JP6003073B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus including a fixing device that thermally fixes a developer image on a recording sheet and a control device that controls the fixing device.

  2. Description of the Related Art Conventionally, there is known an image forming apparatus including a heating member heated by a heat source, a temperature sensor that detects the temperature of the heating member, and a control device that controls the heat source based on the temperature detected by the temperature sensor ( Patent Document 1).

JP 2010-181513 A

  By the way, in the image forming apparatus as described above, when the temperature gradient detected by the temperature sensor is not the assumed gradient, it is conceivable to perform control for determining that the temperature sensor or the control device is abnormal. In this case, the temperature gradient may be compared with a predetermined threshold value. However, since the temperature gradient varies depending on whether the heat source is on or off, it is desirable to change the threshold based on whether the heat source is on or off.

  However, if the threshold value is changed based on ON / OFF of the heat source, in the fixing control for maintaining the temperature of the heating member at the fixing temperature (temperature for thermally fixing the developer image on the recording sheet), the heat source is frequently turned ON / OFF. When it is turned off, the threshold value is frequently switched, and there is a possibility that the abnormality determination is erroneously determined.

  Therefore, an object of the present invention is to improve the accuracy of temperature sensor abnormality determination.

In order to solve the above problems, an image forming apparatus according to the present invention includes a heating member heated by a heat source, a backup member for sandwiching a recording sheet between the heating members, and a temperature sensor that detects the temperature of the heating member. A fixing device, a motor that rotates at least one of the heating member and the backup member, and a control device that controls the heat source and the motor.
The control device is configured to be able to execute abnormality determination control that determines that the temperature sensor, the control device, or the like is abnormal, on the condition that the temperature gradient of the heating member exceeds the upper threshold.
In the abnormality determination control, the control device sets the upper limit threshold to the first upper limit threshold according to the driving of the motor, and sets the upper limit threshold to the second upper limit threshold smaller than the first upper limit threshold according to the motor stop. To do.

  Here, if the heat source is turned on without rotating the backup member, the temperature of the nip portion between the backup member and the heating member increases, and the portion forming the nip portion is damaged. In general, the driving / stopping of the motor substantially corresponds to the ON / OFF of the heat source.

  According to this configuration, the upper threshold value is switched according to the driving / stopping of the motor substantially corresponding to the ON / OFF of the heat source, so that the motor driving state (heat source ON state) and the motor stopping state (heat source OFF state) ) And an upper limit threshold different from each other can be accurately determined. Further, in the mode in which the upper limit threshold value is switched according to the ON / OFF of the heat source, the upper limit threshold value is frequently switched during the fixing control in which the temperature of the heating member is maintained at the fixing temperature. Since the motor continues to be driven, the upper threshold value can be kept constant, and the accuracy of abnormality determination for the temperature sensor, the control device, etc. can be improved.

  In the above-described configuration, the control device further includes a function of determining that the temperature sensor is abnormal in the abnormality determination control on condition that the temperature gradient of the heating member falls below a lower limit threshold, and according to driving of the motor. It is desirable to set the lower limit threshold value to the first lower limit threshold value and set the lower limit threshold value to a second lower limit threshold value that is larger than the first lower limit threshold value according to the stop of the motor.

  According to this, since the lower limit threshold value is referred to in addition to the upper limit threshold value, it is possible to more accurately determine an abnormality such as a temperature sensor or a control device.

  In the abnormality determination control, the control device sets the upper limit threshold to the first upper limit threshold after the first predetermined time from the start of driving of the motor, and sets the upper limit threshold to the first upper limit threshold after the second predetermined time after the motor stops. You may be comprised so that it may set to 2 upper limit threshold values.

  Further, in the abnormality determination control, the control device sets the lower limit threshold to the first lower limit threshold after the third predetermined time from the start of the motor drive, and sets the lower limit threshold to the first lower limit after the fourth predetermined time after the motor stops. You may be comprised so that it may set to 2 lower limit threshold values.

  In the above-described configuration, the heating member may include a cylindrical fixing belt that surrounds the heat source and a nip member that sandwiches the fixing belt with the backup member.

  In the above configuration, the nip member may be made of metal, and the temperature sensor may be configured to detect the temperature of the nip member.

  In the above-described configuration, the control device may be configured to turn on the heat source when receiving a print command, and turn off the heat source until the next print command is received after the print control is completed. Good.

