Patent Publication Number: US-9417607-B2

Title: Image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus which forms an image by an electrophotographic method such as a copying machine and a printer, in particular to an image forming apparatus having a cooling portion for preventing a temperature rise in the apparatus. 
     2. Description of the Related Art 
     When an image forming apparatus operates continuously, a temperature of parts of the apparatus rises. Parts of which temperature rises include a developing device where a friction heat is easily produced by toner agitation, a fixing device for fixing toner to a sheet, a power supply, a motor unit, and the like. 
     Currently, a toner having thermal fixing property is generally used. When dealing with toner of heat fixing type, a temperature suitable for optimally forming an image before fixing is necessary and acceptable temperature range is generally narrow. Therefore, when the temperature of the toner is increased by the influence of frictional heat of the toner itself and a temperature rise of the parts in the apparatus, a print may be degraded due to a low charge amount of the toner and a low image density. 
     For this kind of thermal management, U.S. Patent Application Publication No. 2012/0301159 A1 discloses that a toner temperature sensor is set up in the apparatus and during image formation, image density and so on is controlled by adjusting the charge amount of the toner depending on a detected temperature rise of the toner. 
     However, in recent years, downsizing and cost reduction of image forming apparatuses are progressing and it has been difficult to provide a toner temperature sensor for directly detecting the temperature of the toner in view of space and costs. Therefore, in recent years, as described in Japanese Patent Laid-Open No. 2007-322539, a temperature detecting sensor is disposed at a place to which the temperature of toner is indirectly transmitted and the temperature of the toner is predicted by using a prediction equation based on the temperature of the place. 
     However, when placing a toner temperature sensor away from the toner as described in Japanese Patent Laid-Open No. 2007-322539, it takes time for the heat of the toner to transmit and reach the sensor. As a result, a temperature change detected by the sensor follows an actual temperature change of the toner with delay. 
     For example, in the case where a temperature difference between the inside of the apparatus and the outside air due to a temperature rise of the toner inside the image forming apparatus during a long time continuous printing, when air flow rate of a cooling fan is increased in order to cool the toner, flowing of the outside air of relatively low temperature into the apparatus increases. Then, a temperature change of the toner becomes gentle. At this time, the toner temperature detecting sensor follows an actual temperature change of the toner with delay. Thus, an error occurs between the temperature predicted from the detection results of the sensor and the actual temperature. 
     When the error occurs, the controlling in accordance with the image temperature as described in U.S. Patent Application Publication No. 2012/0301159 A1 is difficult. For example, a large image density change occurs on a replication print of the same original when a flow rate of the cooling fan changes. Because of this, a quality difference occurs among the prints which are printed at the same time or productivity of prints is lowered by adjusting images in order to prevent the quality difference. Thus, in the prior art, there is a problem that maintaining both quality and productivity is difficult. 
     When this quality difference occurs in prints of the same original, a user may easily recognize it even if the difference is slight. However, when a large air flow rate is set in advance to prevent the quality difference, anticipating a large safety factor, operating noise and power consumption are increased. Further, in this case, switching an air flow rate with the provision of a temperature detecting sensor is meaningless. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to suppress the difference in quality among prints as well as to maintain productivity by suppressing the difference between the predicted temperature and the actual temperature when air flow rate of the cooling fan is changed. 
     A representative configuration of the present invention for achieving the above object is an image forming apparatus, comprising: 
     an image bearing member; 
     a developing device which supplies toner to an electrostatic latent image formed on the image bearing member; 
     a first temperature detecting unit which measures an ambient temperature of the developing device; 
     a second temperature detecting unit which measures an atmospheric temperature of a main body of the image forming apparatus; 
     a cooling unit which cools the inside of the image forming apparatus by blowing; and 
     a controller which controls an operation of the cooling unit based on detection results of the first temperature detecting unit and the second temperature detecting unit, 
     wherein when the detection result of the second temperature detecting unit remains the same temperature, the controller increases an air flow rate of the cooling unit such that the air flow rate is equal to or higher than a predetermined rate if the detection result of the first temperature detecting unit is equal to or higher than a first temperature during a non image forming operation and the controller increases an air flow rate of the cooling unit such that the air flow rate is equal to or higher than the predetermined rate if the detection result of the first temperature detecting unit is equal to or higher than a second temperature which is higher than the first temperature during an image forming operation. 
