Patent Publication Number: US-9405240-B2

Title: Image forming apparatus with a heat-controllable fixing unit and computer readable medium for image forming apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2014-093872 filed on Apr. 30, 2014, the entire subject matter of which is incorporated herein by reference. 
     BACKGROUND 
     1. Technical Field 
     An aspect of the present invention relates to an image forming apparatus with a fixing unit, a method to control the image forming apparatus, and a computer readable medium to store a computer readable program to control the image forming apparatus. 
     2. Related Art 
     An image forming apparatus having a fixing unit, in which an image formed in a developer agent on a sheet is thermally fixed thereon, is known. The fixing unit may include a heating member to convey heat to the sheet, a heater to heat the heating member, and a heat sensor to detect a temperature in the heating member. In particular, the heat sensor may detect a temperature in an outer part of the heating member, which is laterally outside a pathway for the sheet being conveyed. 
     In the conventional fixing unit, heat in the heating member in an inner part coincident with the pathway for the sheet may be absorbed by the sheet being conveyed, while heat in the outer part of the heating member coincident with the laterally outside area of the pathway may accumulate. As the image forming apparatus continues forming images, the outer part of the heating member coincident with the laterally outside area of the pathway may be heated excessively. Therefore, the image forming apparatus may take an action to switch the heater off and abort feeding sheets when the heat sensor detects the temperature in the outer part of the heating member being higher than a predetermined degree so that the heating member may be prevented from being overheated in the outer part. 
     SUMMARY 
     According to the known image forming apparatus mentioned above, when a width of the sheet being conveyed is greater, the heat in the outer part of the heating member may tend to be absorbed by the sheet; therefore, the heat may accumulate in a smaller range in the heating member compared to the heating member when the width of the sheet being conveyed is smaller, and influence of the accumulated heat on neighboring parts in the vicinity of the outer part of the heating member may be less. Meanwhile, according to the known image forming apparatus mentioned above, the action to prevent overheat in the outer part of the heating member may be taken regardless of the width of the sheet. In other words, even when the width of the sheet being conveyed is greater, and when a smaller range of the heating member is subject to the increase of the temperature, the action to lower the temperature in the outer part of the heating member may be taken. 
     The present invention is advantageous in that an image forming apparatus, in which the temperature in the outer part of the heating member may be lowered depending on the width of the sheet being conveyed, is provided. Further, a method to control the image forming apparatus, and a computer usable storage device to store a program to control the image forming apparatus are provided. 
     According to an aspect of the present invention, an image forming apparatus, including an image forming unit configured to form an image in a developer agent on a sheet; a fixing unit including a heater and being configured to fix the image in the developer agent on the sheet; an end temperature sensor disposed on an end area of the fixing unit with regard to a direction of width of the sheet; and a controller, is provided. The controller is configured to perform a width obtaining step, in which the controller obtains the width of the sheet; a temperature obtaining step, in which the controller obtains a temperature of the fixing unit detected by the end temperature sensor; a comparison step, in which the controller compares the detected temperature with a threshold; a cooling step, in which the controller manipulates the heater to let the end area of the fixing unit cool down when the temperature detected by the end temperature sensor is one of higher and equal to the threshold; and a threshold-setting step, in which the controller sets a first value to be the threshold when the width of the sheet obtained in the width obtaining step is one of greater and equal to a predetermined width, and in which the controller sets a second value being different from the first value to be the threshold when the width of the sheet obtained in the width obtaining step is smaller than the predetermined width. 
     According to another aspect of the present invention, a method configured to be implemented on a processor to control an image forming apparatus, which includes an image forming unit to form an image in a developer agent on a sheet; a fixing unit including a heater and being configured to fix the image in the developer agent on the sheet; and an end temperature sensor disposed on an end area of the fixing unit with regard to a direction of width of the sheet, is provided. The method includes a width obtaining step, in which the controller obtains the width of the sheet; a temperature obtaining step, in which the controller obtains a temperature of the fixing unit detected by the end temperature sensor; a comparison step, in which the controller compares the detected temperature with a threshold; a cooling step, in which the controller manipulates the heater to let the end area of the fixing unit cool down when the temperature detected by the end temperature sensor is one of higher and equal to the threshold; and a threshold-setting step, in which the controller sets a first value to be the threshold when the width of the sheet obtained in the width obtaining step is one of greater and equal to a predetermined width, and in which the controller sets a second value being different from the first value to be the threshold when the width of the sheet obtained in the width obtaining step is smaller than the predetermined width. 
