Patent Publication Number: US-8983326-B2

Title: Image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2012-030510 filed on Feb. 15, 2012, the entire subject matter of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     This disclosure relates to an image forming apparatus including a control unit for controlling a heat source and a feeding unit. 
     BACKGROUND 
     It is known that an image forming apparatus includes a heating member that is heated by a heat source, and a backup member that forms a nip portion between the backup member and the heating member, and includes a heating unit that heats a recording sheet in the nip portion, and a feeding unit that feeds the recording sheet toward the nip portion (refer to JP-A-2007-261154). Specifically, in this technology, the control unit is configured to appropriately control the feeding unit when performing duplex printing, thereby alternately conveying recording sheets at a first interval and a second interval smaller than the first interval. 
     SUMMARY 
     As a method of controlling the heat source of the above-mentioned heating unit, there is a method, in which the temperature of the heating member is detected, in which the output of the heat source is decreased in a case where the temperature of the heating member exceeds a target temperature, and in which the output of the heat source is increased in a case where the temperature of the heating member is equal to or lower than the target temperature. However, in a case of applying this method to the above-mentioned technology, if two recording sheets are conveyed to the nip portion at the small second interval while the output of the heat source decreases, heat from the nip portion may be taken away by the preceding recording sheet such that the temperature of the nip portion excessively decreases, so that heating on the subsequent recording sheet following immediately after the preceding recording sheet may not be sufficiently performed. 
     In view of the above, this disclosure provides at least an improvement of heating performance in an image forming apparatus which conveys recording sheets at a first interval and a second interval smaller than the first interval. 
     An image forming apparatus of this disclosure may include: a heating unit including: a heating member; a heat source configured to heat the heating member; and a backup member facing the heating member, a nip portion being defined between the heating member and a backup member; a feeding unit configured to feed a recording sheet toward the nip portion; and a control unit. The control unit is configured to: control the feeding unit such that each of a plurality of recording sheets is fed to the nip portion at one of a first interval and a second interval smaller than the first interval, some of the plurality of the recording sheets being sequentially conveyed at the second interval to each other, a first recording sheet of the some of the plurality of the recording sheets being fed to have the first interval with respect to a previously conveyed recording sheet, and increase the output of the heat source when a front end of the first recording sheet reaches a periphery of the nip portion. 
     Here, ‘a periphery of the nip portion’ includes even positions slightly distant from the nip portion toward the upstream side or downstream side in a conveyance direction. 
     According to this configuration, the output of the heat source increases on the timing when the front end of the first recording sheet of the some of the plurality of recording sheets conveyed at the small second interval reaches the periphery of the nip portion. Therefore, it is possible to suppress the temperature of the nip portion from excessively decreasing due to the first recording sheet, and it is possible to heat (thermally fix) the second recording sheet well. 
     According to this disclosure, it is possible to improve heating performance in an image forming apparatus which conveys recording sheets at a first interval and a second interval smaller than the first interval. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and additional features and characteristics of this disclosure will become more apparent from the following detailed descriptions considered with the reference to the accompanying drawings, wherein: 
         FIG. 1  is a sectional side view illustrating an embodiment of a laser printer as an example of an image forming apparatus of this disclosure; 
         FIGS. 2A to 2C  are views illustrating a control to alternately convey a plurality of sheets at a first interval and a second interval; 
         FIG. 3  is a time chart illustrating changes of individual parameters during duplex printing; 
         FIG. 4  is a flow chart illustrating the operation of a control unit during duplex printing; 
         FIG. 5  is a flow chart illustrating a method of setting a target temperature during duplex printing; and 
         FIG. 6  is a view illustrating a modification of a heating member. 
     
    
    
     DETAILED DESCRIPTION 
     Overall Configuration of Laser Printer 
     First, the overall configuration of a laser printer  1  will be described in brief as an example of an image forming apparatus of this disclosure. 