     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 cross-sectional view of the main body of a full-color laser image forming apparatus. 
         FIG. 2  is a schematic diagram of the cooling system of the process cartridge. 
         FIG. 3  is a detailed diagram of a contact part of the process cartridge having an internal sensor. 
         FIG. 4  is a block diagram of a controller. 
         FIG. 5  is a graph showing transition of the temperature of toner during image formation when an air flow rate of the cooling fan is not changed. 
         FIG. 6  is a graph showing the difference between an actual temperature and a predicted temperature of the toner during image formation when an air flow rate of the cooling fan is changed. 
         FIG. 7  is a graph showing the difference between an actual temperature and a predicted temperature of the toner when a cooling fan control of the first embodiment is performed. 
         FIG. 8  is a flowchart showing a control of an air flow rate of the cooling fan of the first embodiment. 
         FIG. 9  is a graph showing the difference between an actual temperature and a predicted temperature of the toner when a cooling fan control of the second embodiment is performed. 
         FIG. 10  is a flowchart showing a control of an air flow rate of the cooling fan of the second embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Preferred embodiments of the present invention will be described in detail referring to the drawings. However, dimensions, materials, shapes and relative positions of the components described in the following embodiments are to be appropriately changed depending on the configuration and various conditions of the apparatus to which the present invention is applied. Therefore, unless specifically stated, it is not intended to limit the scope of the present invention to them. 
     Image Forming Apparatus 
     An embodiment of an image forming apparatus of the present invention will be described with reference to figures.  FIG. 1  is a sectional view of a full-color laser image forming apparatus. The main body (apparatus main body) of the image forming apparatus  500  has an image forming unit  1  for forming a toner image. 
     The image forming unit  1  is of a four drum full color type. The image forming unit  1  has the process cartridges  10  ( 10 Y,  10 M,  10 C and  10 K) for forming a toner image of four colors including yellow (Y), magenta (M), cyan (C) and black (K). The configurations for forming toner images of these colors are similar and an explanation will be made by omitting suffixes Y, M, C and K except when necessary. 
     Other than the image forming unit, the main body of the image forming apparatus  500  has the feeding device  2  for feeding the sheet S to the image forming unit  1 , the fixing device  3  constituting a fixing unit, the discharging unit  4  for conveying and discharging the sheet after fixing, the sheet stacking unit  5  for stacking the discharged sheets and the original reading device  7  for reading an original. 
     Then, detail configurations of the respective parts will be described from the image forming unit  1 . The image forming unit  1  has fresh toner storing portions FT (FTY, FTM, FTC and FTK) which correspond to process cartridges (developing devices)  10 , respectively. 
     The process cartridge  10  has the photosensitive drum  11  (image bearing member) and the following process means acting on the photosensitive drum  11 . The process means includes a charging unit (not shown) for charging the photosensitive drum  11  by applying a predetermined voltage thereto and a developing unit (not shown) for developing an electrostatic latent image formed on the photosensitive drum  11  by adhering toner to the electrostatic latent image. A cleaning unit (not shown) is provided for cleaning toner which has not been transferred on the photosensitive drum  11   
     The laser scanner  12  is disposed under the process cartridge  10 . The laser scanner  12  draws an electrostatic latent image on the photosensitive drum  11 . The intermediate transfer unit  13  is disposed above the process cartridge  10 . The intermediate transfer unit  13  includes the intermediate transfer belt  13   a , the driving roller  13   b , the tension roller  13   c , the four primary transfer rollers  13   d  to make the intermediate transfer belt  13   a  in contact with the photosensitive drum  11 , the idler roller  13   e  and the idler roller  13   f.    
     The intermediate transfer belt  13   a  is formed of a film-like member and is rotated in the direction of the arrow by the driving force of the driving roller  13   b . Application of a predetermined transfer bias by the primary transfer roller  13   d  sequentially transfers each color toner image on the photosensitive drum  11  to the intermediate transfer belt  13   a  on a multiple fashion. As a result, a full-color toner image is formed on the intermediate transfer belt  13   a.    
     In parallel to this toner image forming operation, the sheet S is fed by the feeding device  2  and is conveyed to the sheet conveying path  20 . Skew feeding correction of the sheet S is performed by a registration roller (not shown) provided in the sheet conveying path  20  and the sheet S is aligned with the toner images on the intermediate transfer belt  13   a.    
