Patent Publication Number: US-9835990-B2

Title: Image forming apparatus, image forming apparatus and non-transitory computer readable medium

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2016-005355 filed Jan. 14, 2016. 
     BACKGROUND 
     Technical Field 
     Exemplary embodiments of the invention relate to an image forming apparatus. 
     SUMMARY 
     According to an aspect of the invention, an image forming apparatus is provided with an image processing unit and a heating controller. The image processing unit includes a charging unit that charges a photoconductor, an electrostatic latent image forming unit that forms an electrostatic latent image by light scanning on the charged photoconductor based on image information, a developing unit that develops the electrostatic latent image, a transfer unit that transfers the developed image to a recording medium, and a fixing unit that fixes the transferred image at least by heating the transferred image. Upon receiving an instruction of execution of recovery processing for recovering sensitivity of the photoconductor that is light-fatigued, the heating controller executes a control so that a temperature of the fixing unit rises at a different time from a time at which the image processing unit performs an image processing operation 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a schematic view illustrating a configuration of an image forming apparatus according to an exemplary embodiment which is viewed from a front side; 
         FIG. 2  is a block diagram illustrating a control system of the image forming apparatus according to the exemplary embodiment; 
         FIG. 3  is a flowchart illustrating an optical fatigue recovering process control routine which is executed by a main controller according to the exemplary embodiment; and 
         FIGS. 4A and 4B  are enlarged views of a toner image forming unit according to the exemplary embodiment,  FIG. 4A  illustrating a position of a state where a photoconductor drum is exposed to light, and  FIG. 4B  illustrating a position of a state where the photoconductor drum executes the optical fatigue recovering process. 
     
    
    