     In the following description, directions of the laser printer  1  refer to the directions as seen from a user facing the laser printer during its use. To be more specific, referring to  FIG. 1 , a right-side direction and a left-side direction of the drawing sheet are referred to as a “front side” and a “rear side” of the laser printer, respectively. Also, a direction toward the viewer of  FIG. 1  is referred to as a “left side”, and a direction away from a viewer of  FIG. 1  as a “right side”. An upper direction and a lower direction in  FIG. 1  are referred to as an “upper-lower direction”. 
     As shown in  FIG. 1 , the laser printer  1  includes a feeder unit  4  and an image forming unit  5  inside a main body  2 . The feeder unit  4  feeds a sheet  3  which is an example of a recording sheet, and the image forming unit  5  forms an image onto the fed sheet  3 . 
     The feeder unit  4  includes a sheet feed tray  11  which is mounted at the bottom of the inside of the main body  2  such that the sheet feed tray is detachable, and a sheet pressing plate  12  that is provided inside the sheet feed tray  11 . Further, the feeder unit  4  includes a sheet feeding roller  13  and a sheet feeding pad  14  that are provided on the front end portion of the sheet feed tray  11 , and paper dust removing rollers  15  and  16  that are provided on the downstream side relative to the sheet feeding roller  13  in the conveyance direction of the sheet  3 . Furthermore, the feeder unit  4  includes registration rollers  17  that are provided on the downstream side relative to the paper dust removing rollers  15  and  16 . 
     Between the registration rollers  17  and a photosensitive drum  33  to be described below, a sheet passage sensor  90  for detecting existence or non-existence of the sheet  3  is provided. Also, the sheet passage sensor  90  has a known structure. In brief, the sheet passage sensor  90  is configured by a detecting arm  91  movable by abutting on the sheet  3 , and an optical sensor for detecting movement of the detecting arm. 
     In the feeder unit  4 , the sheets  3  in the sheet feed tray  11  are pulled toward the sheet feeding roller  13  by the sheet pressing plate  12 , are fed one by one by the sheet feeding roller  13  and the sheet feeding pad  14 , and are conveyed to the image forming unit  5  through various rollers  13  to  16 . 
     The image forming unit  5  includes a scanner unit  20 , a processing cartridge  30 , and a fixing device  40  which is an example of a heating unit. 
     The scanner unit  20  is provided at the upper portion of the inside of the main body  2  and includes a laser-beam emitting unit (not shown), a polygon mirror  21  which is driven to rotate, lenses  22  and  23 , and reflective mirrors  24 ,  25 , and  26 . Further, in the scanner unit  20 , a laser beam is irradiated onto the surface of the photosensitive drum  33  of the inside of the processing cartridge  30  through a path shown by a chain line in  FIG. 1 , so that high-speed scanning is performed. 
     The processing cartridge  30  is provided below the scanner unit  20 , and it is attachable to and detachable from the main body  2 . Further, the processing cartridge  30  includes the photosensitive drum  33 , a scorotron charger  34 , a transfer roller  35 , a developing roller  36 , a layer-thickness regulating blade  37 , a feeding roller  38 , and a toner hopper  39 . 
     In the processing cartridge  30 , the surface of the photosensitive drum  33  charged by the scorotron charger  34  is exposed by the laser beam from the scanner unit  20 , so that an electrostatic latent image is formed on the photosensitive drum  33 . Toner in the toner hopper  39  is fed to the electrostatic latent image through the feeding roller  38  and the developing roller  36 , so that a toner image is formed on the photosensitive drum  33 . Then, when the sheet  3  is conveyed between the photosensitive drum  33  and the transfer roller  35 , the toner image carried on the photosensitive drum  33  is transferred onto the sheet  3 , so that an image is formed on the sheet  3 . 
     The fixing device  40  is a device for fixing thermally the toner image transferred on the sheet  3 , and it is disposed on the downstream side of the processing cartridge  30  and includes a heating roller  41  which is an example of a heating member, a pressing roller  42  which is an example of a backup member, and a center thermistor  44  which is an example of a temperature detecting member. 
     The heating roller  41  is a cylindrical member for heating the sheet  3 , and it is configured to be heated by a halogen lamp  43  which is an example of a heat source provided inside the heating roller  41 . 