     According to still another aspect of the present invention, a non-transitory computer readable medium storing computer readable instructions that are executable by a computer to control an image forming apparatus, when executed by the computer, is provided. While the image forming apparatus includes an image forming unit to form an image in a developer agent on a sheet, a fixing unit including a heater and being configured to fix the image in the developer agent on the sheet, and an end temperature sensor disposed on an end area of the fixing unit with regard to a direction of width of the sheet, the computer readable instructions are configured to cause the computer to control the image forming apparatus by performing a width obtaining step, in which the controller obtains the width of the sheet; a temperature obtaining step, in which the controller obtains a temperature of the fixing unit detected by the end temperature sensor; a comparison step, in which the controller compares the detected temperature with a threshold; a cooling step, in which the controller manipulates the heater to let the end area of the fixing unit cool down when the temperature detected by the end temperature sensor is one of higher and equal to the threshold; and a threshold-setting step, in which the controller sets a first value to be the threshold when the width of the sheet obtained in the width obtaining step is one of greater and equal to a predetermined width, and in which the controller sets a second value being different from the first value to be the threshold when the width of the sheet obtained in the width obtaining step is smaller than the predetermined width. 
    
    
     
       BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS 
         FIG. 1  is an illustrative cross-sectional view of a laser printer according to an embodiment of the present invention. 
         FIG. 2A  is a perspective view of a feeder tray in the laser printer according to the embodiment of the present invention.  FIG. 2B  is a plan view of a side-guide in the laser printer according to the embodiment of the present invention. 
         FIG. 3  is a cross-sectional view of a fixing unit in the laser printer according to the embodiment of the present invention. 
         FIG. 4  is a perspective view of a nipper plate, a side thermistor, a thermostat, a central thermistor, and a sheet sensor in the laser printer according to the embodiment of the present invention. 
         FIG. 5  is a block chart to illustrate a configuration of a controller and related components in the laser printer according to the embodiment of the present invention. 
         FIG. 6  is a flowchart to illustrate a flow of steps to be taken by the controller in the laser printer according to the embodiment of the present invention. 
         FIG. 7  is a flowchart to illustrate a first modified example of the flow of steps to be taken by the controller in the laser printer according to the embodiment of the present invention. 
         FIG. 8  is a flowchart to illustrate a second modified example of the flow of steps to be taken by the controller in the laser printer according to the embodiment of the present invention. 
         FIG. 9  is a graph to illustrate shifts of thresholds in the second modified example of the flow of steps to be taken by the controller in the laser printer according to the embodiment of the present invention. 
         FIG. 10A  is a cross-sectional view of the fixing unit in the laser printer according to another modified example of the present invention.  FIG. 10B  is a plan view of a heat roller, a side thermistor, a thermostat, and a central thermistor in the laser printer according to the modified example of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, an exemplary configuration of a laser printer  1  according to an embodiment of the present invention will be described with reference to the accompanying drawings. First, an overall configuration of the laser printer  1  will be described, and second, detailed configurations of a fixing unit and a controller in the laser printer  1  will be described. 
     In the following description, directions concerning the laser printer  1  will be referred to in accordance with a user&#39;s ordinary position to use the laser printer  1 , as indicated by arrows. For example, a viewer&#39;s left-hand side appearing in  FIG. 1  is referred to as a front side of the laser printer  1 , and a right-hand side in  FIG. 1  opposite from the front side is referred to as a rear side. A side which corresponds to the viewer&#39;s nearer side is referred to as a right-hand side for the user, and an opposite side from the right, which corresponds to the viewer&#39;s farther side is referred to as a left-hand side for the user. An up-down direction in  FIG. 1  corresponds to a vertical direction of the laser printer  1 . Further, the right-to-left or left-to-right direction of the laser printer  1  may be referred to as a widthwise direction, and the front-to-rear or rear-to-front direction may be referred to as a direction of depth. The widthwise direction and the direction of depth are orthogonal to each other. Furthermore, directions of the drawings in  FIGS. 2A, 2B, 3-4 and 10A, 10B  are similarly based on the orientation of the laser printer  1  as defined above and correspond to those with respect to the laser printer  1  shown in  FIG. 1  even when the drawings are viewed from different angles. 
     As shown in  FIG. 1 , the laser printer  1  includes a feeder unit  3  to feed sheets S, an image forming unit  10  to form toner images on the sheets S, a fixing unit  100  to thermally fix the images on the sheets S, and a controller  8 , which are accommodated in a main chassis  2 . 
     The feeder unit  3  is disposed in a lower position in the chassis  2  and includes a feeder tray  31  and a feeder device  33 . 
     The feeder tray  31  is a top-open box to accommodate the sheets S and is attachable to the main chassis  2 . The feeder tray  31  may be pulled forward to be drawn out of the main chassis  2  and pushed rearward to be attached to the main chassis  2 . 
     The feeder tray  31  includes, as shown in  FIG. 2A , a bottom  31 A, on which the sheets S are stacked, a sheet-pressing board  32 , and a pair of side-guides  34 , which are arranged on the bottom  31 A. 
     The sheet-pressing board  32  is swingably supported at a rear end thereof by the feeder tray  31 . When a front end of the sheet-pressing board  32  moves upward, the sheets S stored in the feeder tray  31  are uplifted. 
