Patent Publication Number: US-2010111566-A1

Title: Photoconductive drums controller

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from provisional U.S. Application 61/112,108 filed on Nov. 6, 2008, the entire contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an image forming apparatus which reduces adverse effects of the environment on an image without generating any color shift of the image in a color printer, multi-function peripheral (MFP) or the like. 
     BACKGROUND 
     In some of color image forming apparatuses as such MFPs and printers, the phases of plural photoconductive drums are aligned with each other in order to prevent a color shift of an image. When the plural photoconductive drums are stopped at the same position in order to align the phases, image quality may be lowered by the deterioration of a specified part on each photoconductive drum. 
     Thus, for example, JP-A-2008-76546 discloses an apparatus which stops plural photoconductive drums with a shift of a predetermined angle from the position where the photoconductive drums are previously stopped. Also, JP-A-2007-164136 discloses an apparatus which stops a monochrome photoconductive drum in line with the phase of color photoconductive drums after a monochrome print mode. 
     However, in the conventional apparatuses, when the plural photoconductive drums are stopped with their phases aligned at the end of printing, the position is only shifted by a predetermined angle from the previous stop position. After the plural photoconductive drums are stopped once with their phases aligned at the finishing of printing, the plural photoconductive drums continue stopping at the same position. Therefore, only a specified part on the plural photoconductive drums may be affected by ozone products generated in a charging device, a transfer device and so on, and by changes in temperature and humidity within the apparatus body, and image quality may be lowered. 
     In the color image forming apparatus, it is desirable to reduce the influence of ozone products and changes in temperature and humidity within the apparatus body on a specified part on the photoconductive drums, thus restrain deterioration of the specified part on the photoconductive drums, and acquire good image quality. 
     SUMMARY 
     According to an aspect of the invention, the influence of ozone products and changes in temperature and humidity to photoconductive drums within the apparatus body are made even. Deterioration of the photoconductive drums is made even, and reduction in image quality is prevented. 
     According to an embodiment, an image forming apparatus includes plural image carriers, and a control unit which carries out phase alignment control to align phases of the plural image carriers and idling control to move the plural image carriers from a first stop position to a second stop position in a ready mode (The ready mode is a state where printing can immediately start.). 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an overall view of configuration showing an image forming apparatus according to an embodiment; 
         FIG. 2  is a schematic view of configuration showing an image forming station according to the embodiment; 
         FIG. 3  is a schematic perspective view showing a driving unit of photoconductive drums according to the embodiment; 
         FIG. 4  is a schematic perspective view showing a phase detection unit according to the embodiment; 
         FIG. 5  is a schematic explanatory view showing the operation of photoconductive drums in a monochrome mode according to the embodiment; 
         FIG. 6  is a schematic explanatory view showing the operation of photoconductive drums in a color mode according to the embodiment; 
         FIG. 7  is a graph showing continuous print time and ozone concentration according to the embodiment; 
         FIG. 8  is a graph showing image defect level with respect to leaving time of photoconductive drums according to the embodiment; 
         FIG. 9  is a block diagram showing a control system of photoconductive drums according to the embodiment; 
         FIG. 10  is a flowchart showing the start of a photoconductive drum idling process after the finishing of printing according to the embodiment; 
         FIG. 11  is a schematic explanatory view showing a first stop position and a second stop position, and the state until a photoconductive drum is stopped after a trigger according to the embodiment; 
         FIG. 12  is a flowchart showing the process of stopping the photoconductive drum at the second stop position according to the embodiment; 
         FIG. 13  is a schematic explanatory view showing the state until the photoconductive drum is stopped after a trigger of ( 1 ) according to the embodiment; 
         FIG. 14  is a schematic explanatory view showing the state until the photoconductive drum is stopped after a trigger of ( 2 ) according to the embodiment; and 
         FIG. 15  is a schematic explanatory view showing the state until the photoconductive drum is stopped after a trigger of ( 3 ) according to the embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment will be described hereinafter.  FIG. 1  is a schematic view of configuration of a color printer  1  as an image forming apparatus according to an embodiment. The color printer  1  has a printer unit  2  which forms an image, a paper discharge unit  3  which accumulates sheets P discharged from the printer unit  2 , a scanner unit  4  which scans an original image, and a paper supply device  7  and a bypass paper supply device  8  which supply sheets P. 
