Patent Publication Number: US-11644771-B2

Title: Image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present disclosure relates to an image forming apparatus having a fixing device to fix an unfixed toner image on a recording material such as a sheet. 
     Description of the Related Art 
     The fixing device heats and melts, for example, as to a recording material supporting an unfixed toner image, the unfixed toner image to pressurize the fused toner image and the recording material. Thereby the toner image is fixed on the recording material. Such a fixing device includes a fixing roller, in which a heater is installed for heating the toner image, and a pressing roller. The pressing roller is pressed against the fixing roller to form a nip portion between the pressing roller and the fixing roller. A fixing method of fixing the toner image by the fixing device having the fixing roller and the pressing roller is referred to as a roller fixing method. 
     In recent years, a fixing device in which a fixing roller is formed by using a belt-shaped film may also be used. Such a fixing method for fixing the toner image using the fixing device is referred to as a belt fixing method. The belt fixing method can form a wide nip portion without increasing the size of the device, as compared with the roller fixing method. Therefore, the belt fixing method can shorten the waiting time during the fixing process, reduce the size of the device, and perform the fixing process at increased speed. 
     U.S. Pat. No. 6,671,488 discloses a fixing device which employs a film heating method as the belt fixing method. This fixing device sandwiches a heat-resistant resin film (hereinafter referred to as “fixing film”) as a heating member by a ceramic heater (heating pair) and a pressing roller (pressurizing member) to form a nip portion (hereinafter referred to as “fixing nip portion”). The printing media carrying the toner image is sandwiched and transported together with the fixing film at the fixing nip portion, thus, the heat of the ceramic heater is given to the toner image via the fixing film. Further, the fused toner image and the printing media are pressurized at the fixing nip portion to thereby fix the toner image on the printing media. 
     The fixing performance of the fixing device is affected by the environmental condition. For example, in the fixing device, the optimum fixing temperature for heating and melting the toner image changes as the environmental state changes. Therefore, in an electrophotographic type image forming apparatus having the fixing device, an environment sensor which detects environmental information such as temperature and humidity is provided. The environment sensor is provided in the image forming apparatus and includes an environment detection element such as a temperature detection element and a humidity detection element. The analog signal (detection result) output from the environment detection element is A/D converted and processed as environment data such as temperature data and humidity data. The processing result (for example, the environmental temperature) is used in a feedback process for determining the optimum fixing temperature (U.S. Pat. No. 5,138,379, Japanese Patent Application Laid-open No. 4-104284). 
     As to an image forming apparatus which is installed in a humid area, a heater (environmental heater) may be installed in its housing for preventing moisture absorption of the printing media and dew condensation on the photoconductor. The environmental temperature detected by the environment sensor becomes higher than the actual environmental temperature due to the heat generated by the environmental heater. In a case where the environment sensor is arranged at a position away from the fixing device, there may be a difference between the environmental temperature detected by the environment sensor and the environmental temperature around the fixing device. This makes it difficult to determine the optimum fixing temperature w % ben controlling the fixing temperature based on the detection result of the environment sensor. Therefore, in view of the above, at least one object of the present disclosure is to suppress the influence of the heater to appropriately control the fixing temperature. 
     SUMMARY OF THE INVENTION 
     An image forming apparatus of the present disclosure includes: a first sensor configured to detect a temperature; an image forming unit configured to form an image; a sheet container configured to contain sheets; a conveyance roller configured to convey a sheet from the sheet container; a transfer unit configured to transfer the image on the sheet conveyed by the conveyance roller; a fixing unit configured to fix the image on the sheet by heating the image on the sheet, the fixing unit having a fixing heater; a second sensor configured to detect a temperature of the fixing unit; a sheet container heater configured to heat the sheet in the sheet container, wherein the sheet container heater is provided at a position where a distance from the first sensor to the sheet container heater is closer than a distance from the fixing unit to the sheet container heater; a switch provided in a supply line which supplies an electric power to the sheet container heater; and a controller configured to control a power supply to the fixing heater based on a detection result of the first sensor, a detection result of the second sensor, and a state of the switch, wherein the controller is configured to control whether or not to supply the electric power to the sheet container heater in a case where the switch is in a first state, and wherein the electric power is not supplied to the sheet container heater in a case where the switch is in a second state. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic sectional view of an image forming apparatus. 
