Abstract:
An image forming apparatus is provided with: a controller capable of changing a developing bias, which is to be applied to a developing roller configured to carry developer thereon; a peripheral speed ratio of a developing roller to a photosensitive member; a control method by the controller, and a program for operating the controller, the disclosure directed to a configuration where the developing roller is contacted to the photosensitive member when a developing bias is made to be lower during the non-developing than during a developing phase, the configuration can suppress press fogging at room temperature and low humidity conditions, therefore an object of the present disclosure is to provide an image forming apparatus, a control method and a program capable of favorably suppressing press fogging during non-developing.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priorities from Japanese Patent Application No. 2014-133610 filed on Jun. 30, 2014, the entire subject matters of which is incorporated herein by reference. 
       TECHNICAL FIELD 
       [0002]    The present disclosure relates to an image forming apparatus having a controller capable of changing a developing bias, which is to be applied to a developing roller configured to carry developer thereon, and a peripheral speed ratio of a developing roller to a photosensitive member, a control method by the controller, and a program for operating the controller. 
       BACKGROUND 
       [0003]    An image forming apparatus has been known which includes a photosensitive member, on which an electrostatic latent image is to be formed, and a developing roller arranged to be spaced from the photosensitive member and is configured to lower a developing bias when a non-image area of the photosensitive member passes through a developing unit, i.e., during non-developing. An example of such image forming apparatus is disclosed in JP-A-2001-166573. 
       SUMMARY 
       [0004]    The inventors found in a test that in a configuration where the developing roller is contacted to the photosensitive member, when a developing bias is made to be lower during the non-developing than during developing, toner movement from the developing roller to the non-image area of the photosensitive member, which is called press fogging, could be suppressed at room temperature and low humidity conditions. Also, the inventors found in the test that when a predetermined control is performed during the non-developing, in addition to the control of lowering the developing bias, the press fogging could be further suppressed. 
         [0005]    The present disclosure has been made in view of the above circumstances, and one of objects of the present disclosure to provide an image forming apparatus, a control method and a program capable of satisfactory suppressing press fogging during non-developing. 
         [0006]    According to an illustrative embodiment of the present disclosure, there is provided an image forming apparatus including: an image forming unit including: a photosensitive member on which an electrostatic latent image is to be formed; and a developing roller configured to contact the photosensitive member and to supply developer to the electrostatic latent image formed on the photosensitive member; a peripheral speed setting mechanism configured to set a peripheral speed ratio of the developing roller to the photosensitive member to at least a small peripheral speed ratio and a large peripheral speed ratio; a developing bias applying circuit configured to selectively apply a low developing bias or high developing bias to the developing roller; and a controller. The controller is configured to: control the peripheral speed setting mechanism to set the peripheral speed ratio to the small peripheral speed ratio in rotating the developing roller; control the developing bias applying circuit to apply the low developing bias to the developing roller for a predetermined time period; control the developing bias applying circuit to operate in a state in which the high developing bias is applied to the developing roller and the peripheral speed setting mechanism to set to the large peripheral speed ratio after controlling the peripheral speed setting mechanism to set the peripheral speed ratio to the small peripheral speed ratio and the developing bias applying circuit to apply the low developing bias to the developing roller for the predetermined period; and control the image forming unit to supply the developer to the electrostatic latent image formed on the photosensitive member and to transfer the developer on the photosensitive member to a sheet after controlling the developing bias applying circuit to operate in a state in which the high developing bias is applied to the developing roller and the peripheral speed setting mechanism to set to the large peripheral speed ratio. The low developing bias is set to have an absolute value smaller than the high developing bias and to be larger than zero. The small peripheral speed ratio is set to be smaller than the large peripheral speed ratio and to be larger than zero. 
         [0007]    According to another illustrative embodiment of the present disclosure, there is provided a method for controlling an image forming apparatus that is provided with an image forming unit including: a photosensitive member on which an electrostatic latent image is to be formed; and a developing roller configured to contact the photosensitive member and to supply developer to the electrostatic latent image formed on the photosensitive member. The method includes: setting a peripheral speed ratio of the developing roller to the photosensitive member to a small peripheral speed ratio in rotating the developing roller; applying a low developing bias to the developing roller for a predetermined time period; applying a high developing bias to the developing roller while setting the peripheral speed ratio to a large peripheral speed ratio after setting the peripheral speed ratio to the small peripheral speed ratio and applying the low developing bias to the developing roller for the predetermined time period; and controlling the image forming unit to supply the developer to the electrostatic latent image formed on the photosensitive member and to transfer the developer on the photosensitive member to a sheet after applying the high developing bias to the developing roller and setting the large speed ratio to the peripheral speed setting mechanism. The low developing bias is set to have an absolute value smaller than the high developing bias and to be larger than zero. The small peripheral speed ratio is set to be smaller than the large peripheral speed ratio and to be larger than zero. 
         [0008]    According to still another illustrative embodiment of the present disclosure, there is provided a non-transitory computer-readable recording medium storing computer-readable instructions for an image forming apparatus that is provided with an image forming unit including: a photosensitive member on which an electrostatic latent image is to be formed; a developing roller configured to contact the photosensitive member and to supply developer to the electrostatic latent image formed on the photosensitive member; and a processor. The instructions, when executed by the processor, cause the image forming apparatus to perform: setting a peripheral speed ratio of the developing roller to the photosensitive member to a small peripheral speed ratio in rotating the developing roller; applying a low developing bias to the developing roller for a predetermined time period; applying a high developing bias to the developing roller while setting the peripheral speed ratio to a large peripheral speed ratio after setting the peripheral speed ratio to the small peripheral speed ratio and applying the low developing bias to the developing roller for the predetermined time period; and controlling the image forming unit to supply the developer to the electrostatic latent image formed on the photosensitive member and to transfer the developer on the photosensitive member to a sheet after applying the high developing bias to the developing roller and setting the peripheral speed ratio to the large peripheral speed ratio. The low developing bias is set to have an absolute value smaller than the high developing bias and to be larger than zero. The small peripheral speed ratio is set to be smaller than the large peripheral speed ratio and to be larger than zero. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    In the accompanying drawings: 
           [0010]      FIG. 1  is a side sectional view illustrating an image forming apparatus according to an illustrative embodiment of the present disclosure; 
           [0011]      FIG. 2  illustrates components of the image forming apparatus, such as a process cartridge and a controller; 
           [0012]      FIG. 3  is a block diagram showing a configuration of the controller; 
           [0013]      FIG. 4  is a flowchart showing operations of the controller; 
           [0014]      FIGS. 5A to 5E  illustrate a change in a surface potential of a photosensitive drum at the start of print; 
           [0015]      FIG. 6  is a flowchart showing an ending mode; 
           [0016]      FIGS. 7A to 7C  illustrate a change in the surface potential of the photosensitive drum during the ending mode; 
           [0017]      FIG. 8  is a timing chart showing switching timings of a peripheral speed, a developing bias, the surface potential at a nip portion, and the like; 
           [0018]      FIG. 9  is a table showing a test result checking whether press fogging and reverse polarity fogging occurs or not; 
           [0019]      FIG. 10  illustrates a modified embodiment in which a laser printer is provided with a temperature sensor; 
           [0020]      FIG. 11  is a flowchart showing operations of the controller of the modified embodiment; and 
           [0021]      FIG. 12  is a flowchart showing the ending mode of the modified embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    Hereinafter, a laser printer  1 , which is an example of the image forming apparatus according to an illustrative embodiment of the present disclosure, will be described in detail with reference to the drawings. In the following descriptions, an overall configuration of the laser printer  1  will be briefly described and thereafter, operation of the laser printer  1  will be described in detail. 