     The pair of side-guides  34  are arranged in the feeder tray  31  to face with each other along a direction of width of the sheets S, i.e., along the widthwise direction, across the sheets S (see  FIG. 2B ). Each side-guide  34  includes a guide fence  34 A and a rack gear  34 B, and is movable along the widthwise direction. 
     Each guide fence  34 A stands vertically and has a restrictive face  34 C, which faces inward with regard to the widthwise direction. While the guide fences  34 A are arranged to be spaced apart from each other along the widthwise direction and placed to contact widthwise ends of the sheets S, a position of the sheets with regard to the widthwise direction is restricted. 
     As shown in  FIG. 2B , each rack gear  34 B is formed to extend inward from a lower part of the guide fence  34 A along the widthwise direction. The rack gear  34 B is formed to have gear teeth on a side that faces the other rack gear  34 B, and the gear teeth are meshed with a pinion gear  34 D, which is arranged on the feeder tray  31  at an intermediate position between the rack gears  34 B. The pinion gear  34 D links the paired side-guides  34  to be movable in conjunction with each other. Thereby, when one of the paired side-guides  34  is moved along the widthwise direction to fit with the widthwise end of the sheets S, the other of the paired side-guides  34  is moved along symmetrically in the widthwise direction. Thus, the paired side-guides  34  are arranged on each widthwise side of the sheets S to restrict a position of the sheets S interposed there-between and gather the sheets S at a widthwise center in the feeder tray  31 . 
     The paired side-guides  34  are, when widened to a maximum width, i.e., when placed at outermost positions along the widthwise direction, capable of restricting the position of the sheets S with a maximum width usable in the laser printer  1 . Meanwhile, when narrowed to a minimum width, i.e., when placed at innermost positions along the widthwise direction, the paired side-guides  34  are capable of restricting the position of the sheets S with a minimum width usable in the laser printer  1 . In other words, when the paired side-guides  34  are widened at the maximum width, the restrictive faces  34 C are placed in positions corresponding to widthwise ends of a maximum-width sheet pathway Wmax, which is an area for the sheet S with the maximum width is allowed to pass through, and when the paired side-guides  34  are narrowed at the minimum width, the restrictive faces  34  are placed in positions corresponding to widthwise ends of a minimum-width sheet pathway Wmin, which is an area for the sheet S with the minimum width is allowed to pass through. 
     In the feeder unit  3  configured as above, the sheets S stored in the feeder tray  31  are uplifted by the sheet-pressing board  32  and fed to the image forming unit  10  by the feeder device  33 . 
     Referring back to  FIG. 1 , the image forming unit  10  includes an exposure device  4  and a processing cartridge  5 . 
     The exposure device  4  is disposed in an upper position in the main chassis  2  and includes a laser emitter (not shown), a polygon mirror (unsigned), lenses (unsigned), and reflection mirrors (unsigned). In the exposure device  4 , a laser beam is emitted from the laser emitter and transmitted to a surface of a photosensitive drum  61  in the processing cartridge  5  via the polygon mirrors, the lenses, and the reflection mirrors to scan the surface of the photosensitive drum  61 . 
     The processing cartridge  5  is disposed in a position between the exposure device  4  and the feeder tray  31 . The processing cartridge  5  is detachably attached to the main chassis  2  through an opening (unsigned), which is exposed or covered by a front cover  21 , while the front cover  21  is pivotable on a front side of the main chassis  2 . The processing cartridge  5  includes a drum unit  6  and a developer unit  7 . 
     The drum unit  6  includes the photosensitive drum  61 , a charger  62 , and a transfer roller  63 . The developer unit  7  is removably attached to the drum unit  6  and includes a developer roller  71 , a supplier roller  72 , a toner-spreader blade  73 , a toner container  74  to store toner, and an agitator  75  to agitate the toner in the toner container  74 . 
     In the processing cartridge  5 , as the photosensitive drum  61  rotates, a surface of the photosensitive drum  61  is electrically charged evenly by the charger  62  and partly exposed to the laser beam emitted from the exposure device  4  so that electrical charges of the areas exposed to the laser beam are lowered and a latent image according to image data is formed to be carried on the surface of the photosensitive drum  61 . Meanwhile, the toner in the toner container  74  is supplied to the developer roller  71  by the supplier roller  72  and enters a position between the developer roller  71  and the toner-spreader blade  73  to be spread on the developer roller  71  to form an evenly-thinned layer. 
     Meanwhile, the toner on the developer roller  71  is supplied to the latent image on the photosensitive drum  61  while the developer roller  71  is rotated so that the latent image on the photosensitive drum  61  is developed to be a toner image. In the meantime, as the sheet S is conveyed through an intermediate position between the photosensitive drum  61  and the transfer roller  63 , the toner image carried on the surface of the photosensitive drum  61  is transferred onto the sheet S. 