     DETAILED DESCRIPTION 
     &lt;Outline of Image Forming Apparatus  10 &gt; 
       FIG. 1  illustrates an image forming apparatus  10  according to an exemplary embodiment. Arrow H illustrated in  FIG. 1  indicates a vertical direction and arrow W indicates a horizontal direction and an apparatus width direction. 
     As illustrated in  FIG. 1 , the image forming apparatus  10  has an image forming apparatus body  12  that is covered by an apparatus housing  10 A and forms an image on a recording medium P such as a sheet by an electrophotographic system, a transporting device  42  that transports the recording medium P, and a control device  70  that controls an operation of each unit of the image forming apparatus  10 . Moreover, an opening and closing door  10 B is provided in a position facing the image forming apparatus body  12  in the apparatus housing  10 A. 
     As illustrated in  FIG. 1 , the transporting device  42  includes a container  43  in which the recording medium P is contained and has plural transport rolls  44  that are disposed along a transporting path for discharging the recording medium P from the container  43  via a secondary transfer position NT and a fixing device  40  belong to the image forming apparatus body  12 , transporting belts  45 , and a transporting belt  46 . 
     The image forming apparatus body  12  includes plural toner image forming units  20  for forming a toner image for each color. For example, the toner image forming unit  20  may be required to be replaced mainly caused by fatigue over time, the opening and closing door  10 B of the apparatus housing  10 A is opened during replacement, and the replacement is able to be performed by pulling out the toner image forming unit  20 . 
     In the exemplary embodiment, the toner image forming units  20  of total six colors of a first spot color (violet “V”) and a second spot color (green “G”), normal color yellow “Y”, magenta “M”, cyan “C”, and black “K” are provided in order from an upstream side. In  FIG. 1 , it is distinguished by adding a symbol of each color at an end of the toner image forming unit  20 . Moreover, since the toner image forming unit  20  of each color has the same structure, in the following description, the description may be given by omitting the symbol of each color. 
     In addition to the toner image forming unit  20 , the image forming apparatus body  12  has a transfer device  30  that transfers the toner image formed by the toner image forming unit  20  onto the recording medium P and the fixing device  40  that fixes the toner image, which is transferred onto the recording medium P, to the recording medium P by heating and a pressure. 
     The toner image forming unit  20  of each color has the basically same configuration except for toner used in the toner image forming unit  20  of each color. Specifically, the toner image forming unit  20  of each color has a photoconductor drum  21  as an example of an image holding element which is rotated in the clockwise direction in  FIG. 1  and a charger  22  that charges the photoconductor drum  21 . Furthermore, the toner image forming unit  20  of each color has an exposure device  23  that exposures the photoconductor drum  21  charged by the charger  22  and forms an electrostatic latent image on the photoconductor drum  21 , and a developing device  24  as an example of a developing unit that develops the electrostatic latent image formed on the photoconductor drum  21  by the exposure device  23  and forms a toner image. 
     The transfer device  30  primarily transfers the toner image in a primary transfer position T by superposing the toner image of the photoconductor drum  21  of each color on an intermediate transfer belt  31  as an example of an intermediate transfer body and secondarily transfers the superposed toner image to the recording medium P in a secondary transfer position NT. Moreover, the intermediate transfer body may be a drum type (intermediate transfer drum). 
     Specifically, as illustrated in  FIG. 1 , the transfer device  30  includes the intermediate transfer belt  31 , a primary transfer roll  33  as an example of a primary transfer unit, a secondary transfer roll  34 , a cleaning device  47  of the secondary transfer roll  34 , a and cleaning device  35  of the intermediate transfer belt  31 . 
     As illustrated in  FIG. 1 , the intermediate transfer belt  31  is an endless type and a posture thereof is determined by being wound around a plural rolls  32 . 
     In the exemplary embodiment, the intermediate transfer belt  31  has a posture of an inverted obtuse triangle which is long in an apparatus width direction in a front view. a roll  32 D illustrated in  FIG. 1  among the plural rolls  32  functions as a driving roll for circulating the intermediate transfer belt  31  by power of a motor (not illustrated) in an arrow A direction. The intermediate transfer belt  31  circulates in the arrow A direction and thereby the image which is primarily transferred is transported to the secondary transfer position NT. 
     In addition, a roll  32 T illustrated in  FIG. 1  among the plural rolls  32  functions as a tension applying roll for applying a tension to the intermediate transfer belt  31 . a roll  32 B illustrated in  FIG. 1  among the plural rolls  32  functions as a facing roll  32 B of the secondary transfer roll  34 . A top portion of the intermediate transfer belt  31  on a lower end side forming an obtuse angle of the intermediate transfer belt  31 , which is the posture of the inverted obtuse triangle as described above, is wound around the facing roll  32 B. The intermediate transfer belt  31  is in contact with the photoconductor drum  21  of each color from below on an upper side extending in the apparatus width direction in the posture described above. 
     The primary transfer roll  33  is a roll, which transfers an image (hereinafter, may be referred to as “toner image”) developed by supplying toner as an example of developer to each photoconductor drum  21  to the intermediate transfer belt  31  and as illustrated in  FIG. 1 , is disposed on an inward side of the intermediate transfer belt  31 . Each primary transfer roll  33  is disposed to face the photoconductor drum  21  of corresponding color in which the intermediate transfer belt  31  is interposed therebetween. In addition, a primary transfer voltage having an opposite polarity to a toner polarity is applied to the primary transfer roll  33  by a power supply unit (not illustrated). The toner image, which is formed on the photoconductor drum  21  by the application of the primary transfer voltage, is transferred to the intermediate transfer belt  31  in the primary transfer position T. 
