Patent Publication Number: US-11662676-B2

Title: Image forming apparatus and control method

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims priority from Japanese Patent Application No. 2021-086030 filed on May 21, 2021, Japanese Patent Application No. 2021-200661 filed on Dec. 10, 2021, and Japanese Patent Application No. 2022-070648 filed on Apr. 22, 2022, the disclosures of which are incorporated herein by reference in their entirety. 
     TECHNICAL FIELD 
     This disclosure relates to an image forming apparatus with a controller for controlling rotations of a photoconductor drum and a heating member, and a method of control implemented therein. 
     BACKGROUND ART 
     An image forming apparatus may include a controller capable of executing a preliminary (pre-process) rotation process of causing a photoconductor drum to make a predetermined number of (or more) rotations while causing a charger to operate, for a period of time after receipt of a printing instruction before start of feed of a first sheet, so as to stabilize a surface potential of the photoconductor drum. In this technical scheme, typically, a fuser motor is activated to start rotation of a heating roller (heating member) at a time when the temperature of a fuser assembly has reached a fuser motor rotation permissible temperature after receiving a printing instruction, and thereafter, a process motor is activated to start the preliminary rotation process (to start rotation of the photoconductor drum) at a time when the temperature of the fuser assembly has reached a process motor rotation permissible temperature. After activating the process motor, a laser diode for exposure of the photoconductor drum is caused to emit light, and after the output of the laser diode is stabilized, a sheet feed is started, so that a sheet is fed after the temperature of the fuser assembly has reached an image quality assurance temperature. 
     SUMMARY 
     Determination of sheet feed timing based on activation of the process motor depending upon the temperature of the fuser assembly would be subject to variation by the influence of the ambient temperature or the temperatures of the heating roller or the like at the time of receipt of the printing instruction. On the other hand, the preliminary rotation process is not affected by the influence of temperatures and performed for a fixed period as predetermined. 
     For adequate printing, the preliminary rotation process should be completed after receipt of the printing instruction before the leading edge of the first sheet arrives at the photoconductor drum. On this account, the start of the preliminary rotation process may be set at an earlier time with allowance made for the variation of the sheet feed timing to ensure that the preliminary rotation process is completed before the leading edge of the first sheet arrives at the photoconductor drum. However, setting such an earlier time for starting the preliminary rotation process would result in unnecessary rotation of the photoconductor drum between the time of completion of the preliminary rotation process and the arrival of the leading edge of the first sheet at the photoconductor drum. 
     There is a need to obviate unnecessary rotation the photoconductor drum would make after receipt of the printing instruction before the leading edge of the first sheet arrives at the photoconductor drum. 
     In one aspect, an image forming apparatus comprising a sheet feed mechanism, a conveyor belt, a first photoconductor drum, a second photoconductor drum, a heating member, and a controller is disclosed herein. The sheet feed mechanism is provided to feed a sheet. The conveyor belt is provided to convey the sheet fed by the sheet feed mechanism in a direction of conveyance of the sheet. The first photoconductor drum is provided to form a toner image of a first color on the sheet conveyed by the conveyor belt. The second photoconductor drum is provided to form a toner image of a second color on the sheet conveyed by the conveyor belt. The second photoconductor drum is located downstream of the first photoconductor drum in the direction of conveyance of the sheet. The heating member is configured to heat a sheet, the heating member being rotatable. The controller is configured to be operated in one of selectable modes including a multicolor mode in which a toner image is formed using both of toner of the first color and toner of the second color and a monochrome mode in which a toner image is formed using toner of the second color only. The controller is configured to exercise, upon receipt of a printing instruction, control which comprises: starting rotation of the heating member; starting rotation of the first and second photoconductor drums when a first standby time has elapsed from a time of the starting of the rotation of the heating member if the controller is operated in the monochrome mode, or when a second standby time longer than the first standby time has elapsed from the time of the starting of the rotation of the heating member if the controller is operated in the multicolor mode; causing the sheet feed mechanism to feed a sheet after the starting of the rotation of the first and second photoconductor drums; and forming a toner image on the sheet after causing the sheet feed mechanism to feed the sheet. The standby time adopted when the controller is operated in the multicolor mode is longer than the standby time adopted when the controller is operated in the monochrome mode. 