     The pressing roller  42  is disposed to face the heating roller  41 , and it is pressed toward the heating roller  41  such that a nip portion is formed between the pressing roller  42  and the heating roller  41 . 
     The center thermistor  44  is a sensor for detecting the temperature of the center portion of the heating roller  41 , and it is provided to face the center portion of the heating roller  41 . 
     In the fixing device  40 , when the sheet  3  passes through the nip portion between the heating roller  41  and the pressing roller  42 , the toner image on the sheet  3  is thermally fixed by the heating roller  41 . Then, the sheet  3  thermally fixed by the fixing device  40  is discharged to a sheet discharge tray  53  of the outside of the main body  2  by a discharging roller  52  rotating normally. 
     Also, during duplex printing, the discharging roller  52  rotates reversely before the entire sheet  3  is discharged onto the sheet discharge tray  53 , thereby returning the sheet into the main body  2 . The sheet  3  returned into the main body  2  is sent to a double-side conveyance path unit  60  through the rear side of the fixing device  40  by switching of a flapper  54 . 
     The double-side conveyance path unit  60  is a device for double-side conveyance, and it is disposed between the fixing device  40  and the processing cartridge  30 , and the sheet feed tray  11 . Here, ‘the double-side conveyance’ means a conveyance which is performed to return the sheet  3  to the upstream side of the processing cartridge  30  with the sheet  3  upside down, thereby performing the rear surface of the sheet  3  whose front surface has been printed. 
     The double-side conveyance path unit  60  includes a guide member  61  that switches the direction of the sheet  3  conveyed downward through the rear side of the fixing device  40  to the front side, and multiple returning rollers  62  that are arranged in the front/rear direction for returning the sheet  3  guided by the guide member  61  to the upstream side of the photosensitive drum  33  (the registration rollers  17 ). Then, the sheet  3  discharged from the double-side conveyance path unit  60  is guided toward the registration rollers  17  with the sheet  3  upside down by a guide  55  which is on the front side of the double-side conveyance path unit  60 . Therefore, after the front end of the sheet  3  is straightened by the registration rollers  17 , the sheet  3  is sent to the photosensitive drum  33 , and then the toner image of the photosensitive drum  33  is transferred onto the rear surface of the sheet  3 . 
     In other words, in the present embodiment, a feeding unit  70  for feeding the sheet  3  toward the nip portion between the heating roller  41  and the pressing roller  42  is configured by the above-mentioned feeder unit  4 , the double-side conveyance path unit  60 , and other rollers (such as the discharging roller  52 ) contributing to the conveyance of the sheet  3 . Further, the feeding unit  70  and the halogen lamp  43  of the fixing device  40  are controlled by a control unit  80 . 
     Control Unit 
     Now, the control unit  80  will be described in detail. 
     As shown in  FIG. 1 , the control unit  80  includes, for example, a CPU, a RAM, a ROM, and an input/output circuit, and performs arithmetic processing based on inputs from the above-mentioned sheet passage sensor  90  and the center thermistor  44 , the contents of a print instruction, programs and data stored in the ROM, and the like, thereby performing print control, control on the feeding unit  70 , and the halogen lamp  43 , and the like. 
     Specifically, the control unit  80  is configured to control the feeding unit  70  such that the plurality of sheets  3  is fed to the nip portion at a first interval and a second interval smaller than the first interval. In detail, the control unit  80  is configured to alternately perform a conveyance at the first interval and a conveyance at the second interval when performing duplex printing on the sheets  3 . 
     This control will be described below with reference to  FIGS. 2A to 2C  in brief, not in detail. 
     As shown in  FIG. 2A , if receiving an instruction to perform duplex on the plurality of sheets  3 , the control unit  80  first controls the feeding unit  70  such that a first sheet  3 A is conveyed from the sheet feeding roller  13  toward the nip portion of the fixing device  40 . If printing on the front surface of the first sheet  3 A terminates, the control unit  80  controls the feeding unit  70  such that the sheet  3 A passes a route shown by a broken line in  FIG. 2A , so that it proceeds rear-surface print control. 
