Patent Publication Number: US-8983332-B2

Title: Image forming apparatus and method of controlling the same

Description:
This application is based on Japanese Patent Application No. 2012-62032 filed with the Japan Patent Office on Mar. 19, 2012, the entire content of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure relates to an image forming apparatus and a method of controlling the same, and particularly to an image forming apparatus including a fixing unit for fixing a toner image on a recording medium and a method of controlling the same. 
     2. Description of the Related Art 
     Various techniques have been conventionally studied to avoid emissions of ozone, toner dust and the like generated in an electrophotographic image forming apparatus to the outside of the apparatus. 
     It has been discovered in recent years that not only ozone and toner dust, but also ultra fine particles (hereinafter also referred to as “UFP” when appropriate) having a particle size of 100 nm or less are generated in such image forming apparatus. A technique of suppressing UFP emissions to the outside of an apparatus is proposed in Japanese Laid-Open Patent Publication No. 2010-002803, for example. In this document, an image forming apparatus adopts means for collecting dust electrostatically and means for suctioning dust by a cyclone. 
     One problem suffered by the conventional technique such as described above, however, was that the provision of the collecting means and suctioning means as described above results in increase in structural complexity and size of the apparatus. 
     SUMMARY OF THE INVENTION 
     The present disclosure has been made in view of this situation, and an object of the present disclosure is to provide an image forming apparatus capable of achieving suppressed emission of dust to the outside of the apparatus while avoiding increase in structural complexity and size of the apparatus, and a method of controlling the same. 
     In accordance with one aspect, an image forming apparatus having an outline surrounded by a casing, for forming an image on a recording medium, is provided. The casing has an opening. The image forming apparatus includes a fixing unit configured to fix an image formed on the recording medium. The fixing unit includes a heating fixing member. The image forming apparatus further includes a control unit configured to control an opening degree of the opening, and a first detection unit configured to detect temperature of a surface of or in the vicinity of the heating fixing member. The control unit is configured to control the opening degree of the opening based on the temperature detected by the first detection unit, so as to prevent emission of a substance by an air current to outside of the casing through the opening, during an image forming operation. 
     Preferably, in a case where the temperature detected by the first detection unit exceeds a first set value, the control unit is configured to reduce the opening degree of the opening compared to the opening degree when the temperature is equal to or lower than the first set value, and after the opening degree of the opening has been reduced due to the temperature detected by the first detection unit exceeding the first set value, in a case where the temperature detected by the first detection unit reaches a second set value or lower which is a value equal to or lower than the first set value, the control unit is configured to cancel the reduction of the opening degree of the opening. 
     Still preferably, the control unit is configured not to reduce the opening degree of the opening even in a case where the temperature detected by the first detection unit exceeds the first set value, when a particular condition is satisfied. 
     Still preferably, the image forming apparatus further includes a second detection unit configured to detect temperature in the vicinity of the opening, in which the particular condition is that the temperature detected by the second detection unit exceeds a particular temperature. 
     Still preferably, the particular condition is set based on a number of recording media on which an image is formed by the image formation unit. 
     Still preferably, the particular condition is set based on a period of time during which the control unit continues the reduction of the opening degree of the opening. 
     Preferably, the control unit is configured to reduce the opening degree of the opening until the opening is closed. 
     Preferably, the image forming apparatus further includes a member for varying the opening degree of the opening. The control unit is configured to control the opening degree of the opening by displacing the member. The member contains metal. 
     In accordance with another aspect, a method of controlling an image forming apparatus having an outline surrounded by a casing, for forming an image on a recording medium, is provided. The image forming apparatus includes a fixing unit configured to fix an image formed on the recording medium and a control unit configured to control operations of the image forming apparatus. The casing has an opening. The fixing unit includes a heating fixing member. The control method includes the steps of detecting, by the image forming apparatus, a temperature of a surface of or in the vicinity of the heating fixing member during an image forming operation, and controlling, by the image forming apparatus, an opening degree of the opening based on the temperature of a surface of or in the vicinity of the heating fixing member, so as to prevent emission of a substance by an air current to outside of the casing through the opening. 
     According to the present disclosure, emission of dust to the outside of the image forming apparatus can be suppressed while increase in structural complexity and size of the image forming apparatus is avoided. 