         [0023]    Also, in the following descriptions, directions are described from a viewpoint of a user who uses the laser printer  1 . That is, in  FIG. 1 , a right side is referred to as a ‘front side’, a left side is referred to as a ‘rear side’, a front side of the drawing sheet is referred to as a ‘left side’ and an inner side of the drawing sheet is referred to as a ‘right side’. Also, an upper-lower direction of the drawing sheet is referred to as an ‘upper-lower direction.’ 
         [0024]    As shown in  FIG. 1 , the laser printer  1  has, in a main body casing  2 , a feeder unit  4  configured to feed a sheet  3 , and an image forming unit  5  configured to form an image on the sheet  3 . 
         [0025]    The feeder unit  4  has a sheet feeding tray  6  detachably mounted to a lower part in the main body casing  2 , a sheet pressing plate  7  provided in the sheet feeding tray  6 , and a variety of rollers  11  configured to convey the sheet  3  and the like. The sheet  3  accommodated in the sheet feeding tray  6  is inclined upwards by the sheet pressing plate  7  and is conveyed to the image forming unit  5  by the various rollers  11 . 
         [0026]    The image forming unit  5  has a scanner unit  16 , a process cartridge  17 , and a fixing unit  18 . 
         [0027]    The scanner unit  16  is provided at an upper part in the main body casing  2 . The scanner unit  16  is provided with a light emitting unit (not shown), a polygon mirror  19 , lenses  20 ,  21 , reflectors  22 ,  23 ,  24 , and the like. In the scanner unit  16 , a laser light based on image data passes through a route shown with a dashed-two dotted line, and is illuminated onto a surface of a photosensitive drum  27  by high speed scanning. 
         [0028]    The process cartridge  17  is configured to be detachable from the main body casing  2 . The process cartridge  17  can be mounted and demounted to and from the main body casing  2  by opening a front cover  2 A provided at a front side of the main body casing  2 . The process cartridge  17  is provided with a developing cartridge  28 , and a drum unit  39 . 
         [0029]    The developing cartridge  28  is configured to be mounted and demounted to and from the main body casing  2  in a state being mounted to the drum unit  39 . The developing cartridge  28  may be configured to be mounted and demounted to and from the drum unit  39  fixed to the main body casing  2 . As shown in  FIG. 2 , the developing cartridge  28  has a housing  50 , a developing roller  100 , a layer thickness regulation blade  32  and a supply roller  33 , and the housing  50  is formed with a toner accommodation chamber  34 . The developing roller  100  has a rotary shaft  110  made of metal, and an elastic layer  120  configured to cover an outer periphery of the rotary shaft  110 , and the elastic layer  120  is pressed and contacted to the photosensitive drum  27 . 
         [0030]    In the developing cartridge  28 , positively charged toner in the toner accommodation chamber  34 , which is an example of the developer, is stirred with an agitator  34 A, and is then supplied to the developing roller  100  by the supply roller  33 . At this time, the toner is positively friction-charged between the supply roller  33  and the developing roller  100 . As the developing roller  100  is rotated, the toner supplied onto the developing roller  100  is introduced between the layer thickness regulation blade  32  and the developing roller  100 , is further friction-charged and is carried on the developing roller  100 , as a thin layer having a predetermined thickness. 
         [0031]    The drum unit  39  is provided with the photosensitive drum  27 , which is an example of the photosensitive member, a scorotron-type charger  29  and a transfer roller  30  to which a transfer bias is to be applied. In the drum unit  39 , a surface of the photosensitive drum  27  is uniformly positively charged by the charger  29 , and is then exposed by the high speed scanning of the laser light emitted from the scanner unit  16 . Thereby, a potential of the exposed part is lowered, so that an electrostatic latent image based on the image data is formed. 
         [0032]    Subsequently, as the developing roller  100  is rotated, the positively charged toner carried on the surface of the developing roller  100  is supplied to the electrostatic latent image formed on the surface of the photosensitive drum  27 , so that a toner image is formed on the surface of the photosensitive drum  27 . After that, the sheet  3  is conveyed between the photosensitive drum  27  and the transfer roller  30 , so that the toner image carried on the surface of the photosensitive drum  27  is transferred to the sheet  3 . 
         [0033]    As shown in  FIG. 1 , the fixing unit  18  has a heating roller  41  and a pressing roller  42  that is configured to press the heating roller  41 . In the fixing unit  18 , the toner transferred to the sheet  3  is heat-fixed while the sheet  3  passes between the heating roller  41  and the pressing roller  42 . The sheet  3  heat-fixed in the fixing unit  18  is conveyed to sheet discharge rollers  45  arranged downstream of the fixing unit  18  and is sent to a sheet discharge tray  46  from the sheet discharge rollers  45 . 
         [0034]    In the following, a controller  300 , which is an example of the controller becoming a feature of the present disclosure, is described in detail. 
         [0035]    As shown in  FIG. 2 , the laser printer  1  has a motor  210 , a peripheral speed setting mechanism  220 , which is an example of the peripheral speed setting mechanism, a developing bias applying circuit  230 , which is an example of the developing bias applying unit, a charging bias applying circuit  240 , which is an example of the charging bias applying unit, and a controller  300 . 
         [0036]    The motor  210  is a driving source for supplying a driving force to the photosensitive drum  27 , the developing roller  100  and the like, and is connected to the developing roller  100  via the peripheral speed setting mechanism  220 . 
         [0037]    The peripheral speed setting mechanism  220  is a mechanism for setting a peripheral speed v of the developing roller  100  to at least a high peripheral speed v 1  and a low peripheral speed v 2  lower than the high peripheral speed v 1  and larger than zero (0). In this way, the peripheral speed v of the developing roller  100  is set to the low peripheral speed v 2  by the peripheral speed setting mechanism  220 , so that it is possible to prolong the lifetime of the toner. 
         [0038]    Here, in this illustrative embodiment, the rotating speed of the photosensitive drum  27  is set to be constant (to be the same) during developing and during non-developing. For this reason, in response to switching the peripheral speed v of the developing roller by the peripheral speed setting mechanism  220 , a peripheral speed ratio (peripheral speed of the developing roller  100 /peripheral speed of the photosensitive drum  27 ) of the developing roller  100  to the photosensitive drum  27  is changed. That is, the cases where the peripheral speed v is the high peripheral speed v 1  and the low peripheral speed v 2  correspond to cases where the peripheral speed ratio of the developing roller  100  to the photosensitive drum  27  is large and small, respectively. 
         [0039]    The high peripheral speed v 1  may be set to a speed higher than a peripheral speed v 3  of the photosensitive drum  27 , and the low peripheral speed v 2  may be set to a speed lower than the peripheral speed v 3  of the photosensitive drum  27 . For example, a ratio of the high peripheral speed v 1 , the peripheral speed v 3  and the low peripheral speed v 2  may be set to 1.3:1:0.3. Also, both the high peripheral speed v 1  and the low peripheral speed v 2  may be set to be higher or lower than the peripheral speed v 3 . 