     The fixing unit  100  is disposed in a rearward position with respect to the processing cartridge  5 , and the toner image transferred to the sheet S is thermally fixed on the sheet S as the sheet S passes through the fixing unit  100 . Thereafter, the sheet S is ejected by conveyer rollers  23 ,  24  to a sheet outlet  22 . 
     As shown in  FIG. 3 , the fixing unit  100  includes a fixing belt  100 , a heater  120 , a nipper board  130 , a pressure roller  140 , a reflector board  150 , and a stay  160 . 
     The fixing belt  110  transmits heat to the sheet S having the toner images. The fixing belt  110  is an endless belt made of a stainless steel with a heat-resistance property and flexibility. Inside the fixing belt  110 , arranged are the heater  120 , the nipper board  130 , the reflector board  150 , and the stay  160 . 
     The heater  120  may be, for example a halogen lamp, and radiates heat to warm the nipper board  130  and the fixing belt  100 . The heater  120  is disposed in a position spaced apart for a predetermined distance from an inner surface of the nipper board  130 . 
     The nipper board  130  is disposed to slidably contact an inner circumferential surface of the fixing belt  110 . The nipper board  130  is arranged to be exposed to the heat from the heater  120  and conducts the heat to the fixing belt  110 . The nipper board  130  may be a bent-formed aluminum board having a higher heat conductivity than, for example, the stay  160 , which may be made of stainless steel. The nipper board  130  includes, as shown in  FIG. 4 , a straight part  131 , a front bent-part  132 , a rear bent-part  133 , and three (3) detectable parts, which are first detectable part  134 A, a second detectable part  134 B, and a third detectable part  134 C. The straight part  131  is elongated along the widthwise direction and arranged to contact the fixing belt  110 . The front bent-part  132  extends upward from a front end of the straight part  131 , and the rear bent-part  133  extends upward from a rear end of the straight part  131 . 
     The first, second, and third detectable parts  134 A,  134 B,  134 C are pieces, on which a side thermistor  400 A, a thermostat  400 B, and a central thermistor  400 C are placed respectively. The first, second, and third detectable parts  134 A,  134 B,  134 C are formed to extend rearward from an upper edge  133 A of the rear bent-part  133 . 
     The first detectable part  134 A, which is in a rightmost position among the three detectable parts  134 A,  134 B,  134 C, is formed on a rightward end of the nipper board  130  to be arranged laterally outside the maximum-width sheet pathway Wmax with regard to the widthwise direction. The second detectable part  134 B is formed in a leftward position with regard to the first detectable part  134 A, inside the maximum-width sheet pathway Wmax, and laterally outside the minimum-width sheet pathway Wmin, with regard to the widthwise direction. The third detectable part  134 C is formed in a widthwise central position in the nipper board  130  and inside the minimum-width sheet pathway Wmin, with regard to the widthwise direction. 
     The side thermistor  400 A to detect temperature is arranged to contact the first detectable part  134 A. In other words, the side thermistor  400 A is disposed laterally outside the maximum-width sheet pathway Wmax with regard to the widthwise direction. The side thermistor  400 A is disposed on an outer side of the restrictive face  34 C (see  FIG. 2B ) of the side-guide  34  in the feeder tray  31  with regard to the widthwise direction. 
     The side thermistor  400 A is configured to output a signal corresponding to a temperature in the first detectable part  134 A to the controller  8 . Therefore, a temperature in the fixing belt  110  at a rightward end, more specifically, a part of the fixing belt  110  outside the maximum-width sheet pathway Wmax with regard to the widthwise direction, may be indirectly detected by the side thermistor  400 A through the first detectable part  134 A. 
     The thermostat  400 B is disposed to contact the second detectable part  134 B. In other words, the thermostat  400 B is disposed laterally inside the maximum-width sheet pathway Wmax and laterally outside the minimum-width sheet pathway Wmin with regard to the widthwise direction. 
     The thermostat  400 B detects a temperature in the second detectable part  134 B, when a temperature in a part of the fixing belt  110  corresponding to the second detectable part  134 B is increased to be greater than or equal to a third threshold, which will be described later in detail, and shuts down electricity to the heater  120  without being controlled by the controller  8 . 
     The central thermistor  400 C is disposed on the third detectable part  134 C. In other words, the center thermistor  400 C is disposed inside the minimum-width sheet pathway Wmin with regard to the widthwise direction. 
     The central thermistor  400 C is configured to output a signal corresponding to a temperature in the third detectable part  134 C to the controller  8 . Therefore, a temperature in a part of the fixing belt  110  inside the minimum-width sheet pathway Wmax with regard to the widthwise direction may be indirectly detected by the central thermistor  400 C through the third detectable part  134 C. 
     In a position upstream of the nipper board  130  with regard to a direction to convey the sheet S, disposed is a sheet sensor  9 . The sheet sensor  9  is disposed laterally outside the minimum-width sheet pathway Wmin and laterally inside the maximum-width sheet pathway Wmax with regard to the widthwise direction. The sheet sensor  9  may be, for example, in a same position as the thermostat  400 B along the widthwise direction. The sheet sensor  9  is configured to output different signals depending on presence or absence of the sheet S at a position corresponding to the sheet sensor  9 . 