     The secondary transfer roll  34  is a roll that transfers the toner image, which is superposed on the intermediate transfer belt  31 , to the recording medium P. The secondary transfer roll  34  is disposed so as to cause the intermediate transfer belt  31  to interpose between the facing roll  32 B and the secondary transfer roll  34 . The secondary transfer roll  34  and the intermediate transfer belt  31  are in contact with each other with a predetermined load. 
     As described above, the secondary transfer position NT is provided between the secondary transfer roll  34  and the intermediate transfer belt  31  which are in contact with each other as described above. The recording medium P is timely supplied on the secondary transfer position NT from the container  43 . The secondary transfer roll  34  is driven to be rotated in the clockwise direction in  FIG. 1 . Moreover, in the exemplary embodiment, the position between the secondary transfer roll  34  and the intermediate transfer belt  31  also including a case where the secondary transfer roll  34  and the intermediate transfer belt  31  are separated is referred to as the secondary transfer position NT. 
     The cleaning device  47  is configured with a blade as a removing member that removes foreign matters (toner and powder of the recording medium P) adhered to a surface of the secondary transfer roll  34  from the secondary transfer roll  34  by being in contact with the rotating secondary transfer roll  34 . 
     As illustrated in  FIG. 1 , the cleaning device  35  is disposed on a downstream side of the secondary transfer position NT and on an upstream side of the primary transfer position T (V) in the circulation direction of the intermediate transfer belt  31 . 
     &lt;Engine Unit Control System&gt; 
       FIG. 2  is a block diagram illustrating an example of the control device  70  of the image forming apparatus  10 . 
     The control device  70  includes a main controller  72  having a main control function of the image forming apparatus  10 . A user interface  74  (not illustrated in  FIG. 1 ) is connected to the main controller  72 . The user interface  74  includes an input unit for inputting an instruction regarding the image formation and the like, and an output unit for notifying information during the image formation and the like with display or voice. 
     A network line with an external host computer (not illustrated) is connected to the main controller  72  and image data is input into the main controller  72  via the network line. 
     If the image data is input, in the main controller  72 , for example, print instruction information included in the image data and the image data are analyzed, are converted into a type (for example, bitmap data) suitable for the image forming apparatus  10 , and the converted image data is delivered to an the image formation processing control unit  78  functioning as a part of a MCU  76 . 
     The image formation processing control unit  78  synchronously controls a driving system control section  80 , a charging control section  82 , an exposure control section  84 , a transfer control section  86 , a fixing control section  88 , a discharging control section  90 , a cleaner control section  92 , and a developing control section  94  which respectively function as the MCU  76 , together with the image formation processing control unit  78  based on the input image data, to execute image formation. Moreover, in the exemplary embodiment, the functions performed in the MCU  76  are classified into blocks and are described, and it does not limit a hardware configuration of the MCU  76 . 
     Moreover, a temperature sensor  96  and a humidity sensor  98  are connected to the main controller  72 , and environmental temperature and humidity within the housing of the image forming apparatus  10  may be detected based on the temperature sensor  96  and the humidity sensor  98 . 
     In addition, a cooling fan  10 C mounted on the apparatus housing  10 A is connected to the main controller  72 . The cooling fan  10 C has a function of exhausting the atmosphere within the apparatus housing  10 A by being driven. In other words, if driving of the cooling fan  10 C is stopped, a temperature of an inside of the apparatus housing  10 A tends to be higher than a normal atmosphere temperature (for example, +20° C.). 
     &lt;Light Exposure Measures&gt; 
     In the image forming apparatus  10 , if a certain processing amount (processing number of sheets) of sheets is exceeded, it is necessary to replace (hereinafter, including when failure and damage occur, and referred to as “consumption article replacement”) components which are deteriorated over time. A representative component is the photoconductor drum  21  of the toner image forming unit  20 . The photoconductor drum  21  is unitized with the charger  22 , the exposure device  23 , and the developing device  24  which are other components. Therefore, the consumption article replacement is performed by a unit of the toner image forming unit  20 . 
     In this case, the opening and closing door  10 B of the apparatus housing  10 A is opened and the toner image forming unit  20  is pulled out. In this case, the surface of the photoconductor drum  21  is exposed to light (sunlight, fluorescent light, and the like). 
     For example, trap of charges occurs and the optical fatigue (phenomenon in which light sensitivity becomes dull) occurs in an overcoat layer of the surface of the photoconductor drum  21 . 
     Since the optical fatigue significantly affects the image quality, a replacement operation of the toner image forming unit  20  is required to perform in a situation of not being exposed to light and in a short time. However, in a current installation place, the light exposure may occur due to various factors. 
     For example, in order to ensure the situation of not being exposed to light, an operation environment, in which the replacement is performed under light of red or yellow wavelength, may be established. 
     However, the operation environment may not be established depending on the installation place. 
     In addition, if the replacement is performed while covering the toner image forming unit  20  with a black curtain cover to block the light during the replacement operation of the toner image forming unit  20 , the replacement operation that is substantially exposed to the light can be reduced, but the replacement operation may be not efficiently performed. 