     After the alignment, the sheet S is fed to the secondary transfer portion formed by a nip between the secondary transfer outer roller  21  and the driving roller  13   b . At the secondary transfer portion, by the secondary transfer bias applied to the secondary transfer outer roller  21 , the toner image is transferred from the intermediate transfer belt  13   a  to the sheet S. 
     Then, the sheet S is conveyed by the driving force of the driving roller  13   b  and is fed to the fixing device  3 . Heat and pressure is added to the sheet S which is sent to the fixing device  3 , thereby color toner is melted and a full color visible image is fixed on the sheet S. 
     The discharge portion  4  is disposed above the fixing device  3 . The discharge portion  4  has the discharge passage  40 , the discharge roller pair  41 , the both sides reversing path  42  and the reverse roller pair  43 . The sheet discharged from the discharge portion  4  is stacked with the image side down in the sheet stacking portion  5 . 
     The second sensor  101  (second temperature detection unit) is provided at the front portion of the main body of the image forming apparatus  500  for measuring an ambient temperature of the main body of the image forming apparatus  500 . Data of installation environment temperature detected by the second sensor  101  are used for air flow rate control of the cooling fan  80  (cooling portion) which cools the interior of the apparatus by blowing and image formation control such as toner density control. 
     Next, an image forming operation of the image forming apparatus  500  will be explained. Firstly, the image data read by the original reading device  7  is transmitted to the controller  300  and the image data are stored as image information. 
     In the image forming unit  1 , the surface of the photosensitive drum is uniformly charged to a predetermined polarity and potential by a charging unit (not shown). Then, a laser beam is emitted from the laser scanner  12  on the basis of the image information stored in the control unit  300 . The emitted laser beam is scanned over the photosensitive drum  11 ; thereby an electrostatic latent image is formed on the photosensitive drum  11 . 
     Thereafter, toner is supplied to the electrostatic latent image from the developing unit supplied with fresh toner by the fresh toner reservoir FT; thereby an electrostatic latent image is developed and a toner image is obtained. The development of the electrostatic latent image is carried out at a developing position, that is, at the developing nip formed where the photosensitive drum  11  and the developing sleeve are opposed. 
     The developer in which toner and carrier are mixed with a predetermined ratio is accommodated in the developing unit of the process cartridge  10 . The toner and carrier are frictionally charged by a stirring screw and are provided to a developing sleeve. The developing unit has a regulating blade (not shown) for making the thickness of the developer coated on the developing blade a predetermined value. Thus, the developer having a predetermined thickness in which toner and carrier are mixed is coated on the developing sleeve. 
     The toner image borne on the photosensitive drum  11  by the developing sleeve is conveyed to the primary transfer portion of the contact portion between the intermediate transfer belt  13   a  and the photosensitive drum  11  with the rotation of the photosensitive drum  11 . A primary transfer bias is applied to the primary transfer roller  13   d . Therefore, the toner image is transferred to the intermediate transfer belt  13   a  at the primary transfer portion. The operation is sequentially performed in the four process cartridges  10 . Then, a full-color toner image is formed by multiply transferring toner images on the intermediate transfer belt  13   a . Toner which remains without being transferred is scraped from the photosensitive drum surface by a cleaning unit (not shown) of the image forming unit. 
     Factors that cause the temperature of the image forming unit  1  to increase include frictional heat between the bearing which supports the photosensitive drum  11  and the photosensitive drum  11  and frictional heat generated by rubbing of the photosensitive drum  11  and the cleaning unit. Other factors include frictional heat of the bearings for supporting the screw which charges the developing sleeve and the developer and supplies toner to the sleeve and frictional heat generated between the developing sleeve and the photosensitive drum  11 . 
     Therefore, the explanation will be made to the image forming unit  1  which has the photosensitive drum  11 , the cleaning unit, the developing sleeve, the developing screw, the bearings for support, and the developing container for containing the toner in the developing unit. 
     Cooling of the Process Cartridge 
     An arrangement of the temperature detecting unit having a characteristic configuration of this embodiment and a cooling method of the process cartridge  10  will be explained with reference to figures.  FIG. 2  is a schematic diagram showing a method of cooling the process cartridge.  FIG. 3  is a detailed view of a contact portion of the process cartridge having an internal sensor. 
