from becoming longer.SOLUTION: A heating roller stretches a fixing belt. A stretching unit including a stay stretches the fixing belt. The heating roller and the stay are heated respectively by halogen heaters. The heating roller has a smaller heat capacity than that of the stretching unit. At the start of a fixing device 8, the fixing device starts heating of the halogen heater that heats the heating roller before the halogen heater that heats the stay, and after the temperature of the heating roller reaches a target temperature, starts heating of the halogen heater that heats the stay.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to a fixing device that heats a toner image carried on a recording material to fix it to the recording material, and to an image forming apparatus that is equipped with a fixing device, such as a copier, printer, facsimile, or multifunction machine that has multiple functions of these. [Background technology]

[0002] The image forming apparatus has a fixing device that fixes a toner image carried on a recording material to the recording material. A fixing device that includes an endless belt member, a plurality of tension members that tension the belt member, a heater that heats the tension members, and a rotating body that forms a nip between the belt member and the rotating body, and that heats and conveys the toner image on the recording material in the nip, has been known in the art (see Patent Documents 1 and 2).

[0003] Patent document 2 also discloses a configuration having, as multiple tension members, a heating roller that contacts the inner or outer surface of a belt member, and a pad member that is arranged inside the belt member so as to sandwich the belt member between it and a rotating body and forms a nip portion between the belt member and the rotating body, and is equipped with heaters that heat the heating roller and the pad member, respectively.

[0004] In the configuration of Patent Document 2, when starting up the fixing device, the heaters of the heating roller and the pad member start heating simultaneously, and if they are heated until they each reach their target temperature, there is a risk that the belt member being heated by the heating roller, which has a smaller heat capacity than the pad member, will melt. Therefore, heating is started first from the heater that heats the pad member, which has a larger heat capacity than the heating roller and whose belt member heats up at a slower rate than the heating roller. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-228765 [Patent Document 2] Japanese Patent Application Laid-Open No. 2013-156334 Summary of the Invention [Problem to be solved by the invention]

[0006] In an image forming apparatus, the fixing device is generally the device that consumes the most power. However, depending on the environment and circumstances in which the image forming apparatus is used, the amount of power available to the fixing device may be limited. When power is limited, it is possible to reduce the power used by the heaters that generate the most heat among the heaters that heat the multiple tension members and start heating each heater simultaneously. However, if the power used by the heaters that generate the most heat is reduced and heating each heater simultaneously, it takes time for the belt member to reach the target temperature at which the toner image can be fixed to the recording material. In other words, the start-up time of the fixing device is extended, and the time from input of the image formation start signal to the actual start of image formation is also extended.

[0007] Furthermore, in the configuration described in Patent Document 2, the heater that heats the pad member starts heating first to prevent the temperature of the heating roller from rising, but then in addition to heating the heater of the pad member, the heater of the heating roller also starts heating. Therefore, if it is assumed that there is a power limit in the configuration described in Patent Document 2, it is necessary to reduce the power supplied to the heater of the heating roller, and as described above, it takes time for the belt member to reach the target temperature at which the toner image can be fixed on the recording material.

[0008] On the other hand, in a fixing device having a plurality of heaters that respectively heat a plurality of tension members, image formation may be able to start once one tension member reaches the target temperature, depending on the conditions of use.

[0009] SUMMARY OF THE INVENTION An object of the present invention is to provide a configuration that can prevent the time from the start of startup of a fixing device to the start of image formation from becoming long even when there is a power limit. [Means for solving the problem]

[0010] One aspect of the present invention is a fixing device that heats a toner image carried on a recording material and fixes it to the recording material, comprising: an endless belt member; a rotating body that contacts the outer surface of the belt member and forms a nip portion between the belt member and the rotating body to sandwich and transport the recording material; a first tension member that contacts the inner or outer surface of the belt member and tensions the belt member; a first heater that heats the first tension member; a second tension member that contacts the inner or outer surface of the belt member and tensions the belt member; and a second heater that heats the second tension member, wherein the first tension member has a smaller heat capacity than the second tension member, and when the fixing device is started up, the first heater starts heating before the second heater, and the second heater starts heating after the temperature of the first tension member reaches a target temperature.

[0011] One aspect of the present invention is a fixing device that heats a toner image carried on a recording material and fixes it to the recording material, the fixing device comprising: an endless belt member; a rotating body that contacts the outer peripheral surface of the belt member and forms a nip portion between the belt member and the rotating body to sandwich and convey the recording material; a first tension member that contacts the inner peripheral surface or the outer peripheral surface of the belt member and stretches the belt member; a first heater that heats the first tension member; a second tension member that contacts the inner peripheral surface or the outer peripheral surface of the belt member and stretches the belt member; and a second heater that heats the second tension member. and wherein, when a first temperature rise rate is defined as a rate at which the temperature of the belt member rises as a result of the first heater heating the first tension member and a second temperature rise rate is defined as a rate at which the temperature of the belt member rises as a result of the second heater heating the second tension member, the first temperature rise rate is faster than the second temperature rise rate, and when the fixing device is started up, the first heater starts heating before the second heater, and starts heating after the temperature of the first tension member reaches a target temperature.

[0012] One aspect of the present invention is a printer including an image forming unit that forms a toner image on a recording material, a fixing device that heats the recording material on which the toner image has been formed by the image forming unit to fix the toner image on the recording material, a power supply unit, and a control unit that controls the power supply unit, wherein the fixing device includes an endless belt member, a rotating body that contacts the outer peripheral surface of the belt member and forms a nip portion between the belt member and the rotating body to sandwich and convey the recording material, a first tension member that contacts the inner peripheral surface or outer peripheral surface of the belt member and stretches the belt member, a first heater that heats the first tension member, a second tension member that contacts the inner peripheral surface or outer peripheral surface of the belt member and stretches the belt member, and a front and a second heater that heats the second tension member, wherein the first tension member has a smaller heat capacity than the second tension member, the power supply unit supplies power to the first heater and the second heater, and the control unit is capable of executing a mode in which the fixing device is started up with power that is lower than the power that would be used if the first heater and the second heater were each made to generate heat at their maximum heat output, and when executing the mode, the image forming apparatus is characterized in that it controls the power supply unit to start heating the first heater before the second heater, and to start heating the second heater after the temperature of the first tension member reaches a target temperature.