  According to the present invention, it is possible to improve the accuracy of temperature sensor abnormality determination.

1 is a cross-sectional view illustrating a laser printer according to an embodiment of the present invention. 2 is a cross-sectional view illustrating a fixing device. It is a perspective view which shows a nip plate and a temperature sensor. It is a flowchart which shows abnormality determination control. It is a flowchart which shows threshold value setting control.

  Next, embodiments of the present invention will be described in detail with reference to the drawings as appropriate. In the following description, a schematic configuration of a laser printer 1 as an example of an image forming apparatus according to an embodiment of the present invention will be briefly described, and then a fixing device and a control device will be described in detail.

  In the following description, the direction will be described with reference to the user who uses the laser printer 1. That is, the left side in FIG. 1 is “front”, the right side is “rear”, the back side is “left”, and the front side is “right”. Also, the vertical direction in FIG.

<Schematic configuration of laser printer>
As shown in FIG. 1, a laser printer 1 includes a main body housing 2 that feeds a sheet S as an example of a recording sheet, an exposure device 4, and a toner image (developer) on the sheet S. And a fixing device 100 that thermally fixes the toner image on the paper S.

  The paper feed unit 3 is provided at a lower portion in the main body housing 2 and mainly includes a paper feed tray 31, a paper pressing plate 32, and a paper feed mechanism 33. The paper S stored in the paper feed tray 31 is moved upward by the paper pressing plate 32 and supplied toward the process cartridge 5 (between the photosensitive drum 61 and the transfer roller 63) by the paper feed mechanism 33.

  The exposure apparatus 4 is disposed in the upper part of the main body housing 2 and includes a laser light emitting unit (not shown), a polygon mirror, a lens, a reflecting mirror, and the like that are not shown. In this exposure device 4, the surface of the photosensitive drum 61 is exposed by scanning the surface of the photosensitive drum 61 at high speed with laser light (see the chain line) based on the image data emitted from the laser light emitting unit.

  The process cartridge 5 is disposed below the exposure apparatus 4 and is detachably mounted on the main body housing 2 through an opening formed when the front cover 21 provided on the main body housing 2 is opened. Yes. The process cartridge 5 includes a drum unit 6 and a developing unit 7.

  The drum unit 6 mainly includes a photosensitive drum 61, a charger 62, and a transfer roller 63. The developing unit 7 is configured to be detachably attached to the drum unit 6, and includes a developing roller 71, a supply roller 72, a layer thickness regulating blade 73, and a toner that contains toner (developer). The container mainly includes a container 74 and an agitator 75 that stirs the toner in the toner container 74.

  In the process cartridge 5, the surface of the photosensitive drum 61 is uniformly charged by the charger 62 and then exposed by high-speed scanning of the laser light from the exposure device 4, whereby an image is formed on the photosensitive drum 61. An electrostatic latent image based on the data is formed. Further, the toner in the toner container 74 is supplied to the developing roller 71 via the supply roller 72 and enters between the developing roller 71 and the layer thickness regulating blade 73 to form a thin layer of a certain thickness on the developing roller 71. It is carried on.

  The toner carried on the developing roller 71 is supplied from the developing roller 71 to the electrostatic latent image formed on the photosensitive drum 61. As a result, the electrostatic latent image is visualized and a toner image is formed on the photosensitive drum 61. Thereafter, the sheet S is conveyed between the photosensitive drum 61 and the transfer roller 63 so that the toner image on the photosensitive drum 61 is transferred onto the sheet S.

  The fixing device 100 is provided behind the process cartridge 5. The toner image transferred onto the sheet S is thermally fixed onto the sheet S by passing through the fixing device 100. Thereafter, the paper S is discharged onto the paper discharge tray 22 by the transport rollers 23 and 24.

<Detailed configuration of fixing device>
As shown in FIG. 2, the fixing device 100 includes a fixing belt 110 and a nip plate 130 (nip member) as an example of a heating member, a halogen lamp 120 as an example of a heat source, and a pressure roller as an example of a backup member. 140, a reflecting plate 150, and a stay 160.

  The fixing belt 110 is an endless (cylindrical) stainless steel belt having heat resistance and flexibility, and a halogen lamp 120, a nip plate 130, a reflecting plate 150, and a stay 160 are provided therein. ing.

  The halogen lamp 120 is a member that emits radiant heat to heat the toner on the sheet S by heating the nip plate 130 and the fixing belt 110 (nip portion N), and is spaced from the inner surface of the nip plate 130 by a predetermined interval. Has been placed.