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram schematically showing an internal structure of a black-and-white printer in one embodiment of an image forming apparatus. 
         FIG. 2  is a diagram showing a hardware configuration of the printer shown in  FIG. 1 . 
         FIG. 3  is a diagram schematically showing an example of an opened/closed state of a louver. 
         FIG. 4  is a diagram schematically showing an example of a state where an opening degree of an air discharge opening has been reduced from the state shown in  FIG. 3 . 
         FIG. 5  is a flowchart of a process executed during an image forming operation in the printer shown in  FIG. 1 . 
         FIG. 6A  is a diagram for explaining an effect of controlling opening/closing of the louver in the printer shown in  FIG. 1 . 
         FIG. 6B  is a diagram for explaining an effect of controlling opening/closing of the louver in the printer shown in  FIG. 1 . 
         FIG. 7  is a diagram for explaining a manner in which a casing is cooled by a fan provided in a modification of the printer shown in  FIG. 1 . 
         FIG. 8  is a diagram showing a hardware configuration of the modification of the printer shown in  FIG. 1 . 
         FIG. 9  is a flowchart of a process of controlling opening/closing of the louver in a modification of the printer shown in  FIG. 1 . 
         FIG. 10  is a flowchart of a process of controlling opening/closing of the louver in a modification of the printer shown in  FIG. 1 . 
         FIG. 11  is a flowchart of a process of controlling opening/closing of the louver in a modification of the printer shown in  FIG. 1 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of an image forming apparatus will be described hereinafter with reference to the drawings. In the following description, the same components and elements are designated by the same characters, and redundant description will not be repeated. 
     &lt;General Structure of Image Forming Apparatus&gt; 
     Referring to  FIG. 1 , a structure of an image forming apparatus is generally described.  FIG. 1  is a diagram schematically showing an internal structure of a black-and-white printer (hereinafter referred to simply as “printer”) in one embodiment of the image forming apparatus. 
     A printer  10  has an outline surrounded by a casing  10 A. Casing  10 A includes therein a charging device  11 , a developing device  12 , a transferring device  13 , a photoreceptor  14 , and a fixing device  30 . An arrow A 2  indicates a delivery path of a sheet (one example of a recording medium) on which an image is formed in printer  10 . 
     In printer  10 , charging device  11  forms an electrostatic latent image on a surface of photoreceptor  14 . Photoreceptor  14  is rotated in a direction of an arrow A 11  by a not-shown rotation mechanism (which is included in an image formation unit  110  to be described later). Developing device  12  supplies toner to the surface of photoreceptor  14  on which the electrostatic latent image has been formed, to form a toner image on the surface of photoreceptor  14 . The sheet passes between photoreceptor  14  and transferring device  13 , to transfer the toner image on the surface of photoreceptor  14  onto the sheet. An image is thus formed on the sheet. Fixing device  30  heats the sheet that has been delivered to fixing device  30 , to fix the image formed on the sheet. 
     Fixing device  30  includes a fixing roller  31 , and a temperature sensor  32  arranged in the vicinity of fixing roller  31  for detecting temperature in the vicinity of fixing roller  31 . Fixing roller  31  is heated by a not-shown heater (which is included in image formation unit  110  to be described later). 
     Casing  10 A is provided with an air discharge opening  20 . A filter and a louver are provided in the vicinity of air discharge opening  20  (the portion of an ellipse R indicated by a chain-dotted line in  FIG. 1 ), as will be described later with reference to  FIG. 3 . Printer  10  includes a fan (fan  160 ) for blowing air into casing  10 A, as will be described later with reference to  FIG. 2 . The wind from the fan causes the air in casing  10 A to be discharged through the filter and louver to the outside of casing  10 A. An arrow A 1  indicates a flow of air to be discharged. 
     &lt;Hardware Configuration of Image Forming Apparatus&gt; 
     Referring to  FIG. 2 , an example of hardware configuration of the image forming apparatus is described.  FIG. 2  is a diagram showing an example of hardware configuration of printer  10  shown in  FIG. 1 . 