[0013] One aspect of the present invention includes an image forming unit that forms a toner image on a recording material, a fixing device that heats the recording material on which the toner image has been formed by the image forming unit to fix the toner image on the recording material, a power supply unit, and a control unit that controls the power supply unit, wherein the fixing device includes an endless belt member, a rotating body that contacts the outer peripheral surface of the belt member and forms a nip portion between the belt member and the rotating body to sandwich and convey the recording material, a first tension member that contacts the inner peripheral surface or outer peripheral surface of the belt member and stretches the belt member, a first heater that heats the first tension member, a second tension member that contacts the inner peripheral surface or outer peripheral surface of the belt member and stretches the belt member, and a second heater that heats the second tension member, wherein the first heater heats the first tension member. The image forming apparatus is characterized in that, when the rate at which the temperature of the belt member rises is defined as a first temperature rise rate and the rate at which the temperature of the belt member rises by heating the second tension member with the second heater is defined as a second temperature rise rate, the first temperature rise rate is faster than the second temperature rise rate, the power supply unit supplies power to the first heater and the second heater, and the control unit is capable of executing a mode in which the fixing device is started up with power lower than the power when the first heater and the second heater are each made to generate heat at their maximum heat output, and when the mode is executed, the power supply unit is controlled to start heating the first heater before the second heater, and to start heating the second heater after the temperature of the first tension member has reached a target temperature. [Effects of the Invention]

[0014] According to the present invention, even if there is a power limit, it is possible to prevent the time from the start of startup of the fixing device to the start of image formation from becoming long. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a cross-sectional view showing a schematic configuration of an image forming apparatus according to an embodiment. [Figure 2] FIG. 2 is a cross-sectional view showing a schematic configuration of a fixing device according to the embodiment. [Figure 3]6 is a graph showing temperature transitions of the heating roller and stay and power transitions of each heater that heats the heating roller and stay when the fixing device is started up without power restrictions. [Figure 4] A graph showing the temperature changes of the heating roller and the stay and the power changes of each heater that heats the heating roller and the stay when starting up a fixing device with power restrictions and when heating of each heater that heats the heating roller and the stay starts simultaneously. [Figure 5] 10 is a graph showing the temperature transition of the heating roller and the stay and the power transition of each heater that heats the heating roller and the stay when a fixing device with power restrictions is started up and the heating start timing of the heating roller heater and the stay heater are shifted. [Figure 6] FIG. 2 is a control block diagram showing a part of the control configuration of the image forming apparatus according to the embodiment. [Figure 7] 6 is a flowchart illustrating a start-up process of the fixing device according to the embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0016] The embodiment will be described with reference to Figures 1 to 6. First, the schematic configuration of an image forming apparatus according to the present embodiment will be described with reference to Figure 1.

[0017] [Image forming equipment] Image forming apparatus 1 is an electrophotographic full-color printer having four image forming units Pa, Pb, Pc, and Pd corresponding to the four colors of yellow, magenta, cyan, and black. In this embodiment, the image forming units Pa, Pb, Pc, and Pd are arranged in tandem along the rotation direction of an intermediate transfer belt 204 (described later). Image forming apparatus 1 forms a toner image (image) on a recording material in response to an image signal from an image reading unit (document reading device) 2 connected to image forming apparatus main body 3 or from a host device such as a personal computer connected to image forming apparatus main body 3 so as to be able to communicate with the image forming apparatus main body 3. Examples of recording materials include sheet materials such as paper, plastic film, and cloth.

[0018] The image forming apparatus 1 comprises an image reading unit 2 and an image forming apparatus main body 3. The image reading unit 2 reads an original placed on a platen glass 21. Light emitted from a light source 22 is reflected by the original and forms an image on a CCD sensor 24 via optical components 23 such as a lens. This optical unit scans in the direction of the arrow, converting the original into a line-by-line electrical signal data stream. The image signal obtained by the CCD sensor 24 is sent to the image forming apparatus main body 3, where it is subjected to image processing in accordance with each image forming unit (described later) by a control unit 30. The control unit 30 also receives external inputs as image signals from external host devices such as a print server.

[0019] The image forming apparatus main body 3 includes multiple image forming units Pa, Pb, Pc, and Pd, and each image forming unit forms an image based on the image signal. That is, the image signal is converted into a laser beam that is PWM (pulse width modulation) controlled by a control unit 30. A polygon scanner 31 serving as an exposure device scans the laser beam in accordance with the image signal. The laser beam is then irradiated onto photosensitive drums 200a to 200d serving as image carriers of each image forming unit Pa to Pd.

[0020] Note that Pa is a yellow (Y) image forming unit, Pb is a magenta (M) image forming unit, Pc is a cyan (C) image forming unit, and Pd is a black (Bk) image forming unit, each forming an image of the corresponding color. Since the image forming units Pa to Pd are substantially identical, the Y image forming unit Pa will be described in detail below, and descriptions of the other image forming units will be omitted. In the image forming unit Pa, a toner image is formed on the surface of the photosensitive drum 200a based on an image signal, as will be described below.

[0021] A charging roller 201a, which serves as a primary charger, charges the surface of the photosensitive drum 200a to a predetermined potential to prepare for the formation of an electrostatic latent image. A laser beam from a polygon scanner 31 forms an electrostatic latent image on the surface of the photosensitive drum 200a, which has been charged to a predetermined potential. A developing unit 202a develops the electrostatic latent image on the photosensitive drum 200a to form a toner image. A primary transfer roller 203a discharges electricity from the back surface of the intermediate transfer belt 204 and applies a primary transfer bias of opposite polarity to the toner, transferring the toner image on the photosensitive drum 200a onto the intermediate transfer belt 204. After transfer, the surface of the photosensitive drum 200a is cleaned by a cleaner 207a.

[0022] The toner image on the intermediate transfer belt 204 is then conveyed to the next image forming station, where the toner images of each color formed at each image forming station are transferred in the order of Y, M, C, and Bk, forming a four-color image on the surface. The toner image that has passed through the Bk image forming station Pd, which is located at the most downstream side in the rotation direction of the intermediate transfer belt 204, is conveyed to a secondary transfer station made up of a pair of secondary transfer rollers 205 and 206. In the secondary transfer station, a secondary transfer electric field of opposite polarity to the toner image on the intermediate transfer belt 204 is applied, thereby causing the toner image to be secondarily transferred onto the recording material P.

[0023] The recording material P is stored in a cassette 9, and the recording material P fed from the cassette 9 is conveyed to a registration unit 208, which is made up of, for example, a pair of registration rollers, and waits at the registration unit 208. Thereafter, the timing of the registration unit 208 is controlled to align the position of the toner image on the intermediate transfer belt 204 with the position of the recording material P, and the recording material is conveyed to a secondary transfer unit.