         [0040]    Specifically, the peripheral speed setting mechanism  220  has a first transmission mechanism  221  configured to have a first speed transmission ratio for rotating the developing roller  100  with the high peripheral speed v 1 , a second transmission mechanism  222  configured to have a second speed transmission ratio for rotating the developing roller  100  with the low peripheral speed v 2 , and an electromagnetic clutch  223  configured to switch a transmission route of the driving force from the motor  210  to the first transmission mechanism  221  or second transmission mechanism  222 . In the peripheral speed setting mechanism  220 , when the electromagnetic clutch  223  is OFF, the driving force from the motor  210  is transmitted to the developing roller  100  via the second transmission mechanism  222 , and when the electromagnetic clutch  223  is ON, the driving force from the motor  210  is transmitted to the developing roller  100  via the first transmission mechanism  221 . 
         [0041]    The developing bias applying circuit  230  is a circuit for applying a positive developing bias Vb to the developing roller  100 , and is appropriately controlled by the controller  300 . Specifically, the developing bias applying circuit  230  is controlled by the controller  300 , so that the developing bias Vb, which is to be applied to the developing roller  100 , is switched to a high developing bias Vb 1  and a low developing bias Vb 2  lower than the high developing bias Vb 1  and greater than zero (0). 
         [0042]    The charging bias applying circuit  240  is a circuit for applying a positive charging bias Vc to the charger  29 , and is appropriately controlled by the controller  300 . Specifically, the charging bias applying circuit  240  is controlled by the controller  300 , so that the charging bias Vc, which is to be applied to the charger  29 , is switched to a high charging bias Vc 1  and a low charging bias Vc 2  lower than the high charging bias Vc 1  and greater than zero (0). In this way, the charging bias Vc is set to the low charging bias Vc 2 , so that it is possible to prolong the lifetime of the photosensitive drum  27 . 
         [0043]    The respective biases may be controlled based on a voltage or current. Also, when the charging bias Vc is set to the high charging bias Vc 1 , a surface potential V 0  of the photosensitive drum  27  becomes a positive high surface potential V 01 , and when the charging bias Vc is set to the low charging bias Vc 2 , the surface potential V 0  of the photosensitive drum  27  becomes a positive low surface potential V 02  lower than the high surface potential V 01 . 
         [0044]    The controller  300  is configured by electrical components such as a CPU (Central Processing Unit), a storage having a RAM (Random Access Memory), a ROM (Read Only Memory) and the like, and an input/output circuit. The controller  300  is configured to mainly control the motor  210 , the peripheral speed setting mechanism  220 , the developing bias applying circuit  230  and the charging bias applying circuit  240 . 
         [0045]    Specifically, as shown in  FIG. 3 , the controller  300  has a first control unit  310 , a second control unit  320 , and a storage  330 . In other words, the controller  300  is configured to operate based on a program stored in the storage  330 , thereby functioning as the first control unit  310  and the second control unit  320 . 
         [0046]    The first control unit  310  has a function of executing control processing under developing of setting the developing bias Vb to the high developing bias Vb 1 , the peripheral speed v to the high peripheral speed v 1  and the charging bias Vc to the high charging bias Vc 1  during the developing. Also, the first control unit  310  has a function of executing upshift processing for shifting from control processing under non-developing to the control processing under developing, which will be described later. Specifically, in the upshift processing, the first control unit  310  is configured to switch the developing bias Vb from the low developing bias Vb 2  to the high developing bias Vb 1 , the peripheral speed v from the low peripheral speed v 2  to the high peripheral speed v 1 , and the charging bias Vc from the low charging bias Vc 2  to the high charging bias Vc 1 . The timings at which the respective values are switched will be described in detail later. 
         [0047]    Specifically, the first control unit  310  is configured to control the developing bias applying circuit  230  so that the developing bias Vb becomes the high developing bias Vb 1 , to turn on the electromagnetic clutch  223  so that the peripheral speed v becomes the high peripheral speed v 1 , and to control the charging bias applying circuit  240  so that the charging bias Vc becomes the high charging bias Vc 1 . 
         [0048]    The second control unit  320  has a function of executing the control processing under non-developing of setting the developing bias Vb to the low developing bias Vb 2 , the peripheral speed v to the low peripheral speed v 2  and the charging bias Vc to the low charging bias Vc 2  for a predetermined time period during the non-developing. Also, the second control unit  320  has a function of executing downshift processing for shifting from the control processing under developing to the control processing under non-developing. Specifically, in the downshift processing, the second control unit  320  is configured to switch the developing bias Vb from the high developing bias Vb 1  to the low developing bias Vb 2 , the peripheral speed v from the high peripheral speed v 1  to the low peripheral speed v 2 , and the charging bias Vc from the high charging bias Vc 1  to the low charging bias Vc 2 . The timings at which the respective values are switched will be described in detail later. 
         [0049]    Specifically, the second control unit  320  is configured to control the developing bias applying circuit  230  so that the developing bias Vb becomes the low developing bias Vb 2 , to turn off the electromagnetic clutch  223  so that the peripheral speed v becomes the low peripheral speed v 2 , and to control the charging bias applying circuit  240  so that the charging bias Vc becomes the low charging bias Vc 2 . 
         [0050]    Further, the second control unit  320  has a function of switching the peripheral speed v from zero (0) to the low peripheral speed v 2 , the developing bias Vb from zero (0) to the low developing bias Vb 2  and the charging bias Vc from zero (0) to the low charging bias Vc 2  when it shifts to the control processing under non-developing from a state such as a sleep mode and a standby mode where the operation of the motor  210  is stopped. Specifically, the second control unit  320  is configured to drive the motor  210  at a state where the electromagnetic clutch  223  is OFF, so as to switch the peripheral speed v from zero (0) to the low peripheral speed v 2 . Also, the second control unit  320  has a function of switching the peripheral speed v, the developing bias Vb and the charging bias Vc to zero (0) from the control processing under non-developing. 
         [0051]    Here, the sleep mode is a mode that is set when an instruction and the like are not received for a predetermined time period in the standby mode (which will be described later), for example. In the sleep mode, the energization to the motor  210  and the heating roller  41  is OFF, and the bias applying to the charger  29 , the developing roller  100  and the like is also OFF. Also, the standby mode is a mode that is set after an ending mode is over (which will be described in detail later), for example. In the standby mode, the energization to the motor  210  is OFF, the bias applying to the charger  29 , the developing roller  100  and the like is OFF, and the heating roller  41  is kept at a preliminary temperature lower than a fixing temperature (temperature for heat-fixing). 
         [0052]    In the storage  330 , a program as shown with flowcharts shown in  FIGS. 4 and 6  is stored. 
         [0053]    Subsequently, the operations of the first control unit  310  and second control unit  320  of the controller  300  are described in detail. In the following descriptions, since the well-known methods are preferred to be adopted in regards to the sheet feeding control, the exposure control and the fixing control in the printing control, the descriptions thereof are omitted. 
         [0054]    The flowchart shown in  FIG. 4  is implemented by a shift instruction from the sleep mode or standby mode, for example. As shown in  FIG. 4 , the second control unit  320  first determines whether a print command is received by receiving a signal from the user interface  410  (a button, a touch panel and the like provided for the laser printer  1 ) or network interface  420  (S 1 ). In response to receiving the signal and determining that a print command is received (S 1 : Yes), the second control unit  320  turns on the motor  210  at a state where the electromagnetic clutch  223  is OFF (S 2 ). Thereby, the photosensitive drum  27 , the developing roller  100 , the agitator  34 A and the like start to rotate. In the meantime, at this time, since the second control unit  320  does not turn on the electromagnetic clutch  223 , the developing roller  100  is rotated at the low peripheral speed v 2 . 