     Referring back to  FIG. 3 , the pressure roller  140  is arranged in a lower position with regard to the nipper board  130  and forms a nipping section N to nip the fixing belt  110  in conjunction with the straight part  131  of the nipper board  130 . In order to nip the fixing belt  110  in the nipping section N, one of the nipper board  130  and the pressure roller  140  is urged against the other of the nipper board  130  and the pressure roller  140 . The pressure roller  140  is driven to be rotated by a driving force from a driving source, which is disposed in the main chassis  2  but not shown. With the fixing belt  110  and the sheet S nipped between the pressure roller  140  and the nipper board  130 , when the pressure roller  140  is driven to rotate, the fixing belt  110  is rolled along with the rotation of the pressure roller  140 , and the sheet S is conveyed rearward. 
     The reflector board  150  is a board, which has a characteristic of a higher reflection rate for infrared and far-infrared rays, to reflect the radiation heat from the heater  120  toward the nipper board  130  and may be, for example, an aluminum board. The reflector board  150  is formed in curve to have a cross-sectional approximate shape of a U, which surrounds the heater  120 , inside the belt  110 . 
     The stay  160  supports the nipper board  130  through the reflector board  150  to bear the pressure from the pressure roller  140 . The stay  160  is arranged to surround the heater  120  and the reflector board  150  inside the fixing belt  110 . The stay  160  may be, for example, a stainless sheet, which has greater rigidity than the nipper board  130 . Thus, the nipper board  130  may be restrained from being deformed by the pressure from the pressure roller  140 . 
     The controller  8  (see  FIG. 8 ) is a computing device which includes a central processing unit (CPU), a read-only memory (ROM), and a random access memory (RAM) (not shown). The controller  8  is configured to execute a cooling operation, according to the output from the side thermistor  400 A and a preinstalled program, to let a lateral end area of the fixing belt  110  cool down when a temperature in the lateral end area of the fixing belt is at a threshold TA. 
     The controller  8  includes, as shown in  FIG. 5 , a heater-controlling unit  81 , a temperature-obtaining unit  82 , a sheet-width obtaining unit  83 , a threshold-setting unit  84 , and a cooling-commander unit  85 . 
     The heater-controlling unit  81  is configured to control the heater  120 , according to the output from the central thermistor  400 C, to maintain the temperature in a widthwise central area of the fixing belt  110  at a predetermined fixing temperature TF. 
     The temperature-obtaining unit  82  is configured to obtain a temperature T in a rightward end area of the fixing belt  110  according to the output from the side thermistor  400 A. 
     The sheet-width obtaining unit  83  is configured to execute an obtaining operation, according to the output from the sheet sensor  9 , to obtain a width W of the sheet S being conveyed. For example, in the obtaining operation, the sheet-width obtaining unit  83  may determine that the width W of the sheet S being conveyed is greater than or equal to a predetermined width WA when the sheet sensor  9  detects the sheet S and that the width W of the sheet S being conveyed is smaller than the predetermined width WA when the sheet sensor  9  does not detect the sheet S. The obtaining operation may be executed at a predetermined timing, in which the sheet S being conveyed passes through the position corresponding to the sheet sensor  9 . 
     Optionally, the sheet-width obtaining unit  83  may determine the width W of a first sheet S being conveyed based on information concerning the sheet S, which may be input by the user, and the width W of a second and succeeding sheets S being conveyed based on the output from the sheet sensor  9 . 
     The threshold-setting unit  84  is configured to execute a threshold-setting operation, in which a first threshold T 1  is set to be the threshold TA when the width W of the sheet S obtained in the obtaining operation is greater than or equal to the predetermined width WA, and a second threshold T 2  is set to be the threshold TA when the width W of the sheet S obtained in the obtaining operation is smaller than the predetermined width WA. In this regard, the second threshold T 2  is smaller than the first threshold T 1 . In the meantime, the above-mentioned third threshold is greater than the second threshold T 2  and smaller than the first threshold T 1 . 
     The cooling-commander unit  85  is configured to execute a cooling-commander operation, in which a command to execute the cooling operation is issued when the temperature T in the rightward end area of the fixing belt  110  obtained by the temperature-obtaining unit  82  is greater than or equal to the threshold TA. Upon executing the cooling operation, the cooling-commander unit  85  may issue a command to let the temperature in the central area of the fixing belt  110  be lower than a regular-driving temperature, which is a temperature in the central area of the fixing belt  110  when the cooling operation is not performed, and to reduce a conveying velocity to convey the sheet S to be lower than a regular-driving velocity, which is a velocity to convey the sheet S when the cooling operation is not performed. 