[0024] The recording material P onto which the toner image has been transferred in the secondary transfer section is conveyed to a fixing device 8, where the toner image carried on the recording material P is fixed to the recording material P by heating and pressing. The recording material P that has passed through the fixing device 8 is discharged onto a discharge tray 7. When forming images on both sides of the recording material P, once the toner image has been transferred and fixed onto the first side (front side) of the recording material P, the recording material P is turned over via a reversing conveyance section 10, and the toner image is transferred and fixed onto the second side (rear side) of the recording material P, and the recording material P is then stacked on the discharge tray 7.

[0025] As described above, the control unit 30 controls the entire image forming apparatus 1. The control unit 30 can also perform various settings based on input from the operation unit 4 of the image forming apparatus 1. The control unit 30 has a CPU (Central Processing Unit) 30a, a ROM (Read Only Memory) 30b, and a RAM (Random Access Memory) 30c (see FIG. 6, which will be described later). The CPU 30a controls each unit by reading a program corresponding to a control procedure stored in the ROM 30b. The RAM 30c stores working data and input data, and the CPU 30a performs control by referring to the data stored in the RAM 30c based on the aforementioned programs, etc.

[0026] [Fusing device] Next, the configuration of the fixing device 8 in this embodiment will be described with reference to FIG. 2. FIG. 2 is a cross-sectional view that schematically shows the general configuration of the fixing device 8. In this embodiment, a belt heating type fixing device using an endless belt is adopted. In FIG. 2, the X direction indicates the conveyance direction of the recording material P, the Y direction indicates the width direction that intersects with the conveyance direction of the recording material P (orthogonal in this embodiment), and the Z direction indicates the pressure direction of the pressure roller 305, which will be described later. These directions are perpendicular to each other. Furthermore, one side of the width direction (Y direction) is the front side of the image forming apparatus 1, for example, the side where the operation unit 4 is located and operated by the user. On the other hand, the other side of the width direction is the rear side of the image forming apparatus 1.

[0027] The fixing device 8 includes a heating unit 300 having a fixing belt 301 as an endless rotatable belt member, and a pressure roller 305 as a rotating body that contacts the fixing belt 301 and forms a nip portion N together with the fixing belt 301.

[0028] Heating unit 300 includes the above-mentioned fixing belt 301, fixing pad 303 as a nip portion forming member and pad member, stay 302 as a support member, heating roller 307 as a tension roller, and steering roller 308. Heating unit 300 also includes halogen heaters 306a and 306b for heating fixing belt 301, and as will be described later, halogen heater 306b as a first heater is disposed inside heating roller 307, and halogen heater 306a as a second heater is disposed inside stay 302. Pressure roller 305 rotates in contact with the outer peripheral surface of fixing belt 301, and also serves as a driving rotor that applies driving force to fixing belt 301.

[0029] The fixing belt 301 has thermal conductivity, heat resistance, and other properties, and is a thin cylindrical shape. In this embodiment, it has a three-layer structure consisting of a base layer, an elastic layer on the outer periphery of the base layer, and a release layer on the outer periphery of the elastic layer. The base layer is 80 μm thick and made of polyimide resin (PI), the elastic layer is 300 μm thick and made of silicone rubber, and the release layer is 30 μm thick and made of PFA (tetrafluoroethylene-perfluoroalkoxyethylene copolymer resin) as a fluororesin. The fixing belt 301 is tensioned by a heating roller 307 as a first tensioning member, a steering roller 308 as a tensioning member, and a tensioning unit 304 as a second tensioning member. The tensioning unit 304 has a stay 302 and a fixing pad 303.

[0030] The fixing pad 303 contacts the inner circumferential surface of the fixing belt 301 and stretches the fixing belt 301. The fixing pad 303 is disposed inside the fixing belt 301 and is non-rotating so as to face the pressure roller 305 with the fixing belt 301 sandwiched therebetween. The fixing pad 303 forms a nip portion N between the fixing belt 301 and the pressure roller 305, which sandwiches and conveys the recording material. In this embodiment, the fixing pad 303 is a substantially plate-shaped member that is long along the width direction of the fixing belt 301 (the longitudinal direction intersecting the rotation direction of the fixing belt 301, i.e., the direction of the rotation axis of the heating roller 307). The fixing pad 303 is pressed against the pressure roller 305 with the fixing belt 301 sandwiched therebetween, thereby forming the nip portion N. The fixing pad 303 is made of LCP (liquid crystal polymer) resin. That is, in this embodiment, the fixing pad 303 is made of resin. The material of the fixing pad 303 is not limited to LCP, but may be other resins or metals.

[0031] The fixing pad 303 is supported by a stay 302, which serves as a support member and is disposed inside the fixing belt 301. The stay 302, which constitutes a second tension member together with the fixing pad 303, is disposed on the opposite side of the fixing pad 303 from the pressure roller 305 and supports the fixing pad 303. The stay 302 is a reinforcing member having long rigidity along the longitudinal direction of the fixing belt 301, and contacts the fixing pad 303 to back it up. When the fixing pad 303 is pressed by the pressure roller 305, the stay 302 provides strength to the fixing pad 303, ensuring the application of pressure at the nip N. The stay 302 is made of stainless steel and has a thickness of 3 mm. In this embodiment, the stay 302 is made of metal. However, other metals such as iron or aluminum, or resin, may also be used.

[0032] Stay 302 is a square pipe having a space formed therein along the width direction, and halogen heater 306a serving as a second heater for heating fixing pad 303 is arranged in the space inside stay 302 along the width direction. Stay 302 can be heated to a predetermined temperature by halogen heater 306a. Fixing belt 301 is heated by halogen heater 306a arranged inside stay 302 via stay 302 and fixing pad 303.

[0033] That is, halogen heater 306a heats fixing pad 303 via stay 302. Then, halogen heater 306a heats fixing pad 303, thereby heating fixing belt 301 stretched by fixing pad 303. A thermistor Th2 serving as a temperature detection unit that detects the surface temperature of stay 302 is in contact with or close to the outer surface of stay 302. Based on the temperature detected by thermistor Th2, control unit 30 controls (temperature adjustment control) the power supply to halogen heater 306a so that the surface temperature of stay 302 reaches a target temperature (stay target temperature).

[0034] 2, one halogen heater 306a is shown, but two or more may be used. Also, the heater that heats the fixing pad 303 is not limited to a halogen heater, and may be another heater that can heat the fixing pad 303, such as a carbon heater.