     The printer unit  2  has four image forming stations  11 Y,  11 M,  11 C and  11 K of Y (yellow), M (magenta), C (cyan) and K (black) arranged in parallel along the lower side of an intermediate transfer belt  10 . The image forming stations  11 Y,  11 M,  11 C and  11 K have photoconductive drums  12 Y,  12 M,  12 C and  12 K as image carriers, respectively. Each of the image forming stations  11 Y,  11 M,  11 C and  11 K forms Y (yellow), M (magenta), C (cyan) and K (black) toner images on the photoconductive drums  12 Y,  12 M,  12 C and  12 K, respectively. Each of the photoconductive drums  12 Y,  12 M and  12 C forms a color image carrier. The photoconductive drum  12 K forms a black image carrier. 
     The photoconductive drums  12 Y,  12 M,  12 C and  12 K rotate in the direction of arrow m. Around the photoconductive drums  12 Y,  12 M,  12 C and  12 K, chargers  13 Y,  13 M,  13 C and  13 K, developing devices  14 Y,  14 M,  14 C and  14 K, and photoconductor cleaners  16 Y,  16 M,  16 C and  16 K are arranged, respectively, along the direction of rotation. 
     As shown in  FIG. 2 , in the image forming stations  11 Y,  11 M,  11 C and  11 K, photoconductive drums  12 Y,  12 M,  120  and  12 K, the chargers  13 Y,  13 M,  13 C and  13 K, the developing devices  14 Y,  14 M,  14 C and  14 K, and the photoconductor cleaners  16 Y,  16 M,  16 C and  16 K may be integrated to form process cartridges. If process cartridges are formed, each process cartridge is made separate and integrally attached to and removed from the body of the color printer  1 . 
     Between each chargers  13 Y,  13 M,  13 C and  13 K and each developing devices  14 Y,  14 M,  14 C and  14 K around each photoconductive drums  12 Y,  12 M,  12 C and  12 K, corresponding exposure light is emitted by a laser exposure device  17 . The laser exposure device  17  scans the photoconductive drums  12 Y,  12 M,  12 C and  12 K in the axial direction with a laser beam emitted from a semiconductor laser element. By the emitting of the corresponding exposure light from the laser exposure device  17 , electrostatic latent images are formed on the photoconductive drums  12 Y,  12 M,  12 C and  12 K, respectively. The developing devices  14 Y,  14 M,  14 C and  14 K supplies a toner to the electrostatic latent image on the photoconductive drum  12 Y,  12 M,  12 C and  12 K and visualizes the electrostatic latent image respectively. Each of the developing devices  14 Y,  14 M, 14 C and  14 K carries out development using a two-component developer including a Y, M, C or K toner and a carrier. 
     Above the developing devices  14 Y,  14 M,  14 C and  14 K, toner cartridges  26 Y,  26 M,  26 C and  26 K are arranged, respectively, which house toners as Y, M, C and K replenishment developers for the developing devices  14 Y,  14 M,  14 C and  14 K. The toner cartridges  26 Y,  26 M,  26 C and  26 K has toner augers  36 Y,  36 M,  36 C and  36 K which carries the toner to the direction of the developing devices  14 Y,  14 M,  14 C and  14 K respectively. 
     The intermediate transfer belt  10  is tensioned by a backup roller  20 , a driven roller  21 , and the first to third tension rollers  22  to  24 . The intermediate transfer belt  10  is turned in the direction of arrow n. The intermediate transfer belt  10  faces and contacts the photoconductive drums  12 Y,  12 M,  12 C and  12 K. At the positions facing the photoconductive drums  12 Y,  12 M,  12 C and  12 K on the intermediate transfer belt  10 , primary transfer rollers  18 Y,  18 M,  18 C and  18 K are provided. Each primary transfer rollers  18 Y,  18 M,  18 C and  18 K performs primary transfer of each toner images formed on each photoconductive drums  12 Y,  12 M,  12 C and  12 K to the intermediate transfer belt  10 . Each photoconductor cleaners  16 Y,  16 M,  16 C and  16 K removes and collects residual toners on each photoconductive drums  12 Y,  12 M,  12 C and  12 K after primary transfer. 
     A secondary transfer roller  27  faces a secondary transfer section on the intermediate transfer belt  10  supported by the backup roller  20 . In the secondary transfer section, a predetermined secondary transfer bias is applied to the backup roller  20 . In the secondary transfer section, the secondary transfer roller  27  transfers the toner image on the intermediate transfer belt  10  to the sheet P passes between the intermediate transfer belt  10  and the secondary transfer roller  27 . The sheet P is supplied from a paper supply cassette  7   a  or  7   b  or the bypass paper supply device  8 . After secondary transfer, the intermediate transfer belt  10  is cleaned by a belt cleaner  10   a.    