     More specifically, when the temperature T in the rightward end area of the fixing belt  110  is determined to be lower than the threshold TA, the cooling-commander unit  85  sets a first fixing temperature TF 1  to be a fixing temperature TF and a first conveying velocity V 1  to be a conveying velocity VA to convey the sheet S. On the other hand, when the temperature T in the rightward end area of the fixing belt  110  is determined to be higher than or equal to the threshold TA, the cooling-commander unit  85  sets a second fixing temperature TF 2 , which is lower than the first fixing temperature TF 1 , to be the fixing temperature TF and a second velocity V 2 , which is slower than the first velocity V 1 , to be the conveying velocity VA to convey the sheet S. Further, the cooling-commander unit  85  manipulates the heater-controlling unit  81  to control the heater  120  so that the temperature in the central area of the fixing belt  110  is maintained at the fixing temperature TF. Furthermore, the cooling-commander unit  85  controls the image forming unit  10  and the pressure roller  140  to convey the sheet S at the conveying velocity VA. 
     Next, referring to  FIG. 6 , detailed behaviors of the controller  8  will be described. When the controller  8  receives a print job (S 10 ), the controller  8  obtains the width W of the sheet S being conveyed (S 11 ). 
     In S 12 , the controller  8  determines if the obtained width W of the sheet S is smaller than the predetermined width WA (S 12 ). When the width W of the sheet S being conveyed is determined to be greater than or equal to the predetermined width WA (S 12 : NO), the controller  8  sets the first threshold T 1  to be the threshold TA (S 13 ). On the other hand, when the width W of the sheet S being conveyed is determined to be smaller than the predetermined width WA (S 12 : YES), the controller  8  sets the second threshold T 2  to be the threshold TA (S 14 ). 
     After setting the threshold TA in S 13  or S 14 , the controller  8  determines if the temperature T in the rightward end of the fixing belt  110  detected by the side thermistor  400 A is higher than or equal to the threshold TA (S 15 ). 
     In S 15 , when the temperature T of the rightward end of the fixing belt  110  is determined to be lower than the threshold TA (S 15 : NO), the controller  8  sets the first fixing temperature TF 1  to be the fixing temperature TF and the first velocity V 1  to be the conveying velocity VA to convey the sheet S (S 16 ). On the other hand, when the temperature T of the rightward end of the fixing belt  110  is determined to be higher than or equal to the threshold TA (S 15 : YES), the controller  8 , more specifically, the cooling-commander unit  85 , sets the second fixing temperature TF 2  to be the fixing temperature TF and the second velocity V 2  to be the conveying velocity VA to convey the sheet S (S 17 ). 
     After setting the conveying velocity VA to convey the sheet S in S 16  or S 17 , the controller  8  manipulates the feeder unit  3  to convey the sheet S at the conveying velocity VA and the image forming unit  10  to form the image based on the print job (S 18 ). After forming a first page of the print job, the controller  8  determines if the print job should end thereat (S 19 ). When the print job is determined to end thereat (S 19 : YES), the controller  8  finishes the printing operation. On the other hand, when the print job is determined to be continued (S 19 : NO), the controller  8  returns to S 11  and continues the print job. 
     According to the laser printer  1  configured as above, when the sheet S passes through the fixing unit  100 , the sheet S absorbs the heat in the fixing belt  110 . Therefore, a temperature in the part of the fixing belt  110  that contacts the sheet S is lowered. When the temperature in the fixing belt  100  is lowered, the heater-controlling unit  81  in the controller  8  manipulates the heater  120  to heat the fixing belt  110  so that the temperature in the central area of the fixing belt  110  is maintained at the first fixing temperature TF 1 . In this regard, however, in lateral end areas of the fixing belt  110 , where the sheet S being conveyed does not contact, the heat is not absorbed by the sheet S but accumulates. Therefore, the temperature in the lateral end areas may increase to be higher than the central part, where the sheet S being conveyed contacts. 
     As the temperature in the lateral end areas increases, when the side thermistor  400  detects the temperature T in the rightward end area of the fixing belt  110  being higher than or equal to the threshold TA, the controller  8  activates the cooling operation. 
     When the cooling operation is performed, the heater  120  is controlled to adjust the temperature T at the central area of the fixing belt  110  to be the second fixing temperature TF  2 , which is lower than the first fixing temperature TF 1  of the regular-driving temperature. In other words, the output from the heater  120  is reduced. Thus, with the reduced output from the heater  120 , the heat to be conducted from the heater  120  to the lateral end areas of the fixing belt  110  may be reduced, and the fixing belt  110  may be restrained from being heated up at the lateral end areas. 
     In the meantime, while the cooling operation is performed, the sheet S is conveyed at the second velocity V 2 , which is slower than the first velocity V 1  of the regular-driving conveying velocity. Therefore, although the temperature in the fixing belt  110  is at the second fixing temperature TF 2 , which is lower than the first fixing temperature TF 1  being the regular-driving temperature, the toner image may be substantially thermally fixed on the sheet S. 