     Referring to  FIG. 2 , printer  10  includes a CPU (Central Processing Unit)  101  for controlling the entire apparatus, a RAM (Random Access Memory)  103  for temporarily storing data, a ROM (Read Only Memory)  105  for storing a program, a constant and the like, a storage unit  107  for storing image data and the like, an operation panel  130  for receiving operations by a user, image formation unit  110  for printing image data on a sheet, a delivery unit  120  having a motor and the like for delivering the sheet in the direction of arrow A 2  in printer  10 , and a communication unit  140  for controlling communication between components in printer  10  and communication with equipment present outside of printer  10 . 
     Operation panel  130  includes a display screen  131  for displaying a state of printer  10  and command options, and an input key  132  for inputting a numerical value and the like. Display screen  131  and input key  132  may be implemented as a touch panel. The touch panel includes a liquid crystal display and a touch sensor mounted thereon, for example. 
     Printer  10  includes temperature sensor  32 , a temperature sensor  29 , a motor for louver  150 , and fan  160 . Temperature sensor  29  detects a temperature of the louver provided in the vicinity of air discharge opening  20 , as will be described later with reference to  FIG. 3  and the like. Motor for louver  150  is a motor for varying an opening degree of the louver. Fan  160  is a fan for discharging the air in casing  10 A to the outside, as described above. 
     During an image forming operation in printer  10 , CPU  101  controls driving of motor for louver  150  based on the detected temperature from temperature sensor  32  and the like. As such, the opening degree of the louver is controlled based on the detected temperature from temperature sensor  32  and the like. In printer  10 , the opening degree of the louver is controlled to control an opening degree of air discharge opening  20 . 
     In this embodiment, “during an image forming operation” refers to a state where printer  10  has been turned on and is operating to form an image. That is, “during an image forming operation” includes not only a period of time during which an image is formed on a sheet that has been delivered into casing  10 A in printer  10 , but also a warm-up period during recovery from a sleep state or immediately after turn-on of printer  10 , a period of time between completion of the image forming operation and transition to a sleep state, and the like. 
     &lt;Opening/Closing of Louver&gt; 
     As described above, in printer  10 , opening/closing of the louver provided in the vicinity of air discharge opening  20  is controlled based on the temperature in the vicinity of fixing roller  31  and the like. Referring to  FIGS. 3 and 4 , an operation of the louver is described. 
       FIG. 3  is a diagram schematically showing an example of an opened/closed state of air discharge opening  20  (louver).  FIG. 4  is a diagram schematically showing an example of a state where the opening degree of air discharge opening  20  (louver) has been reduced from the state shown in  FIG. 3 . It is noted that each of  FIGS. 3 and 4  corresponds to an enlarged view of ellipse R and its vicinity shown in  FIG. 1 . 
     Referring first to  FIG. 3 , air discharge opening  20  is provided with a louver  21  including one or more blades. Temperature sensor  29  is arranged on part of a blade of louver  21 . In  FIG. 3 , an arrow A 3  indicates a flow of air delivered from the inside of casing  10 A (see  FIG. 1 ) to the outside. A filter  22  is provided upstream of louver  21  in this flow. Since an ambient temperature in fixing device  30  becomes relatively high, filter  22  is formed of a material resistant to damage such as melting due to the air of relatively high temperature flowing from fixing device  30 . The air indicated by arrow A 3  passes through filter  22  and a region provided with louver  21 , and is delivered to the outside of casing  10 A. 
     In  FIG. 4 , the blades of louver  21  are rotated to move from the state shown in  FIG. 3 , to close air discharge opening  20 . As a result, even if the air is delivered by fan  160  as indicated by an arrow A 4 , the path of the air to the outside of casing  10 A is interrupted (or blocked). 
     During the image forming operation in printer  10 , louver  21  is basically in the state shown in  FIG. 3 . When fixing roller  31  reaches a high temperature, louver  21  is closed to reduce the opening degree of air discharge opening  20 , as shown in  FIG. 4 . As a result, emissions of ozone, toner dust and the like, as well as UFPs and the like considered to be generated in casing  10 A at a high temperature, to the outside of casing  10 A is avoided, and condensation of the UPFs in casing  10 A and adhesion of the UFPs to an inner wall of casing  10 A are facilitated. 