         [0055]    After step S 2 , the second control unit  320  switches the charging bias Vc from zero (0) to the low charging bias Vc 2  (S 3 ). Thereby, the surface potential V 0  of a part of the photosensitive drum  27 , which faces the charger  29 , is switched from zero (0) to the low surface potential V 02  (refer to  FIG. 5A ). In  FIGS. 5A-5E  and in  FIGS. 7A-7C , the broken line indicates the surface potential of the photosensitive drum  27 , and the potential is higher as it is more distant from the photosensitive drum  27 . 
         [0056]    After step S 3 , the second control unit  320  determines whether a first time period T 1  elapses from the setting of the charging bias Vc in step S 3  (S 41 ). 
         [0057]    In response to determining in step S 41  that the first time period T 1  elapses from the setting of the charging bias Vc in step S 3 , the second control unit  320  switches the developing bias Vb from zero (0) to the low developing bias Vb 2 , thereby starting the control processing under non-developing (S 42 ). Here, the first time period T 1  is set as a time period or longer necessary for a part (refer to a charging start part P 1  shown with the thick line in  FIGS. 5A and 5B ) of the surface of the photosensitive drum  27 , which faces the charger  29  at a point of time that the charging bias Vc is switched from zero (0) to the low charging bias Vc 2 , to move from a position at which the part faces the charger  29  to a nip portion NP between the developing roller  100  and the photosensitive drum  27 . Thereby, it is possible to suppress movement of the toner on the developing roller  100  to a non-charged part P 2  of the photosensitive drum  27 , which is caused due to the applying of the developing bias Vb. 
         [0058]    After step S 42 , the second control unit  320  keeps the current state for a predetermined time period, thereby preliminarily rotating the photosensitive drum  27 , the developing roller  100 , the agitator  34 A and the like for the predetermined time period (S 5 ). Thereby, the toner in the toner accommodation chamber  34  is stirred by the agitator  34 A. 
         [0059]    After step S 5 , the first control unit  310  switches the charging bias Vc from the low charging bias Vc 2  to the high charging bias Vc 1  (S 6 ). Thereby, the surface potential V 0  of the photosensitive drum  27  is switched from the low surface potential V 02  to the high surface potential V 01  (refer to  FIG. 5C ). Also, the charging bias Vc is switched, so that the upshift processing is enabled to start and the control processing under non-developing is over. 
         [0060]    After step S 6 , the first control unit  310  determines whether a second time period (first preset time period) T 2  elapses from the switching of the charging bias Vc in step S 6  (S 71 ). 
         [0061]    In response to determining in step S 71  that the second time period T 2  elapses from the switching of the charging bias Vc in step S 6 , the first control unit  310  switches the developing bias Vb from the low developing bias Vb 2  to the high developing bias Vb 1  (S 72 ). 
         [0062]    Here, the second time period T 2  is set as a time period shorter than the time period necessary for a part (refer to a high potential part P 3  shown with the thick line in  FIG. 5C ) of the surface of the photosensitive drum  27 , which faces the charger  29  at a point of time that the charging bias Vc is switched from the low charging bias Vc 2  to the high charging bias Vc 1 , to move from a position at which the part faces the charger  29  to the nip portion NP between the developing roller  100  and the photosensitive drum  27  (refer to  FIG. 5D ). 
         [0063]    That is, when shifting from the control processing under non-developing to the control processing under developing, the first control unit  310  first switches the charging bias Vc from the low charging bias Vc 2  to the high charging bias Vc 1 , and then switches the developing bias Vb from the low developing bias Vb 2  to the high developing bias Vb 1  before the high potential part P 3  of the surface of the photosensitive drum  27 , which faces the charger  29  upon the switching, reaches the developing roller  100 . In other words, the first control unit  310  switches the developing bias Vb from the low developing bias Vb 2  to the high developing bias Vb 1  before the surface potential V 0  of the photosensitive drum  27  at the nip portion NP between the photosensitive drum  27  and the developing roller  100  is switched from the low surface potential V 02  to the high surface potential V 01 . 
         [0064]    After step S 72 , the first control unit  301  determines whether a third time period (second preset time period) T 3  elapses from the switching of the charging bias Vc in step S 6  (S 81 ). 
         [0065]    In response to determining in step S 81  that the third time period T 3  elapses from the switching of the charging bias Vc in step S 6 , the first control unit  301  turns on the electromagnetic clutch  223  (S 82 ). Thereby, the peripheral speed v of the developing roller  100  is switched from the low peripheral speed v 2  to the high peripheral speed v 1 . Here, the third time period T 3  is set as a time period longer than the time period necessary for the high potential part P 3  to move from the position at which it faces the charger  29  to the nip portion NP between the developing roller  100  and the photosensitive drum  27  (refer to  FIG. 5E ). 
         [0066]    That is, when shifting from the control processing under non-developing to the control processing under developing, the first control unit  310  first switches the charging bias Vc from the low charging bias Vc 2  to the high charging bias Vc 1 , and then switches the peripheral speed v from the low peripheral speed v 2  to the high peripheral speed v 1  after the high potential part P 3  of the surface of the photosensitive drum  27 , which faces the charger  29  upon the switching, reaches the developing roller  100  (a downstream end of the high potential part P 3  with respect to a rotating direction of the photosensitive drum  27  exits from the nip portion NP). In other words, the first control unit  310  switches the peripheral speed v from the low peripheral speed v 2  to the high peripheral speed v 1  after the surface potential V 0  of the photosensitive drum  27  at the nip portion NP between the photosensitive drum  27  and the developing roller  100  is switched from the low surface potential V 02  to the high surface potential V 01 . 
         [0067]    The processing of steps S 6  to S 82  is executed in this way, so that the switching is made in order of the charging bias Vc--&gt;the developing bias Vb--&gt;the surface potential V 0  at the nip portion NP (the high potential part P 3  reaches the nip portion NP)--&gt;the peripheral speed v. The peripheral speed v is switched, so that the upshift processing is over and the control processing under developing is enabled to start. 
         [0068]    After step S 82 , the first control unit  310  executes the printing control for one sheet  3  of the number of sheets to be printed, which is designated in the print command (S 9 ). Specifically, in step S 9 , when printing a first sheet  3 , the controller  300  emits the laser light from the scanner unit  16  if a predetermined standby time period elapses from the ON setting of the electromagnetic clutch  223  in steps S 81  and S 82 . Here, the standby time period is a time period or longer necessary for the peripheral speed v to stabilize to the high peripheral speed v 1 . Thereby, when shifting from the control processing under non-developing to the control processing under developing, the first control unit  310  can complete the switching of the peripheral speed v before an electrostatic latent image forming area on the photosensitive drum  27  reaches the developing roller  100 . 
         [0069]    After step S 9 , the first control unit  310  determines whether the printing is performed for all the number of sheets to be printed, which is designated in the print command (S 10 ). In response to determining that the printing is not over (S 10 : No), the first control unit  310  returns to the processing of step S 9 , and in response to determining that the printing is over (S 10 : Yes), the first control unit  310  shifts to the ending mode (S 11 ). 