     A pickup roller  7   e,  a separation roller  7   c,  a carrying roller  7   d  and a registration roller pair  28  are provided between the paper supply device  7  and the secondary transfer roller  27 . A manual pickup roller  8   b , a manual separation roller  8   c  and a manual carrying roller  8 d are provided between a manual insertion tray  8   a  in the bypass paper supply device  8  and the registration roller pair  28 . Along the direction of carrying the sheet P, a fixing device  30  is provided downstream of the secondary transfer roller  27 . The fixing device  30  fixes the toner image transferred to the sheet Pin the secondary transfer section, to the sheet P. A gate  33  which allocates a sheet to the direction of a paper discharge roller  31  or to the direction of a re-carrying unit  32  is provided downstream of the fixing device  30 . A sheet guided to the paper discharge roller  31  is discharged to the paper discharge unit  3 . A sheet guided to the re-carrying unit  32  is guided again to the direction of the secondary transfer roller  27 . 
     When the color printer  1  is in a monochrome mode, only the black photoconductive drum  12 K is rotated in the direction of arrow m to form a monochrome image. When the color printer  1  is in a color mode, all the photoconductive drums  12 Y,  12 M,  12 C and  12 K are rotated to form a color image. In the color mode, all phases of the photoconductive drums  12 Y,  12 M,  12 C and  12 K are aligned. 
     For example, as a print process is started in the color mode, all the photoconductive drums  12 Y,  12 M,  12 C and  12 K start rotating in the direction of arrow m with the same phase. After each photoconductive drums  12 Y,  12 M,  12 C and  12 K is charged by each chargers  13 Y,  13 M,  13 C and  13 K, each exposure lights is emitted onto each photoconductive drums  12 Y,  12 M,  12 C and  12 K by the laser exposure device  17  and each electrostatic latent images corresponding to each exposure lights is formed on each photoconductive drums  12 Y,  12 M,  12 C and  12 K. Each toners is applied by each developing devices  14 Y,  14 M,  14 C and  14 K to each electrostatic latent images formed on each photoconductive drums  12 Y,  12 M,  12 C and  12 K, thus visualizing each electrostatic latent images. Each toner images formed on each photoconductive drums  12 Y,  12 M,  12 C and  12 K is transferred to the sheet P at the secondary transfer section via the intermediate transfer belt  10 . 
     After being supplied from either the paper supply device  7  or the bypass paper supply device  8 , the sheet P reaches the secondary transfer position synchronously with the toner images on the intermediate transfer belt  10 . The fixing device  30  fixes the toner images on the sheet P. The sheet P after having the toner images fixed thereto is discharged to the paper discharge unit  3  via the paper discharge roller  31 , or is carried again to the secondary transfer roller  27  via the re-carrying unit  32 . 
     Next, the driving of the photoconductive drums  12 Y,  12 M,  12 C and  12 K will be described. The photoconductive drums  12 Y,  12 M,  12 C and  12 K have a drum motor  40  to the rear side, as shown in  FIG. 3 . The drum motor  40  has a first gear  41  which links to a gear  50  of the photoconductive drum  12 K, and a second gear  42  which links to a gear array  60  of the photoconductive drums  12 Y,  12 M and  12 C and transmits the driving of the first gear  41  to the gear array  60 . The first gear  41  rotates in the direction of arrow n. The second gear  42  rotates in the direction of arrow q. 
     In the monochrome mode, the link between the second gear  42  and the first gear  41  is canceled. In the monochrome mode, only the first gear  41  rotates, thus rotating the gear  50  only. In the color mode, the second gear  42  links to the first gear  41 . In the color mode, the first gear  41  and the second gear  42  rotate, thus rotating the gear  50  and the gear array  60 . 
     As the first gear  41  rotates, the gear  50  rotates in the direction of arrow m. The gear  50  rotates the photoconductive drum  12 K connected thereto via a coupling  50   a,  in the direction of arrow m. The gear array  60  has a gear  61  of the photoconductive drum  12 C, a gear  62  of the photoconductive drum  12 M and a gear  63  of the photoconductive drum  12 Y. In the gear array  60 , the gears  61 ,  62  and  63  are connected by connection gears  64  or  65  respectively. 
     As the second gear  42  rotates, the gears  61 ,  62  and  63  of the gear array  60  simultaneously rotate in the direction of arrow m. The gears  61 ,  62  and  63  rotate the photoconductive drums  12 C,  12 M and  12 Y connected thereto via couplings  61   a,    62   a  and  63   a,  respectively, in the direction of arrow m. The connection gear  64  rotates in the direction of arrow r and transmits the rotation of the gear  61  to the gear  62 . The connection gear  65  rotates in the direction of arrow r and transmits the rotation of the gear  62  to the gear  63 . 