     Furthermore, it is also contemplated that sensitivity reduction corresponding to the light exposure is corrected by light amount adjustment during the image formation, but it is not substantial countermeasures to eliminate the optical fatigue. 
     Thus, in the exemplary embodiment, it is noted that the optical fatigue is caused by trap of charges described above and the trap of charges is released by heating and in order to recover the optical fatigue, a function of heating the photoconductor drum  21  is added to the image forming apparatus  10 . 
     Moreover, in the exemplary embodiment, a new device for recovering the optical fatigue is not added by using heat of the fixing device  40  that is an existing heating body. 
     In the exemplary embodiment, the following conditions may be considered as a time at which the optical fatigue recovering process is executed. 
     (Execution Condition 1) A case where a user orders an execution instruction from the user interface  74 . 
     (Execution Condition 2) A case where, for example, a threshold is provided in a processing amount and periodic maintenance is performed whenever the processing amount exceeds the threshold. 
     (Execution Condition 3) A case where the execution is performed when the opening and closing door  10 B of the apparatus housing  10 A is opened including when the consumption article replacement is performed. 
     If any one of Execution Conditions 1 to 3 is satisfied, in the main controller  72  of the exemplary embodiment, an optical fatigue recovering process sequence which is programmed in advance is started. 
     In the optical fatigue recovering process sequence, in order to raise an atmosphere temperature within the apparatus housing  10 A higher than that of a normal state, the following temperature rising processes are executed. 
     (Temperature Rising Process 1) Drive stop of the cooling fan  10 C. 
     (Temperature Rising Process 2) Heating temperature change of the fixing device  40 . 
     The photoconductor drum  21  is rotated (idle) in accordance with the temperature rising process in the optical fatigue recovering process sequence. Therefore, the surface of the photoconductor drum  21  is heated within the apparatus housing  10 A of which the temperature is higher than that of the normal state. 
     Moreover, an execution time of the optical fatigue recovering process (heating process) may be increased in proportion to, for example, the instruction value of the user if it is Execution Condition 1, a certain time if it is Execution Condition 2, and the opening time of the opening and closing door  10 B if it is Execution Condition 3. 
     In addition, in a form of a time adjustment, in a normal replacement operation time in a state where apparatus power supply is turned on, an optical fatigue recovering process time may be set to one to two minutes and the replacement operation is determined to be a long period of time in a state where apparatus power supply is turned off, thereby the optical fatigue recovering process time may be set to five to ten minutes. 
     Hereinafter, operations according to the exemplary embodiment will be described. 
     &lt;Flow of Normal Imaging Process&gt; 
     If the control device  70  receives an image formation command (print command), the control device  70  operates the toner image forming units  20 Y to  20 K, the toner image forming units  20 V and  20 G of spot colors as required, the transfer device  30 , and the fixing device  40  (see  FIG. 1 ). 
     Therefore, in the toner image forming unit  20 , the toner image is formed in the following image forming step (process). That is, the photoconductor drum  21  of each color is charged by the charger  22  while being rotated. Each charged photoconductor drum  21  is exposed by each exposure device  23  and the electrostatic latent image is formed on the surface of each photoconductor drum  21 . The electrostatic latent image formed on each photoconductor drum  21  is developed by developer supplied from the developing device  24 . Therefore, the toner images of yellow (Y), magenta (M), cyan (C), and black (K) are formed on the photoconductor drum  21  in order from the upstream side. In a case where the spot colors are applied, the toner images of violet (V) and green (G) are formed from further upstream side, and then the toner images of yellow (Y), magenta (M), cyan (C), and black (K) are formed. 
     The toner image formed on the photoconductor drum  21  of each color is sequentially transferred and superposed in a transfer image region of the circulating intermediate transfer belt  31  by the primary transfer roll  33  of each color. 
     The toner image of each color, which is overlapped and superposed to the intermediate transfer belt  31 , is transported to the secondary transfer position NT by the circulation of the intermediate transfer belt  31 . 
     The recording medium P is supplied to the secondary transfer position NT by the transport roll  44  in synchronization with timing of transporting of the toner image. When the recording medium P and the transfer image region (superposed toner image region) pass through the secondary transfer position NT, a secondary transfer voltage (positive voltage) is applied to the secondary transfer roll  34 . Therefore, the toner image is transferred from the intermediate transfer belt  31  to the recording medium P. 
     The recording medium P to which the toner image is transferred is transported from the secondary transfer position NT to the fixing device  40  by the transporting belt  45 . In the fixing device  40 , the toner image on the recording medium P is fixed to the recording medium P and is discharged by the transporting belt  46 . 
     &lt;Flow of Optical Fatigue Recovering Process Sequence&gt; 
       FIG. 3  is a flowchart illustrating an optical fatigue recovering process control routine in the main controller  72 . 
     In step  100 , a flag F, which is applied when overheating within the apparatus housing  10 A is detected, is reset (F←0) and then the procedure proceeds to step  102 , and it is determined whether or not it is a time at which an optical fatigue recovering process is executed. The determination whether or not any of Execution Condition 1 to Execution Condition 3 which are described above is satisfied is performed. Moreover, the execution conditions are not limited to Execution Condition 1 to Execution Condition 3 and other conditions, in which a light detecting sensor is provided within the apparatus housing  10 A and a condition is satisfied when a light amount is deviated from a light amount (light intensity×time) that is set in advance in the light detecting sensor, may be set. 