     In another aspect, an image forming apparatus comprising a photoconductor drum, a heating member, and a controller is disclosed herein. The photoconductor drum is provided to form a toner image on a sheet. The heating member is configured to heat a sheet, and comprises a heating roller that is rotatable. The controller is configured to execute, upon receipt of a printing instruction, processes comprising a heating process, a pre-fixing rotation process, a pre-process rotation process, and a sheet feed process. The heating process is a process of heating the heating roller. The pre-fixing rotation process is a process of starting rotation of the heating roller and causing the heating roller to rotate for a first execution period TMp from a time of the starting of the rotation of the heating roller. The pre-process rotation process is a process of starting rotation of the photoconductor drum when a standby time TMw has elapsed from the time of the starting of the rotation of the heating roller and causing the photoconductor drum to rotate for a second execution period TMd from a time of starting of rotation of the photoconductor drum. The second execution period TMd is shorter than the first execution period TMp. The sheet feed process is a process of starting feed of a sheet if both of a first condition and a second condition are satisfied where the first condition is satisfied if the pre-fixing rotation process has been completed, and the second condition is satisfied if a temperature of the heating roller has become equal to or higher than a predetermined temperature. The standby time TMw satisfies an equation:
 
 TMw =( TMp+TMf )− TMd  
 
where TMf is a time period from a time of the starting of the feed of the sheet until a leading edge of the sheet arrives at the photoconductor drum.
 
     In still another aspect, a method of control implemented in an image forming apparatus including a sheet feed mechanism provided to feed a sheet, a conveyor belt provided to convey the sheet fed by the sheet feed mechanism in a direction of conveyance of the sheet, a first photoconductor drum provided to form a toner image of a first color on the sheet conveyed by the conveyor belt, a second photoconductor drum provided to form a toner image of a second color on the sheet conveyed by the conveyor belt, and a heating member configured to heat a sheet is disclosed herein. The second photoconductor drum is located downstream of the first photoconductor drum in the direction of conveyance of the sheet. The heating member is rotatable. The control is exercised in one of selectable modes including a multicolor mode in which a toner image is formed using both of toner of the first color and toner of the second color and a monochrome mode in which a toner image is formed using toner of the second color only. The method to be executed upon receipt of a printing instruction comprises: starting rotation of the heating member; starting rotation of the first and second photoconductor drums when a first standby time has elapsed from a time of the starting of the rotation of the heating member if the controller is operated in the monochrome mode, or when a second standby time longer than the first standby time has elapsed from the time of the starting of the rotation of the heating member if the controller is operated in the multicolor mode; causing the sheet feed mechanism to feed a sheet after the starting of the rotation of the first and second photoconductor drums; and forming a toner image on the sheet after causing the sheet feed mechanism to feed the sheet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, their advantages and further features will become more apparent by describing in detail illustrative, non-limiting embodiments thereof with reference to the accompanying drawings, in which: 
         FIG.  1    is a section view of a color printer; 
         FIG.  2    is a schematic diagram for explaining contact/separation states of photoconductor drums and development rollers; 
         FIG.  3    is a schematic diagram showing paths of transmission of driving forces of motors or the like; 
         FIG.  4    is a flowchart showing an operation of a controller; 
         FIG.  5    is a flowchart showing a pre-process rotation process; 
         FIG.  6    is a flowchart showing an example in which a detected temperature reaches a pickup permissible temperature during execution of the pre-fixing rotation process; and 
         FIG.  7    is a time chart showing an example in which a detected temperature reaches a pickup permissible temperature after completion of the pre-fixing rotation process. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     As shown in  FIG.  1   , a color printer  1  as an example of an image forming apparatus includes a housing  2 , a sheet feeder unit  3 , an image forming unit  4 , a fixing device  8 , and a conveyor unit  9 . The housing  2  includes a sheet output tray  21  provided in an upper surface thereof. 
     The sheet feeder unit  3  is provided in a lower space inside the housing  2 , and includes a sheet feed tray  31  configured as a receptacle to hold and serve sheets S, and a sheet feed mechanism  32  configured to feed a sheet S held in the sheet feed tray  31 , to the image forming unit  4 . The sheet feed mechanism  32  includes a pickup roller  33 , a separation roller  34 , a separation pad  35 , and a registration roller  36 . 
     In the sheet feeder unit  3 , sheets S in the sheet feed tray  31  are fed by the pickup roller  33  toward the separation roller  34 , and separated one from the others between the separation roller  34  and the separation pad  35 . The registration roller  36  aligns the edge of the sheet S and conveys the sheet S toward the image forming unit  4 . 
     The image forming unit  4  includes an exposure device  5 , four process units  6 , and a transfer unit  7 . 
     The exposure device  5  is provided in an upper space inside the housing  2 , and includes a light source, a polygon mirror and other components which are not illustrated. The exposure device  5  is configured to scan a surface of each photoconductor drum  61  with a light beam, to thereby expose the surface of the photoconductor drum  61  to the light beam. 
     Each of the four process units  6  includes a photoconductor drum  61 , a charger  62 , a development roller  63 , a drum cleaner  64 , and a static eliminator lamp  65 . The drum cleaner  64  is configured to clean the surface of the corresponding photoconductor drum  61 , and the static eliminator lamp  65  is configured to irradiate the surface of the corresponding photoconductor drum  51  with light to thereby eliminate static electricity. Each of the four process units  6  contains, in its inside space, toner as an example of a developer. 