     The gear  50  has a first phase detection unit  70  as a detection mechanism. The gear  61  has a second phase detection unit  71  as a detection mechanism. The first phase detection unit  70  and the second phase detection unit  71  have the same structure, though reversed in terms of the front and rear. Therefore, these phase detection units will be described with reference a common diagram of  FIG. 4 . The first and second phase detection units  70  and  71  have ribs  75  and  76  in an area covering ½ (180 degrees) of the circumferential edge of wheels  72  and  73  which rotate integrally with the gears  50  and  61 , respectively. The ribs  75  and  76  forms an operating unit respectively. Each photo-sensor  77  and  78  which is switched by each ribs  75  and  76  is provided respectively around each wheels  72  and  73  as rotating body. 
     The photo-sensor  77  detects a leading end  75   a  and a terminal end  75   b  of the rib  75  and detects the phase of the photoconductive drum  12 K. The photo-sensor  78  detects a leading end  76   a  and a terminal end  76   b  of the rib  76  and detects the phase of the photoconductive drums  12 Y,  12 M and  12 C. 
     While the color printer  1  forms an image, the phase of the black photoconductive drum  12 K detected by the first phase detection unit  70  coincides with the phase of the color photoconductive drums  12 Y,  12 M and  12 C detected by the second phase detection unit  71 . As shown in  FIG. 5  and  FIG. 6 , at the time of starting printing, for example, the black photoconductive drum  12 K is located at the position of phase A where the photo-sensor  77  detects the leading end  75   a  of the rib  75 . The color photoconductive drums  12 Y,  12 M and  12 C are located at the position of phase A where the photo-sensor  78  detects the leading end  76   a  of the rib  76 . All the photoconductive drums  12 Y,  12 M,  12 C and  12 K are located at the position of phase A. The phases of the photoconductive drums are aligned. 
     As printing in the monochrome mode is started, only the black photoconductive drum  12 K rotates. At the finishing of printing, the black photoconductive drum  12 K stops at the position of phase A in line with the phase of the color photoconductive drums  12 Y,  12 M and  12 C. As printing in the color mode is started, all the photoconductive drums  12 Y,  12 M,  12 C and  12 K rotate. At the finishing of printing, all the photoconductive drums  12 Y,  12 M,  12 C and  12 K stop at the position of phase A+W degrees, which is a position shifted from phase A by a predetermined angle (W degrees). In the color mode, all the photoconductive drums  12 Y,  12 M,  12 C and  12 K are aligned in phase though the phase at the finishing of printing is shifted from the phase at the start of printing. 
     In the color mode, the phase at the finishing of printing is shifted from the phase at the start of printing by W degrees. The next printing starts at phase A+W deg. The next shift angle is (W degrees)+(W degrees). If the shift angle exceeds 360 degrees because of add the shift angle sequentially, the phase is shifted by an angle as a result of subtracting 360 degrees from the shift angle, and the operation time is thus reduced. As the phase of the photoconductive drums  12 Y,  12 M,  12 C and  12 K at the stop position is shifted for every printing sequentially, the use of a specified part on the photoconductive drums  12 Y,  12 M,  12 C and  12 K at the print operation is avoided. 
     At the time of the above printing, the chargers  13 Y,  13 M,  13 C and  13 K generate ozone (O 3 ) between each chargers  13 Y,  13 M,  13 C and  13 K and each photoconductive drums  12 Y,  12 M,  12 C and  12 K because of charging. The generated ozone (O 3 ) is constantly discharged from the color printer  1 . However, if a large number of sheets are printed and a large quantity of ozone (O 3 ) is generated, it takes time to discharge the ozone (O 3 ). The ozone (O 3 ) stays between the photoconductive drums  12 Y,  12 M,  12 C and  12 K and the chargers  13 Y,  13 M,  13 C and  13 K. The ozone (O 3 ) concentration in the color printer  1  in the case of continuous printing is, for example, as shown in  FIG. 7 . At the continuous printing time reaches approximately  25  seconds, the ozone (O 3 ) concentration exceeds  12 . (The unit expressing ozone concentration is ppm. Using an ozone meter, the atmosphere is absorbed at the rate of 1.5 liter per minute and the ozone concentration is measured every 12 seconds.) 