     Therefore, a time interval from the rear end of the sheet  3 A conveyed for front-surface print being passed the sheet passage sensor  90  such that the sheet passage sensor  90  is turned off to the front end of the sheet  3 A re-conveyed for rear-surface print being reached the sheet passage sensor  90  as shown in  FIG. 2B  such that the sheet passage sensor  90  is turned on is lengthen. 
     In other words, an interval (sheet interval) between the rear end of the sheet  3 A conveyed for front-surface print and the front end of the sheet  3 A newly re-conveyed for rear-surface print is set to a very long first interval T 1  (see  FIG. 3 ) corresponding to the route shown by the broken line. Here, the sheet interval is intended to include not only an interval between two sheets  3  actually conveyed but also an interval between the rear end of the sheet  3 A conveyed for front-surface print and the front end of the sheet  3 A conveyed for rear-surface print as described above. 
     Next, the control unit  80  controls the feeding unit  70  such that the a second sheet  3 B is conveyed from the sheet feeding roller  13  toward the nip portion of the fixing device  40  at an appropriate timing during the rear-surface print on the first sheet  3 A. Therefore, a sheet interval between the rear end of the first sheet  3 A for rear-surface print and the front end of the second sheet  3 B for front-surface print is set to a second interval T 2  (see  FIG. 3 ) smaller than the first interval T 1 . 
     In the following description, for the sake of convenience, the first to last sheets  3  for front-surface print are referred to as sheets SX 1 , SX 2 , . . . , and the first to last sheets  3  for rear-surface print are referred to as sheets DX 1 , DX 2 , . . . . 
     Also, as shown in  FIG. 3 , the control unit  80  is configured to increase the output of the halogen lamp  43  in a case where the current value of the detected temperature detected by the center thermistor  44  is lower than the target temperature (times t 4  and t 11 ), and decrease the output of the halogen lamp  43  in a case where the current value is higher than the target temperature (times t 3  and t 10 ). 
     Specifically, in the case where the detected temperature is higher than the target temperature, the control unit  80  turns off the halogen lamp  43 , and in the case where the detected temperature is lower than the target temperature, the control unit  80  performs duty control to increase or decrease the output based on a difference between the detected temperature and the target temperature. For example, in a case where the difference between the detected temperature and the target temperature is equal to or greater than a predetermined value, the control unit  80  performs control such that the duty ratio becomes 80%, and in a case where the difference between the detected temperature and the target temperature is less than the predetermined value, the control unit  80  performs control such that the duty ratio becomes 60%. 
     Further, the control unit  80  is configured to increase the output of the halogen lamp  43  at the timing (around a time t 5 ) when the front end of a first sheet (DX 1 ) of two sheets (DX 1  and SX 2 ) sequentially conveyed at the second interval T 2  with the first interval T 1  relative to the previously conveyed sheet (for example, SX 1 ) reaches the periphery of the nip portion. 
     Therefore, the output of the halogen lamp  43  increases on the timing (around the time t 5 ) when the front end of the first sheet (DX 1 ) of the two sheets (DX 1  and SX 2 ) conveyed at the small second interval T 2  reaches the periphery of the nip portion. As a result, it is possible to suppress the temperature of the nip portion from excessively decreasing by the first sheet (DX 1 ), and it is possible to thermally fix the second sheet (SX 2 ) well. 
     In  FIG. 3 , a times t 1 , t 5 , or t 7  represents a time when the front end of a sheet  3  reaches the nip portion, that is, a time that is after a time a from when the front end of the sheet  3  contacts the sheet passage sensor  90  and the sheet passage sensor  90  is turned on and to when the front end reaches the nip portion. Also, a time t 2 , t 6 , or t 8  represents a time when the rear end of a sheet  3  reaches the nip portion, that is, a time that is after a time β from when the rear end of the sheet  3  leaves the sheet passage sensor  90  and the sheet passage sensor  90  is turned off and to when the rear end reaches the nip portion. 
     Specifically, in the present embodiment, the control unit  80  is configured to increase the output of the halogen lamp  43  (the time t 4 ) immediately before the time (the time t 5 ) when the front end of the first sheet (DX 1 ) reaches the nip portion. 