     When the opening degree of air discharge opening  20  is reduced on condition that fixing roller  31  reaches a high temperature as described above, a situation is envisioned where the temperature in casing  10 A increases. Thus, in order to be able to efficiently release heat in casing  10 A to the outside even when air discharge opening  20  has a low opening degree, louver  21  is preferably made of a material having a high thermal conductivity such as metal including iron, copper, aluminum, or their alloys. 
     &lt;Control of Opening/Closing of Louver&gt; 
     Referring to  FIG. 5 , control of opening/closing of louver  21  is described.  FIG. 5  is a flowchart of the process of controlling opening/closing of louver  21  executed by CPU  101  during the image forming operation in printer  10 . 
     Referring to  FIG. 5 , upon turn-on of printer  10 , in step S 10 , CPU  101  drives motor for louver  150  so that louver  21  is in the state such as shown in  FIG. 3 , and proceeds the control process to step S 20 . 
     In step S 20 , CPU  101  determines whether or not a surface temperature of fixing roller  31  derived based on a detected temperature from temperature sensor  32  (hereinafter referred to simply as “surface temperature of fixing roller  31 ”) has exceeded 170° C. If it is determined that the surface temperature has exceeded 170° C. (YES in step S 30 ), CPU  101  proceeds the control process to step S 30 . On the other hand, if it is determined that the surface temperature is equal to or lower than 170° C. (NO in step S 30 ), CPU  101  proceeds the control process to step S 40 . 
     In this embodiment, the surface temperature of fixing roller  31  may be derived based on the temperature in the vicinity of fixing roller  31  detected by temperature sensor  32 . Specifically, the surface temperature of fixing roller  31  is derived by calculating the product of the temperature in the vicinity of fixing roller  31  and a correction coefficient, for example. The surface temperature of fixing roller  31  may be obtained by detecting the temperature in the vicinity of the surface of fixing roller  31  using a noncontact sensor, for example. 
     In step S 40 , CPU  101  drives motor for louver  150  so that louver  21  is in the state where the air flows through opening  20  (see  FIG. 3 ), and returns the control process to step S 20 . In step S 40 , CPU  101  checks the state of louver  21  at that point in time. Then, if louver  21  is already in the state shown in  FIG. 3 , CPU  101  does not drive motor for louver  150  in step S 40  and returns the control process to step S 20 . 
     In step S 30 , on the other hand, CPU  101  drives motor for louver  150  so that louver  21  is in the state where the air is less likely to flow through opening  20  than in the state shown in  FIG. 3  (see  FIG. 4 ), and returns the control process to step S 20 . In step S 30 , CPU  101  checks the state of louver  21  at that point in time. Then, if louver  21  is already in the state shown in  FIG. 4 , CPU  101  does not drive motor for louver  150  in step S 30  and returns the control process to step S 20 . 
     In the embodiment described above, when the surface temperature of fixing roller  31  is equal to or lower than 170° C., louver  21  is controlled to be in the state shown in  FIG. 3 . When the surface temperature exceeds 170° C., on the other hand, louver  21  is controlled to be in the state where the air is less likely to flow through opening  20  than in the state shown in  FIG. 3  (see  FIG. 4 ), that is, a state where the opening degree of air discharge opening  20  is reduced compared to that when the surface temperature is equal to or lower than 170° C. 
     &lt;Example of Control Result&gt; 
       FIGS. 6A and 6B  are diagrams for explaining an effect of the control of opening/closing of louver  21  in printer  10 .  FIG. 6A  shows a temporal variation in temperature of fixing roller  31 .  FIG. 6B  shows a temporal variation in UFP concentration outside of casing  10 A. In each of  FIGS. 6A and 6B , a horizontal axis represents time that has elapsed since printer  10  was turned on. 
     Referring to  FIG. 6A , after turn-on of printer  10 , a warm-up operation is performed (“WU” in  FIG. 6A ). During this period of time, fixing roller  31  is heated, causing increase in surface temperature of fixing roller  31  as well. 
     At the start of the warm-up operation, louver  21  is controlled to be in the state shown in  FIG. 3 . 
     Then, when the surface temperature of fixing roller  31  exceeds 170° C., louver  21  is closed to change the state of louver  21  from that shown in  FIG. 3  to that shown in  FIG. 4 . During the state change of louver  21 , printer  10  makes a transition to a standby state. Then, when the surface temperature of fixing roller  31  becomes equal to or lower than 170° C., louver  21  is opened again. That is, louver  21  is controlled to make a transition to the state shown in  FIG. 3 . 