     The staying ozone (O 3 ) generates oxides. If the photoconductive drums  12 Y,  12 M,  12 C and  12 K keep stopping at the same position, the influence of ozone (O 3 ) products and temperature and humidity changes is unevenly concentrated at a specified position on the photoconductive drums  12 Y,  12 M,  12 C and  12 K. The oxides generated by ozone (O 3 ) unevenly concentrate to adhere to the specified position on the photoconductive drums  12 Y,  12 M,  12 C and  12 K and create an oxide film on the surface of the specified position on the photoconductive drums  12 Y,  12 M,  12 C and  12 K. On the photoconductive drums  12 Y,  12 M,  12 C and  12 K, the electrostatic property in the oxide film part changes and may generate unevenness in image and hence an image defect. 
     For example, under the condition that the ozone (O 3 ) concentration is  12  in an area [A] (shaded in  FIG. 2 ) immediately below the chargers  13 Y,  13 M,  13 C and  13 K, the relation between the leaving time of the photoconductive drums  12 Y,  12 M,  12 C and  12 K and the image defect level can be found as shown in  FIG. 8 . (In  FIG. 8 , image defect levels are defined as follows: image defect level  0  means that no defect can be found; image defect level  1  means that a professional user may notice the uneven concentration; image defect level  2  means that a general user may notice the uneven concentration but there is no practical problem; and image defect level  3  means that the uneven concentration is not practical at all.) 
     In an environment where the ozone (O 3 ) concentration is 12, according to  FIG. 8 , if the leaving time of the photoconductive drums  12 Y,  12 M,  12 C and  12 K is about 6 minutes or shorter, no adhesion of an oxide film to the surface of the photoconductive drums  12 Y,  12 M,  12 C and  12 K is observed and a satisfactory image having no fogging and no uneven concentration can be acquired. If the leaving time exceeds 6 minutes, uneven concentration gradually occurs in the image. Before the leaving time reaches 10 minutes, the image defect level reaches 3 and an unpractical level of image defect occurs. 
     In the embodiment, when the color printer  1  is in a ready mode, the photoconductive drums  12 Y,  12 M,  12 C and  12 K are idled by a predetermined angle according to the need. As the photoconductive drums  12 Y,  12 M,  12 C and  12 K are idled, the influence of ozone products generated by the staying ozone (O 3 ) and temperature and humidity changes on the photoconductive drums  12 Y,  12 M,  12 C and  12 K is made even. 
     In the color printer  1 , for example, when the continuous printing time, as the amount of continuous image formation immediately before the transition from the print mode to the ready mode, exceeds time T 2 , and the continuous printing time after the transition to the ready mode exceeds time T 1 , the photoconductive drums  12 Y,  12 M,  12 C and  12 K are idled by W degrees. Also, the number of continuously printed sheets may be observed and the idling of the photoconductive drums  12 Y,  12 M,  12 C and  12 K may be carried out when the number of continuously printed sheets reaches a predetermined number of sheets or greater. 
       FIG. 9  shows a block diagram of a control system  80  mainly for driving control to idle the photoconductive drums  12 Y,  12 M,  12 C and  12 K. A CPU  81  which controls the entire color printer  1  has the photo-sensors  77  and  78 , a motor driver  45  which drives the drum motor  40 , and a static random access memory (SRAM)  83  and a read only memory (ROM)  84  as storage units, via an input-output (I/O) interface  82 . The CPU  81  has a timer  81   a,  an operation unit  81   b  and a counter unit  81   c.    
     The ROM  84  stores, for example, an idling program to idle the photoconductive drums  12 Y,  12 M,  12 C and  12 K. The SRAM  83  stores, for example, the first stop position of the photoconductive drums  12 Y,  12 M,  12 C and  12 K, the idling angle (W degrees) of the photoconductive drums  12 Y,  12 M,  12 C and  12 K, a threshold value T 2  of continuous printing time, the elapsed time T 1  in the ready mode, the time (D) required for the drum motor  40  to stop, and so on. 
     The start of the process of idling the photoconductive drums  12 Y,  12 M,  12 C and  12 K after the finishing of printing will be described with reference to the flowchart shown in  FIG. 10 . The elapsed time T 1  in the ready mode, which is a condition that requires the idling of the photoconductive drums  12 Y,  12 M,  12 C and  12 K in the ready mode, is set to be shorter than the leaving time which causes an oxide film to be generated on the photoconductive drums  12 Y,  12 M,  12 C and  12 K and causes the generation an image defect to be started. However, if in case that the photoconductive drums  12 Y,  12 M,  12 C and  12 K are idled, the life of the photoconductive drums  12 Y,  12 M,  12 C and  12 K may be reduced by the influence of peripheral devices. Therefore, the idling of the photoconductive drums  12 Y,  12 M,  12 C and  12 K at a high frequency influences the life of the photoconductive drums  12 Y,  12 M,  12 C and  12 K. Thus, it is desirable that the elapsed time T 1  has as large a value as possible within a period before an oxide film is generated on the photoconductive drums  12 Y,  12 M,  12 C and  12 K. 