     Therefore, for example, as compared to a form in which the output of the halogen lamp  43  increases immediately after the front end of the first sheet (DX 1 ) passes through the nip portion, it is possible to further suppress the temperature of the nip portion from excessively decreasing by the first sheet (DX 1 ). Also, in the present embodiment, the control unit  80  raises (changes) the target temperature at the time t 4 , thereby increasing (changing) the output of the halogen lamp  43 . 
     Also, the control unit  80  is configured to decrease the output of the halogen lamp  43  (target temperature) on a timing when the rear end of the first sheet (DX 1 ) reaches the periphery of the nip portion (around a time t 6 ). In other words, the control unit  80  steps down the target temperature having been raised at the time t 4  (to a temperature higher than an initial value) at the time t 6 , thereby performing control such that the output of the halogen lamp  43  decreases. 
     Also, in the example shown in  FIG. 3 , at the timing (around the time t 6 ) when the rear end of the first sheet (DX 1 ) reaches the periphery of the nip portion, since the detected temperature is sufficiently low, the actual output of the halogen lamp  43  does not decease. If the detected temperature around the time t 6  is high, the above-mentioned configuration is effective. In other words, if the detected temperature around the time t 6  is slightly lower than the target temperature having been raised at the time t 4 , the target temperature is stepped down, so that the target temperature becomes lower than the detected temperature, and thus it is possible to decrease the output of the halogen lamp  43 . Therefore; it is possible to suppress the temperature of the nip portion from overshooting after the rear end of the first sheet (DX 1 ) leaves the nip portion. 
     Also, the control unit  80  is configured to decease the output of the halogen lamp  43  (target temperature) on a timing (around the time t 8 ) when the rear end of the last sheet (SX 2 ) of the two sheets (DX 1  and SX 2 ) sequentially conveyed at the second interval T 2  reaches the periphery of the nip portion. Therefore, it is possible to suppress the temperature of the nip portion from overshooting after the rear end of the last sheet (SX 2 ) leaves the nip portion (after the time t 8 ). 
     Specifically, in the present embodiment, the control unit  80  is configured to decrease the output of the halogen lamp  43  after the rear end of the last sheet (SX 2 ) of the two sheets (DX 1  and SX 2 ) sequentially conveyed at the second interval T 2  leaves the nip portion (a time t 9 ). Therefore, the output of the halogen lamp  43  does not decrease until the rear end of the last sheet (SX 2 ) passes through the nip portion, and thus it is possible to surely suppress fixation defects. 
     Also, in the present embodiment, the target temperature lowers to the initial value at the time t 9 , so that the output of the halogen lamp  43  decreases. In other words, the control unit  80  is configured to gradually (stepwise) lower the target temperature to the initial value as the target temperature having been greatly changed from the initial value at the time t 4  lowers. 
     Therefore, as compared to a form in which the target temperature lowers to the initial value at once, for example, even in a case where the halogen lamp  43  is turned off before the last sheet (SX 2 ) enters the nip portion, the temperature of the nip portion lowering while the last sheet (SX 2 ) passes through the nip portion reaches a second target value (a target value between the initial value and a maximum value) before reaching the initial value of the target temperature, and thus it is possible to early turn on the halogen lamp  43  again. Therefore, it is possible to suppress fixation defects. 
     The control unit  80  configured as described above performs control according to flow charts shown in  FIGS. 4 and 5 . During print control, the control unit  80  repeatedly performs the flow charts shown in  FIGS. 4 and 5 . 
     In the control of  FIG. 4 , first, the control unit  80  detects the temperature by the center thermistor  44  (step S 1 ). After step S 1 , the control unit  80  determines whether the detected temperature is higher than the target temperature (step S 2 ). The target temperature can be appropriately changed by the flow chart of  FIG. 5  to be described below. 