     When the image forming operation is performed (“PRINT” in  FIG. 6A ) and completed in printer  10 , the surface temperature of fixing roller  31  exceeds 170° C. again in  FIG. 6A . Louver  21  is closed accordingly. Then, when the surface temperature of fixing roller  31  becomes equal to or lower than 170° C., louver  21  is opened again. 
     In  FIG. 6B , a solid line indicates a detected amount of UFPs when the control described with reference to  FIG. 5  was performed. On the other hand, a chain-dotted line in  FIG. 6B  indicates a detected amount of UFPs when louver  21  was kept in the opened state (see  FIG. 3 ) regardless of the surface temperature of fixing roller  31 . 
     The detected concentration of UFPs indicated by the chain-dotted line in  FIG. 6B  varies in a manner substantially similar to that of the temperature variation shown in  FIG. 6A . That is, the detected concentration of UFPs increases as the surface temperature of fixing roller  31  increases. Particularly, (during standby) after completion of the warm-up and after completion of the image forming operation in printer  10 , an overshoot (abrupt increase) is temporarily seen. 
     On the other hand, as indicated by the solid line in  FIG. 6B , the overshoot seen (during standby) after completion of the warm-up and after completion of the image forming operation in printer  10  is eliminated when the control in this embodiment ( FIG. 5 ) is performed. Specifically, although the UFP concentration exhibits increase in accordance with the behavior of the surface temperature of fixing roller  31  while louver  21  is opened, the increase stops when louver  21  is closed. 
     In view of the above, an overshoot in UFP concentration can be avoided by controlling opening/closing of louver  21  in accordance with the surface temperature of fixing roller  31  in this embodiment. In printer  10 , toner and ozone not emitted to the outside of casing  10 A adheres to a bottom surface and inner wall of casing  10 A. As a result, an amount of these emissions to the outside of casing  10 A can be reduced. 
     &lt;Modification (1)&gt; 
     Printer  10  in a modification (1) can have a configuration similar to that of printer  10  described above, except for the following features described below. The difference of printer  10  in the modification (1) from above-described printer  10  is described below. 
     During the control of opening/closing of louver  21  described with reference to  FIG. 5 , louver  21  is closed when the surface temperature of fixing roller  31  exceeds 170° C. and opened when the surface temperature becomes equal to or lower than 170° C. In this case, if the surface temperature of fixing roller  31  varies at short intervals in the vicinity of 170° C., a situation is envisioned where louver  21  is constantly switched between the opened state and the closed state. 
     In order to avoid such situation, during the control of opening/closing of louver  21 , the states of louver  21  may be switched so that louver  21  is closed when the surface temperature of fixing roller  31  exceeds a first temperature and louver  21  is opened when the surface temperature becomes equal to or lower than a second temperature lower than the first temperature. The control of opening/closing of louver  21  employing the first and second temperatures can handle a time lag between the variation in detected temperature of temperature sensor  32  and the variation in the detected amount of UFPs outside of casing  10 A. 
     During the opening/closing control in the modification (1), if switching from the opened state to the closed state and switching from the closed state to the opened state of fixing roller  31  are performed at the same temperature, that is, if the first temperature and the second temperature are identical to each other, a situation is envisioned where louver  21  is constantly switched between the opened state and the closed state. In order to avoid the constant switching, CPU  101  may switch between the opening and closing of fixing roller  31  by acquiring temperature detected by temperature sensor  32  at regular time intervals, or by utilizing an average value of a temperature detected by temperature sensor  32  after a certain period of time. 
     &lt;Modification (2)&gt; 
     Printer  10  in a modification (2) can have a configuration similar to that of above-described printer  10 , except for the following features described below. The difference of printer  10  in the modification (2) from above-described printer  10  is described below. 
     In printer  10 , air discharge opening  20  is closed when fixing roller  31  reaches a high temperature. In order to suppress temperature increase in casing  10 A resulting from such control, printer  10  may further include another fan separate from fan  160 . 
       FIG. 7  is a diagram for explaining a manner in which casing  10 A is cooled by another fan separate from fan  160 .  FIG. 8  is a diagram showing a hardware configuration of printer  10  in the modification (2). 