     When the ozone concentration in the color printer is low, an image defect does not easily occur and therefore the photoconductive drums  12 Y,  12 M,  12 C and  12 K need not be idled in the ready mode. The continuous printing time T 2  immediately before the transition to the ready mode, which is a condition that requires the idling of the photoconductive drums  12 Y,  12 M,  12 C and  12 K in the ready mode, is set in consideration of the influence of the idling on the life of the photoconductive drums  12 Y,  12 M,  12 C and  12 K and in consideration of the influence of ozone (O 3 ) in the color printer  1  on the electrostatic property of the photoconductive drums  12 Y,  12 M,  12 C and  12 K. The continuous printing time T 2  may also be set separately for printing in the color mode and printing in the monochrome mode. 
     As the idling of the photoconductive drums  12 Y,  12 M,  12 C and  12 K is started and printing is finished in the color printer  1  (ACT  100 ), the stop position of the photoconductive drums  12 Y,  12 M,  12 C and  12 K at the finishing of printing is set as a first stop position. The first stop position is stored into the SRAM  83  and the timer  81   a  is reset once and then restarted (ACT  101 ). In ACT  102 , it is determined whether the conditions (the elapsed time T 1  in the ready mode and the continuous printing time T 2 ) for executing the idling of the photoconductive drums  12 Y,  12 M,  12 C and  12 K are satisfied. In case that the continuous printing time immediately before exceeds T 2  and the elapsed time in the ready mode exceeds T 1  (Yes in ACT  102 ), the idling of the photoconductive drums  12 Y,  12 M,  12 C and  12 K is executed in ACT  103  and the processing returns to ACT  101 . While the ready mode is maintained, when the execution conditions in ACT  102  are satisfied, the idling in ACT  103  is repeated. 
     In case that the conditions for executing the idling of the photoconductive drums  12 Y,  12 M,  12 C and  12 K are not satisfied in ACT  102  (No in ACT  102 ), it is determined whether transition to a power-saving mode should be made (the power-saving mode is a mode in which power supplied to constantly powered components such as the fixing device is lowered or cut, and the ready mode cannot be restored unless predetermined restoration procedures such as warm-up are taken in the power-saving mode.) Even without the lapse of the elapsed time T 1 , if transition to the power-saving mode is made (Yes in ACT  106 ), the idling of the photoconductive drums  12 Y,  12 M,  12 C and  12 K is executed in ACT  107 . Then, the color printer  1  is shifted to the power-saving mode (ACT  108 ) and the idling of the photoconductive drums  12 Y,  12 M,  12 C and  12 K is finished. 
     As the idling is executed in ACT  103  or ACT  107 , the photoconductive drums  12 Y,  12 M,  12 C and  12 K are rotated from the current stop position by W degrees and then stopped. For example, an arbitrary position F of the photoconductive drums  12 Y,  12 M,  12 C and  12 K located at the current first stop position S 1  is idled to a second stop position S 2  as a result of rotating in the direction of arrow m by W degrees, and then stops there, as shown in  FIG. 11 . 
     In  FIG. 11 , the timing when the photo-sensors  77  and  78  detect the leading ends  75   a  and  76   a  of the ribs  75  and  76 , respectively, is defined as ta, and the timing when the photo-sensors  77  and  78  detect the terminal ends  75   b  and  76   b  of the ribs  75  and  76 , respectively, is defined as tb. It is now assumed that a trigger for the photoconductive drums  12 Y,  12 M,  12 C and  12 K to start idling and stop at the position where the arbitrary position F reaches the second stop position S 2  is, for example, the timing ta. The angle from the leading ends  75   a  and  76   a  of the ribs  75  and  76  to the second stop position S 2  of the arbitrary position F in the timing ta is α. The photoconductive drums  12 Y,  12 M,  12 C and  12 K are rotated by the angle α from the timing ta and then stopped. 
     However, for the drum motor  40 , an angle β is needed for a movement from the start of the stop of the drum motor  40  until the drum motor  40  stops. Therefore, the motor driver  45  makes the start of stop of the drum motor  40  earlier in consideration of the angle β necessary for the stop of the drum motor  40 . The motor driver  45  starts the stop of the drum motor  40  after the lapse of the time (D) required for the photoconductive drums  12 Y,  12 M,  12 C and  12 K to rotate by an angle (α−β) from the timing ta. 