     In a case where the detected temperature is higher than the target temperature in step S 2  (Yes), the control unit  80  decreases the output of the halogen lamp  43  in step S 3 , and terminates the present control. Meanwhile, in a case where the detected temperature is not higher than the target temperature in step S 2  (No), the control unit  80  increases the output of the halogen lamp  43  and terminates the present control (step S 4 ). Specifically, in step S 4 , the control unit  80  necessarily increases the output at the time when the target temperature becomes the target temperature for the first time, and after that controls the output of the halogen lamp  43  according to the above-mentioned duty control. 
     In the control of  FIG. 5 , first, the control unit  80  determines whether the print instruction is an instruction to perform duplex printing on the plurality of sheets  3  (step S 11 ). In a case where the print instruction is for duplex printing in step S 11  (Yes), the control unit  80  determines whether the front end of the sheet DX 1  for rear-surface print has reached the sheet passage sensor  90  (step S 12 ). 
     Here, the determination of step S 12  can be performed, for example, based on the ON/OFF history of the sheet passage sensor  90  after receiving the duplex printing instruction. 
     In a case where it is determined in step S 12  that the sheet DX 1  has reached the sheet passage sensor  90  (Yes), the control unit  80  raises the target temperature after a first time from that time point (in other words, immediately before the front end of the sheet DX 1  reaches the nip portion)(step S 13 ). After step S 13 , the control unit  80  steps down the target temperature after a second time from that time point (in other words, after the rear end of the sheet DX 1  leaves the nip portion) (step S 14 ). 
     After step S 14 , the control unit  80  returns the target temperature to the initial value after a third time from that time point (in other words, after the rear end of the sheet SX 2  leaves the nip portion) (step S 15 ). After step S 15 , the control unit  80  determines whether duplex printing on all of the plurality of sheets  3  has terminated (step S 16 ). 
     In a case where the duplex printing has not entirely terminated in step S 16  (No), or in a case where the detection result of step S 12  is ‘No’, the control unit  80  returns to the process of step S 12 . In a case where the duplex printing has entirely terminated in step S 16  (Yes), or in a case where the detection result of step S 11  is ‘No’, the control unit  80  terminates the present control. 
     Also, this disclosure is not limited to the above-mentioned embodiment, but can be used in various forms as exemplified below. 
     In the present embodiment, the control unit  80  grasps the timing when the front end of the sheet DX 1  for rear-surface print will reach the nip portion, by the processes of steps S 12  and S 13 . However, this disclosure is not limited thereto. For example, since the control unit controls the double-side conveyance path unit  60 , the registration rollers  17 , and the like for conveying the sheet DX 1  for rear-surface print, the control unit may determine the timing when the sheet DX 1  for rear-surface print will reach the nip portion, for example, by the time from when the double-side conveyance path unit  60  is switched from a stop state to a driven state, or based on the stop/rotation history of the registration rollers  17 . 
     In the present embodiment, the number of sheets  3  to be sequentially conveyed at the second interval T 2  has been set to 2. However, this disclosure is not limited thereto. This disclosure can be applied to a form in which three or more sheets are conveyed at the second interval. 
     In the above-mentioned embodiment, as an example of the heat source, the halogen lamp  120  has been exemplified. However, this disclosure is not limited thereto. The heat source may be, for example, a heat element, an IH heat source, or the like. Here, the IH heat source refers to a heat source which does not produce heat by itself but makes a roller or a metal belt produce heat according to an electromagnetic-induction heating scheme. 
     In the above-mentioned embodiment, as the heating member, the heating roller  41  has been exemplified. However, this disclosure is not limited thereto. For example, the heating member may be a nip plate  83  which is heated by the halogen lamp  82  as shown in  FIG. 6 . This disclosure is specifically effective in a belt fixing scheme in which the heat capacity of a heating unit (the nip plate  83 ) as shown in  FIG. 6  is small. Hereinafter, a fixing device  8  shown in  FIG. 6  will be described in brief. 
     The fixing device  8  mainly includes a fixing belt  81 , the halogen lamp  82 , the nip plate  83 , a reflective plate  84 , a backup roller  85 , a stay  86 , and a thermistor  87 . 
     The fixing belt  81  is an endless (cylindrical) belt having heat resistance and flexibility, and both end portions of the fixing belt  81  are guided to be rotatable by a guide member (now shown). 