  The nip plate 130 is a plate-like member that receives radiant heat from the halogen lamp 120, and is arranged so that the lower surface thereof is in sliding contact with the inner peripheral surface of the fixing belt 110. In the present embodiment, the nip plate 130 is made of metal, and is formed, for example, by bending an aluminum plate or the like having a higher thermal conductivity than a steel stay 160 described later. If the nip plate 130 is made of aluminum, the thermal conductivity of the nip plate 130 can be improved.

  As shown in FIGS. 2 and 3, the nip plate 130 includes a plate-like portion 131, a front bent portion 132, a rear bent portion 133, and three detected portions 134.

  The plate-like portion 131 is a long plate-like member that is orthogonal to the vertical direction and that is long in the left-right direction. The plate-like portion 131 is sandwiched between the pressure belt 140 and the fixing belt 110 in a vertical direction. The nip portion N is formed. The plate-like portion 131 is arranged below the halogen lamp 120 and transmits heat from the halogen lamp 120 to the toner on the paper S via the fixing belt 110.

  Note that the inner surface (upper surface) of the plate-like portion 131 may be painted black or provided with a heat absorbing member. According to this, the radiant heat from the halogen lamp 120 can be efficiently absorbed.

  The front bent portion 132 is formed so as to be bent in a substantially arc shape upward from the front end side (upstream side in a predetermined direction) of the plate-like portion 131, and is disposed so as to face the halogen lamp 120. As a result, the front bent portion 132 is directly heated by the halogen lamp 120, so the sheet S before entering the nip portion N can be preheated (preheated) by the front bent portion 132, and the heat fixing property is improved. It is possible.

  The rear bent portion 133 is formed to extend upward (inward in the radial direction of the fixing belt 110) from the rear edge of the plate-like portion 131. Specifically, the rear bent portion 133 is formed so as to extend from one end side to the other end side in the left-right direction among the rear end edges of the plate-like portion 131. Thereby, the rear side bent portion 133 effectively suppresses the lubricant G adhering to the inner peripheral surface of the fixing belt 110 from flowing into the upper surface of the plate-like portion 131 (the surface on which black coating or the like is applied). Therefore, it is possible to suppress a reduction in heating efficiency of the nip plate 130.

  The three detected portions 134A, 134B, and 134C are portions where temperatures are detected by the side thermistor 400A, the thermostat 400B, and the center thermistor 400C, respectively. The three detected parts 134A, 134B, and 134C are formed so as to extend from a part of the upper end edge 133A of the rear bent part 133 to the rear side. Specifically, two detected portions 134B and 134C are disposed at a substantially central portion of the rear bent portion 133 extending in the left-right direction, and one detected portion 134A is disposed at one end portion on the outer side in the left-right direction.

Further, as shown in FIG. 3, the detected parts 134B and 134C are arranged within the minimum sheet passing range PR in the left-right direction, and the detected part 134A is arranged outside the minimum sheet passing range PR in the left-right direction. ing. Here, the minimum sheet passing range PR refers to a sheet passing range having a minimum width in the left-right direction among the sheets that can be used in the laser printer 1.
Here, the side thermistor 400A and the center thermistor 400C are temperature sensors for transmitting the detected temperature to the control device 510, and the thermostat 400B mechanically transfers to the halogen lamp 120 when the detected temperature exceeds a predetermined temperature. This is a thermal switch that cuts off the power supply.

The side thermistor 400A may be a contact type thermistor that detects the temperature of the detected part 134A by contacting the detected part 134A, or the side thermistor 400A does not contact the detected part 134A. A non-contact thermistor that detects the temperature may be used.
Similarly, the center thermistor 400C may be a contact type thermistor that detects the temperature of the detected part 134C by contacting the detected part 134C, or the detected part 134C without contacting the detected part 134C. It may be a non-contact thermistor that detects the temperature.

  The pressure roller 140 is a member that forms a nip portion N with the fixing belt 110 by sandwiching the fixing belt 110 with the nip plate 130, and is disposed below the nip plate 130. In order to form the nip portion N, one of the nip plate 130 and the pressure roller 140 is urged toward the other. The pressure roller 140 is configured so as to be rotationally driven by a driving force transmitted from a motor 500 (see FIG. 1) provided in the main body housing 2, and is fixed between the pressure roller 140 and the nip plate 130. By rotating with the belt 110 and the sheet S sandwiched, the sheet S rotates with the fixing belt 110 to convey the sheet S backward.