     When the width W of the sheet S being conveyed is greater, the heat in a larger area in the fixing belt  110  may be absorbed by the sheet S. Therefore, compared to the sheet S with a smaller width, the heat may accumulate in a smaller area in the fixing belt  110 , and influence by the heat on the neighboring parts may be limited. In consideration of that, according to the embodiment described above, the first threshold T 1  is set to be the threshold TA when the width W of the sheet S is greater than or equal to the predetermined width WA, while the second threshold T 2  being lower than the first threshold T 1  is set to be the threshold TA when the width W of the sheet S is smaller than the predetermined width WA. In other words, when the width W of the sheet S is greater than or equal to the predetermined width WA, the cooling operation to let the lateral end areas of the fixing belt  110  cool down may be restrained from being performed. 
     Further, while the side thermistor  400 A is disposed laterally outside the maximum-width sheet pathway Wmax with regard to the widthwise direction, the side thermistor  400 A may detect the temperature in the area of the fixing belt  110 , in which the temperature is not absorbed by the sheet S, so that the cooling operation may be effectively performed depending on the temperature in the lateral end areas. In other words, the lateral end areas of the fixing belt  110  may be efficiently prevented from being overheated. 
     Moreover, while the third threshold is set to be higher than the second threshold T 2 , the cooling operation may be activated before the thermostat  400 B shuts down the electricity to the heater  120 . 
     The sheet S passes through the position corresponding to the thermostat  400 B when the width W of the sheet S is greater than or equal to the predetermined width WA. Therefore, even when the temperature T in the fixing belt  110  detected by the side thermistor  400 A exceeds the third threshold, the temperature in the part of the fixing belt  110  corresponding to the position of the thermostat  400 B may not necessarily exceed the third threshold. In this regard, according to the present embodiment, the third threshold is smaller than the first threshold T 1 . Therefore, it may be prevented that the cooling operation is activated when the thermostat  400 B does not shut down the electricity to the heater  120 . 
     Although an example of carrying out the invention has been described, those skilled in the art will appreciate that there are numerous variations and permutations of the image forming apparatus, the method and the program to control the image forming program that fall within the spirit and scope of the invention as set forth in the appended claims. It is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or act described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims. In the meantime, the terms used to represent the components in the above embodiment may not necessarily agree identically with the terms recited in the appended claims, but the terms used in the above embodiment may merely be regarded as examples of the claimed subject matters. 
     Next, modified examples of the above embodiment will be described below. In the following examples, items or structures which are the same as or similar to the items or the structures described in the previous embodiment will be referred to by the same reference signs, and description of those will be omitted. 
     For example, in the threshold-setting operation, the second threshold T 2  may not necessarily be set to be the threshold TA each time when the width W of the sheet S obtained in the obtaining operation is smaller than the predetermined width WA. But the controller  8  may set the second threshold T 2  to be the threshold TA after holding for a predetermined length of period from a point, at which the width W of the sheet S obtained in the obtaining operation changes from the predetermined width WA or greater to the smaller width which is smaller than the predetermined width WA. 
     More specifically, as shown in  FIG. 7 , in S 12 , when the controller  8  determines that the width W of the sheet S to be smaller than the predetermined width WA (S 12 : YES), in S 21 , the controller  8  may determine if the predetermined length of period elapsed after the width W of the sheet S indicated the smaller width than the predetermined width WA. 
     In S 21 , if the controller  8  does not determine that the predetermined length of period elapsed after the width W of the sheet S indicated the smaller width than the predetermined width WA (S 21 : NO), in S 23 , the controller  8  may set the first threshold T 1  to be the threshold TA. On the other hand, in S 21 , if the controller  8  determines that the predetermined length of period elapsed after the width W of the sheet S indicated the smaller width than the predetermined width WA (S 21 : YES), in S 22 , the controller  8  may set the second threshold T 2  to be the threshold TA. After setting the threshold TA in S 23  or S 22 , the flow may proceed to S 15 . 
     Thus, by maintaining the threshold TA at the first threshold TA for the predetermined length of period after the width W of the sheet S changes from the greater width to the smaller width, the cooling operation may be prevented from being activated too early after the threshold TA is switched to the smaller value. 
     For another example, in the threshold-setting process, the threshold TA may be decreased gradually from the first threshold T 1  to the second threshold T 2  when the width W of the sheet S obtained in the obtaining operation changes from the value greater than or equal to the predetermined width WA to the value smaller than the predetermined width WA. 
     In particular, as shown in  FIG. 8 , after determining that the width W of the sheet S is smaller than the predetermined width WA in S 12  (S 12 : YES), in S 31 , the controller  8  may subtract a constant C 1  from the current threshold TA. In S 32 , the controller  8  may compare the subtracted difference with the second threshold T 2  and set a larger value to be a new threshold TA. The flow may thereafter proceed to S 15 . 