         FIG.  2    is an explanatory configuration diagram of a fixing device. 
         FIG.  3    is an exemplary diagram of a control unit. 
         FIG.  4    is an explanatory diagram of a power supply control for a fixing heater. 
         FIG.  5    is an explanatory diagram of an electric power supply to a built-in heater (environmental heater). 
         FIG.  6    is an explanatory diagram of relation between an environmental temperature and a fixing temperature. 
         FIG.  7    is a flow chart representing a control process of the fixing heater. 
         FIG.  8    is an explanatory diagram of relation between the environmental temperature and the fixing temperature. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     At least one embodiment of the present invention is described below in detail with reference to the accompanying drawings. 
     Image Forming Apparatus 
       FIG.  1    is a sectional view illustrating an example of an image forming apparatus of the present embodiment. The image forming apparatus  1  transfers the toner image formed by using electrophotographic process technology onto printing media P, such as a sheet to heat and fix the toner image transferred onto the printing media P. The image forming apparatus  1  of the present embodiment includes an operation unit  102 , an image reading unit  1 R, and an image output unit  1 P. The image reading unit  1 R optically reads the image of an original to convert the read original image into an electric signal (hereinafter referred to as “image data”) and transmits the image data to the image output unit  1 P. The image output unit  1 P performs an image forming process based on the image data received from the image reading unit  1 R to form an image on the printing media P. As described above, the image forming apparatus  1  has a copying ability for copying the original using the image reading unit  1 R. Further, the image forming apparatus  1  has a print function of forming an image on the printing media P according to the image data stored in a portable storage medium, and a print job obtained from an external device via a network or the like. 
     As a physical key for allowing a user to make various inputs and settings, the operation unit  102  includes a numeric keypad for inputting the number of images to be formed and a copy magnification, etc., a start button for starting image forming, and a setting button for setting a paper type and a size of the printing media P. Further, the operation unit  102  includes a display capable of displaying guidance for assisting various operations of the image forming apparatus  1 . On the display, an image, or a message for notifying the user of the status of the image forming apparatus  1  is displayed. Through the display, the user is notified, for example, that the image forming apparatus  1  is forming an image and that an error such as a jam has occurred in the image forming apparatus  1 . 
     The image output unit  1 P includes four image forming units  10   a ,  10   b ,  10   c , and  10   d , an intermediate transfer belt  14 , a sheet cassette  18 , and a detachable fixing device  20 . Each of the image forming unit  10   a .  10   b ,  10   c , and  10   d  has the same configuration and operates in the same manner except that the colors of the images formed are different. The image forming unit  10   a  forms a yellow toner image. The image forming unit  10   b  forms a magenta toner image. The image forming unit  10   c  forms a cyan toner image. The image forming unit  10   d  forms a black toner image. The alphabet a, b, c, and d at the end of the reference numerals correspond to the colors to be formed. In the following description, when it is not necessary to distinguish the colors, a, b, c, and d at the end of the reference numerals are omitted. 
     The image forming unit  10  includes a photosensitive drum  11 , a charger  12 , a developing device  13 , a drum cleaner  16 , and a laser scanner unit  17 . The photosensitive drum  11  has a built-in drum heater  152 . The drum heater  152  is an environmental heater which raises a temperature inside the housing of the image output unit  1 P to prevent a dew condensation on the photosensitive drum  11 . 
     The photosensitive drum  11  is a photosensitive member having a photosensitive layer on its surface. The charger  12  uniformly charges the surface of the photosensitive drum  11 . The laser scanner unit  17  irradiates the charged surface of the photosensitive drum  11  with light which is modulated according to the image data through a mirror. As a result, an electrostatic latent image of the corresponding color component is formed on the surface of the photosensitive drum  11 . The developing device  13  develops the electrostatic latent image on the photosensitive drum  11  using the toner of the corresponding color accumulated inside. As a result, the toner image of the corresponding color component is formed in the photosensitive drum  11 . The image forming unit  10  functions as an image forming means for forming the toner image. 