[0035] Between the fixing pad 303 and the fixing belt 301, a sliding member (not shown), such as a lubricating sheet whose surface is coated with PTFE (polytetrafluoroethylene), and silicone oil (hereinafter referred to as oil) as a lubricant, are interposed, allowing the fixing belt 301 to slide smoothly against the fixing pad 303. The sliding member is formed, for example, by coating the surface of a 70 μm-thick polyimide substrate with PTFE. Note that the sliding member is arranged to improve the sliding properties between the fixing pad 303 and the fixing belt 301, and therefore, it is possible to substitute a coating that improves sliding properties for the surface layer of the fixing pad 303 instead of the sliding member.

[0036] Heating roller 307 as a first tension member is disposed inside fixing belt 301, contacts the inner circumferential surface of fixing belt 301, and tensions fixing belt 301 together with fixing pad 303 and steering roller 308. Heating roller 307 is formed into a cylindrical shape from a metal such as aluminum or stainless steel, and is provided therein with halogen heater 306b as a first heater for heating fixing belt 301. That is, halogen heater 306b is disposed inside hollow cylindrical heating roller 307 along the width direction. Heating roller 307 is heated to a predetermined temperature by halogen heater 306b.

[0037] That is, halogen heater 306b heats heating roller 307. Then, by heating heating roller 307, halogen heater 306b heats fixing belt 301, which is stretched around heating roller 307. A thermistor Th1, which serves as a temperature detection unit that detects the surface temperature of heating roller 307, is in contact with or close to the outer circumferential surface of heating roller 307. Based on the temperature detected by thermistor Th1, control unit 30 controls (temperature adjustment control) the power supply to halogen heater 306b so that the surface temperature of heating roller 307 reaches a target temperature (target heating roller temperature).

[0038] In this embodiment, the heating roller 307 is formed, for example, from an aluminum alloy pipe having a thickness of 2 mm. Considering the temperature rise at the end of the recording material as it passes through the nip, the material for the heating roller 307 is preferably one with high thermal conductivity, and other metals such as stainless steel alloys may be used in addition to aluminum alloys. The number of halogen heaters 306b may be one or more. In this embodiment, three halogen heaters 306b are used. The heater for heating the heating roller 307 is not limited to a halogen heater, and may be another heater capable of heating the heating roller 307, such as a carbon heater. In this embodiment, the fixing belt 301 is heated by the halogen heaters 306a and 306b.

[0039] The steering roller 308 is disposed inside the fixing belt 301, contacts the inner circumferential surface of the fixing belt 301, stretches the fixing belt 301 together with the fixing pad 303 and the heating roller 307, and is rotated by the fixing belt 301. The steering roller 308 is disposed so as to be tiltable in a direction parallel to the rotation axis direction (width direction, longitudinal direction) of the heating roller 307, and controls the position (offset position) of the fixing belt 301 with respect to this rotation axis direction by tilting. That is, the steering roller 308 has a rotation center at the center or one end of the rotation axis direction (longitudinal direction) of the steering roller 308, and tilts with respect to the longitudinal direction of the heating roller 307 by swinging around this rotation center. This generates a tension difference between one side and the other side of the fixing belt 301 in the longitudinal direction, moving the fixing belt 301 in the longitudinal direction.

[0040] Fixing belt 301 tends to shift to one end during rotation depending on the outer diameter accuracy of the rollers that tension it and the alignment accuracy between the rollers. For this reason, such shifting is controlled by steering roller 308. Note that steering roller 308 may be oscillated by a drive source such as a motor, or may be configured to oscillate by automatic centering.

[0041] In this embodiment, the steering roller 308 is biased by a spring supported by the frame of the heating unit 300, and also serves as a tension roller that applies a predetermined tension to the fixing belt 301. The steering roller 308 is formed into a cylindrical shape from a metal such as aluminum or stainless steel. In this embodiment, the steering roller 308 is formed from a stainless steel pipe with an outer diameter of 20 mm. The steering roller 308 may be provided with a rubber material or the like on its surface to increase its grip on the fixing belt 301.

[0042] The pressure roller 305 rotates in contact with the outer peripheral surface of the fixing belt 301, applying a driving force to the fixing belt 301. In this embodiment, the pressure roller 305 is a roller having an elastic layer formed on the outer periphery of a shaft (core metal), and a release layer formed on the outer periphery of the elastic layer. The shaft is made of stainless steel with a diameter of 44 mm, the elastic layer is made of conductive silicone rubber with a thickness of 8 mm, and the release layer is made of PFA (tetrafluoroethylene-perfluoroalkoxyethylene copolymer resin) as a fluororesin with a thickness of 100 μm. The pressure roller 305 is rotatably supported by a pressure arm that is movable relative to the fixing frame (not shown) of the fixing device 8, and has a gear fixed to one end thereof. The pressure roller 305 is connected to a motor (not shown) via the gear and rotates.

[0043] In the fixing device 8 configured as above, the toner image is heated in the nip portion N formed between the fixing belt 301 and the pressure roller 305 while the recording material P carrying the toner image is nipped and conveyed. The toner image is then fixed to the recording material P. Therefore, the fixing device 8 needs to have both the function of applying heat and pressure and the function of conveying the recording material P. The pressure roller 305 is pressed against the fixing pad 303 via the fixing belt 301 by a drive source (not shown). In this embodiment, the pressure force (NF) at the nip portion N during image formation is set to 1600 N, and the length of the nip portion N in the X direction (conveyance direction) is set to 20 mm and the length in the Y direction (width direction) is set to 350 mm.

[0044] [Relationship between the heating element and the temperature rise rate of the fixing belt] Here, the relationship between the heating member heated by the heater and the temperature rise rate of the fixing belt 301 will be described. First, the temperature rise rate of the heating member will be described. As described above, the stay 302 and the heating roller 307 as heating members each have halogen heaters 306a and 306b disposed therein as heaters for heating the fixing belt 301. Here, the temperature rise rate of the heating member is determined by the heat capacity of the member used and the amount of heat provided by the heater. Specifically, the greater the heat capacity of the heating member, the slower the temperature rise rate, and the greater the heat output of the heater, the faster the temperature rise rate.