     As shown in  FIG. 2 , the cooling fan  80  is a suction fan for taking the outside air into the inside of the apparatus. The outside air taken by the cooling fan  80  flows into the duct  81 . The duct  81 , as indicated by the arrow  82 , is configured to guide the outside air to the lower surface of each process cartridge  10 . Thus, by taking the outside air by the cooling fan  80  to form a flow of air, and by introducing it to the lower surface of the process cartridge  10 , the toner in the process cartridge  10  is indirectly cooled. 
     A memory tag (not shown in  FIG. 2 ) is mounted on the rear surface of the process cartridge  10 . The contact portion  90  of the image forming apparatus is in contact with the memory tag. As shown in  FIG. 3 , the contact portion  90  has the contact boards  91  ( 91 Y,  91 M,  91 C,  91 K). The memory tag records individual information for each process cartridge  10 , such as a number of used sheets. The image forming apparatus  500  receives the information of each process cartridge  10  from the memory tag through each contact substrate  91 . 
     As shown in  FIG. 3 , the first sensor  100  (first temperature detector) is disposed on the contact substrate  91 K for black color and measures the ambient temperature of the process cartridge  10 . The first sensor  100  is disposed outside the process cartridge and is adjacent to the process cartridge  10 . Thus, the first sensor  100  is never in contact with the toner in the process cartridge  10  and it does not detect the toner temperature directly. A toner temperature is predicted by using the toner temperature prediction method which will be described later based on the temperature detected by the first sensor  100 . 
     Cooling Fan and Image Formation Control 
     The control of the cooling fan will be described with reference to figures.  FIG. 4  is an explanatory diagram of the controller. The image forming apparatus  500  performs an image formation control such as an image density control of the image forming unit  1  and an air flow rate control of the cooling fan  80  based on the results of the toner temperature prediction control. 
     As shown in  FIG. 4 , the controller  300  comprising the CPU  301  and the memory  302  issues an instruction of the image forming operation of the main body of the image forming apparatus  500  and controls the cooling fan  80 . Detected temperature signals from the first sensor  100  and the second sensor  101  and a print job from the external PC  200  are input to the controller  300 . 
     The temperature prediction equation including temperatures detected by the first sensor  100  and the second sensor  101  is stored in the memory  302 . The temperature prediction equation is used for calculating the predicted temperature T from the detected temperature Ts of the first sensor  100  and the detected temperature Te of the second sensor  101 . 
     The CPU  301  calculates the predicted temperature T using the stored temperature prediction equation and thereafter issues an instruction of the command value of the rotational speed (air flow rate) of the cooling fan  80  and an instruction of the image formation control such as image density control. 
     In this embodiment, an input voltage to the cooling fan  80  is virtually controlled by using a PWM control which can vary pulse modulation width. During the image forming operation, the air flow rate of the cooling fan  80  is selected from three stages of M 0  (zero air flow rate), M 1  (half of the maximum air flow rate of the fan) and M 2  (the maximum air flow rate of the fan) in the order of magnitude of the air flow rate. 
     When the toner temperature predicted by the prediction equation described below is less than or equal to a previously defined threshold value, the air flow rate of the cooling fan  80  is made zero for suppressing noise of the operation sound of the fan. Conversely, when the toner temperature increases and exceeds a predetermined threshold temperature, the air flow rate of the cooling fan  80  is increased for suppressing temperature rise of the toner. Further, the image formation control adjusts the charge amount of the toner at all times based on the results of the toner temperature prediction. 
     Toner Temperature Prediction 
     The prediction equation of the toner temperature of this embodiment will be described.  FIG. 5  is a graph showing changes in the toner temperature at the time of image formation when the air flow rate of the cooling fan is not changed. In  FIG. 5 , the state of the image forming apparatus  500  during a printing operation is indicated when the air flow rate of the cooling fan  80  is not changed. Specifically, the transition (L 1 ) of the detected temperature Te of the second sensor  101 , the transition (L 2 ) of the detected temperature Ts of first sensor  100 , the transition (L 3 ) of the actual temperature of the toner of the inside of the process cartridge and the transition (L 4 ) of prediction temperature T are shown. Note that the predicted temperature T of the toner is a value obtained by predicting the actual temperature of the toner from the values of the temperature sensors. The vertical axis represents the toner temperature, and the horizontal axis represents the print time. 