     The process units  6  containing toner of respective colors of yellow, magenta, cyan, and black, as designated by reference characters  6 Y,  6 M,  6 C, and  6 K, respectively, are arranged in this sequence in a direction of conveyance of a sheet S. In the following description and associated drawings, when the photoconductor drums  51 , the development rollers  61  or other members are specified for respective colors of toner, signs of Y, M, C, and K are appended to reference numerals of the respective members for yellow, magenta, cyan, and black. 
     As shown in  FIG.  2   , the development roller  63  is movable between a contact position in which the development roller  63  is in contact with the photoconductor drum  61  and a separate position in which the development roller  63  is separate from the photoconductor drum  61 . The color printer  1  further includes a contact/separation mechanism  300  and a controller  100 . The contact/separation mechanism  300  is a mechanism that causes the development roller  63  to move between the contact position and the separate position. The controller  100  includes a central processing unit (CPU), a read-only memory (ROM), a random access memory (RAM), etc., and is configured to execute various processes, in response to receipt of a printing instruction, or the like, according to programs prepared in advance. 
     The controller  100  is configured to be operated in one of selectable modes including a multicolor mode in which a toner image is formed using toner of a plurality of colors, a monochrome mode in which a toner image is formed using toner of a single color, and an all-separate mode in which all the development rollers  63  are separated from the corresponding photoconductor drums  61 . To switch the modes, the controller  100  causes the contact/separation mechanism  300  to operate to move one or more of the development rollers  63 . In the multicolor mode, all the development rollers  63  are kept in contact with the corresponding photoconductor drums  61 . In the monochrome mode, only the development roller  63 K for black is kept in contact with the corresponding photoconductor drum  61 K, and the other three development rollers  63 Y,  63 M,  63 C for yellow, magenta, and cyan are kept out of contact with the corresponding photoconductor drums  61 Y,  61 M,  61 C. In the all-separate mode, all the development rollers  63  are kept out of contact with the corresponding photoconductor drums  61 . The all-separate mode is adopted, for example, when the photoconductor drums  61  are subject to cleaning. 
     In the following description, “first” will be prefixed to the legends of members to be used in the multicolor mode and not to be used in the monochrome mode, which are specified with signs of Y, M and C appended to their reference numerals, and “second” will be prefixed to the legends of members to be used in both of the multicolor mode and the monochrome mode, which are specified with a sign of K appended to their reference numerals, as the case may be. For example, the photoconductor drums  61 Y,  61 M and  61 C may be expressed as the first photoconductor drums  61 Y,  61 M and  61 C. Similarly, the photoconductor drums  61 K may be expressed as the second photoconductor drum  61 K. 
     As shown in  FIG.  1   , the transfer unit  7  includes a drive roller  71 , a follower roller  72 , a conveyor belt  73 , and four transfer rollers  74 . The conveyor belt  73  is an endless belt, and stretched between and looped over the drive roller  71  and the follower roller  72 . Inside the conveyor belt  73 , the transfer rollers  74  are located in positions corresponding to the photoconductor drums  61 . The conveyor belt  73  is nipped between the transfer rollers  74  and corresponding photoconductor drums  61 . 
     The charger  62  is a member that charges a surface of the corresponding photoconductor drum  61 . In other words, for example, the first charger  62 Y charges a surface of the first photoconductor drum  61 Y. The second charger  62 K charges a surface of the second photoconductor drum  61 K. The exposure device  5  exposes the charged surface of each photoconductor drum  61  to light, and an electrostatic latent image as formulated according to image data is formed on the surface of the photoconductor drum  61 . 
     The development roller  63  is a member that supplies toner to an electrostatic latent image formed on the corresponding photoconductor drum  61 . Thus, a toner image is formed on the photoconductor drum  61 . Thereafter, a sheet S is conveyed to an interface between the photoconductor drum  61  and the corresponding transfer roller  74 , and the toner image on the photoconductor drum  61  is transferred to the sheet S. 
     The fixing device  8  is a device configured to thermally fix a toner image on a sheet S. The fixing device  8  includes a heating roller  81  configured to heat a sheet S, and a pressure roller  82  configured to nip a sheet S in combination with the heating roller  81 . The heating roller  81  has a shape of a circular cylinder, and contains a heater H in its cylindrical inside space. The pressure roller  82  is pressed against the heating roller  81 , and configured to rotate by following the rotational motion of the rotating heating roller  81 . 