         [0070]    In the meantime, after the ending mode is over or when a print command is not received in step S 1  (No), the second control unit  320  ends this control. 
         [0071]    As shown in  FIG. 6 , in the ending mode, the second control unit  320  first switches the charging bias Vc from the high charging bias Vc 1  to the low charging bias Vc 2  (S 101 ). The charging bias Vc is switched, so that the downshift processing is enabled to start and the control processing under developing is over. 
         [0072]    After step S 101 , the second control unit  320  determines whether a fourth time period (fourth preset time period) T 4  elapses from the switching of the charging bias Vc in step S 101  (S 1021 ). 
         [0073]    In response to determining in step S 1021  that the fourth time period (fourth preset time period) T 4  elapses from the switching of the charging bias Vc in step S 101 , the second control unit  320  turns off the electromagnetic clutch  223  (S 1022 ). Thereby, the peripheral speed v of the developing roller  100  is switched from the high peripheral speed v 1  to the low peripheral speed v 2 . Here, the fourth time period T 4  is set as a time period shorter than a time period necessary for a part (low potential part P 4  shown with the thick line in  FIG. 7A ) of the surface of the photosensitive drum  27 , which faces the charger  29  at a point of time that the charging bias Vc is switched from the high charging bias Vc 1  to the low charging bias Vc 2 , to move from a position at which the part faces the charger  29  to the nip portion NP between the developing roller  100  and the photosensitive drum  27  (refer to  FIG. 7B ). 
         [0074]    That is, when shifting from the control processing under developing to the control processing under non-developing, the second control unit  320  first switches the charging bias Vc from the high charging bias Vc 1  to the low charging bias Vc 2 , and then switches the peripheral speed v from the high peripheral speed v 1  to the low peripheral speed v 2  before the low potential part P 4  of the surface of the photosensitive drum  27 , which faces the charger  29  upon the switching, reaches the developing roller  100 . In other words, the second control unit  320  switches the peripheral speed v from the high peripheral speed v 1  to the low peripheral speed v 2  before the surface potential V 0  of the photosensitive drum  27  at the nip portion NP between the photosensitive drum  27  and the developing roller  100  is switched from the high surface potential V 01  to the low surface potential V 02 . 
         [0075]    The processing of steps S 1021  and S 1022  is executed after the printing is performed for all the number of sheets to be printed, which is designated in the print command. Therefore, substantially, when shifting from the control processing under developing to the control processing under non-developing, the second control unit  320  switches the peripheral speed v after the electrostatic latent image forming area on the photosensitive drum  27 , which corresponds to the image forming area of the sheet  3 , exits from the developing roller  100 . 
         [0076]    After step S 1022 , the second control unit  302  determines whether a fifth time period (third preset time period) T 5  elapses from the switching of the charging bias Vc in step S 101  (S 1031 ). 
         [0077]    In response to determining in step S 1031  that the fifth time period (third preset time period) T 5  elapses from the switching of the charging bias Vc in step S 101 , the second control unit  302  switches the developing bias Vb from the high developing bias Vb 1  to the low developing bias Vb 2  (S 1032 ). Here, the fifth time period T 5  is set as a time period longer than a time period necessary for the low potential part P 4  of the surface of the photosensitive drum  27 , which faces the charger  29  at a point of the time that the charging bias Vc is switched from the high charging bias Vc 1  to the low charging bias Vc 2 , to move from the position at which the part faces the charger  29  to the nip portion NP between the photosensitive drum  27  and the developing roller  100  (refer to  FIG. 7C ). 
         [0078]    That is, when shifting from the control processing under developing to the control processing under non-developing, the second control unit  320  first switches the charging bias Vc from the high charging bias Vc 1  to the low charging bias Vc 2 , and then switches the developing bias Vb from the high developing bias Vb 1  to the low developing bias Vb 2 , after the low potential part P 4  of the surface of the photosensitive drum  27 , which faces the charger  29  upon the switching, reaches the developing roller  100  (a downstream end of the low potential part P 4  with respect to a rotating direction of the photosensitive drum  27  exits from the nip portion NP). In other words, the second control unit  320  switches the developing bias Vb from the high developing bias Vb 1  to the low developing bias Vb 2  after the surface potential V 0  of the photosensitive drum  27  at the nip portion NP between the photosensitive drum  27  and the developing roller  100  is switched from the high surface potential V 01  to the low surface potential V 02 . 
         [0079]    The processing of steps S 101  to S 1032  is executed in this way, so that the switching is made in order of the charging bias Vc--&gt;the peripheral speed v--&gt;the surface potential V 0  at the nip portion NP (the low potential part P 4  reaches the nip portion NP)--&gt;the developing bias Vb. The developing bias Vb is switched, so that the downshift processing is over and the control processing under non-developing is enabled to start. 
         [0080]    After step S 1032 , the second control unit  320  executes an ending rotation mode for a predetermined time period, in which the photosensitive drum  27 , the developing roller  100 , the agitator  34 A and the like are rotated (S 104 ). After step S 104 , the second control unit  320  turns off the motor  210  and also turns off the applying of the respective biases to the developing roller  100  and the charger  29  (S 105 ). The processing of step S 105  is executed, so that the control processing under non-developing is over. 
         [0081]    Subsequently, the timings of the respective processing are described in detail with reference to a timing chart shown in  FIG. 8 . In  FIG. 8 , for convenience sake, the upshift processing and the downshift processing are enlarged in terms of time for comparison with the control processing under developing and the control processing under non-developing. 
         [0082]    As shown in  FIG. 8 , in response to receiving the print command, the second control unit  320  first turns on the motor  210  to switch the peripheral speed v from zero (0) to the low peripheral speed v 2  (time t 1 ). After that, the second control unit  320  switches the charging bias Vc from zero (0) to the low charging bias Vc 2  (time t 2 ). 
         [0083]    In response to determining that the first time period T 1  elapses from time t 2 , the surface potential V 0  of the photosensitive drum  27  at the nip portion NP is switched from zero (0) to the low surface potential V 02 , and the second control unit  320  switches the developing bias Vb from zero (0) to the low developing bias Vb 2  (time t 3 ). Thereby, a preliminary rotation mode (control processing under non-developing) is enabled to start. 
         [0084]    When the preliminary rotation mode is over, the first control unit  310  switches the charging bias Vc from the low charging bias Vc 2  to the high charging bias Vc 1  (time t 4 ). In response to determining that the second time period T 2  elapses from time t 4 , the first control unit  310  switches the developing bias Vb from the low developing bias Vb 2  to the high developing bias Vb 1  (time t 5 ). In response to determining that time (T 1 −T 2 ) elapses from time t 5 , i.e., in response to determining that the first time period T 1  from time t 4  to time at which the high potential part P 3  reaches the nip portion NP elapses, the surface potential V 0  of the photosensitive drum  27  at the nip portion NP is switched from the low surface potential V 02  to the high surface potential V 01  (time t 6 ). 
         [0085]    In response to determining that time (T 3 −T 1 ) elapses from time t 6 , i.e., in response to determining that the third time period T 3  elapses from time t 4 , the first control unit  310  turns on the electromagnetic clutch  223  to switch the peripheral speed v from the low peripheral speed v 2  to the high peripheral speed v 1  (time t 7 ). Thereby, the control processing under developing including the printing control (developing control) is enabled to start. 