     In  FIG. 5 , the detected temperature Te (detection result) of the second sensor  101  is substantially the same temperature. In this case, the temperature T of the toner in the process cartridge  10  of this embodiment is predicted by using the following prediction equation (Equation 1),
 
 T =α( Ts−Te )+ Te+t   (Equation 1)
 
     wherein t denotes any offset temperature, α denotes an arbitrary temperature coefficient. The offset temperature t and the temperature coefficient α are determined arbitrarily by the configuration of the image forming apparatus. In this embodiment, t=2° C., α=2. However, the prediction equation has been created based on the configuration of this embodiment and is not intended to limit the scope of the invention. 
     The second sensor  101 , as shown in  FIG. 1 , is disposed at a position away from the toner in the process cartridge. Therefore, a rise in the temperature of the second sensor  101  is lower than that of the actual temperature of the toner in the process cartridge during the same print time. In this case, the toner temperature in the process cartridge  10  is predicted by using the prediction equation (Equation 1). 
     Mechanism of Detection Errors Occurring Due to Air Flow Rate Control of the Cooling Fan 
     Then the temperature transition of the image forming apparatus in the case of changing the air flow rate of the cooling fan  80  will be described with reference to figures.  FIG. 6  is a graph showing the difference between the actual temperature and the predicted temperature of the toner at the time of image formation in the case of changing the air flow rate of the cooling fan. Indicated in  FIG. 6  are the transition (L 1   a ) of the detected temperature Te of the second sensor  101 , the transition (L 2   a ) of the detected temperature Ts of the first sensor  100 , the transition (L 3   a ) of the actual temperature of the toner in the process cartridge and transition (L 4   a ) of the predicted temperature T of the toner. 
     Next, the operation of the image forming apparatus  500  in the case of changing the air flow rate of the cooling fan will be described. The controller  300  of the image forming apparatus  500  receives a print job from the external PC  200  and starts the image forming operation. 
     In the following description, the start of the image forming operation is set at any timing between reception of a print job and the time when a top edge of an image of the first sheet in the job reaches the development nip. In the present embodiment, the CPU determines the start of the image forming operation in response to the print job. Also, the end of the image forming operation is set at any timing between the time when the last image of the job passes through the development nip and the time when the post-rotation is completed. In the present embodiment, the CPU determines the end of the image forming operation according to a post-rotation end signal. 
     Until the time S 1  shown in  FIG. 6 , the predicted temperature of the toner is low. Therefore, the cooling fan  80  is stopped and the image formation control for the low-temperature is performed. Thereafter, at time S 1  when the predicted temperature of the toner becomes the threshold temperature T 1  (the first temperature) during printing, the controller  300  determines that heating of the toner in the process cartridge  10  advances. At this time, the controller  300  increases the air flow rate of the cooling fan  80  to run the apparatus at half the maximum air flow rate. As explained above, the suppression of the toner temperature rise begins. This operation is called an image formation control for medium high temperature. Thereafter, at the time S 3 , the image forming operation of the print job received from the external PC  200  is ended. 
     As shown in  FIG. 6 , during the time period (Δs) from the time S 1  when the air flow rate of the cooling fan  80  is increased until the time S 2 , an error occurs between the transition (L 3   a ) of the actual temperature and transition (L 4   a ) of the predicted temperature. This is because it takes time for the temperature to transfer to the sensor due to the structure where the sensor is disposed away from the toner and a change in temperature detected by the sensor follows with delay in a change in the air flow rate. 
     If an error occurs between the actual temperature and the predicted temperature as explained above, the image formation control is influenced. As a result, when printing the same content for example, a difference in density and quality of the print occurs before and after the air flow rate changes. 
     This error appears more remarkably especially when increasing the air flow rate of the cooling fan  80  from the state where the cooling fan  80  is stopped. This is because when a temperature difference between the outside air and the inside air becomes remarkable after a temperature rise by an image forming operation, relatively cold outside air is rapidly introduced into the apparatus by the start of the operation of the cooling fan  80 . Namely, relatively cold air rapidly flows into the apparatus, so that the toner is rapidly cooled and a difference between the temperature of the first sensor  100  and that of the second sensor  101  is widened. 
     Air Flow Rate Control of the Cooling Fan 
     First Embodiment 
     The next, an embodiment of air flow rate control of the cooling fan  80  which is a feature of the present invention will be described with reference to figures.  FIG. 7  is a graph showing a difference between the actual temperature and the predicted temperature of the toner in the case of performing the cooling fan control of the first embodiment. Indicated in  FIG. 7  are the transition (L 1   b ) in temperature detected by the second sensor  101 , the transition (L 2   b ) in temperature detected by the first sensor  100 , the transition (L 3   b ) in the actual temperature of the toner of the process cartridge and the transition (L 4   b ) of the temperature T which predicts the actual temperature of the toner. 