     The conveyor unit  9  is, principally, configured to convey a sheet S outputted from the fixing device  8  toward the outside of the housing  2 . The conveyor unit  9  mainly includes a conveyor roller  91 , and an output roller  92 . A sheet S outputted from the fixing device  8  is conveyed by the conveyor roller  91  to the output roller  92 , and ejected by the output roller  92  onto the sheet output tray  21 . 
     As shown in  FIG.  3   , the color printer  1  further includes a main motor M 1  as an example of a first motor, a process motor M 2  as an example of a second motor, a sheet feed clutch C 1 , a development clutch C 2 , a contact/separation clutch C 3 , charging bias supply circuits V, and a temperature sensor  83 . 
     The main motor M 1  is a motor that supplies driving force to the sheet feed mechanism  32 , the heating roller  81 , the development roller  63 K (i.e., second development roller  63 K for black color, for use in the monochrome mode and the multicolor mode), and the contact/separation mechanism  30 . The main motor M 1  is connected via gearing (not shown) and the sheet feed clutch C 1  to the pickup roller  33 . The sheet feed clutch C 1  is a clutch capable of switching between an engagement state in which a driving force of the main motor M 1  is transmitted to the pickup roller  33  and a disengagement state in which transmission of the driving force from the main motor M 1  to the pickup roller  33  is interrupted. 
     The main motor M 1  is connected via gearing (not shown) to the heating roller  81 . The main motor M 1  is connected via gearing (not shown) and the development clutch C 2  to the development roller  63 K. The development clutch C 2  is a clutch capable of switching between an engagement state in which the driving force of the main motor M 1  is transmitted to the development roller  63 K and a disengagement state in which transmission of the driving force from the main motor M 1  to the development roller  63 K is interrupted. 
     The main motor M 1  is connected via gearing (not shown) and the contact/separation clutch C 3  to the contact/separation mechanism  300 . The contact/separation clutch C 3  is a clutch capable of switching between an engagement state in which the driving force of the main motor M 1  is transmitted to the contact/separation mechanism  300  and a disengagement state in which transmission of the driving force from the main motor M 1  to the contact/separation mechanism  300  is interrupted. 
     The process motor M 2  is a motor that supplies driving force to the development rollers  63 Y,  63 M and  63 C (i.e., first development rollers for yellow, magenta and cyan, for use in the multicolor mode), the four photoconductor drums  61 , and the drive roller  71 . The process motor M 2  is connected via gearing (not shown) to the three development rollers  63 Y,  63 M and  63 C, the four photoconductor drums  61 , and the drive roller  71 . 
     The charging bias supply circuit V is a circuit that provides a charging bias to the charger  62 . Four charging bias supply circuits V are provided one for each of the four chargers  62 . For example, the first charging bias supply circuit VY for yellow color, (i.e., one of the three first charging bias supply circuits V) is provided for the first charger  62 Y, and the second charging bias supply circuit VK is provided for the second charger  62 K. 
     The temperature sensor  83  is a sensor configured to detect a temperature of the heating roller  81 . The temperature detected by the temperature sensor  83  is provided to the controller  100 . 
     The controller  100  is configured to execute, after receipt of instruction, several processes which include a heating process, a pre-fixing rotation process, a pre-process rotation process and a sheet feed process. The heating process is a process of heating the heating roller  81 . More specifically, in the heating process, the heating roller  81  is heated by the heater H to increase the temperature of the heating roller  81  to a fixing temperature To (see also  FIG.  6   ). To start the heating process, the controller  100  activates the heater H before activating the main motor M 1 . The pre-fixing rotation process is a process of rotating the heating roller  81  and the pressure roller  82  before starting an image forming process for making the conditions of the heating roller  81  and the pressure roller  82  suitable for the fixing process. To be more specific, the pre-fixing rotation process is a process of starting rotation of the heating roller  81  and causing the heating roller  81  to rotate for a first execution period TMp from a time of the starting of the rotation of the heating roller  81 . To start the pre-fixing rotation process, the controller  100  activates the main motor M 1  before activating the process motor M 2 . 
     The controller  100  is configured to change the first execution period TMp according to an ambient temperature, and a type of a sheet S. Specifically, the controller  100  sets the first execution period TMp on the basis of a first table stored in a memory such as RAM. The first table contains information of several parameters including the ambient temperatures and the types of a sheet S, each combination of which is associated with the first execution period TMp, showing a relationship between each combination of the parameters and the first execution period TMp. 
     The lower the ambient temperature, the longer the first execution period TMp the controller  100  sets. The thicker the thickness of the sheet S and the larger the area of the sheet S, the longer the first execution period TMp the controller  100  sets. 
     The pre-process rotation process is a process of rotating the photoconductor drums  61  and the like before starting the image forming process for making the conditions of the photoconductor drums  61  or the like suitable for the image forming process. To be more specific, the pre-process rotation process is a process of starting rotation of the photoconductor drums  61  and causing the photoconductor drums  61  to rotate for a second execution period TMd from a time of starting the rotation of the photoconductor drums  61 . In the pre-process rotation process, the photoconductor drums  61  are caused to rotate while the chargers  62  are being operated. To start the pre-process rotation process, the controller  100  activates the process motor M 2 . 