     The time (D) is calculated using the following equation (1). 
       Time( D )=diameter of photoconductive drum×π×{angle(α−β)}/360/circumferential speed of drum motor   (1) 
     The second stop position S 2  is found by adding W degrees to the first stop position S 1  stored in the SRAM  83 . In case of repeating ACT  101  to ACT  103  of  FIG. 10 , the second stop position S 2  found in a previous idling is set as the first stop position in a subsequent idling. Therefore, in the subsequent idling, (W degrees×2) is added to the first stop position S 1  of the previous idling, thus finding a subsequent second stop position S 2 . In case that the angle added to the first stop position exceeds 360 degrees, an angle obtained by subtracting 360 degrees from the original angle is added and the second stop position S 2  is thus found. With the subtraction of 360 degrees, the operation time of the idling is reduced and the influence of the idling on the life of the photoconductive drums  12 Y,  12 M,  12 C and  12 K is reduced. 
     The process of starting the rotation of the arbitrary position F on the photoconductive drums  12 Y,  12 M,  12 C and  12 K from the first stop position S 1  and then stopping the arbitrary position Fat the second stop position S 2  reached by rotating W degrees will be described with reference to the flowchart shown in  FIG. 12 . The black photoconductive drum  12 K stops, based on the detection of the leading end  75   a  or the terminal end  75   b  of the rib  75  by the first photo-sensor  77  in the first phase detection unit  70 , as a trigger. The color photoconductive drums  12 Y,  12 M and  12 C stop, based on the detection of the leading end  76   a  or the terminal end  76   b  of the rib  76  by the second photo-sensor  78  in the second phase detection unit  71 . 
     In stopping the photoconductive drums  12 Y,  12 M,  12 C and  12 K, the angle required for the stop of the drum motor is taken into account. The time (D) until the photoconductive drums  12 Y,  12 M,  12 C and  12 K start the stop of the drum motor  40  after the photo-sensors  77  and  78  detects the ends of the ribs  75  and  76 respectively is set to be earlier by the angle β required for the stop of the drum motor  40 . 
     A W degrees is added to the current first stop position S 1  and the target second stop position S 2  is thus set (ACT  110 ). It is set whether the leading ends  75   a ,  76   a  or the terminal ends  75   b  and  76   b  of the respective ribs  75  and  76  are used as the trigger for stopping the arbitrary position F on the photoconductive drums  12 Y,  12 M,  12 C and  12 K at the second stop position S 2  (ACT  111 ). 
     In ACT  111 , ( 1 ) at the photo-sensors  77  and  78  detect the leading ends  75   a  and  76   a  of the ribs  75  and  76 , respectively, as shown in  FIG. 13 , the angle from the leading ends  75   a  and  76   a  of the respective ribs  75  and  76  to the second stop position S 2  is assumed α 1 . In case that the angle α 1  is greater than the angle β, the motor driver  45  stops the drum motor  40 , based on the detection of the leading ends  75   a  and  76   a  by the photo-sensors  77  and  78 , respectively, as a trigger. It is assumed that the timing when the photo-sensors  77  and  78  detect the leading ends  75   a  and  76   a  of the respective ribs  75  and  76  is ta. The motor driver  45  starts the stop of the drum motor  40  after the elapse of the time (D) required for the rotation of the photoconductive drums  12 Y,  12 M,  12 C and  12 K by the angle (α 1 −β) after the timing ta. 
     In ACT  111 , ( 2 ) at the photo-sensors  77  and  78  detect the leading ends  75   a  and  76   a  of the ribs  75  and  76 , respectively, as shown in  FIG. 14 , angle from the leading ends  75   a  and  76   a  of the respective ribs  75  and  76  to the second stop position is assumed α 2 . If the angle α 2  is smaller than the angle β, the motor driver  45  cannot be stop the drum motor  40  at the second stop position S 2  after the photo-sensors  77  and  78  detect the leading ends  75   a  and  76   a,  respectively. In the case of  FIG. 14 , the motor driver  45  stops the drum motor  40 , based on the detection of the terminal ends  75   b  and  76   b  of the respective ribs  75  and  76  by the photo-sensors  77  and  78 , as a trigger. It is assumed that the timing when the photo-sensors  77  and  78  detect the terminal ends  75   b  and  76   b  of the respective ribs  75  and  76  is tb. At the photo-sensors  77  and  78  detect the terminal ends  75   b  and  76   b,  respectively, the angle from the terminal ends  75   b  and  76   b  of the respective ribs  75  and  76  to the second stop position S 2  is α 3 . The motor driver  45  starts the stop of the drum motor  40  after the elapse of the time (D 2 ) required for the rotation of the photoconductive drums  12 Y,  12 M,  12 C and  12 K by the angle (α 3 −β) after the timing tb. 