       FIG. 8  is a flowchart showing the air flow rate control of the cooling fan of the first embodiment. As shown in  FIG. 8 , when running an image forming apparatus  500  (F 101 ), the controller  300  has received a print job from the external PC  200 . Thereafter, the controller  300  has received detected temperature signals indicating the detected temperature Ts of the first sensor  100  and the detected temperature Te of the second sensor  101 . The CPU  301  of the controller  300  calculates the predicted temperature T of the toner using the prediction equation stored in the memory  302  (F 102 ). 
     The controller  300  starts the operation of the cooling fan  80  according to the following procedure based on the predicted temperature T by calculation. The threshold temperature T 1  as a reference for switching the air flow rate of the cooling fan  80  is set in the controller  300  in advance. 
     When the predicted temperature T&lt;T 1  (F 103 ), it is recognized that the toner temperature has not yet risen, the air flow rate of the cooling fan  80  is set to M 0  (zero air flow rate) (F 104 ), thereby suppressing the operation of the cooling fan  80  to a minimum and suppressing the operation sound. 
     On the other hand, when the predicted temperature T≧T 1  (F 105 ), the air flow rate is set to M 1  (half of the maximum air flow rate of the fan) and the cooling fan  80  runs at half the maximum air flow rate (F 106 ). This makes it possible to perform adequate cooling of the image forming apparatus  500 . As described above, an upper limit of the air flow rate of the cooling fan  80  is provided, thereby suppressing the operation sound of the cooling fan  80 . As explained later, the air flow rate is not set to M 2  in the present embodiment. 
     Even after the image forming operation starts, the operation of the image forming apparatus  500  is the same as that of  FIG. 6  until the predicted temperature T reaches the threshold temperature T 1  (until the time S 1 ). The controller  300  compares the predicted temperature T with the threshold temperature T 2  that is set in advance for controlling the air flow rate of the cooling fan  80 . The threshold temperature T 2  may be set in advance, or it may be changed according to the use of the image forming apparatus. 
     As shown in the flowchart of  FIG. 8 , after the image formation operation is started (F 107 ), the predicted temperature T of the toner is calculated again (F 108 ). When T&lt;T 2  (F 109 ), the value the setting of the cooling fan  80  is maintained. Thus, the cooling fan control during the image forming operation is suppressed and it is possible to suppress the occurrence of the difference between the actual temperature and the predicted temperature of the toner. On the other hand, unless T&lt;T 2 , the air flow rate of the cooling fan  80  is set to M 1 . As a result, it is possible to reduce the difference in quality in the print material and to suppress the air flow. 
     When the image forming process is terminated (F 111 ), the predicted temperature T of the toner is calculated again (F 112 ). If the predicted temperature T of the toner is less than the threshold temperature T 1 , the air flow rate is set to M 0  (F 114 ). On the other hand, if the predicted temperature T of the toner meets T 1 ≦T (F 115 ), the air flow rate is set to M 1  (F 116 ). 
     Thereafter, the image forming apparatus  500  enters into a standby state (F 117 ) waiting for the next print job while maintaining the fan operation during a predetermined period of time (F 118 ). Then the operation of the image forming apparatus  500  is terminated (F 119 ). When a print job is started again during waiting, the sequence returns to the beginning of the flowchart. If the temperature exceeds the threshold temperature T 1  prior to the start of the image formation process, a print operation is performed with the half of the maximum air flow rate of the cooling fan  80 . This operation is performed in order to avoid a change in the air flow rate of the fan during printing by increasing the air flow rate prior to the image forming operation. 
     The temperature setting of the threshold temperatures T 1  and T 2  can be set as appropriate depending on the situation. For example, the threshold temperature T 2  may be set as follows. The maximum number of prints in a single image forming operation from the envisioned number of printed sheets a day is assumed and an amount of the temperature rise at the number of printed sheets is added to the threshold temperature T 1 . 
     In the first embodiment, during the period where an image forming operation is not performed prior to an image forming operation, the driving of the cooling fan  80  is started when the temperature is equal to or higher than the threshold temperature (first temperature). On the other hand, during an image forming operation, when the temperature is equal to or higher than the threshold temperature T 2  (second temperature) which is higher than the threshold temperature T 1 , the cooling fan is operated during the image forming operation. 