     The intermediate transfer belt  14  is an endless intermediate transfer member stretched and supported by a plurality of rollers. The intermediate transfer belt  14  is moved in a direction of an arrow T by the rotational drive of each roller. A belt cleaner  19  is provided in the vicinity of the intermediate transfer belt  14 . The toner images of the color components formed on the respective photosensitive drum  11   a ,  11   b ,  11   c , and  11   d  are transferred on the intermediate transfer belt  14  in an overlapping fashion. As a result, a full-color toner image is formed on the intermediate transfer belt  14 . The remaining toner, which is not transferred from the photosensitive drum  11  to the intermediate transfer belt  14 , on the photosensitive drum  11  is removed by the drum cleaner  16 . 
     The sheet cassette  18  functions as a sheet container for accommodating the printing media P. The printing media P is, for example, a sheet-like paper material having a size such as A4 size, letter size, A5 size, A4R size, or the like. A cassette heater  151  is provided below the sheet cassette  18 . The cassette heater  151  is the environmental heater, which raises the temperature inside a housing of the image output unit  1 P to suppress the moisture absorption of the printing media P. 
     The printing media P is fed from the sheet cassette  18  by the pickup roller  90  and is conveyed along the conveying path by the conveyance roller  91 . The printing media P is conveyed to the transfer roller  15  by adjusting the timing so as to contact with the toner image formed on the intermediate transfer belt  14 . The transfer roller  15  transfers the toner image on the intermediate transfer belt  14  on the printing media P by applying a transfer voltage in a state in which the toner image on the intermediate transfer belt  14  and the printing media P are in contact with each other. The remaining toner, which is not transferred from the intermediate transfer belt  14  to the printing media P, on the intermediate transfer belt  14  is removed by the belt cleaner  19 . 
     The printing media P on which the full-color toner image has been transferred is conveyed to the fixing device  20 , which is provided on the downstream side of the transfer roller  15  and separated as a unit. The toner image on the printing media P is melted by heat in the fixing device  20  and fixed to the printing media P by pressure. The printing media P on which the toner image is fixed is discharged to a discharge tray  22  of the image forming apparatus by a discharge roller. 
     The image forming apparatus  1  of the present disclosure has an environment sensor  150  (first sensor) for detecting temperature and humidity (environmental information). The environment sensor  150  is provided in the image forming apparatus  1  away from the fixing device  20  having a heat source, and the environment sensor  150  includes an environment detection element such as a temperature detection element and a humidity detection element. The target value of the fixing temperature of the fixing device  20  is controlled according to a detection result of the environmental temperature by the environment sensor  150 . 
     Fixing Device 
       FIG.  2    is a configuration diagram of the fixing device  20 . The fixing device  20  includes a fixing heater  111 , a fixing film  201 , a pressing roller  202 , a stay  204 , and a thermistor  210  (second sensor). The fixing heater  111  is a heating element which functions as a heating means. The thermistor  210  is an example of a temperature sensor which detects the temperature of the fixing heater  111 . 
     The fixing film  201  is a cylindrical heat-resistant film material having a thickness of 40 μm to 100 μm. The pressing roller  202  is a roller on which a heat-resistant elastic layer  207 , which is made of silicon rubber, for example, on an outer circumference of the core metal  203 . The stay  204  is a member having heat resistance property and heat insulating property and presses the fixing heater  111  toward the pressing roller  202  side via the fixing film  201 . By pressing the fixing film  201  toward the pressing roller  202 , a fixing nip portion N (area surrounded by a broken line in  FIG.  2   ) is formed. The printing media P on which the toner image has been transferred passes through the fixing nip portion N. The fixing device  20  heats and melts the toner image with the fixing heater  111  at the fixing nip portion N and pressurizes the printing media P with the pressing roller  202  to fix the toner image on the printing media P. 
     The pressing roller  202  rotates in the direction of arrow B. As a result, the fixing film  201  is driven and rotated by the pressing roller  202  in the direction of arrow C. The printing media P is sandwiched at the fixing nip portion N and is conveyed in the arrow direction A, by rotating the pressing roller  202 . 
     Control Unit 
       FIG.  3    is an explanatory configuration diagram of a control unit which comprehensively controls the operation of the image forming apparatus  1 . A control unit  350  is comprised of one board and is provided in a housing of the image forming apparatus  1 . The control unit  350  drives each load (motors  341  and clutch/solenoid  342 ) in the image forming apparatus  1 , collects and analyzes detection results of the sensors  344 , and exchanges data with the operation unit  102 . 