     With this flow, as shown in  FIG. 9 , after the width W of the sheet S obtained in the obtaining operation changes from the predetermined width WA or greater to the width smaller than the predetermined width WA (time: t 1 ) and until the predetermined length of period elapses (time: t 2 ), a plurality of thresholds, which decrease gradually by the constant C 1  from the first threshold T 1  to the second threshold T 2 , may be set to be the threshold TA. After the predetermined length of period (time: t 2  and onward), the second threshold T 2  should be greater than the difference calculated by subtracting the constant C 1  from the threshold TA; therefore, the threshold TA may stay at the second threshold T 2 . 
     For another example, the fixing unit  100  may not necessarily be equipped with the fixing belt  110  that is heated by the heater  120  to fix the image on the sheet S, but a fixing unit  200  may be equipped with, for example, a heat roller  210  as shown in  FIG. 10A . 
     In the fixing unit  200 , the heat roller  210  may be formed in a cylindrical shape, in which a heater  220  to heat the heat roller  210  is disposed, and a pressure roller  230  may be urged against the heat roller  210 . As the sheet S passes through an intermediate position between the heat roller  210  and the pressure roller  230 , the sheet S may be heated. 
     In the previous embodiment, the lateral thermistor  400 A detects the temperature in the first detectable part  134 A of the nipper board  130  to detect the lateral end part of the fixing belt  110  indirectly. Meanwhile, according to the above-mentioned configuration as shown in  FIG. 10A , a side thermistor  250  may directly contact the lateral end part of the heat roller  210  to detect the temperature therein. 
     In particular, the lateral thermistor  250  may be supported by a frame  240 , which may accommodate the heat roller  210  and the pressure roller  230 , and may contact a surface of the heat roller  210 . In this regard, as shown in  FIG. 10B , the side thermistor  250  may be disposed laterally outside the maximum width sheet pathway Wmax with regard to the widthwise direction. Meanwhile, a thermostat  260  and a central thermistor  270  may be arranged to face with and spaced apart from the heat roller  210 , as shown in  FIG. 10A . 
     As shown in  FIGS. 10A-10B , the thermostat  260  may be fixed to the frame  240  in an upper position with respect to the heat roller  210 , inside the maximum width sheet pathway Wmax and laterally outside the minimum width sheet pathway Wmi with regard to the widthwise direction. The thermostat  260  may be configured to shut down the electricity to the heater  220  when the temperature in a part of the heat roller  210  facing with the thermostat  260 , which may be estimated based on a temperature detected by the thermostat  260 , is determined to be higher than or equal to the third threshold. 
     Meanwhile, the central thermistor  270  may be supported by a supporting member  241 , which may be fixed to the frame  240 , in a position laterally inside the minimum width sheet pathway Wmin with regard to the widthwise direction, and may be configured to detect a temperature in a part of the heat roller  210  facing with the central thermistor  270 . 
     In the previous embodiment, the temperature in the lateral end part of the fixing belt  110  may be lowered by lowering the temperature in the central part of the fixing belt  110  and lowering the conveying velocity VA to convey the sheet S in the cooling operation, which may be activated by the output from the cooling-commander unit  85 . However, a cooling method to lower the temperature in the fixing belt  110  may not necessarily be limited to that described above. For example, the temperature in the lateral end part of the fixing belt  110  may be lowered by stopping the printing operation for a predetermined length of period or by placing a longer interval between the sheets S being fed according to output from the cooling-commander unit  85 . Additionally or alternatively, a cooling device such as a fan may be employed. 
     For another example, in the obtaining operation, the width W of the sheet S may not necessarily be determined based on the output from the sheet sensor  9  but may be determined by output from a sensor (not shown), which detects a position of the paired side-guides  34 . For another example, the width W of the sheet S being conveyed may be determined based on changes of the temperatures in the central thermistor  400 C and the side thermistor  400 A. 
     For another example, the width W of the sheet S may not necessarily be determined between the two values: the value greater than or equal to the predetermined width WA; or the value smaller than the predetermined width WA. But the width W of the sheet S may be determined among three or more width ranges. In this regard, the threshold TA may be set among three or more values, each of which corresponds to one of the three or more width ranges. Additionally or alternatively, the methods to lower the temperature in the fixing belt  110  may be varied depending on the thresholds TA. 
     For another example, the paired side-guides  34  may not necessarily be configured to move to be closer to each other toward the widthwise center to restrict the positions of the widthwise ends of the sheets S, but one of the paired side-guides may be movable to be closer to the other of the paired side-guides along the widthwise direction so that one of the widthwise ends of the sheet S on the movable one of the paired side-guides may be restricted. In this regard, the side thermistor may be arranged to detect a temperature in the lateral end part of the fixing belt  110  on the side of the movable one of the paired side-guides with regard to the widthwise direction. 
     For another example, the sheet S may not necessarily be paper but may be, for example, an OHP sheet.