         [0086]    After the printing control is over, the second control unit  320  switches the charging bias Vc from the high charging bias Vc 1  to the low charging bias Vc 2  (time t 8 ). In response to determining that the fourth time period T 4  elapses from time t 8 , the second control unit  320  turns off the electromagnetic clutch  223  to switch the peripheral speed v from the high peripheral speed v 1  to the low peripheral speed v 2  (time t 9 ). 
         [0087]    In response to determining that time (T 1 −T 4 ) elapses from time t 9 , i.e., in response to determining that the first time period T 1  elapses from time t 8 , the surface potential V 0  of the photosensitive drum  27  at the nip portion NP is switched from the high surface potential V 01  to the low surface potential V 02  (time t 10 ). In response to determining that time (T 5 −T 1 ) elapses from time t 10 , i.e., in response to determining that the fifth time period T 5  elapses from time t 8 , the second control unit  320  switches the developing bias Vb from the high developing bias Vb 1  to the low developing bias Vb 2  (time t 11 ). Thereby, the ending rotation mode (control processing under non-developing) is enabled to start. 
         [0088]    When ending the ending rotation mode, the second control unit  320  turns off the motor  210  and sets the respective biases to zero (0) (time t 12 ). 
         [0089]    According to the above illustrative embodiment, it is possible to accomplish the following effects. In the following descriptions, the effects are described with reference to a test result shown in  FIG. 9 . 
         [0090]    The test result shown in  FIG. 9  indicates whether press fogging occurs or not at a room temperature and low humidity (NL) environment and reverse polarity fogging occurs or not at a high temperature and high humidity (HH) environment when the surface potential V 0  of the photosensitive drum  27 , the developing bias Vb, and the peripheral speed v are appropriately changed. Here, the reverse polarity fogging indicates a phenomenon that the negatively charged toner due to the friction charging partially occurs and moves from the developing roller  100  to the non-image area (area in which the electrostatic latent image is not formed) of the photosensitive drum  27 . The negatively charged toner due to the friction charging is increased in the high temperature and high humidity environment. 
         [0091]    The description ‘the peripheral speed v is rapid (high)’ indicates the ‘rapid peripheral speed’ in terms of the peripheral speed of the photosensitive drum  27 , and the description ‘the peripheral speed v is slow (low)’ indicates the ‘slow peripheral speed’ in terms of the peripheral speed of the photosensitive drum  27 . Also, the room temperature is within a range of 15° C. or higher and lower than 28° C. In the test, the room temperature is set to 25° C. Also, the low humidity is a humidity of 30% or lower. In the test, the low humidity is set to 10%. Also, the high temperature is a temperature of 28° C. or higher. In the test, the high temperature is set to 32.5° C. Also, the high humidity is a humidity of 60% or higher. In the test, the high humidity is set to 80%. 
         [0092]    Also, the press fogging and the reverse polarity fogging are evaluated by rotating the photosensitive drum  27  and the developing roller  100  for a predetermined time period at a state where the photosensitive drum  27  is not exposed and then visually inspecting the non-image area of the photosensitive drum  27 . In  FIG. 9 , a symbol “∘-(one circle symbol and one dash symbol)” indicates a boundary line of a limit within which the influence of the press fogging or the reverse polarity fogging on the image formation is allowed. Based on this, the more the number of the symbols “∘ (circle symbol)”, such as “∘, ∘∘, ∘∘∘”, indicates that the press fogging or the reverse polarity fogging has less influence on the image formation. Also, a symbol “x” indicates that influence of the press fogging or the reverse polarity fogging on the image formation is high. 
         [0093]    Also, in  FIG. 9 , states C 1  to C 8  indicate states of the surface potential V 0 , the developing bias Vb and the peripheral speed v. For example, in the state C 1 , the surface potential V 0  is the high surface potential V 01  (850V), the developing bias Vb is the high developing bias Vb 1  (400V), and the peripheral speed v is the high peripheral speed v 1  (rapid). That is, the state C 1  is a state during the control processing under developing, the state C 8  is a state during the control processing under non-developing, and the states C 2  to C 7  are respective states during the upshift processing or during the downshift processing (hereinafter, the time period during the upshift processing and the time period during the downshift processing are collectively referred to as a time period during shift processing). 
         [0094]    In the test result, it is confirmed that the influence of the press fogging is less in the lower developing bias Vb (200V) than in the higher developing bias Vb (400V) and the influence of the press fogging is less in the slow peripheral speed v than in the rapid peripheral speed v. For this reason, like the above illustrative embodiment, when the developing bias Vb is set to the low developing bias Vb 2  and the peripheral speed of the developing roller  100  is set to the low peripheral speed v 2  for a predetermined time period during the non-developing, it is possible to favorably suppress the press fogging for a predetermined time period during the non-developing, as compared to a configuration where the peripheral speed of the developing roller  100  is maintained at the high peripheral speed v 1  for the predetermined time period during the non-developing, for example. 
         [0095]    Also, in the test result, it is confirmed that when the surface potential V 0  is 850V and the developing bias Vb is 200V in the high temperature and high humidity environment, the influence of the reverse polarity fogging is high. Thereby, it is confirmed that it is preferable not to form the states C 3 , C 4  during the shift processing in the high temperature and high humidity environment. 
         [0096]    Also, in the test result, it is confirmed that when the surface potential V 0  is 650V, the developing bias Vb is 400V and the peripheral speed v is high in the room temperature and normal humidity environment, the influence of the press fogging is high. Thereby, it is confirmed that it is preferable not to form the state C 5  during the shift processing in the room temperature and normal humidity environment. 
         [0097]    Considering the above results, in the illustrative embodiment, when shifting from the control processing under non-developing (state C 8 ) to the control processing under developing (state C 1 ), the switching is made in order of the developing bias Vb--&gt;the surface potential V 0  at the nip portion NP--&gt;the peripheral speed v. Thereby, when shifting from the control processing under non-developing to the control processing under developing, the switching is made in order of the state C 8 --&gt;C 6 --&gt;C 2 --&gt;C 1 , so that it is possible to avoid the states C 3  to C 5  and to suppress the press fogging and the reverse polarity fogging. 
         [0098]    Also, according to the illustrative embodiment, when shifting from the control processing under developing to the control processing under non-developing, the switching is made in order of the peripheral speed v--&gt;the surface potential V 0  at the nip portion NP--&gt;the developing bias Vb. Thereby, when shifting from the control processing under developing to the control processing under non-developing, the switching is made in order of the state C 1 --&gt;C 2 --&gt;C 6 --&gt;C 8 , so that it is possible to avoid the states C 3  to C 5  and to suppress the press fogging and the reverse polarity fogging. 
         [0099]    Also, in the test result, it is confirmed that as a potential difference (V 0 −Vb) between the surface potential V 0  of the photosensitive drum  27  and the developing bias Vb increases, the press fogging is more difficult to occur and the reverse polarity fogging is more likely to occur. Also, it is confirmed that as the potential difference (V 0 −Vb) decreases, the press fogging is more likely to occur and the reverse polarity fogging is more difficult to occur. 
         [0100]    According to the illustrative embodiment, when shifting from the control processing under non-developing to the control processing under developing, the switching of the peripheral speed v is completed before the electrostatic latent image forming area on the photosensitive drum  27  reaches the developing roller  100 . Therefore, it is possible to suppress the unfavorable supply of the toner from the developing roller  100  to the electrostatic latent image on the photosensitive drum  27 , which is caused due to the switching of the peripheral speed v, so that it is possible to suppress the deterioration of an image quality of an image to be formed on the sheet  3 . 