     As shown in  FIG.  6   , the second execution period TMd includes a first period TM 1 , a second period TM 2 , a third period TM 3 , a fourth period TM 4 , and a fifth period TM 5 . The first period TM 1  is a time period elapsed from a time of starting of rotation of the process motor M 2  until a lock signal for the process motor M 2  turns on. The second period TM 2  is a time period elapsed from a time of turning-on of the lock signal for the process motor M 2  until rotation of the photoconductor drum stabilizes. The third period TM 3  is a time period elapsed from a time of starting of supply of the charging bias to the charger  62  by the charging bias supply circuit V until the supply of the charging bias by the charging bias supply circuit V stabilizes. The fourth period TM 4  is a time period elapsed from a time of completion of stabilization of the charging bias until the photoconductor drum  61  makes N rotations where N is a natural number indicative of the number of full rotations. The fifth period TM 5  is a time period elapsed from a time at which the development roller has been pressed into contact with the photoconductor drum until the development roller  63  makes M rotations where M is a natural number indicative of the number of full rotations. Each of the number of rotations N, M may be set at one or any number more than one. Each of the first period TM 1 , second period TM 2 , third period TM 3 , fourth period TM 4  and fifth period TM 5  is predetermined as a fixed value determined by means of experiment, simulation, etc. Therefore, the second execution period TMd takes on a fixed value. 
     The sheet feed process is a process of starting feed of a sheet S if both of a first condition and a second condition are satisfied where the first condition is satisfied if the pre-fixing process has been completed, and the second condition is satisfied if a temperature of the heating roller  81  has become equal to or higher than a predetermined temperature. In the following description, the predetermined temperature will also be called a pickup permissible temperature Tp. 
     The controller  100  is configured to start the pre-process rotation process when a standby time TMw has elapsed from the time of starting of the pre-fixing rotation process. 
     The standby time TMw is a value that satisfies the following inequality:
 
 A−α≤TMw≤A  
 
where A=(TMp+TMf)−TMd, TMf is a feed time period from a time of the starting of the feed of a sheet S until a leading edge of the sheet S arrives at the photoconductor drum  61 , and α is a time period expended for the photoconductor drum  61  to make a half turn.
 
     Specifically, the controller  100  sets the standby time TMw on the basis of a second table stored in a memory such as RAM. The second table contains a list of items of information on the first execution periods TMp, the feed time periods TMf and the second execution periods TMp, each combination of which is associated with the standby time TMw, showing a relationship between each combination of the periods and the standby time TMw. 
     Herein, α takes on a fixed value. It is to be understood that α may be changed where appropriate as long as it is not longer than a time period expended for the photoconductor drum  61  to make a half turn. For example, α may be a time period expended for the photoconductor drum  61  to rotate a one-third turn, a one-quarter turn, a one-fifth turn, or through any angle less than 180 degrees. α may be set at 0. That is, A may be equal to TMw (A=TMw). In the present embodiment, the standby time TMw is set with α being 0, to satisfy the equation, TMw=(TMp+TMf)−TMd. 
     The feed time period TMf takes on a value determined by a distance for a sheet S to be conveyed from the pickup roller  33  to the photoconductor drum  61  (conveyance distance), and a speed at which the sheet S is to be conveyed (conveyance speed). 
     The conveyance distance from the pickup roller  33  to one photoconductor drum  61  located in a position upstreammost in the direction of conveyance of a sheet S among photoconductor drum(s)  61  used for the image forming process varies with the mode selected, i.e., the conveyance distance in the multicolor mode and the conveyance distance in the monochrome mode are different from each other. To be more specific, the conveyance distance in the multicolor mode is a distance from the pickup roller  33  to the first photoconductor drum  61 Y; the conveyance distance in the monochrome mode is a distance from the pickup roller  33  to the second photoconductor drum  61 K. Accordingly, the feed time period TMf is set at a value greater in the monochrome mode than in the multicolor mode. 
     The controller  100  sets the feed time period TMf on the basis of a third table stored in a memory such as RAM. The third table contains information on the feed time periods TMf associated with the modes (the multicolor mode and the monochrome mode), showing a relationship between each of the modes and the feed time period TMf. 
     Next, a detailed description will be given of an operation of the controller  100  with reference to the flowchart shown in  FIG.  4   . In the initial state before execution of the process of  FIG.  4   , the clutches C 1 , C 2  and C 3  are disengaged, and the four development rollers  63  are separate from the corresponding photoconductor drums  61 . 