  The reflection plate 150 is a member that reflects the radiant heat from the halogen lamp 120 toward the nip plate 130, and is disposed at a predetermined interval from the halogen lamp 120 so as to surround the halogen lamp 120 inside the fixing belt 110. ing. The reflecting plate 150 is formed by curving an aluminum plate or the like in a U shape in a sectional view, for example, having high infrared and far-infrared reflectance.

  The stay 160 is a member that receives a load from the pressure roller 140 by supporting the nip plate 130 via the reflection plate 150, and is arranged so as to surround the halogen lamp 120 and the reflection plate 150 inside the fixing belt 110. ing. Note that the load referred to here is the reaction force of the force with which the nip plate 130 biases the pressure roller 140 in the configuration in which the nip plate 130 biases the pressure roller 140. Such a stay 160 has relatively high rigidity, for example, is formed by bending a steel plate or the like.

  The halogen lamp 120 of the fixing device 100 configured as described above, the motor 500 for driving the pressure roller 140 and the like are controlled by the control device 510 shown in FIG. The motor 500 supplies driving force to the pressure roller 140 via a gear mechanism (not shown), and also supplies driving force to the developing roller 71, the supply roller 72, and the agitator 75 via a gear mechanism (not shown). Is configured to do. That is, when the motor 500 is driven, the pressure roller 140, the developing roller 71, the supply roller 72, and the agitator 75 are rotated simultaneously.

  In addition, driving / stopping of the motor 500 substantially corresponds to ON / OFF of the halogen lamp 120. That is, when the motor 500 is driven, the halogen lamp 120 is substantially ON, and when the motor 500 is stopped, the halogen lamp 120 is substantially OFF.

<Control device>
Next, the control device 510 will be described in detail.
As shown in FIG. 1, the control device 510 includes, for example, a CPU, a RAM, a ROM, and an input / output circuit. The input from the center thermistor 400C and the side thermistor 400A described above, the contents of the print command, and the ROM The halogen lamp 120 and the motor 500 are controlled by performing arithmetic processing based on stored programs and data. The temperature sensor used in the following control may be either the side thermistor 400A or the center thermistor 400C, but in this embodiment, the temperature sensor used in the following control is the center thermistor 400C.

  The control device 510 is configured to turn on the halogen lamp 120 when receiving a print command, and to turn off the halogen lamp 120 until the next print command is received after the print control is completed. That is, the control device 510 performs a standby mode in which the halogen lamp 120 is maintained at a preparation temperature lower than the fixing temperature (a temperature at which the toner image can be thermally fixed on the paper S) after the end of the print control. It is configured not to run.

  The controller 510 determines that the center thermistor 400C is abnormal on the condition that the temperature gradient of the nip plate 130 detected by the center thermistor 400C exceeds the upper threshold or the temperature gradient is lower than the lower threshold. The determination control is configured to be executable. Then, in the abnormality determination control, control device 510 sets the upper limit threshold to the first upper limit threshold according to driving of motor 500, and sets the upper limit threshold to a second upper limit that is smaller than the first upper limit threshold according to stop of motor 500. The threshold is set.

  That is, since the halogen lamp 120 is substantially ON when the motor 500 is driven, the gradient of the temperature (detected temperature of the center thermistor 400C) tends to increase, and the halogen lamp 120 is substantially OFF when the motor 500 is stopped. For this reason, the gradient of the temperature (the detected temperature of the center thermistor 400C) tends to be small. Therefore, according to this phenomenon, the upper limit threshold value is set to a high value (first upper limit threshold value) when the motor 500 is driven, and is set to a low value (second upper limit threshold value) when the motor 500 is stopped, so that the abnormality determination is accurately performed. It is possible to do. In the present embodiment, the first upper limit threshold is 60 ° C./second, and the second upper limit threshold is 30 ° C./second.

  Control device 510 is configured to set the lower limit threshold to the first lower limit threshold when motor 500 is driven, and to set the lower limit threshold to a second lower limit threshold that is greater than the first lower limit threshold when motor 500 is stopped. Yes. Here, when the motor is driven, the temperature (detected temperature of the center thermistor 400C) tends to be lower because the heat of the nip plate 130 is absorbed by the rotating pressure roller 140 than when the motor is stopped. In the present embodiment, the first lower limit threshold is −40 ° C./sec, and the second lower limit threshold is −25 ° C./sec.