     Second Embodiment 
     The air flow rate control of the cooling fan  80  of the second embodiment will be described with reference to figures.  FIG. 9  is a graph showing a difference between the actual temperature and the predicted temperature of the toner in the case of performing the cooling fan control of the second embodiment. Indicated in  FIG. 9  are the transition (L 1   c ) of the temperature detected by the second sensor  101 , the transition (L 2   c ) of the temperature detected by the first sensor  100 , the transition (L 3   c ) of the actual temperature of the toner in the process cartridge and the transition (L 4   c ) of the predicted temperature which predicts the actual temperature of the toner from values of temperature sensors. 
     With reference to the temperature of  FIG. 9 , the air flow rate control of the cooling fan  80  will be described with reference to  FIG. 10 .  FIG. 10  is a flowchart showing an air flow rate control of the cooling fan of the second embodiment. In the second embodiment, setting of the air flow rate M 2  is added to the first embodiment and the air flow rate is selected from three air flow rates. Further, the threshold temperatures T 3  and T 4  are additionally set. 
     As shown in  FIG. 10 , when the operation of the image forming apparatus is started (F 201 ), the controller  300  receives a print job from the external PC  200 . Thereafter, the controller  300  receives detected temperature signals indicating the detected temperature Ts of the first sensor  100  and the detected temperature Te of the second sensor  101 . 
     The CPU  301  of the controller  300  calculates the predicted temperature T of the toner on the basis of the detected temperature signal (F 202 ). The calculation of predicted temperature T of the toner is performed using the prediction equation stored in the memory  302  based on the detected temperature signal. The controller  300  controls the operation of the cooling fan  80  based on the calculated predicted temperature of the toner as follows. 
     When the predicted temperature T&lt;T 1  (F 203 ), the controller  300  recognizes that the toner temperature has not yet increased. In this case, the air flow rate of the cooling fan  80  is set to M 0  (zero air flow rate) (F 204 ), thereby suppressing the operation of the cooling fan  80  to a minimum and suppressing the operation sound. 
     When the predicted temperature T meets T 1 ≦T&lt;T 3  (F 205 ), the air flow rate is set to M 1  (F 206 ) and the cooling fan  80  runs at half the maximum air flow rate. Thus, the cooling of the image forming apparatus is started. Also, when the predicted temperature T meets T 3 &lt;T (F 220 ), the air flow rate is set to M 2  and the cooling fan  80  runs at the maximum air flow rate (F 221 ). Only in this case, the cooling capability of the image forming apparatus is set to be maximum. 
     Then, the operation of the image forming apparatus  500  after starting the image forming operation is the same as that of  FIG. 6  until the predicted temperature T reaches the threshold temperature T 1  (until the time S 1 ). Specifically, as shown in the flowchart of  FIG. 10 , after starting the image forming process (F 207 ), the controller  300  compares the predicted temperature T with the threshold temperature T 4  that is set in advance for controlling the air flow rate of the cooling fan  80 . The relationship of the threshold temperatures is T 1 &lt;T 3 &lt;T 4 . 
     Then, during the image formation, the air flow rate of the cooling fan  80  is maintained until the temperature reaches the threshold temperature T 4  (F 208 , F 209 , F 211 ). Thus, the operation of the cooling fan  80  during the image formation is suppressed. Further, by suppressing the operation of the cooling fan  80 , a rapid decrease in the temperature detected by the first sensor  100  is prevented. Therefore, it is possible to suppress the occurrence of the difference between the actual temperature and the predicted temperature of the toner. As a result, it is possible to reduce the difference in quality in the printed material. 
     When the predicted temperature T exceeds the threshold temperature T 4  during image formation processing (F 209 ), the air flow rate of the cooling fan  80  is increased even during printing. In this case, the air flow rate is set to M 2  regardless of the previous air flow rate (F 210 ). Thus, if the temperature exceeds the threshold temperature T 4 , the inside of the apparatus is cooled at the maximum air flow rate of the cooling fan  80 . Alternatively, when the previous air flow rate is M 0 , the air flow rate may be changed in a stepwise way from M 1  to M 2 . In this case, it is sufficient that the threshold temperature of the switching temperature is set to be higher than the temperature during the non image forming. 