     As shown in  FIG.  4   , the controller  100  first determines whether or not a printing instruction has been received. (S 1 ). If it is determined in step S 1  that no printing instruction has been received (No), then the controller  100  brings the process to an end. 
     If it is determined in step S 1  that a printing instruction has been received (Yes), then the controller  100  sets a first execution period TMp, a second execution period TMd, and a feed time period TMf (S 2 ). Specifically, in step S 2 , the controller  100  sets the first execution period TMp based on the first table. The controller  100  retrieves a fixed value corresponding to the second execution period TMd from the memory and sets the retrieved fixed value as the second execution period TMd. The controller  100  sets a feed time period TMf based on the third table. 
     After step S 2 , the controller  100  sets a standby time TMw based on the second table (S 3 ). Specifically, in step  3 , the controller  100  sets the standby time TMw which satisfies the equation:
 
 TMw =( TMp+TMf )− TMd.  
 
     After step S 3 , the controller  100  turns on the heater H (S 4 ), and determines whether or not the temperature T detected by the temperature sensor  83  is equal to or higher than a main motor rotation start temperature Tm (S 5 ). If it is determined in step S 5  that T is not equal to or higher than Tm (No), then the controller  100  repeats the process of step S 5 . 
     If it is determined in step S 5  that T≥Tm (Yes), then the controller  100  starts causing the main motor M 1  to operate and starts causing the heating roller  81  to rotate, to thereby start the pre-fixing rotation process (S 6 ). After step S 6 , the controller  100  determines whether or not the standby time TMw has elapsed from the time of the starting of the pre-fixing rotation process (S 7 ). 
     If it is determined in step S 7  that the standby time TMw has not elapsed yet (No), then the controller  100  repeats the process of step S 7 . If it is determined in step S 7  that the standby time TMw has elapsed (Yes), then the controller  100  starts the pre-process rotation process shown in  FIG.  5    (S 8 ). 
     As shown in  FIG.  5   , in the pre-process rotation process, the controller  100  first starts causing the process motor M 2  to operate, and causes the development rollers  63 Y,  63 M and  63 C and the photoconductor drums  61  to rotate (S 31 ). In step S 31 , additionally, the controller  100  turns on the development clutch C 2  to cause the development roller  63 K to rotate as well. 
     After step S 31 , the controller  100  determines whether or not the rotation of the process motor M 2  has stabilized (S 32 ). Herein, after the process motor M 2  is caused to operate, the rotational speed of the process motor M 2  increases, and when the rotational speed of the process motor M 2  has reached a predetermined rotational speed, the process motor M 2  outputs a process motor lock signal to the controller  100 . Upon receipt of the process motor lock signal, the controller  100  determines that the process motor M 2  has stabilized. 
     It is understood that the time period expended from the starting of the rotation of the process motor M 2  to the outputting of the process motor lock signal is an approximately fixed period, more specifically, equivalent to the first period TM 1  described above, irrespective of the ambient temperature, or the like. Therefore, the controller  100  may be configured to determine, in step S 32 , whether or not the first period TM 1  has elapsed from the time of the starting of the rotation of the process motor M 2  to thereby determine whether or not the rotation of the process motor M 2  has stabilized. 
     If it is determined in step S 32  that the rotation of the process motor M 2  has not stabilized yet (No), then the controller  100  repeats the process of step S 32 . If it is determined in step S 32  that the rotation of the process motor M 2  has stabilized (Yes), then the controller  100  determines whether or not the rotations of the photoconductor drums  61  have stabilized (S 33 ). To be more specific, the controller  100  determines, in step S 33 , whether or not the second period TM 2  has elapsed from a time of completion of stabilization of the rotation of the process motor M 2  to thereby determine whether or not the rotations of the photoconductor drums  61  have stabilized. 
     If it is determined in step S 33  that the rotations of the photoconductor drums  61  have not stabilized yet (No), then the controller  100  repeats the process of step S 33 . If it is determined in step S 33  that the rotations of the photoconductor drums  61  have stabilized (Yes), then the controller  100  turns on the static eliminator lamps  65 , and starts supplying charging biases from the charging bias supply circuits V to the chargers  62  to turn on the chargers  62  (S 34 ). 
     After step S 34 , the controller  100  turns on the contact/separation clutch C 3  at a predetermined point in time, to start bringing the development rollers  63  into contact with the corresponding photoconductor drums  61  (S 35 ). To be more specific, the contact/separation clutch C 3  is turned on at such a point in time that the development rollers  63  being pressed in contact with the photoconductor drums  61  finish making M rotations at a time t 8  that is, as shown in  FIG.  6   , a time just after a lapse of a time period (TM 3 +TM 4 ) from a time t 5  at which the chargers  62  have been turned on. In other words, the turning-on of the contact/separation clutch C 3  is timed to complete causing the development rollers  63  to be pressed in contact with the photoconductor drums  61  at a time t 7  that is a time just after a lapse of a time period (TM 3 +TM 4 −TM 6 ) from the time t 5 . 