[0045] Next, the temperature rise rate of fixing belt 301 will be described. The temperature rise rate of fixing belt 301 is determined by the temperature rise rate of the heating member, as well as the thermal properties of the heating member, such as the thermal conductivity, and the contact state between fixing belt 301 and the heating member. Here, the rate at which fixing belt 301 rises in temperature as a result of halogen heater 306b heating heating roller 307 is referred to as the first temperature rise rate, and the rate at which fixing belt 301 rises in temperature as a result of halogen heater 306a heating stay 302 and fixing pad 303 is referred to as the second temperature rise rate. In this case, the first temperature rise rate is determined by the heat capacity of heating roller 307, the heat generation amount of halogen heater 306b, and the thermal resistance and contact area between fixing belt 301 and heating roller 307. In addition, the second temperature rise rate is determined by the heat capacity of the stay 302, the heat generation amount of the halogen heater 306a, the thermal properties and thermal conduction characteristics of the fixing pad 303 placed between the fixing belt 301 and the stay 302, the thermal properties of the sliding member placed between the fixing pad 303 and the fixing belt 301, and the thermal resistance and contact area between each member.

[0046] In the configuration of this embodiment, stay 302 and heating roller 307 are designed with the heat capacities shown in Table 1, with halogen heater 306a generating 300 W and halogen heater 306b generating 1400 W. That is, in this embodiment, heating roller 307 has a smaller heat capacity than stay 302. Since tension unit 304 is composed of stay 302 and fixing pad 303, it naturally has a larger heat capacity than heating roller 307. [Table 1]

[0047] Therefore, the temperature rise rate of the heating roller 307 is faster than that of the stay 302, which in turn is faster than that of the tension unit 304. The heating roller 307 is in contact with the fixing belt 301 over a range of 60 mm in the circumferential direction. On the other hand, the tension unit 304, which is made up of the stay 302 and the fixing pad 303, is in contact with the fixing belt 301 over a range of 20 mm in the conveyance direction (X direction) of the recording material P. That is, since the length of the nip portion N in the X direction is 20 mm as described above, the effective length over which the fixing pad 303 is in contact with the fixing belt 301 in the circumferential direction of the fixing belt 301 is approximately equal to the length of the nip portion N in the X direction.

[0048] Note that fixing pad 303 may come into contact with fixing belt 301 at locations other than nip N, such as when steering roller 308 is steered or when pressure roller 305 starts or stops driving. However, the area where heat from halogen heater 306a is transferred to fixing belt 301 via stay 302 and fixing belt 301 is mainly nip N. Therefore, in this embodiment, the length over which tension unit 304 comes into contact with fixing belt 301 in the circumferential direction of fixing belt 301 is the length of nip N. Even if fixing pad 303 comes into contact at a location other than nip N, the length over which heating roller 307 comes into contact with fixing belt 301 in the circumferential direction of fixing belt 301 is longer than the length over which tension unit 304 (specifically fixing pad 303) comes into contact with fixing belt 301.

[0049] In this way, the temperature rise rate of heating roller 307 is faster than the temperature rise rate of tension unit 304, and the length of contact between heating roller 307 and fixing belt 301 in the circumferential direction of fixing belt 301 is longer than the length of contact between tension unit 304 and fixing belt 301, so the first temperature rise rate (the temperature rise rate of fixing belt 301 caused by heating roller 307) is faster than the second temperature rise rate (the temperature rise rate of fixing belt 301 caused by stay 302 and fixing pad 303). In the configuration of this embodiment, the temperature rise rate of fixing belt 301 caused by heating roller 307 is approximately 2°C / sec, and the temperature rise rate of fixing belt 301 caused by stay 302 and fixing pad 303 is approximately 0.05°C / sec.

[0050] Here, the role of each heating element from the viewpoint of control will be explained. The heating roller 307, as the main heating element for the fixing belt 301, has the role of successively adjusting the temperature of the fixing belt 301. On the other hand, the stay 302 and fixing pad 303 have the role of auxiliary heating elements for the fixing belt 301. For example, when the recording material P is thick paper that requires a large amount of heat and this thick paper is continuously passed through the nip portion N during continuous image formation (continuous printing), the halogen heater 306b inside the heating roller 307 alone may not be able to heat the fixing belt 301. In this case, by heating the stay 302 and fixing pad 303 with the halogen heater 306a, the temperature of the internal components of the fixing belt 301 is raised overall, helping to maintain the temperature of the fixing belt 301. Therefore, when printing content that does not require a large amount of heat, such as a sheet with a basis weight of 128 g / m, is used, the halogen heater 306b alone may not be able to heat the fixing belt 301. 2 When fixing a toner image on the following plain paper or thin paper, or when the number of images formed is small, the ability to raise the temperature of the fixing belt 301 by the auxiliary stay 302 and fixing pad 303 may not be necessary, and the ability to raise the temperature of the fixing belt 301 by the heating roller 307 alone may be sufficient.

[0051] [Upper limit of power that can be used by the fixing unit] In the image forming apparatus of this embodiment, the upper limit of the power available to the fixing device 8 varies depending on the situation. For example, the power available to the entire image forming apparatus is 2000 W, of which the upper limit of the power available to the fixing device varies between 1400 W and 1700 W depending on the situation. This is because, for example, the power required for the operation of the image reading unit 2 and the image forming apparatus main body 3 varies between 300 W and 600 W. Here, since the heat output of the halogen heaters 306a and 306b is designed based on a situation where 1700 W can be used, a situation where the upper limit is 1400 W can be said to be a situation where the power available to the fixing device 8 is limited. That is, in this embodiment, 1700 W is the power when the halogen heaters 306a and 306b are each operated at their maximum heat output, i.e., in a situation where there is no power limit. Furthermore, 1400 W is a lower power, i.e., the power when the halogen heaters 306a and 306b are operated under a power limit.

[0052] [Start-up control of the fixing device] Next, we will explain the control when starting up the fixing device 8 in a situation where there is no power limit as described above and a situation where there is a power limit. Starting up the fixing device 8 is an operation in which, when an image formation start signal is input to the control unit 30, power is supplied to the halogen heaters 306a and 306b of the fixing device 8, and the pressure roller 305 is driven to rotate, thereby starting the rotation of the fixing belt 301, and the recording material P is ready to be conveyed to the nip portion N (i.e., printing can start).

[0053] 3 shows the transition of the temperatures of stay 302 and heating roller 307 and the power of halogen heaters 306a and 306b when starting up fixing device 8 under conditions where there are no restrictions on the power available to fixing device 8. Under conditions where there are no restrictions on power, heating roller 307 and stay 302 start heating simultaneously. At this time, the time it takes for heating roller 307 to reach its target temperature (heating roller target temperature) is t1, and the time it takes for stay 302 to reach its target temperature (stay target temperature) is t2. This is the condition under which fixing device 8 of this embodiment can start up the fastest.