     The threshold temperature T 4  is set to be lower than the temperature at which toner aggregates increase due to a temperature rise. This configuration is made in order to prioritize preventing damage of the process cartridge due to toner aggregates generated by high temperature more than maintaining the quality of printed material in the case of the temperature exceeding the threshold temperature T 4 . 
     During the image forming process, when the predicted temperature T is lower than the threshold temperature T 4 , the air flow rate before the image formation is maintained in order to keep the quality of a toner image. Thus, in the present embodiment, a change in the air flow rate in the image forming process is suppressed as much as possible. 
     Then, when the image processing is temporarily finished (F 211 ), the controller  300  returns the threshold temperatures to T 1  and T 3 . Therefore, when the image processing is completed at the time S 4  beyond the time S 3  of  FIG. 9  (corresponding to the first embodiment), the calculated predicted temperature T of the toner (F 212 ) meets T 3 ≦T. In this case, the air flow rate is set to M 2  (F 223 ), and the cooling fan  80  is operated at the maximum air flow rate. 
     The predicted temperature T is below the threshold temperature T 3  at the time S 5  shown in  FIG. 9 . In this case, the predicted temperature T of the present embodiment meets T 1 ≦T≦T 3  (F 215 ) and the air flow rate is set to M 1  (F 216 ). Namely, the apparatus runs at the half of the maximum air flow rate of the cooling fan  80 . 
     Then, the apparatus enters into a standby state (F 217 ) where the apparatus waits for the next print job while maintaining the fan operation for a predetermined time period (F 218 ). Then, the operation of the image forming apparatus is stopped (F 219 ). 
     When a print job is in the standby state and is started again, the sequence returns to the top of the flowchart. When, for example, the predicted temperature T (F 202 ) of the toner exceeds the threshold temperature T 1  before the image forming process starts (F 205 ), the air flow rate of the cooling fan  80  is set to half of the maximum air flow rate (F 206 ). On the other hand, when the predicted temperature (F 202 ) of the toner exceeds the threshold temperature T 3 , the air flow rate is set to M 2  which indicates the maximum air flow rate and printing is resumed (F 207 ). 
     As described above, in the present embodiment, it is also possible to avoid an air flow rate change of the fan during printing by increasing air flow rate of the cooling fan prior to the image forming. In the first embodiment, it is possible to suppress the quality difference between printed materials and to maintain productivity as well as to reduce operational sound by reducing an error between an actual temperature and a predicted temperature by suppressing an air flow rate change during operation. 
     In the second embodiment, during the non image forming operation prior to the image forming operation, when the temperature is equal to or higher than the first temperature (the threshold temperatures T 1 , T 3 ), the cooling fan  80  is started to be driven or the air flow rate is increased to the second rate M 2  (second flow rate) which is higher than the first rate M 1  (first flow rate). On the other hand, during the image forming operation, when the temperature is equal to or higher than the second temperature (the threshold temperature T 4 ), the cooling fan  80  is started to be driven. 
     Other Embodiments 
     In the above embodiments, although the cooling fan  80  has two steps or three steps of air flow rates selected from M 0  (zero air flow rate), M 1  (half of the maximum air flow rate of the fan), M 2  (the maximum air flow rate of the fan), the invention is not limited thereto and it is sufficient to have plurality of stepwise air flow rates. An air flow rate of each step and the number of steps are arbitrary and the present invention can be realized if the cooling fan  80  has at least two steps of air flow rates. The method for changing the air flow rate is not limited to the pulse width modulation. 
     Further, in the embodiments described above, a new threshold value is provided at the time when the image forming process is started but the present invention is not limited thereto. That is, whether a new threshold is provided in any air flow rate or not is arbitrary. For example, only one among the plurality of threshold temperatures may be changed with a new threshold temperature and a new threshold temperature may be different upon each setting. 
     In the embodiments described above, the air flow rate is controlled based on sensing results of a plurality of temperature sensors. The present invention may be applied to a configuration in which just one temperature sensor is provided. In that case, the threshold temperature during an image formation operation is set to be higher than that during non image forming operation. 
     According to the above configurations, it is possible to achieve both suppression of quality difference between printed materials and maintenance of productivity by suppressing the difference between the predicted temperature and the actual temperature due to an air flow rate change of the cooling fan. 
     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. 2014-036240, filed Feb. 27, 2014 which is hereby incorporated by reference herein in its entirety.