     The control unit  350  is connected to a power supply unit  332  and is configured to control the operation of the power supply unit  332 . The power supply unit  332  generates a predetermined voltage from the electric power supplied from a commercial power source and supplies it to each part in the image forming apparatus  1 . The control unit  350  is connected to a thermistor  210 , an environment sensor  150 , and sensors  344  such as a photo interrupter and a micro switch. The sensors  344  are arranged in various places in the image forming apparatus, and detect the operation of each part in the image forming apparatus, conveyance states of the printing media P, and the like. The control unit  350  obtains the detection results of the thermistor  210 , the environment sensor  150 , and the sensors  344 , and controls the operation of each part in the image forming apparatus  1  according to the detection results. 
     The control unit  350  is connected to a high voltage unit  340  to control the operation of the same. The high voltage unit  340  is a unit including an electric wire and a wire spring for applying a high voltage. The high voltage unit  340  outputs a transfer voltage used when transferring the toner image and a charging bias for charging the surface of the photosensitive drum  11 . The charger  12  charges the surface of the photosensitive drum  11  according to the charging bias. The control unit  350  is connected to DC loads such as motors  341  and a clutch/solenoid  342  arranged in various places in the image forming apparatus  1  and is configured to control the operation of these DC loads. The DC loads are used for controlling the operation of each part in the image forming apparatus  1 . The motors  341  are drive sources for conveying the printing media P, drive sources for rotationally driving the photosensitive drum  11 , drive sources for rotating the intermediate transfer belt  14 , drive sources for the pressing roller  202  of the fixing device  20 , and the like. 
     The control unit  350  is connected to the fixing heater  111  to adjust a fixing temperature of the fixing heater  111  based on a detected temperature detected by the thermistor  210  and the detection result by the environment sensor  150 . The fixing heater  11  is, for example, a ceramic heater. The control unit  350  is connected to the drum heater  152 , the cassette heater  151 , and a built-in heater switch (SW)  343 . The control unit  350  controls the operation of the drum heater  152  to prevent dew condensation on the photosensitive drum  11 . The control unit  350  controls the operation of the cassette heater  151  to prevent the printing media P in the sheet cassette  18  from absorbing moisture. Whether or not to control the drum heater  152  and the cassette heater  151  is determined according to a state of the built-in heater SW  343 . For example, when the built-in heater SW  343  is on, the drum heater  152  and the cassette heater  151  can be controlled by the control unit  350 . 
     The control unit  350  has the following configuration for controlling the operation of these connected components. The control unit  350  includes a CPU (Central Processing Unit)  301   a  and a ROM (Read Only Memory)  301   b . The CPU  301   a  controls the image forming process by the image forming apparatus  1  by executing the computer program stored in the ROM  301   b . The CPU  301   a  has a RAM (Random Access Memory)  301   c . The RAM  301   c  stores data which can be rewritten when the process is executed. In the RAM  301   c , for example, high voltage set value set in the high voltage control unit  304  and input information such as a image formation command information from the operation unit  102  are stored. The operation unit  102  inputs information such as a density setting value and a copy magnification set by the user in the control unit  350 . The operation unit  102  represents to the user the state of the image forming apparatus  1 , for example, information on the number of images formed, information on whether or not an image is being formed, an occurrence of a jam and its location, and the like. 
     The control unit  350  includes a power supply control unit  302  to control the power supply unit  332 . The power supply control unit  302  controls the operation of the power supply unit  332  in response to an instruction from the CPU  301   a . The control unit  350  includes an A/D conversion unit  303  for obtaining detection results from the thermistor  210  and the environment sensor  150 . The A/D conversion unit  303  converts the detection results obtained from the thermistor  210  and the environment sensor  150  into a digital signal and transmits it to the CPU  301   a . The CPU  301   a  detects the temperature of the fixing heater  111  from the detection result of the thermistor  210 . The CPU  301   a  detects the environmental temperature in the housing of the image forming apparatus  1  from the detection result of the environment sensor  150 . The control unit  350  includes a high voltage control unit  304  for controlling the high voltage unit  340 . The high voltage control unit  304  controls the operation of the high voltage unit  340  in response to the instruction from the CPU  301   a.    