     The halogen lamp  82  is a heater for generating heat by electricity and heating the nip plate  83  and the fixing belt  81 , thereby heating toner transferred on a sheet S, and it is disposed inside the fixing belt  81 . 
     The nip plate  83  is a plate-like member which receives radiant heat from the halogen lamp  120 , and it is disposed to be in sliding contact with the inner circumferential surface of the fixing belt  81 . In order to transfer the radiant heat received from the halogen lamp  82  to the toner on the sheet S through the fixing belt  81 , for example, the nip plate  83  is made of an aluminum plate or the like having high heat conductivity. Also, between the nip plate  83  and the fixing belt  81 , heat-resistant lubricant (not shown) such as fluorine grease is held to reduce friction between the nip plate  83  and the fixing belt  81  rotating. 
     The reflective plate  84  is a member for reflecting the radiant heat from the halogen lamp  82  toward the nip plate  83 , and it is disposed on the inside of the fixing belt  81  to surround the halogen lamp  82 . The reflective plate  84  is formed by bending, for example, an aluminum plate having high reflectivity for infrared rays and far infrared rays, almost in a U shape in a cross-sectional view. 
     The backup roller  85  is a roller for pressing toner transferred on the sheet S while conveying the sheet S between the backup roller  85  and the fixing belt  81 , and it is disposed below the nip plate  83  with the fixing belt  81  interposed therebetween. The backup roller  85  is rotated by a driving force transmitted from a motor (not shown) provided inside the main body  2 , and rotates the fixing belt  81  by a frictional force with the fixing belt  81  (or the sheet S) at that time. 
     The stay  86  is a member for supporting the nip plate  83  through the reflective plate  84 , thereby securing the rigidity of the nip plate  83  receiving a load from the backup roller  85 , and it is disposed to cover the reflective plate  84 . The stay  86  is formed by bending a material having relatively high rigidity, for example, a steel plate. 
     The thermistor  87  is a member for detecting the temperature of the fixing device  8 , and it is disposed inside the fixing belt  81  of the fixing device  8 . More specifically, the thermistor  87  is disposed to face the nip plate  83  whose temperature detection surface is heated by the halogen lamp  82 , and detects the temperature of the nip plate  83  as the temperature of the fixing device  8 . A detection signal of the thermistor  87  is output to a control unit  80  configured like the above-mentioned embodiment. 
     In the above-mentioned embodiment, as the backup member, the pressing roller  42  has been exemplified. However, this disclosure is not limited thereto. For example, the backup member may be a belt-like pressing member or the like. 
     In the above-mentioned embodiment, as an example of the recording sheet, the sheets  3  such as thick sheet, card, and thin sheet have been used. However, this disclosure is not limited thereto. For example, the recording sheet may be an OHP sheet. 
     In the above-mentioned embodiment, the output of the halogen lamp  43  increases immediately before the front end of the first sheet DX 1  of the two sheets  3  conveyed at the second interval T 2  reaches the nip portion. However, this disclosure is not limited thereto. The output of the heat source may increase when the front end reaches the nip portion, or immediately after the front end reaches the nip portion, or the like. Also, similarly, the output of the heat source may decrease immediately before the rear end of the last sheet SX 1  of the two sheets  3  conveyed at the second interval T 2  reaches the nip portion, or when the rear end reaches the nip portion, or the like. 
     In the above-mentioned embodiment, as an example of the temperature detecting member, the center thermistor  44  has been exemplified. However, this disclosure is not limited thereto. The temperature detecting member may be a side thermistor for detecting, for example, the temperature of an end portion of the heating roller, or the like. 
     In the above-mentioned embodiment, the target temperature is stepwise lowered to the initial value. However, this disclosure is not limited thereto. For example, the target temperature may be lowered to the initial value with a predetermined gradient. 
     In the above-mentioned embodiment, this disclosure has been applied to the laser printer  1 . However, this disclosure is not limited thereto. This disclosure may be applied to other image forming apparatuses, for example, copy machines, multi-function apparatuses, and so on.