[0054] 4 shows the transition of the temperatures of stay 302 and heating roller 307 and the power of halogen heaters 306a and 306b when starting up fixing device 8 under conditions where the power available to fixing device 8 is limited and this embodiment is not applied. Here, heating roller 307 and stay 302 are heated simultaneously. In this case, halogen heater 306b of heating roller 307 is heated at 1100 W, and halogen heater 306a of stay 302 is heated at 300 W. The reason that halogen heater 306a of stay 302 is heated at 300 W is that in order to minimize the time it takes for both stay 302 and heating roller 307 to reach the target temperature, if stay 302, which has a slower temperature rise rate, is not heated preferentially, the time t2 required to reach the target temperature will be longer.

[0055] Furthermore, the halogen heater 306b is composed of three pieces, and after the target temperature is reached, the temperature of the heating roller 307 can be maintained with power of 1100 W or less by energizing two of the halogen heaters 306b. At this time, the time until the heating roller 307 reaches the target temperature is t1', and the time until the stay 302 reaches the target temperature is t2. Here, since the time t1' is longer than the time t1, 2 Even when fixing a toner image on the following plain paper or thin paper, or when the number of sheets on which images are to be formed is small, the time until image formation can begin (printing can begin) is delayed.

[0056] Therefore, in this embodiment, when starting up the fixing device 8, heating of the halogen heater 306b of the heating roller 307 begins before the halogen heater 306a of the stay 302, and after the temperature of the heating roller 307 reaches the target temperature, heating of the halogen heater 306a of the stay 302 begins. Fig. 5 shows the transition of the temperatures of the stay 302 and the heating roller 307 and the power of the halogen heaters 306a and 306b when starting up the fixing device 8 under the condition that the power available to the fixing device 8 is limited and when this embodiment is applied.

[0057] In FIG. 5, when the fixing device 8 is started up, the halogen heater 306b of the heating roller 307 first starts heating at 1400 W (first power). That is, the control unit 30 controls the power supply 40 (FIG. 7), which will be described later, to supply 1400 W of power to the halogen heater 306b when the heating of the halogen heater 306b starts. At this time, the heating of the halogen heater 306a of the stay 302 has not started. Next, when the temperature of the heating roller 307 reaches the target temperature, the halogen heater 306a of the stay 302 starts heating at 300 W, and the power supplied to the halogen heater 306a of the heating roller 307 is set to a power (second power) lower than 1400 W. That is, when the temperature of the heating roller 307 reaches the target temperature, the control unit 30 controls the power supply 40 to supply a power lower than 1400 W to the halogen heater 306b.

[0058] The power control configuration of fixing device 8 of this embodiment will now be described with reference to FIG. 6. FIG. 6 is a control block diagram illustrating a portion of the control configuration of image forming apparatus 1 of this embodiment. Power is supplied to halogen heater 306a of stay 302 and halogen heater 306b of heating roller 307 from power supply 40, which serves as a power supply unit. Power supply 40 is controlled by control unit 30. In temperature control of fixing device 8, control unit 30 controls power supply 40 based on the temperatures detected by thermistors Th1 and Th2 to change the power supplied to halogen heaters 306a and 306b and their lighting ratios, thereby maintaining the temperatures of heating roller 307 and stay 302 at target temperatures.

[0059] The control unit 30 can also execute a first mode in which the fixing device 8 is started up when there is a power limit, and a second mode in which the fixing device 8 is started up when there is no power limit. That is, the first mode is a mode in which the fixing device 8 is started up with power (1400 W in this embodiment) lower than the power required when the halogen heaters 306a and 306b are each made to generate heat at their maximum heat output. The first mode is the control shown in FIG. 5 above. The second mode is a mode in which the fixing device 8 is started up with power (1700 W in this embodiment) required when the halogen heaters 306a and 306b are each made to generate heat at their maximum heat output.

[0060] The second mode is the control shown in FIG. 3 , and when the second mode is executed, the control unit 30 controls the power supply 40 so that the halogen heater 306b of the heating roller 307 and the halogen heater 306a of the stay 302 start heating simultaneously. Note that the term "simultaneous" here refers not only to "simultaneous" in the strict sense, but also to slight timing errors such as signal delays, and also to cases where heating of one heater starts within a predetermined time after heating of the other heater begins while heating of the other heater is in progress. The predetermined time is, for example, 0.1 seconds, 1.0 seconds, or the like, which is the time during which heating of the halogen heaters 306a and 306b can be considered to start approximately simultaneously.

[0061] The startup control of the fixing device 8 according to this embodiment, including the first and second modes, will be described with reference to FIGS. 3 and 5 and using the flowchart of FIG. 7. When the startup operation of the fixing device 8 is initiated, for example by inputting an image formation start signal to the control unit 30, the control unit 30 determines whether sufficient power can be secured to simultaneously heat the heating roller 307 and the stay 302 (S1). Specifically, the control unit 30 determines whether sufficient power (1700 W in this embodiment) can be secured when the halogen heaters 306a and 306b are each operated at their maximum heat output. As described above, if there is no limit to the power available to the fixing device 8, S1 is "Y." If S1 is "Y," the control unit 30 executes the second mode, simultaneously starting heating of the halogen heaters 306a and 306b and controlling the temperatures of the heating roller 307 and the stay 302 (S2), as shown in FIG. 3.

[0062] If S1 is "N," that is, if the power available to the fixing device 8 is limited to 1400 W and 1700 W cannot be secured, the control unit 30 executes the first mode and determines whether the heating roller 307 has reached the target temperature (S3). Since the heating roller 307 has not reached the target temperature in the initial state (Y in S3), the control unit 30 starts heating the halogen heater 306b of the heating roller 307 at 1400 W (first power) as shown in Fig. 5, and does not heat the halogen heater 306a of the stay 302 (S4).

[0063] S4 continues to be executed until heating roller 307 reaches the target temperature, and when heating roller 307 reaches the target temperature (N in S3, after time t1 in FIG. 5 has elapsed), control unit 30 determines whether stay 302 has reached the target temperature (S5). Since stay 302 has not reached the target temperature in the initial state (Y in S5), control unit 30 starts heating halogen heater 306a of stay 302 and controls the temperature of halogen heater 306b of heating roller 307 with minimum power (S6).