     The control unit  350  includes a motor control unit  305  for controlling the motors  341 . The motor control unit  305  controls the operation of the motors  341  in response to the instruction from the CPU  301   a . The control unit  350  includes a DC load control unit  306  for controlling the clutch/solenoid  342 . The DC load control unit  306  controls the operation of the clutch/solenoid  342  in response to the instruction from the CPU  301   a.    
     The control unit  350  includes an AC driver  307  for controlling the fixing heater  111 , the drum heater  152 , and the cassette heater  151 . The AC driver  307  includes a built-in heater detection unit  308 . The CPU  301   a  instructs the AC driver  307  to control the fixing heater  111  based on the detection result (detection value) obtained from the thermistor  210  and the environment sensor  150 . The AC driver  307  controls the operation of the fixing heater  111  in response to the instruction from the CPU  301   a  to adjust the temperature during the fixing process by the fixing device  20 . 
     The AC driver  307  confirms a state of the built-in heater SW  343  by the built-in heater detection unit  308 . The CPU  301   a  can control the operation of the drum heater  152  and the cassette heater  151  when the built-in heater SW  343  is on. The AC driver  307  controls operations of the drum heater  152  and the cassette heater  151  in response to an instruction from the CPU  301   a.    
     The control unit  350  includes a sensor interface (I/F)  309  for obtaining detection results from the sensors  344 . The sensor I/F  309  transmits the detection results obtained from the sensors  344  to the CPU  301   a . The CPU  301   a  monitors the operations of the motors  341  and the clutch/solenoid  342  based on the detection results of the sensors  344  and controls the operations of the motors  341  and the clutch/solenoid  342 . 
     Fixing Heater Control 
       FIG.  4    is an explanatory diagram of power supply control to the fixing heater  111  of the fixing device  20 .  FIG.  4    represents a case where the fixing device  20  includes two fixing heaters  111   a  and  111   b . Electric power is supplied to the fixing heaters  111   a  and  111   b  from the commercial power supply  32 . The commercial power supply  32  is an AC power supply. A semiconductor switch  37  is provided on the power supply line from the commercial power supply  32  to the fixing heater  111   a . A semiconductor switch  36  is provided in the power supply line from the commercial power supply  32  to the fixing heater  111   b . The thermistor  210  is provided in the vicinity of the fixing heaters  111   a  and  111   b.    
     The power supply unit  332  generates a DC Voltage having predetermined voltage values (for example, 24V or 3.3V) from the AC power supplied from the commercial power supply  32  and supplies it to each part in the image forming apparatus  1  such as CPU  301   a . The CPU  301   a  performs a continuity control of the semiconductor switches  36  and  37  by the AC driver  307  (see  FIG.  3   ) based on the detection results of the environment sensor  150  and the thermistor  210 . Information indicating the relationship between an environmental temperature and an amount of electric power supplied to the fixing heater  111  is stored in the ROM  301   b . The CPU  301   a  calculates an electric power to be supplied to the fixing heaters  111   a  and  111   b  based on the environmental temperature detected by the environment sensor  150 , the detected temperature of the fixing device  20  detected by the thermistor  210 , and the information stored in the ROM  301   b . The CPU  301   a  controls the continuity of the semiconductor switches  36  and  37  according to the calculated electric power. 
     The semiconductor switches  36  and  37  are gate-controlled switches, which are realized by a bidirectional thyristor, for example. The semiconductor switches  36  and  37  supply the electric power from the commercial power supply  32  to the fixing heaters  111   a  and  111   b  by controlled to be in a conductive state by the CPU  301   a.    
     Built-In Heater Control 
       FIG.  5    is an explanatory diagram of the power supply to the built-in heaters (environmental heaters) such as a cassette heater  151 , a drum heater  152 , and the like. A built-in heater SW  343  and a normally closed relay  501  are provided in series on the power supply line from the commercial power supply  32  to the cassette heater  151  and the drum heater  152 . The normally closed relay  501  is normally closed, and is controlled to open and close by the CPU  301   a . A photocoupler  351  is provided between the built-in heater SW  343  and the normally closed relay  501  in order to detect an ON/OFF state of the built-in heater SW  343 . 