     At a time t 9  that is a time just after a lapse of a time period (TM 3 +TM 4 +TM 5 ) from the time t 5  at which the chargers  62  have been turned on, the photoconductor drums  61  charged by the chargers  62  with stable charging biases have made N or more rotations, and the development rollers  63  being pressed against the photoconductor drums  61  have made M rotations. Accordingly, the photoconductor drums  61  and the development rollers  63  come to exhibit good conditions suitable for the image forming process; therefore, the pre-process rotation process is completed at the time t 9 . After the pre-process rotation process is completed (i.e., after the photoconductor drums  61  and the development rollers  63  come to exhibit good conditions suitable for image forming process), the controller  100  may not necessarily bring the pre-process rotation process to an end; rather, after a lapse of time expended for completion of the pre-process rotation process, the controller  100  continues causing the photoconductor drums  61  and the development rollers  63  to rotate and proceeds to a printing process (image forming process) of forming a toner image on a sheet S. 
     Referring back to  FIG.  4   , after starting the pre-process rotation process in step S 8 , the controller  100  determines whether or not both of a first condition and a second condition are satisfied where the first condition is satisfied if the first execution period TMp has elapsed from a time of starting of the main motor M 1  and the second condition is satisfied if a temperature T detected by the temperature sensor  83  is equal to or higher than the pickup permissible temperature Tp (S 9 ). If it is determined in step S 9  that at least one of the first condition and the second condition is not satisfied (No), then the controller  100  repeats the process of step S 9 . If it is determined in step S 9  that the both of the first condition and the second condition are satisfied (Yes), then the controller  100  turns on the sheet feed clutch C 1  to start feeding a first sheet S (S 10 ). 
     After step S 10 , the controller  100  causes the image forming unit  4  to perform an image forming process (S 11 ) to print sheets S of which the number is specified in the printing instruction received in step S 1 , and brings this process of  FIG.  4    to an end. At the end of this process, the controller  100  restores the states of the clutches C 1 , C 2  and C 3  and the contact/separation mechanism  300  to their initial states. 
     Next, a detailed description will be given of examples of an operation of the controller  100 . In one example shown in  FIG.  6   , the detected temperature T reaches the pickup permissible temperature Tp during the first execution period TMp for execution of the pre-fixing rotation process. 
     As shown in  FIG.  6   , the controller  100 , upon receiving the printing instruction (at a time t 1 ), turns on the heater H (not shown). Then, the temperature T of the heating roller  81  increases at a specific rate. When the temperature T of the heating roller  81  reaches the main motor rotation start temperature Tm (at a time t 2 ), the controller  100  turns on the main motor M 1  to start the pre-fixing rotation process. When the pre-fixing rotation process is started, the heating roller  81  and the pressure roller  82  are caused to rotate, and the rotating pressure roller  82  absorbs heat from the heating roller  81 . Accordingly, the temperature T of the heating roller  81  increases at a rate lower than the aforementioned specific rate. 
     In the example of  FIG.  6   , the temperature T of the heating roller  81  reaches the pickup permissible temperature Tp at a time t 21  preceding a time t 11  of completion of the pre-fixing rotation process. If the temperature T of the heating roller  81  reaches the pickup permissible temperature Tp, the second condition is satisfied; however, at the time t 21 , the first condition has not been satisfied yet. Therefore, the controller  100  does not start feeding of a sheet S at this time t 21 . When the temperature T reaches the fixing temperature To, the controller  100  controls the heater H so as to keep the temperature T at the fixing temperature To. 
     When the standby time TMw has elapsed from the time t 2  of starting the pre-fixing rotation process (i.e., at a time t 3 ), the controller  100  turns on the process motor M 2  to start the pre-process rotation process. Accordingly, the photoconductor drums  61  are caused to start rotating. At the time t 3 , as described above, the controller  100  does not only turn on the process motor M 2 , but also turns on the development clutch C 2  to cause the development rollers  63  to rotate. 
     When the first period TM 1  has elapsed from the time t 3 , the rotation of the process motor M 2  stabilizes (at a time t 4 ). When the second period TM 2  has elapsed from the time t 4 , the rotations of the photoconductor drums  61  stabilize (at a time t 5 ). 
     When a time period (TM 1 +TM 2 ) has elapsed from the time t 3  (i.e., at the time t 5 ), the controller  100  turns on the static eliminator lamps  65 , and turns on the chargers  62 . When the third period TM 3  has elapsed from the time t 5 , the charging biases supplied to the chargers  62  stabilize (at a time t 6 ). 