     In a case where a negative determination is made in step  102 , it is determined that the optical fatigue does not occur in the photoconductor drum  21  and the routine is completed. 
     If a positive determination is made in step  102 , it is determined that the optical fatigue may occur in the photoconductor drum  21  and the procedure proceeds to step  104 . 
     In step  104 , a heating process time is set. The heating process time is set by the execution condition. 
     That is, in a case where Execution Condition 1 is satisfied, the heating process time is set to a value which is manually indicated by the user. Moreover, a defined value may be stored as a default value. 
     In a case where Execution Condition 2 is satisfied, the heating process time is set to a defined value (constant time) that is set in advance. 
     In a case where Execution Condition 3 is satisfied, the heating process time is set to an opening time×unit time of the opening and closing door  10 B. The unit time is a heating time with respect to a unit opening time (for example, 10 seconds). 
     Next, in step  106 , drive of the cooling fan  10 C is stopped. Drive of the cooling fan  10 C is stopped and thereby the atmosphere within the apparatus housing  10 A is not discharged and the temperature of the fixing device  40  is raised in a fixing process standby state. Therefore, the temperature thereof is raised higher than that of the normal state. 
     Next, in step  108 , heating of the fixing device  40  is started at a recovering process temperature. The fixing device  40  is controlled to a fixing process standby temperature (for example, 160° C. to 180° C.) in the normal state and the recovering process temperature is to be the fixing process standby temperature of the normal state+α. Therefore, it is possible to quickly raise the atmosphere temperature within the apparatus housing  10 A in cooperation with drive stop of the cooling fan  10 C. 
     A guide of a surface temperature of the photoconductor drum  21  may be approximately 40° C. and, as indicated in the following Table 1, a recovery degree of the optical fatigue is different by the optical fatigue recovering process time (heating process time). 
     Next, in step  110 , the rotation of the photoconductor drum  21  is started. The rotation is a so-called idle and it is a state where each process of charging exposure, image recording, developing, and fixing is not executed. 
     The photoconductor drum  21  is rotated and thereby the photoconductor drum  21  is heated by the atmosphere within the apparatus housing  10 A of which the temperature is raised to the recovering process temperature. 
     The trapped charges generated in the overcoat layer of the photoconductor drum  21  is released by the heating and thereby the optical fatigue is eliminated (recovery of the optical fatigue). 
     Next, in step  112 , a timer is reset and started, and then the procedure proceeds to step  114 , the temperature in the apparatus housing  10 A is detected by the temperature sensor  96 , and the procedure proceeds to step  116 . 
     In step  116 , it is determined whether or not the inside of the apparatus housing  10 A is in an overheated state (temperature exceeding a limited temperature that is set in advance) based on the detected temperature. In a case where a positive determination is made, the flag F is set (F←1) in step  118  and the procedure proceeds to step  122 . 
     In addition, in a case where a negative determination is made in step  116 , the procedure proceeds to step  120  and it is determined whether or not it is time-up, that is, a time of the timer is a time-up value. If a positive determination is made in step  120 , the procedure proceeds to step  122 . 
     In addition, in step  120 , in a case where a negative determination is made, the procedure returns to step  116  and step  116  and step  120  are repeated until the positive determination is made in step  116  or step  120 . 
     In step  122 , heating of the fixing device is completed and then the procedure proceeds to step  124 . The cooling fan  10 C is driven and the procedure proceeds to step  126 . 
     In step  126 , the rotation of the photoconductor drum  21  is stopped, the photoconductor drum  21  is positioned at an initial position, and the procedure proceeds to step  128 . 
     In step  128 , it is determined whether or not the flag F is set (F=1). 
     If a positive determination is made in step  128 , the inside of the apparatus housing  10 A is in the overheated state and thereby it is determined that the optical fatigue recovering process is interrupted and the procedure proceeds to step  130 . The fact that the optical fatigue recovering process is interrupted is notified and the routine is completed. 
     In addition, if a negative determination is made in step  128 , it is determined that the optical fatigue recovering process is completed and the procedure proceeds to step  132 . The fact that the optical fatigue recovering process is completed is notified and the routine is completed. 
     Table 1 is experimental examples indicating the recovery degree of the photoconductor drum  21  which is exposed to the light. 
     In the experiment, light of 600 Lux is applied to the photoconductor drum  21  for three minutes and the optical fatigue occurs. Thereafter, the photoconductor drum  21  is heated to 40° C. and as a result of sensitivity, G level is displayed. The evaluation G indicates grade evaluation from G 0  that is good to G 5  that is bad. G 5  is a level before the heating process. 
     As illustrated in Table 1, in a white spot level (52 mm/s), if the photoconductor drum  21  is heated for one minute, the evaluation becomes G 2 . In addition, the evaluation becomes G 0  of the best thereof by heating for five minutes and ten minutes. 
     On the other hand, as illustrated in Table 1, in a charred black level (165 and 208 mm/s), if the photoconductor drum  21  is heated for one minute, the evaluation becomes G 4 . In addition, the evaluation becomes G 3  of the best thereof by heating for five minutes and ten minutes. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Heating Time 
                   