     Referring to  FIG. 7 , in the modification ( 2 ), a separately provided exterior fan  23  cools louver  21  provided at air discharge opening  20  from outside of casing  10 A. Arrow A 5  indicates a path of air delivered from the inside of casing  10 A to the outside. Arrow A 6  indicates a flow of air delivered from the exterior fan  23  to the louver  21 . If louver  21  is made of a material having a high thermal conductivity as described above, the interior of casing  10 A of printer  10  in this modification is efficiently cooled by exterior fan  23  even while louver  21  is closed. In addition, such cooling can facilitate condensation of UPFs in casing  10 A and adhesion of UFPs to the inner wall of casing  10 A. 
     Referring to  FIG. 8 , the operation of exterior fan  23  is controlled by CPU  101 . CPU  101  causes exterior fan  23  to rotate while printer  10  performs the image forming operation and while louver  21  is closed in step S 30  of  FIG. 5 , for example. 
     &lt;Modification (3)&gt; 
     Printer  10  in a modification (3) can have a configuration similar to that of above-described printer  10 , except for the following features described below. The difference of printer  10  in the modification (3) from above-described printer  10  is described below. 
     During the control of opening/closing of louver  21  in the modification (3), louver  21  is opened even when the surface temperature of fixing roller  31  exceeds 170° C., if a particular condition is satisfied. 
     The particular condition in the modification (3) is that temperature in the vicinity of air discharge opening  20  (hereinafter also referred to as opening-vicinity temperature in casing  10 A) exceeds a particular temperature. That is, in the modification (3), louver  21  is opened even when the surface temperature of fixing roller  31  exceeds 170° C., if the temperature in the vicinity of air discharge opening  20  exceeds a particular temperature. 
       FIG. 9  is a flowchart of the process of controlling opening/closing of louver  21  in this modification. 
     Referring to  FIG. 9 , in the opening/closing process in this modification, if it is determined in step S 20  that the surface temperature of fixing roller  31  has exceeded 170° C., CPU  101  proceeds the control process to step S 21 . 
     In step S 21 , CPU  101  determines whether or not the opening-vicinity temperature has exceeded 80° C. Specifically, in step S 21 , CPU  101  determines whether or not a detected temperature from temperature sensor  29  (see  FIG. 3 ) has exceeded 80° C. If it is determined that the detected temperature from temperature sensor  29  has exceeded 80° C., CPU  101  proceeds the control process to step S 40 . If it is determined that the detected temperature is equal to or lower than 80° C., CPU  101  proceeds the control process to step S 30 . 
     Then, CPU  101  closes louver  21  in step S 30 , and opens louver  21  in step S 40 . 
     As such, in the modification (3), louver  21  is opened even when the surface temperature of fixing roller  31  exceeds 170° C., if the detected temperature from temperature sensor  29  exceeds 80° C. 
     &lt;Modification (4)&gt; 
     Printer  10  in a modification (4) can have a configuration similar to that of above-described printer  10 , except for the following features described below. The difference of printer  10  in the modification (4) from above-described printer  10  is described below. 
     During the control of opening/closing of louver  21  in the modification (4), louver  21  is opened even when the surface temperature of fixing roller  31  exceeds 170° C., if a particular condition is satisfied. 
     The particular condition in the modification (4) is that the number of recording media (sheets) successively subjected to image formation (printing) in printer  10  exceeds a particular number. That is, in the modification (4), louver  21  is opened even when the surface temperature of fixing roller  31  exceeds 170° C., if the number of recording media (sheets) successively subjected to image formation (printing) in printer  10  exceeds a particular number. 
       FIG. 10  is a flowchart of the process of controlling opening/closing of louver  21  in this modification. 
     Referring to  FIG. 10 , in the opening/closing process in the modification (4), if it is determined in step S 20  that the surface temperature of fixing roller  31  has exceeded 170° C., CPU  101  proceeds the control process to step S 22 . 
     In step S 22 , CPU  101  determines whether or not the number of sheets successively subjected to printing in printer  10  has exceeded 200. CPU  101  implements the determination in step S 22  by acquiring, from image formation unit  110 , the number of sheets that have been successively subjected to printing that is counted in image formation unit  110 , for example. If it is determined that the number of successively printed sheets has exceeded 200, CPU  101  proceeds the control process to step S 40 . If it is determined that the number is equal to or lower than 200, CPU  101  proceeds the control process to step S 30 . 