[0064] Here, the minimum power of halogen heater 306b is a power (second power) lower than the first power of 1400 W, and is a power that can maintain the temperature of heating roller 307. Furthermore, as shown in Fig. 5, the minimum power of halogen heater 306b is a power that is equal to or less than the upper limit power (1400 W in this embodiment) due to the power restriction, even when the power (300 W in this embodiment) when halogen heater 306a of stay 302 is made to generate heat at the maximum heat output is added. In this embodiment, the lighting ratio of halogen heater 306b when halogen heater 306b is made to generate heat at the minimum power is set lower than the lighting ratio from the start of heating of halogen heater 306b until heating roller 307 reaches the target temperature (while heating at 1400 W). For example, from the time when the halogen heater 306b starts heating until the heating roller 307 reaches the target temperature, the lighting ratio is set to 100%, and after the heating roller 307 reaches the target temperature, the lighting ratio is changed within a range less than 100% depending on the temperature detected by thermistor Th1.

[0065] S6 continues to be executed until the stay 302 reaches the target temperature, and when the stay 302 reaches the target temperature (N in S5, after time t2' in FIG. 5 has elapsed), the control unit 30 controls the temperature of the halogen heater 306b of the heating roller 307 and the halogen heater 306a of the stay 302 at the minimum power (S7). Here, the minimum power of the halogen heater 306a is lower than the power used from the start of heating of the halogen heater 306a until the stay 302 reaches the target temperature, and is power that can maintain the temperature of the stay 302. In this embodiment, the lighting ratio of the halogen heater 306a when the halogen heater 306a is being driven with the minimum power is set lower than the lighting ratio from the start of heating of the halogen heater 306a until the stay 302 reaches the target temperature (while heating at 300 W). For example, the lighting ratio is set to 100% from the start of heating of the halogen heater 306a until the stay 302 reaches the target temperature, and after the stay 302 reaches the target temperature, the lighting ratio is changed within a range less than 100% according to the temperature detected by thermistor Th2.

[0066] In this embodiment, even with power limitations, the time from the start of startup of the fixing device 8 to the start of image formation can be prevented from becoming too long. Specifically, in the first mode, during startup of the fixing device 8, the halogen heater 306b of the heating roller 307 is heated at 1400 W, while the halogen heater 306a of the stay 302 is not heated. As shown in FIG. 5 , the time it takes for the heating roller 307 to reach the target temperature is t1, and the time it takes for the stay 302 to reach the target temperature is t2'. Here, the time t2' for the stay 302 to reach the target temperature is longer than the time t2 shown in FIG. 4 , but the time t1 until image formation (printing) can begin remains unchanged compared to the case shown in FIG. 3 where power is not limited. This is because, as described above, with the configuration of this embodiment, depending on the print content, heating of the stay 302 is not necessarily required. As a result of examining this configuration, t1 was approximately 90 seconds and t1' was approximately 140 seconds, enabling a reduction in startup time of approximately 50 seconds.

[0067] Note that under the conditions of this embodiment, the time until printing can begin is shortened, but since the stay 302 is not heated, it may be difficult to maintain the temperature of the fixing belt 301 when images are continuously formed (printed) on thick paper, which requires a lot of power. That is, as shown in Figures 3 to 5, the temperature of the stay 302 may not reach a temperature at which thick paper can be continuously printed (minimum stay temperature for thick paper). In this case, it is possible to reduce productivity by extending the image formation interval to allow the temperature of the fixing belt 301 to recover midway through continuous image formation, or by putting the image formation operation on hold (print standby) until the temperature of the stay 302 can rise to a predetermined temperature or higher when the number of sheets of thick paper continuously formed with images exceeds the set number.

[0068] [Other embodiments] In the above embodiment, a configuration has been described in which nip portion N is formed by fixing pad 303, stay 302, and pressure roller 305 via fixing belt 301, and halogen heater 306a is disposed on stay 302, and halogen heater 306b is disposed on heating roller 307. However, the configuration in which nip portion N is formed is not limited to fixing pad 303 and stay 302, and nip portion N may also be formed by a roller member or the like. In this case, the roller member corresponds to the second tension member, and a second heater such as a halogen heater is disposed inside the roller member.

[0069] Furthermore, the heating method for the tension members is not limited to the halogen heater 306a in the stay 302 and the halogen heater 306b in the heating roller 307, but may be other methods such as an electromagnetic induction method or a ceramic heater. The number of tension members, each heated by a heater, may be three or more, instead of just two. For example, a heater may also be provided inside the steering roller 308. In the above-described embodiment, the tension members, each heated by a heater, are arranged inside the belt member and heat the belt member by contacting the inner circumferential surface of the belt member. However, tension members having heaters for heating the belt member may be arranged outside the belt member and heat the belt member by contacting the outer circumferential surface of the belt member. In other words, the tension members may be arranged in contact with the inner or outer circumferential surface of the belt member.

[0070] In the above description, the basis weight is 128 g / m when the stay 302 has not yet reached the target temperature. 2 In this example, when printing on thick paper exceeding 128 g / m2, if the number of sheets of continuous image formation (number of prints) on the thick paper is set to a specified number or more, the printing is put on standby until the temperature of the stay 302 can be raised to a predetermined temperature or higher. However, if the temperature of the stay 302 does not reach the target temperature, and the basis weight is 128 g / m2, 2 When printing on thick paper exceeding this limit, the print interval may be increased immediately after printing begins.

[0071] In the above-described embodiment, the two tension members heated by the heaters have different heat capacities. However, for example, the two tension members may have the same heat capacity. In this case, the present invention is applicable if one of the tension members heats the belt member at a faster rate due to differences in the heater heat generation amount or the contact length with the belt member. Furthermore, the relationship between the heat capacities of the two tension members may be reversed from that of the above-described embodiment. In this case, the present invention is applicable if the tension member with a larger heat capacity heats the belt member at a faster rate than the tension member with a smaller heat capacity, for example, because the tension member with a larger heat capacity has a larger heater heat generation amount or a longer contact length with the belt member. In either case, in the first mode, if the heater that heats the tension member (first tension member) with the faster temperature rise rate is designated as the first heater and the heater that heats the tension member (second tension member) with the slower temperature rise rate is designated as the second heater, when starting up the fixing device, the first heater starts heating before the second heater, and the second heater starts heating after the temperature of the first tension member reaches the target temperature.