                   
               
               
                 (Heating on 
                 White Spot Level 
                 Charred Black Level 
               
               
                 40° C.) 
                 (52 mm/sec) 
                 (165 and 208 mm/sec) 
               
               
                   
               
             
            
               
                 1 minute 
                 G2 
                 G4 
               
               
                  5 minutes 
                 G0 
                 G3 
               
               
                 10 minutes 
                 G0 
                 G3 
               
               
                   
               
               
                 Evaluation ranges 6 grades of from G0 (no unevenness) to G5 (unevenness). G0 to G3 are determined to be good. 
               
            
           
         
       
     
     According to the exemplary embodiment, the recovering process of the optical fatigue can be performed without adding a heating unit. For example, the recovery of the optical fatigue, that is, release of trapped charges is reliably performed rather than the optical fatigue is inconspicuous on the image quality. 
     In addition, since the fixing device  40  is used as the heating unit, it is possible to reduce the heating time of the fixing device  40  when the image forming process is performed after the recovering process of the optical fatigue. In addition, an operation opportunity of an image quality correction sequence due to a temperature difference is reduced and productivity is increased more than that in an imaging process which is performed by warming up from a completely stop state of the apparatus by the heating process by the optical fatigue recovering process. 
     Modification Example 
     In the exemplary embodiment described above, the photoconductor drum  21  is driven to be rotated during the optical fatigue recovering process, but, for example, as illustrated in  FIG. 4A , during the consumption article replacement, the photoconductor drum  21  is exposed to the light and the exposure is often concentrated on an upper surface (see dotted line Z of  FIG. 4A ) of the photoconductor drum  21  (optical fatigue concentration region). 
     Then, as a modification example, when executing the optical fatigue recovering process, as illustrated in  FIG. 4B , a rotational position of the photoconductor drum  21  is determined and the optical fatigue recovering process may be executed in a state of being fixed so that the optical fatigue concentration region of the photoconductor drum  21  faces the fixing device  40 . 
     As illustrated in  FIG. 4B , in a case where the rotational position of the photoconductor drum  21  is the fixed position, since radiant heat from the fixing device  40  directly acts on the optical fatigue concentration region, it is possible to recover the optical fatigue faster than the optical fatigue recovering process with convection heat by the atmosphere within the apparatus housing  10 A. 
     Moreover, as illustrated in  FIG. 1 , since the directions of the toner image forming units  20  are different from each other when viewed from the fixing device  40 , it is preferable that orientation of each toner image forming unit  20  faces an effective range of radiant heat of the fixing device  40 . 
     The foregoing description of the exemplary embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.