     Then, CPU  101  closes louver  21  in step S 30 , and opens louver  21  in step S 40 . 
     As such, in the modification (4), louver  21  is opened even when the surface temperature of fixing roller  31  exceeds 170° C., if the number of successively printed sheets exceeds 200. It is noted that “successively” means that a printing operation on the next sheet is started before a certain period of time has elapsed since a printing operation on a previous sheet in printer  10 , for example. 
     &lt;Modification (5)&gt; 
     Printer  10  in a modification (5) can have a configuration similar to that of above-described printer  10 , except for the following features described below. The difference of printer  10  in the modification (5) from above-described printer  10  described above is described below. 
     During the control of opening/closing of louver  21  in the modification (5), louver  21  is opened even when the surface temperature of fixing roller  31  exceeds 170° C., if a particular condition is satisfied. 
     The particular condition in the modification (5) is that a state where louver  21  is closed due to the surface temperature exceeding 170° C. continues for a prescribed period of time or longer. That is, in the modification (5), louver  21  is opened even when the surface temperature of fixing roller  31  exceeds 170° C., if louver  21  is closed due to the surface temperature of fixing roller  31  exceeding 170° C. and such state continues for a prescribed period of time or longer in printer  10 . 
       FIG. 11  is a flowchart of the process of controlling opening/closing of louver  21  in the modification (5). 
     Referring to  FIG. 11 , in the opening/closing process in the modification (5), if it is determined in step S 20  that the surface temperature of fixing roller  31  has exceeded 170° C., CPU  101  proceeds the control process to step S 23 . 
     In step S 23 , CPU  101  determines whether or not a period of time during which louver  21  is closed in step S 30  (hereinafter referred to as “duration”) in printer  10  has exceeded 3 minutes. If it is determined that the duration has exceeded 3 minutes, CPU  101  proceeds the control process to step S 40 . If it is determined that the duration is equal to or shorter than 3 minutes, CPU  101  proceeds the control process to step S 30 . 
     Then, CPU  101  closes louver  21  in step S 30 , and opens louver  21  in step S 40 . 
     As such, in the modification (5), louver  21  is opened even when the surface temperature of fixing roller  31  exceeds 170° C., if the above “duration” exceeds 3 minutes. 
     &lt;Other Modifications&gt; 
     According to the embodiment and its modifications described above, opening/closing of louver  21  is controlled based on the surface temperature of fixing roller  31 . As such, in printer  10 , emissions of dust including UFPs, ozone and toner to the outside of casing  10 A by an air current in casing  10 A can be suppressed without requiring increase in structural complexity and size of the apparatus. 
     In the embodiment and its modifications, the closed state of louver  21  shown in  FIG. 4  is only required to have an opening degree of air discharge opening  20  lower than at least the opening degree of the opened state shown in  FIG. 3 . Here, the opening degree refers to a degree to which the air passes through air discharge opening  20 . The higher the opening degree, the easier for the air to pass through air discharge opening  20 . 
     In printer  10 , when the surface temperature of fixing roller  31  exceeds 170° C., louver  21  is displaced so that air discharge opening  20  has a relatively low opening degree. In this case, in order to more reliably avoid discharge of air and the like from the inside of casing  10 A to the outside, louver  21  is preferably displaced so that air discharge opening  20  has an opening degree of zero, that is, until a rotational position to close air discharge opening  20 . 
     Moreover, in the embodiment and its modifications described above, the operation of fan  160  is also preferably suspended during the period of time during which louver  21  is closed in the process of step S 30 . 
     While a black-and-white printer has been illustrated as an example of the image forming apparatus in the embodiment and its modifications described above, the image forming apparatus may be an apparatus of another type as long as it includes a fixing unit. 
     For example, the image forming apparatus may be a four-cycle type image forming apparatus including four developing devices of different colors around a rotation axis of photoreceptor  14 , which are successively caused to face an electrostatic latent image carrier to produce a full-color image. 
     Alternatively, the image forming apparatus may be a complex machine further having a facsimile functionality and scan functionality. 
     Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the scope of the present invention being interpreted by the terms of the appended claims.