  Therefore, according to this phenomenon, the lower limit threshold value is set to a low value (first lower limit threshold value) when the motor 500 is driven, and is set to a high value (second lower limit threshold value) when the motor 500 is stopped, so that the abnormality determination can be performed accurately. It is possible to do it. Further, when the temperature rises, the ON / OFF state of the halogen lamp 120 is dominant with respect to the temperature change, whereas when the temperature falls, the drive / stop of the motor 500 (pressure roller 140) changes to the temperature change. Each threshold is set as described above because it is dominant.

  Note that the timing for switching between the upper threshold and the lower threshold may be at the time when the motor 500 is driven or stopped, or may be shifted from the time when the driving or stop is performed.

  That is, for example, in the abnormality determination control, the control device 510 sets the upper limit threshold value to the first upper limit threshold after the first predetermined time from the start of driving of the motor 500, and the second predetermined time after the motor 500 stops. The upper limit threshold may be set to the second upper limit threshold later. Further, in the abnormality determination control, control device 510 sets the lower limit threshold to the first lower limit threshold after a third predetermined time after the start of driving of motor 500, and the lower limit after a fourth predetermined time after motor 500 stops. The threshold may be set to the second lower limit threshold.

  Specifically, control device 510 performs control according to the flowcharts shown in FIGS. 4 and 5. The control device 510 always executes the abnormality determination control shown in FIG. 4 while the laser printer 1 is powered on. In this abnormality determination control, first, the control device 510 detects the temperature of the nip plate 130 at least twice with a predetermined time interval by the center thermistor 400C (S1).

  After step S1, control device 510 calculates a gradient based on the detected temperature (S2). After step S2, control device 510 executes threshold setting control for setting the upper and lower thresholds used for abnormality determination control according to the driving state of motor 500 (S3).

  As shown in FIG. 5, in the threshold setting control, the control device 510 first determines whether or not the motor 500 is driven (S11). In step S11, when the motor 500 is driven (Yes), the control device 510 sets the upper limit threshold to the first upper limit threshold TU1 (S12), and sets the lower limit threshold to the first lower limit threshold TD1 ( S13).

  If the motor 500 is stopped in step S11 (No), the control device 510 sets the upper limit threshold to the second upper limit threshold TU2 that is smaller than the first upper limit threshold TU1 (S14), and the lower limit threshold. Is set to a second lower limit threshold value TD2 larger than the first lower limit threshold value TD1 (S15).

  After setting each threshold value in the threshold setting control (after step S3), control device 510 returns to the flowchart of FIG. 4 and determines whether or not the gradient calculated in step S2 is larger than the upper limit threshold value (S4). ). In step S4, when the gradient is equal to or lower than the upper limit threshold value (No), control device 510 determines whether or not the gradient is smaller than the lower limit threshold value (S5).

  In step S5, when the gradient is equal to or greater than the lower limit threshold value (No), control device 510 determines that center thermistor 400C is normal and ends this control. Further, when the gradient is larger than the upper threshold value in Step S4 (Yes) or when the gradient is smaller than the lower threshold value in Step S5 (Yes), control device 510 determines that center thermistor 400C is abnormal. (S6).

According to the above, the following effects can be obtained in the present embodiment.
Since the upper threshold and lower threshold are switched according to the driving / stopping of the motor 500 that substantially corresponds to the ON / OFF of the halogen lamp 120, the driving state of the motor 500 (ON state of the halogen lamp 120) and the stopping state of the motor 500 ( The abnormality of the center thermistor 400C (or the control device 510) can be accurately determined using a threshold value that is different from that in the halogen lamp 120 OFF state.

  Further, compared to a mode in which the threshold value is switched according to whether the halogen lamp is turned on or off, for example, according to the configuration of the present embodiment, the motor 500 continues to be driven during the fixing control, so that each threshold value can be maintained constant. The accuracy of the abnormality determination of the center thermistor 400C can be improved.

  Since the abnormality determination of the center thermistor 400C is performed with reference to the lower threshold in addition to the upper threshold, the abnormality of the center thermistor 400C can be determined more accurately.

In addition, this invention is not limited to the said embodiment, It can utilize with various forms so that it may illustrate below.
In the above embodiment, the abnormality determination control is executed using both the upper threshold and the lower threshold. However, the present invention is not limited to this. For example, the abnormality determination control may be executed using only the upper threshold.