[0072] In the above-described embodiments, a pressure roller is used as the rotating body. However, the rotating body may be an endless belt that is tensioned by a plurality of tension rollers and driven by one of the tension rollers. In the above-described embodiments, the pressure roller as the rotating body presses against the belt to form the nip, but the belt may also be pressed against the rotating body. [Explanation of symbols]

[0073] 8. Fixing device 30 Control unit 40...Power supply (power supply section) 301 Fixing belt (belt member) 302... Stay (support member) 303 Fixing pad (pad member) 304... Tension unit (second tension member) 305 Pressure roller (rotating body) 306a Halogen heater (secondary heater) 306b Halogen heater (first heater) 307 Heating roller (first tension member)

Claims

1. A fixing device that heats a toner image carried on a recording material and fixes the toner image to the recording material, an endless belt member; a rotating body that contacts the outer peripheral surface of the belt member and forms a nip portion between the rotating body and the belt member to sandwich and convey a recording material; a first tension member that contacts an inner circumferential surface or an outer circumferential surface of the belt member and tensions the belt member; a first heater that heats the first tension member; a second tension member that contacts an inner circumferential surface or an outer circumferential surface of the belt member and tensions the belt member; a second heater that heats the second tension member, the first tension member has a smaller heat capacity than the second tension member; When the fixing device is started up, the first heater starts heating before the second heater, and the second heater starts heating after the temperature of the first tension member reaches a target temperature. A fixing device characterized by:

2. The heat generation amount of the first heater is greater than the heat generation amount of the second heater.

2. The fixing device according to claim 1, wherein the fixing device is a fixing device for fixing a toner image onto a recording medium.

3. In the circumferential direction of the belt member, the length over which the first tension member contacts the belt member is longer than the length over which the second tension member contacts the belt member.

2. The fixing device according to claim 1, wherein the fixing device is a fixing device for fixing a toner image onto a recording medium.

4. the first tension member is a heating roller disposed inside the belt member, The first heater is disposed inside the heating roller.

2. The fixing device according to claim 1, wherein the fixing device is a fixing device for fixing a toner image onto a recording medium.

5. the second tension member includes a pad member disposed inside the belt member so as to sandwich the belt member between itself and the rotating body, and which forms the nip portion between the belt member and the rotating body; and a support member disposed on the opposite side of the pad member from the rotating body, which supports the pad member and has a space formed therein; 2. The fixing device according to claim 1, wherein the second heater is disposed inside the support member.

6. A fixing device that heats a toner image carried on a recording material and fixes the toner image to the recording material, an endless belt member; a rotating body that contacts the outer peripheral surface of the belt member and forms a nip portion between the rotating body and the belt member to sandwich and convey a recording material; a first tension member that contacts an inner circumferential surface or an outer circumferential surface of the belt member and tensions the belt member; a first heater that heats the first tension member; a second tension member that contacts an inner circumferential surface or an outer circumferential surface of the belt member and tensions the belt member; a second heater that heats the second tension member, When a rate at which the temperature of the belt member rises due to heating of the first tension member by the first heater is defined as a first temperature rise rate, and a rate at which the temperature of the belt member rises due to heating of the second tension member by the second heater is defined as a second temperature rise rate, the first temperature rise rate is faster than the second temperature rise rate, When the fixing device is started up, the first heater starts heating before the second heater, and the second heater starts heating after the temperature of the first tension member reaches a target temperature. A fixing device characterized by:

7. an image forming unit that forms a toner image on a recording material; a fixing device that heats a recording material on which a toner image has been formed by the image forming unit, and fixes the toner image to the recording material; a power supply; a control unit that controls the power supply unit, The fixing device an endless belt member; a rotating body that contacts the outer peripheral surface of the belt member and forms a nip portion between the rotating body and the belt member to sandwich and convey a recording material; a first tension member that contacts an inner circumferential surface or an outer circumferential surface of the belt member and tensions the belt member; a first heater that heats the first tension member; a second tension member that contacts an inner circumferential surface or an outer circumferential surface of the belt member and tensions the belt member; a second heater that heats the second tension member, the first tension member has a smaller heat capacity than the second tension member; the power supply unit supplies power to the first heater and the second heater; The control unit a mode in which the fixing device is started up with lower power than the power required when the first heater and the second heater are caused to generate heat at their maximum heat values, When the mode is executed, the power supply unit is controlled so that the first heater starts heating before the second heater starts heating, and the second heater starts heating after the temperature of the first tension member reaches a target temperature. An image forming apparatus characterized by:

8. In the mode, the control unit controls the power supply unit to supply a first power to the first heater when the first heater starts heating, and to supply a second power lower than the first power to the first heater when the temperature of the first tension member reaches a target temperature.

8. The image forming apparatus according to claim 7,

9. the mode is a first mode, The control unit a second mode in which the fixing device is started up with electric power equivalent to that for causing the first heater and the second heater to generate heat at their maximum heat values, When the second mode is executed, the power supply unit is controlled so that the first heater and the second heater start heating simultaneously.

8. The image forming apparatus according to claim 7,

10. The heat generation amount of the first heater is greater than the heat generation amount of the second heater.

8. The image forming apparatus according to claim 7,

11. In the circumferential direction of the belt member, the length over which the first tension member contacts the belt member is longer than the length over which the second tension member contacts the belt member.

8. The image forming apparatus according to claim 7,

12. the first tension member is a heating roller disposed inside the belt member, The first heater is disposed inside the heating roller.

8. The image forming apparatus according to claim 7,

13. the second tension member includes a pad member disposed inside the belt member so as to sandwich the belt member between itself and the rotating body, and which forms the nip portion between the belt member and the rotating body; and a support member disposed on the opposite side of the pad member from the rotating body, which supports the pad member and has a space formed therein; 8. The image forming apparatus according to claim 7, wherein the second heater is disposed inside the support member.

14. an image forming unit that forms a toner image on a recording material; a fixing device that heats a recording material on which a toner image has been formed by the image forming unit, and fixes the toner image to the recording material; a power supply; a control unit that controls the power supply unit, The fixing device an endless belt member; a rotating body that contacts the outer peripheral surface of the belt member and forms a nip portion between the rotating body and the belt member to sandwich and convey a recording material; a first tension member that contacts an inner circumferential surface or an outer circumferential surface of the belt member and tensions the belt member; a first heater that heats the first tension member; a second tension member that contacts an inner circumferential surface or an outer circumferential surface of the belt member and tensions the belt member; a second heater that heats the second tension member, When a rate at which the temperature of the belt member rises due to heating of the first tension member by the first heater is defined as a first temperature rise rate, and a rate at which the temperature of the belt member rises due to heating of the second tension member by the second heater is defined as a second temperature rise rate, the first temperature rise rate is faster than the second temperature rise rate, the power supply unit supplies power to the first heater and the second heater; The control unit a mode in which the fixing device is started up with lower power than the power required when the first heater and the second heater are caused to generate heat at their maximum heat values, When the mode is executed, the power supply unit is controlled so that the first heater starts heating before the second heater starts heating, and the second heater starts heating after the temperature of the first tension member reaches a target temperature. An image forming apparatus characterized by:

Images