     From the outside of the image forming apparatus  1 , a user or a service engineer who maintains the image forming apparatus  1  performs an ON/OFF control of the built-in heater SW  343 . For example, when the image forming apparatus  1  is provided in a high humidity environment, the service engineer turns on the built-in heater SW  343  in order to prevent the printing media P stored in the sheet cassette  18  from absorbing moisture. As a result, the cassette heater  151  is energized. The cassette heater  151  generates heat when energized to dehumidify. 
     The photocoupler  351  notifies the CPU  301   a  of the ON/OFF state of the built-in heater SW  343 . In the photocoupler  351 , an electric current flows through an internal photodiode when the built-in heater SW  343  is in the ON state. This causes the photodiode to emit light. Light emitted from the photodiode is received by the phototransistor inside the photocoupler  351 . The phototransistor conducts by receiving light and transmits the voltage output from the power supply unit  332  to the CPU  301   a . The voltage output from the power supply unit  332  is information indicating energization information to the cassette heater  151  and the drum heater  152  (built-in heater). The CPU  301   a  can detect the ON state of the built-in heater SW  343  by receiving the voltage output from the power supply unit  332  via the photocoupler  351 . When the voltage output from the power supply unit  332  is not received via the photocoupler  351 , the CPU  301   a  detects that the built-in heater SW  343  is in the OFF state. 
     The opening and closing of the normally closed relay  501  are controlled by the CPU  301   a . The normally closed relay  501  is in a closed state when the CPU  301   a  is not operating, and the normally closed relay  501  supplies power from the commercial power supply  32  to the cassette heater  151 . While the image is being formed by the image forming apparatus  1 , the CPU  301   a  controls the normally closed relay  501  in an open state by the AC driver  307  (see  FIG.  3   ) to suppress power consumption to thereby cut off the power supply to the cassette heater  151 . 
     Fixing Heater Control According to Environmental Conditions 
     An optimum value of the fixing temperature when the fixing device  20  performs the fixing process changes according to the environmental temperature around the fixing device  20 .  FIG.  6    is an explanatory diagram of relationship between the environmental temperature and the fixing temperature. The horizontal axis represents the environmental temperature, and the vertical axis represents the optimum fixing temperature during the fixing process. 
     As to the fixing device  20 , its internal atmospheric temperature changes due to the environmental temperature, thus the fixing performance changes. In general, the lower the environmental temperature, the cooler the fixing film  201  is during rotation, thus the temperature is lowered at the fixing nip portion N. Therefore, the toner is less likely to melt at the fixing nip portion N. On the contrary, the higher the environmental temperature, the more difficult it is for the fixing film  201  to be cooled during rotation. Therefore, the temperature at the fixing nip portion N is maintained high. Thus, the toner at the fixing nip portion N is easy to melt. Resultingly, there is a tendency that the higher the environmental temperature around the fixing device  20 , the lower the optimum fixing temperature for the fixing process. 
     The CPU  301   a  sets a target fixing temperature according to the detection value (the environmental temperature) of the environment sensor  150 . Based on the detection temperature of the fixing device  20  (fixing heater  111 ) detected by the thermistor  210  and a target fixing temperature, the CPU  301   a  determines the content of the energization control (power amount to be supplied, etc.) to the fixing heater  111  to thereby control the power supply from the commercial power supply  32  to the fixing heater  111 . In order for the CPU  301   a  to set a target fixing temperature from the environment temperature, a fixing temperature table representing the relationship between the environment temperature and the fixing temperature as shown in  FIG.  6    is previously stored in the ROM  301   b.    
       FIG.  7    is a flowchart representing the control process of the fixing heater  111 . This process is based on the energized state of the built-in heaters such as the cassette heater  151  and the drum heater  152 , the environmental temperature detected by the environment sensor  150 , and the temperature detected by the thermistor  210 . This process is started when the image forming apparatus  1  is powered on or resumed from a sleep state. 
     The CPU  301   a  determines whether or not the built-in heater SW  343  is in the ON state (STEP S 201 ). The CPU  301   a  makes this determination based on whether or not the voltage output from the power supply unit  332  is received via the photocoupler  351 . The CPU  301   a  determines whether or not the built-in heater is generating heat depending on whether or not the built-in heater SW  343  is in the ON state. 