     In ACT  111 , ( 3 ) “in case that the angle α 4  from the leading ends  75   a  and  76   a  to the second stop position S 2  is large at the timing ta of detecting the leading ends  75   a  and  76   a  by the photo-sensors  77  and  78 , respectively” and “in case that the angle α 5  from the terminal ends  75   b  and  76   b  to the second stop position S 2  is greater than the angle β at the timing tb of detecting the terminal ends  75   b  and  76   b  by the photo-sensors  77  and  78 , respectively” as shown in  FIG. 15 , the motor driver  45  stops the drum motor  40 , based on the timing tb as a trigger. The motor driver  45  starts the stop of the drum motor  40  after the elapse of the time (D 3 ) required for the rotation of the photoconductive drums  12 Y,  12 M,  12 C and  12 K by the angle (α 5 −β) after the timing tb. The idling time of the photoconductive drums  12 Y,  12 M,  12 C and  12 K is shortened and the influence on their life is reduced. 
     After the trigger for the motor driver  45  to stop the driver motor  40  is set according to one of the above situations ( 1 ) to ( 3 ) in ACT  111 , the angle α from the photo-sensors  77  and  78  to the second stop position S 2  is found at the timing ta or the timing tb (ACT  112 ). Then, the position of the angle (α−β) is found (ACT  113 ). The time (D) until the stop of the drum motor  40  is started after the ends of the ribs  75  and  76  are detected is found (ACT  114 ). As either the leading ends  75   a  and  76   a  or the terminal ends  75   b  and  76   b  set as the trigger in ACT  111  reach the photo-sensors  77  and  78  (Yes in ACT  115 ), the processing goes to ACT  116 . The time (D) calculated in ACT  114  is passed from the timing to or the timing tb (Yes in ACT  116 ), the motor driver  45  stops the drum motor  40  (ACT  117 ) and the process of stopping the photoconductive drums  12 Y,  12 M,  12 C and  12 K finishes. 
     As the time T 1  passes when the color printer  1  is in the ready mode after the end of printing, the photoconductive drums  12 Y,  12 M,  12 C and  12 K idle by W degrees from the first stop position S 1  and stop at the second stop position S 2 . Both in the monochrome mode shown in  FIG. 5  and in the color mode shown in  FIG. 6 , all the photoconductive drums  12 Y,  12 M,  12 C and  12 K rotate in the direction of arrow m with their phases aligned. The arbitrary position F on the photoconductive drums  12 Y,  12 M,  12 C and  12 K is shifted. 
     In the embodiment, if ozone (O 3 ) stays between the photoconductive drums  12 Y,  12 M,  12 C and  12 K and the chargers  13 Y,  13 M,  13 C and  13 K, the photoconductive drums  12 Y,  12 M,  12 C and  12 K are idled by W degrees every time the time T 1  passes in the ready mode. A specified part on the photoconductive drums  12 Y,  12 M,  12 C and  12 K is thus prevented from being continuously affected by ozone (O 3 ). The influence of ozone (O 3 ) on the photoconductive drums  12 Y,  12 M,  12 C and  12 K is made even and variance in the electrostatic property of the photoconductive drums  12 Y,  12 M,  12 C and  12 K is restrained. Thus, a satisfactory image having uniform image quality is acquired. 
     In the embodiment, when the photoconductive drums  12 Y,  12 M,  12 C and  12 K are idled, either the leading ends  75   a  and  76   a  or the terminal ends  75   b  and  76   b  of the ribs  75  and  76  are set as a trigger to stop the photoconductive drums  12 Y,  12 M,  12 C and  12 K. The idling time to idle the photoconductive drums  12 Y,  12 M,  120  and  12 K is thus reduced and a long life is provided for the photoconductive drums  12 Y,  12 M,  12 C and  12 K. 
     The invention is not limited to the above embodiment and various changes and modifications can be made without departing from the scope of the invention. For example, the continuous printing time T 2  which requires the idling of the image carriers and the time T 1  to start the idling of the image carriers in the ready mode are not limited. The rotation angle in the case of moving the image carriers from the first stop position to the second stop position is not limited, either. Moreover, the mechanism for rotating plural image carriers and aligning their phases is not limited. A phase detection unit may be provided for each of the plural image carriers and the phases of all the image carriers may be thus aligned. The phase alignment of the plural image carriers may be carried out immediately before the start of image formation, instead of at the end of image formation.