         [0101]    Also, when shifting from the control processing under developing to the control processing under non-developing, the peripheral speed v is switched after the electrostatic latent image forming area on the photosensitive drum  27  exits from the developing roller  100 . Therefore, it is possible to suppress the unfavorable supply of the toner from the developing roller  100  to the electrostatic latent image on the photosensitive drum  27 , which is caused due to the switching of the peripheral speed v, so that it is possible to suppress the deterioration of an image quality of an image to be formed on the sheet  3 . 
         [0102]    The present disclosure is not limited to the above illustrative embodiment and can be variously implemented, as exemplified in the following. In the following, the substantially same configurations as the illustrative embodiment are denoted with the same reference numerals and the descriptions thereof are omitted. 
         [0103]    In the above illustrative embodiment, when shifting from the control processing under non-developing to the control processing under developing, the switching is made in order of the developing bias Vb--&gt;the surface potential V 0  at the nip portion NP, and when shifting from the control processing under developing to the control processing under non-developing, the switching is made in order of the surface potential V 0  at the nip portion NP--&gt;the developing bias Vb. However, the present disclosure is not limited thereto. For example, when the temperature or humidity is equal to or lower than a predetermined value, the developing bias Vb and the surface potential V 0  may be switched in an opposite order to the above illustrative embodiment. 
         [0104]    Specifically, as shown in  FIG. 10 , when the laser printer  1  is provided with a temperature sensor  400 , which is an example of the sensor configured to transmit a signal resulting from the detected temperature to the controller  300 , the developing bias Vb and the surface potential V 0  may be switched in an opposite order to the above illustrative embodiment, on condition that a temperature TH detected by the temperature sensor  400  is equal to or smaller than a predetermined value TH 1 . Here, the predetermined value TH 1  may be set to the upper limit of the room temperature range. Specifically, the controller  300  is configured to perform the control in accordance with flowcharts shown in  FIGS. 11 and 12 . 
         [0105]    In the flowchart of  FIG. 11 , new steps S 201  to S 2052  are added to the flowchart of  FIG. 4 . The step S 201  is provided between step S 1  and step S 2 . In step S 201 , the first control unit  310  and the second control unit  320  acquire a temperature TH detected by the temperature sensor  400 . 
         [0106]    The step S 202  is provided between step S 5  and step S 6 . In step S 202 , the first control unit  310  determines whether the temperature TH is larger than the predetermined value TH 1 . 
         [0107]    In response to determining in step S 202  that the temperature TH is larger than the predetermined value TH 1  (S 202 : Yes), the first control unit  310  proceeds to the processing of step S 6 . In response to determining in step S 202  that the temperature TH is equal to or smaller than the predetermined value TH 1  (S 202 : No), the first control unit  310  switches the charging bias Vc from the low charging bias Vc 2  to the high charging bias Vc 1  (S 203 ). 
         [0108]    After step S 203 , the first control unit  310  determines whether the second time period T 2  elapses from the switching of the charging bias Vc in step S 203  (S 2041 ). 
         [0109]    In response to determining in step S 2041  that the second time period T 2  elapses from the switching of the charging bias Vc in step S 203 , the first control unit  310  turns on the electromagnetic clutch  223  to switch the peripheral speed v from the low peripheral speed v 2  to the high peripheral speed v 1  (S 2042 ). Here, the second time period T 2  is the same time as that described in the illustrative embodiment. For this reason, in steps S 2041  and S 2042 , the peripheral speed v is switched before the high potential part P 3  reaches the nip portion NP (refer to  FIG. 5D ). 
         [0110]    After step S 2042 , the first control unit  310  determines whether the third time period T 3  elapses from the switching of the charging bias Vc in step S 203  (S 2051 ). 
         [0111]    In response to determining in step S 2051  that the third time period T 3  elapses from the switching of the charging bias Vc in step S 203 , the first control unit  310  switches the developing bias Vb from the low developing bias Vb 2  to the high developing bias Vb 1  (S 2052 ). Here, the third time period T 3  is the same time as that described in the illustrative embodiment. For this reason, in steps S 2051  and S 2052 , the developing bias Vb is switched after the high potential part P 3  reaches the nip portion NP (refer to  FIG. 5E ). In the meantime, after step S 2052 , the first control unit  310  proceeds to the processing of step S 9 . 
         [0112]    In the flowchart of  FIG. 12 , new steps S 211  to S 2142  are added to the flowchart of  FIG. 6 . The step S 211  is provided before step S 101 . 
         [0113]    In step S 211 , the second control unit  320  determines whether the temperature TH is larger than the predetermined value TH 1 . In response to determining in step S 211  that the temperature TH is larger than the predetermined value TH 1  (S 211 : Yes), the second control unit  320  proceeds to the processing of step S 101 . 
         [0114]    In response to determining in step S 211  that the temperature TH is equal to or smaller than the predetermined value TH 1  (S 211 : No), the second control unit  320  switches the charging bias Vc from the high charging bias Vc 1  to the low charging bias Vc 2  (S 212 ). 
         [0115]    After step S 212 , the second control unit  320  determines whether the fourth time period T 4  elapses from the switching of the charging bias Vc in step S 212  (S 2131 ). 
         [0116]    In response to determining in step S 2131  that the fourth time period T 4  elapses from the switching of the charging bias Vc in step S 212 , the second control unit  320  switches the developing bias Vb from the high developing bias Vb 1  to the low developing bias Vb 2  (S 2132 ). 
         [0117]    Here, the fourth time period T 4  is the same time as that described in the illustrative embodiment. For this reason, in steps S 2131  and S 2132 , the developing bias Vb is switched before the low potential part P 4  reaches the nip portion NP (refer to  FIG. 7B ). 
         [0118]    After step S 2132 , the second control unit  320  determines whether the fifth time period T 5  elapses from the switching of the charging bias Vc in step S 212  (S 2141 ). 
         [0119]    In response to determining in step S 2141  that the fifth time period T 5  elapses from the switching of the charging bias Vc in step S 212 , the second control unit  320  turns off the electromagnetic clutch  223  to switch the peripheral speed v from the high peripheral speed v 1  to the low peripheral speed v 2  (S 2142 ). Here, the fifth time period T 5  is the same time as that described in the illustrative embodiment. For this reason, in steps S 2141  and S 2142 , the peripheral speed v is switched after the low potential part P 4  reaches the nip portion NP (refer to  FIG. 7C ). In the meantime, after step S 2142 , the second control unit  320  proceeds to the processing of step S 104 . 
         [0120]    According to the above example, if the temperature TH is equal to or smaller than the predetermined value TH 1 , i.e., at the room temperature, when shifting from the control processing under non-developing to the control processing under developing, the switching is made in order of the peripheral speed v--&gt;the surface potential V 0  at the nip portion NP--&gt;the developing bias Vb, and when shifting from the control processing under developing to the control processing under non-developing, the switching is made in order of the developing bias Vb--&gt;the surface potential V 0  at the nip portion NP--&gt;the peripheral speed v. Thereby, as shown in  FIG. 9 , at the room temperature, when shifting from the control processing under non-developing to the control processing under developing, the state is switched in order of the state C 8 --&gt;C 7 --&gt;C 3 --&gt;C 1 , and when shifting from the control processing under developing to the control processing under non-developing, the state is switched in order of the state C 1 --&gt;C 3 --&gt;C 7 --&gt;C 8 . For this reason, it is possible to avoid the states C 5 , C 6  at the room temperature, so that it is possible to suppress the press fogging at the room temperature, more favorably. 