     When the built-in heater SW  343  is in the ON state (STEP S 201 : Y), the CPU  301   a  determines whether or not to start the image forming process (STEP S 202 ). The CPU  301   a  makes this determination based on whether or not an image formation instruction has been received from the operation unit  102  or the external device. When the built-in heater SW  343  is in the ON state, the CPU  301   a  supplies power to the built-in heater in a standby mode. When starting the image forming process (STEP S 202 : Y), to reduce the power consumption other than the power required for image formation, the CPU  301   a  cuts off (turns off) the power supply to the built-in heaters such as the cassette heater  151  and the drum heater  152  (STEP S 203 ). The CPU  301   a  cuts off the power supply to the built-in heater by opening the normally closed relay  501 . Further, the CPU  301   a  detects the environmental temperature based on the detection result of the environment sensor  150  (STEP S 204 ). 
     Since the built-in heater SW  343  is in the ON state, the environmental temperature detected by the environment sensor  150  increases due to the influence of the heat from the cassette heater  151  and the drum heater  152 . The cassette heater  151  and the drum heater  152  are provided for the purpose of preventing dew condensation and moisture absorption, however, due to this, the temperature inside the housing of the image forming apparatus  1  is increased. Due to the heat caused by these heaters, the environmental temperature detected by the environment sensor  150  increases. The CPU  301   a  reads out and corrects, considering the increase in the environmental temperature, a fixing temperature table of the fixing device  20  previously stored in the ROM  301   b  in order to calculate the amount of electric power supplied to the fixing heater  111  (STEP S 205 ). 
     The increase in the detection result of the environmental temperature due to the influence of the heat applied to the environment sensor  150  will be described with reference to  FIG.  8   .  FIG.  8    is an explanatory diagram of the relationship between the environmental temperature and the fixing temperature. The horizontal axis represents the environmental temperature, and the vertical axis represents the optimum fixing temperature during the fixing process performed by the fixing heater  111 . 
     For example, the environmental temperature detected by the environment sensor  150  when the built-in heater SW  343  is in the OFF state is set to A° C. The environmental temperature detected by the environment sensor  150  when the built-in heater SW  343  is in the ON state in the same environment becomes B° C., which is higher than A° C. by a predetermined temperature (in this case, about 2° C.). As to the deviation of the environmental temperature detected by the environment sensor  150  caused by the change in the state of the built-in heater SW  343 , it tends to be slightly smaller as the environmental temperature increases. However, the deviation is in the range of around 2° C. 
     The fixing device  20  is configured to be a separated unit, and is arranged at a position away from the environment sensor  150 . Therefore, the fixing device  20  is not easily affected by the heat generated by the built-in heaters such as the cassette heater  151  and the drum heater  152  even when the built-in heater SW  343  is in the ON state. As a result, when the built-in heater SW  343  is in the ON state, a predetermined temperature (about 2° C.) difference occurs between the environmental temperature detected by the environment sensor  150  and the environmental temperature around the fixing device  20 . In  FIG.  8   , the solid line shows the relationship between the environmental temperature around the fixing device  20  and the fixing temperature, and the broken line shows the relationship between the environmental temperature detected by the environment sensor  150  and the fixing temperature. In order to suppress the influence of the difference between the environmental temperature detected by the environment sensor  150  and the environmental temperature around the fixing device  20 , the correction process of STEP S 205  is performed. Specifically, the CPU  301   a  corrects the fixing temperature table (broken line) to the fixing temperature table (solid line). The fixing temperature table (broken line) shows the relationship between the environmental temperature detected by the environment sensor  150  and the fixing temperature, and the fixing temperature table (solid line) shows the relationship between the environmental temperature around the fixing device  20  and the fixing temperature. 