  In the embodiment, the halogen lamp 120 is illustrated as an example of the heat source. However, the present invention is not limited to this, and may be, for example, a heating resistor or an IH heat source. Here, the IH heat source does not generate heat by itself, but generates heat from a roller or a metal belt by an electromagnetic induction heating method.

  In the above-described embodiment, the fixing belt 110 and the nip plate 130 are illustrated as an example of the heating member. However, the present invention is not limited to this, and for example, a heating roller that is a metal tube thicker than the fixing belt 110 may be used. Good.

  In the embodiment, the pressure roller 140 (backup member) is rotated by the motor 500. However, the present invention is not limited to this, and the motor may rotate at least one of the backup member and the heating member. For example, when the heating member is a heating roller, the heating roller may be driven by a motor.

  In the above embodiment, the present invention is applied to the laser printer 1. However, the present invention is not limited to this, and the present invention may be applied to other image forming apparatuses such as a copying machine and a multifunction machine.

  In the embodiment, the paper S such as a thick paper, a postcard, and a thin paper is used as an example of the recording sheet. However, the present invention is not limited to this, and may be an OHP sheet, for example.

  In the embodiment, the pressure roller 140 is exemplified as the backup member. However, the present invention is not limited to this, and may be, for example, a belt-like pressure member.

  In addition, the control apparatus which controls a heat source, the control apparatus which controls a motor, and the control apparatus which performs abnormality determination may be separate, and may be comprised as one control apparatus.

  In the embodiment, the nip plate 130 is illustrated as an example of the nip member, but the present invention is not limited to this, and may be a thick member that is not plate-shaped, for example.

  In the above embodiment, the abnormality determination control is always executed. However, the present invention is not limited to this, and the abnormality determination control may be executed only in a predetermined temperature range, for example.

DESCRIPTION OF SYMBOLS 1 Laser printer 100 Fixing apparatus 110 Fixing belt 120 Halogen lamp 130 Nip plate 140 Pressure roller 400 Temperature sensor 500 Motor 510 Control apparatus S Paper TU1 1st upper limit threshold TU2 2nd upper limit threshold

Claims (7)

A fixing device having a heating member heated by a heat source, a backup member for sandwiching a recording sheet between the heating member, and a temperature sensor for detecting the temperature of the heating member;
A motor for rotating at least one of the heating member and the backup member;
A control device for controlling the heat source and the motor,
An image forming apparatus capable of executing abnormality determination control for determining that the temperature sensor is abnormal, on the condition that the temperature gradient that is a temperature change per unit time of the heating member exceeds an upper limit threshold. There,
In the abnormality determination control, the control device sets the upper limit threshold to a positive first upper limit threshold when the motor is driven while the heat source is substantially ON, and the heat source is approximately OFF. An image forming apparatus, wherein when the motor is stopped , the upper limit threshold value is set to a second upper limit threshold value that is a positive value smaller than the first upper limit threshold value . The controller is
In the abnormality determination control, further comprising an abnormality and determining function of the temperature sensor that the gradient of the temperature of the heating member is below a lower threshold as a condition,
The lower limit threshold is set to a first lower limit threshold according to the driving of the motor, and the lower limit threshold is set to a second lower limit threshold that is larger than the first lower limit threshold according to the stop of the motor. The image forming apparatus according to claim 1.   In the abnormality determination control, the control device sets the upper limit threshold to the first upper limit threshold after a first predetermined time after the start of driving of the motor, and after a second predetermined time after the motor stops. The image forming apparatus according to claim 1, wherein the upper limit threshold is set to the second upper limit threshold.   In the abnormality determination control, the control device sets the lower limit threshold to the first lower limit threshold after a third predetermined time from the start of driving of the motor, and after a fourth predetermined time after the motor stops. The image forming apparatus according to claim 2, wherein the lower limit threshold is set to the second lower limit threshold.   The heating member includes a cylindrical fixing belt that surrounds the heat source, and a nip member that sandwiches the fixing belt with the backup member. The image forming apparatus according to claim 1. The nip member is made of metal,
The image forming apparatus according to claim 5, wherein the temperature sensor detects a temperature of the nip member.   The control device turns on the heat source when receiving a print command, and turns off the heat source until the next print command is received after the print control is completed. The image forming apparatus according to any one of 6.

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