         [0121]    The sensor is not limited to the temperature sensor  400  and may be a humidity sensor configured to detect the humidity, a temperature-humidity sensor configured to detect both the temperature and the humidity, and the like. In the meantime, when using the humidity sensor, the temperature TH of  FIGS. 11 and 12  is changed to the humidity and the predetermined value TH 1  is changed to a value relating to the humidity. 
         [0122]    In the above illustrative embodiment, the developing bias Vb, the peripheral speed v and the surface potential V 0  are not switched at the same time during the shift processing. However, the present disclosure is not limited thereto. For example, at least two parameters of the respective parameters such as the developing bias Vb, the peripheral speed v and the surface potential V 0  may be switched at the same time. 
         [0123]    However, for example, when shifting from the control processing under non-developing to the control processing under developing, in case that the developing bias Vb is switched at a point of time that the surface potential V 0  at the nip portion NP is switched (at a point of time that the downstream end of the high potential part P 3  reaches the developing roller  100 ), if the surface potential V 0  is switched before the developing bias Vb is switched, due to an error and the like, the state may shift from the state C 8  to the state C 4 , so that the reverse polarity fogging may occur. In contrast, according to the order of the above illustrative embodiment, when shifting from the control processing under non-developing to the control processing under developing, the developing bias Vb is switched before the switching of the surface potential V 0 , so that it is possible to favorably suppress the reverse polarity fogging. 
         [0124]    Also, when shifting from the control processing under non-developing to the control processing under developing, for example, in case that the developing bias Vb is switched (state C 8 --&gt;C 6 ) before the surface potential V 0  at the nip portion NP is switched (before the high potential part P 3  reaches the nip portion NP) and then the peripheral speed v is switched (state C 6 --&gt;state C 1 ) at a point of time that the surface potential V 0  is switched, if the peripheral speed v is switched before the switching of the surface potential V 0 , due to an error and the like, the state may shift from the state C 6  to the state C 5 , so that the press fogging may occur. In contrast, according to the order of the above illustrative embodiment, when shifting from the control processing under non-developing to the control processing under developing, the peripheral speed v is switched after the switching of the surface potential V 0 , so that it is possible to favorably suppress the press fogging. 
         [0125]    Also, for example, when shifting from the control processing under developing to the control processing under non-developing, in case that the developing bias Vb is switched (state C 2 --&gt;C 8 ) at a point of time that the surface potential V 0  at the nip portion NP is switched (the downstream end of the low potential part P 4  reaches the developing roller  100 ), if the developing bias Vb is switched before the switching of the surface potential V 0 , due to an error and the like, the state may shift from the state C 2  to the state C 4 , so that the reverse polarity fogging may occur. In contrast, according to the order of the above illustrative embodiment, when shifting from the control processing under developing to the control processing under non-developing, the developing bias Vb is switched after the switching of the surface potential V 0 , so that it is possible to favorably suppress the reverse polarity fogging. 
         [0126]    Also, for example, when shifting from the control processing under developing to the control processing under non-developing, in case that the peripheral speed v is switched (state C 1 --&gt;C 6 ) at a point of time that the surface potential V 0  at the nip portion NP is switched (the low potential part P 4  reaches the developing roller  100 ), if the surface potential V 0  is switched before the switching of the peripheral speed v, due to an error and the like, the state may shift from the state C 1  to the state C 5 , so that the press fogging may occur. In contrast, according to the order of the above illustrative embodiment, when shifting from the control processing under developing to the control processing under non-developing, the peripheral speed v is switched before the switching of the surface potential V 0 , so that it is possible to favorably suppress the press fogging. 
         [0127]    In the meantime, when shifting from the control processing under non-developing to the control processing under developing, if the surface potential V 0  and the developing bias Vb are switched at the same time, it is possible to avoid the states C 3  to C 5  even though the peripheral speed v is switched before the switching thereof. However, also in this case, it is preferable to switch the peripheral speed v at the same time as or after the switching of the surface potential V 0  and the like. The reason is that the influence of the pressing fogging is less at the slow peripheral speed v than at the rapid peripheral speed v (for example, refer to the state C 1  and the state C 2 ), as shown in the test result of  FIG. 9 . For this reason, according to the method of switching the peripheral speed v at the same time as or after the switching of the surface potential V 0  and the like, when shifting from the control processing under non-developing to the control processing under developing, it is possible to delay the timing at which the peripheral speed v becomes fast, as compared to the method of switching the peripheral speed v before the switching of the surface potential V 0  and the like, so that it is possible to suppress the press fogging. 
         [0128]    Also, likewise, when shifting from the control processing under developing to the control processing under non-developing, if the surface potential V 0  and the developing bias Vb are switched at the same time, it is possible to avoid the states C 3  to C 5  even though the peripheral speed v is switched after the switching thereof. However, also in this case, it is preferable to switch the peripheral speed v at the same time as or before the switching of the surface potential V 0  and the like because of the above reason. According to this method, when shifting from the control processing under developing to the control processing under non-developing, it is possible to make the timing at which the peripheral speed v slows down faster, as compared to the method of switching the peripheral speed v after the switching of the surface potential V 0  and the like, so that it is possible to suppress the press fogging. 
         [0129]    In the above illustrative embodiment, the rotating speed of the photosensitive drum  27  is set to be constant in the control processing under developing and the control processing under non-developing. However, the present disclosure is not limited thereto. For example, the rotating speed (peripheral speed) of the photosensitive drum may be changed in two stages or more in the control processing under developing and the control processing under non-developing. Meanwhile, in this case, the peripheral speed of the developing roller or photosensitive drum may be changed so that the peripheral speed ratio of the developing roller to the photosensitive drum becomes a large peripheral speed ratio in the control processing under developing, and becomes a small peripheral speed ratio smaller than the large peripheral speed ratio and larger than zero (0) in the control processing under non-developing. 
         [0130]    In the above illustrative embodiment, the present disclosure is applied to the laser printer  1  in which the positively charged toner is used. However, the present disclosure is not limited thereto. For example, the present disclosure can also be applied to a laser printer in which negatively charged toner is used. That is, the developing bias and the charging bias may be the negative biases. 
         [0131]    In the above illustrative embodiment, the present disclosure is applied to the laser printer  1 . However, the present disclosure is not limited thereto. For example, the present disclosure can also be applied to the other image forming apparatuses such as a copier and a multi function device. 
         [0132]    In the above illustrative embodiment, the photosensitive drum  27  has been exemplified as the photosensitive member. However, the present disclosure is not limited thereto. For example, a belt-type photosensitive member may also be used. The charger is not limited to the scorotron-type charger  29  of the illustrative embodiment, and may be a corotron-type charger, a charging roller configured to contact and charge the photosensitive member, and the like. 
         [0133]    In the above illustrative embodiment, the sheet  3  such as a cardboard, a postcard, thin paper and the like has been exemplified as the recording sheet. However, the present disclosure is not limited thereto. For example, an OHP sheet may also be used.