     After the correction of the fixing temperature table, the CPU  301   a  adjusts the fixing temperature using the corrected fixing temperature table (STEP S 206 ). The CPU  301   a  determines a target fixing temperature based on the corrected fixing temperature table. That is, the CPU  301   a  determines the fixing temperature according to the temperature decreased by a predetermined temperature from the environmental temperature detected by the environment sensor  150 . The CPU  301   a  calculates the amount of electric power supplied to the fixing heaters  111   a  and  111   b  based on the target fixing temperature and the detected temperature of the fixing device  20  detected by the thermistor  210 . The CPU  301   a  controls the semiconductor switches  36  and  37  to supply the calculated electric power to the fixing heaters  111   a  and  111   b . Thus, the electric power supplied to the fixing heaters  111   a  and  111   b  is adjusted so that the temperature of the fixing device  20  becomes the target fixing temperature. 
     The CPU  301   a  performs an image forming process while adjusting the fixing temperature (STEP S 207 ). The CPU  301   a  repeatedly performs a fixing temperature adjustment process until the image forming process is completed (STEP S 208 : N). When the image forming is completed (STEP S 208 : Y), the CPU  301   a  ends the fixing temperature adjustment process for the fixing device  20  (STEP S 209 ). The CPU  301   a  restarts (turns on) the power supply to the built-in heaters such as the cassette heater  151  and the drum heater  152  (STEP S 210 ). The CPU  301   a  restarts the power supply to the built-in heater by closing the normally closed relay  501 . When the built-in heater SW  343  is in the ON state, the fixing temperature is adjusted as described in the above processing. 
     When the built-in heater SW  343  is in the OFF state (STEP S 201 : Y), the CPU  301   a  determines whether or not to start the image forming process, as in the process of STEP S 202  (STEP S 211 ). When starting the image forming process (STEP S 211 : Y), the CPU  301   a  detects the environmental temperature based on the detection result of the environment sensor  150  (STEP S 212 ), as in the process of STEP S 204 . 
     The CPU  301   a  adjusts the fixing temperature using the fixing temperature table of the fixing device  20  previously stored in the ROM  301   b  based on the detected environmental temperature (STEP S 213 ). Since the built-in heater SW  343  is in the OFF state, the detected environmental temperature is not affected by the built-in heater. Therefore, the CPU  301   a  does not need to correct the fixing temperature table. The CPU  301   a  determines the target fixing temperature based on the uncorrected fixing temperature table. The CPU  301   a  calculates the amount of electric power supplied to the fixing heaters  111   a  and  111   b  based on the target fixing temperature and the detected temperature of the fixing device  20  detected by the thermistor  210 . The CPU  301   a  controls the semiconductor switches  36  and  37  to supply the calculated electric power to the fixing heaters  111   a  and  111   b . As a result, the electric power supplied to the fixing heaters  111   a  and  111   b  is adjusted so that the temperature of the fixing device  20  becomes the target fixing temperature. 
     The CPU  301   a  performs an image forming process while adjusting the fixing temperature (STEP S 214 ). The CPU  301   a  repeatedly adjusts the fixing temperature until the image forming process is completed (STEP S 215 : N). When the image forming is completed (STEP S 215 : Y), the CPU  301   a  ends the fixing temperature adjustment process for the fixing device  20  (STEP S 216 ). When the built-in heater SW  343  is in the OFF state, the fixing temperature is adjusted by the above processing. 
     The image forming apparatus  1  described in the above embodiment is configured to correct the fixing temperature table when the energized state of the built-in heater is in the ON state. However, the image forming apparatus  1  may have a plurality of fixing temperature tables corresponding to the energized state, for example. The CPU  301   a  determines the fixing temperature with reference to a Table A as the fixing temperature table when the energized state of the built-in heater is the ON state, and determines the fixing temperature with reference to a Table B as the fixing temperature table when the energized state of the built-in heater is the OFF state. Table A and Table B function as different determinants for determining the fixing temperature. 
     As described above, the image forming apparatus  1  of the present embodiment determines whether or not the power is supplied to the environmental heaters (built-in heaters such as the cassette heater  151  and the drum heater  152 ) to thereby suppress the mismatch of environmental information such as the environmental temperature and the like. As a result, the image forming apparatus  1  can suppress the influence of the environmental heater, and perform the fixing process by the fixing device  20  at the optimum fixing temperature. Therefore, the image forming apparatus  1  can form an image with better image quality on the printing media P as compared to the conventional method. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2020-212661, filed Dec. 22, 2020, which is hereby incorporated by reference herein in its entirety.