     At a predetermined time after the time t 6 , the controller  100  turns on the contact/separation clutch C 3  (at a time t 7 ). When the first execution period TMp has elapsed from the time t 2  of starting the pre-fixing rotation process (i.e., at a time t 11 ), the first condition is satisfied; therefore, the controller  100  turns on the sheet feed clutch C 1  to start feeding a sheet S. 
     When the fourth period TM 4  has elapsed (i.e., the photoconductor drums  61  have made N rotations) from the time t 6  at which the charging biases have stabilized (i.e., at a time t 8 ), the development rollers  63  brought into contact with the corresponding photoconductor drums  61  at the time t 7  have made M rotations while being pressed against the corresponding photoconductor drums  61 . When the fifth period TM 5  has elapsed (i.e., the development rollers  63  pressed against the corresponding photoconductor drums  61  have made M rotations) from the time t 8  (i.e., at a time t 9 ), the photoconductor drums  61  and the development rollers  63  come to exhibit good conditions suitable for the image forming process, and the pre-process rotation process comes to an end. In short, the pre-process rotation process is completed after a lapse of a time period (TMw+TMd) from the time t 2  of the starting of the pre-fixing rotation process. 
     On the other hand, the leading edge of a sheet S started being fed at the time t 11  arrives at the photoconductor drum  61  (at the time t 9 ) after a lapse of the feed time period TMf from the time t 11 . In other words, if the detected temperature T reaches the pickup permissible temperature Tp during the first execution period TMp, the leading edge of the sheet S arrives at the photoconductor drum  61  (at the time t 9 ) after a lapse of a time period (TMp+TMf) from the time t 2  of the starting of the pre-fixing rotation process. 
     The standby time TMw is set at a time period which satisfies the equation TMw=(TMp+TMf)−TMd, as described above. Therefore, the time period (TMw+TMd) elapsed from the time t 2  of starting the pre-fixing rotation process until completion of the pre-process rotation process is equal to the time period (TMp+TMf) elapsed from the time t 2  of starting the pre-fixing rotation process until arrival of the leading edge of a sheet S at the photoconductor drum  61 . Accordingly, the pre-process rotation process is determined to get completed at a time when the leading edge of the sheet S arrives at the photoconductor drum  61 , and thus, unnecessary rotation the photoconductor drum  61  would make after receipt of the printing instruction before the leading edge of the sheet S arrives at the photoconductor  61  can be reduced. 
     In another example shown in  FIG.  7   , the detected temperature T reaches the pickup permissible temperature Tp after completion of the pre-fixing rotation process. The example of  FIG.  7    differs from the example of  FIG.  6    only in the timing of starting the feed of a sheet S, and the following process to be executed in the example of  FIG.  7    is substantially the same as that of  FIG.  6   . 
     In the example of  FIG.  7   , the temperature T of the heating roller  81  reaches a pickup permissible temperature Tp (at a time t 22 ) when a time period (TMp+TMf+TMa) which is longer than the first execution period TMp has elapsed from the time t 2 . Therefore, at the time t 22 , the controller  100  turns on the sheet feed clutch C 1 , and start feeding a sheet S. In this instance, the photoconductor drum  61  rotates unnecessarily during a time period (TMa+TMf) from the time t 9  of completion of the pre-process rotation process (the end of the second execution period TMd) to a time t 23  at which the leading edge of a sheet S arrives at the photoconductor drum  61 . However, in the example of  FIG.  7    as well, the photoconductor drums  61  are not caused to rotate during the standby time TMw; therefore, unnecessary rotation of the photoconductor drums  61  can be reduced in comparison, for example, with a configuration in which the pre-fixing rotation process and the pre-process rotation process are caused to start at the same time. 
     In the present embodiment as described above, the following advantageous effects can be achieved. 
     If the temperature T of the heating roller  81  reaches the pickup permissible temperature Tp before completion of the pre-fixing rotation process, the pre-process rotation process is completed at a time when the leading edge of a sheet S arrives at the photoconductor drum  61 ; thus, unnecessary rotation of the photoconductor drums  61  can be reduced. 
     The above-described embodiment may be modified and implemented in various forms as will be described below. 
     In the above-described embodiment, the color printer  1  is given as an example of an image forming apparatus; however, the image forming apparatus may be a monochrome printer, or other apparatus such as a copier, a multifunction peripheral, etc. In an alternative embodiment, such as a monochrome printer, without any means for rendering a development roller movable between the contact position and the separate position, the second execution period TMd may not include the fifth period TM 5  (time period necessary for the development roller to make M rotations). 
     Parameters such as the first execution period TMp may not be set based on the tables as described above, but may be, for example, computed using predetermined formulae. 
     The elements described in the above embodiment and its modified examples may be implemented selectively and in combination.