Abstract:
An image forming apparatus includes an image forming unit having an image carrier, a developing agent container for holding a developing agent, and a developing agent supplier for supplying the developing agent from the developing agent container to the image carrier. A controller determines the amount of use of the image forming unit. A shaking mechanism shakes the image forming unit from time to time, at intervals determined by the controller according to the amount of use of the image forming unit. The shaking loosens the developing agent so as to maintain its fluidity, thereby avoiding faint image formation.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus that employs a developing agent. 
     2. Description of the Related Art 
     Developing agents are employed in electrophotographic image forming apparatus including printers, copiers, and facsimile machines. A typical apparatus of this type has a photosensitive drum that functions as an image carrier, a developer roller that applies a developing agent known as toner to develop an electrostatic latent image formed on the photosensitive drum, a toner supply roller with a porous surface that supplies toner to the developer roller, and an agitator or the like, disposed upwardly adjacent the toner supply roller, that stirs the toner to maintain a continuous flow of toner to the toner supply roller. A color image forming apparatus may have a plurality of these image forming units with toners of different colors. 
     Japanese Patent Application Publication No. 2005-172842 describes a type of agitator that revolves in a circular orbit, making periodic contact with the toner supply roller, to prevent a loss of fluidity of the toner in the vicinity of the toner supply roller due to the ‘nip’ between the developer roller and toner supply roller. At high printing speeds, however, even this type of agitator may fail to maintain a steady toner flow. The problem is that the rapidly revolving agitator flings toner away from it, so that after a while the agitator is revolving in a hollow space surrounded by compacted layers of toner banked against the walls of the toner container. As a result, the toner supply roller fails to receive an adequate supply of toner and printing becomes faint. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an image forming apparatus with improved printing quality by preventing faint printing due to compaction of a developing agent. 
     The image forming apparatus provided by the present invention includes an image forming unit having an image carrier, a developing agent container for holding a developing agent such as toner, and a developing agent supplier for supplying the developing agent from the developing agent container to the image carrier. A controller monitors the amount of use of the image forming unit. A shaking mechanism shakes the image forming unit from time to time, at intervals determined by the controller according to the amount of use of the image forming unit. 
     The shaking loosens the developing agent so as to maintain its fluidity, even if the developing agent is stirred by a rapidly rotating agitator. As a result, faint printing is avoided and image quality is improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the attached drawings: 
         FIG. 1  is a side sectional view of a color electrophotographic printer embodying the present invention; 
         FIG. 2  is a sectional view of an image forming unit in the color electrophotographic printer in  FIG. 1 ; 
         FIG. 3  is a plan view of the shaking mechanism in  FIG. 1 ; 
         FIGS. 4 and 5  are more detailed side sectional views of the shaking mechanism in  FIG. 1 ; 
         FIG. 6  is a side sectional view illustrating dimensions in the shaking mechanism; 
         FIG. 7  is a block diagram illustrating the shaking control system in a first embodiment of the invention; 
         FIG. 8  is a sectional view of the image forming unit shown in  FIG. 1 , illustrating the direction of shaking and its effect on the toner; 
         FIG. 9  is a flowchart illustrating the shaking control scheme according to the first embodiment; 
         FIG. 10  is a flowchart illustrating the shaking control scheme according to a second embodiment of the invention; 
         FIG. 11  is a block diagram illustrating the shaking control system in a third embodiment; 
         FIG. 12  is a block diagram illustrating the shaking control scheme according to a fourth embodiment; 
         FIG. 13  is a flowchart illustrating the shaking control scheme according to the fourth embodiment; 
         FIG. 14  is a block diagram illustrating the shaking control system in a fifth embodiment; and 
         FIG. 15  is a block diagram illustrating the shaking control scheme according to a fifth embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Printers embodying the present invention will now be described with reference to the attached drawings, in which like elements are indicated by like reference characters. 
     First Embodiment 
     Referring to  FIG. 1 , the printer  1  discussed in the embodiments has a housing comprising a lower cover  3 , a clamshell shaft  5 , an upper cover  7 , and a stacker  9 . The upper cover  7  is pivotably attached to the clamshell shaft  5  so that the upper cover  7  can be opened and closed. The stacker  9  is a part of the upper cover  7  that receives printed pages that have been ejected from the printer  1 . 
     A cassette  11  located at the bottom of the lower cover  3  holds a supply of paper, which is picked up one sheet at a time by a hopping roller  13  and fed by a pair of feed rollers  15  across a media transport path  17  into a belt unit  19  comprising a media transport belt  21  that loops around a driving roller  23  and a following roller  25 . The belt unit  19  carries the paper past a black image forming unit  27 Bk, a yellow image forming unit  27 Y, a magenta image forming unit  27 M, and a cyan image forming unit  27 C, which rest on a shaking mechanism  29 , and past four transfer rollers  31 . Each image forming unit includes a photosensitive drum  33  that functions as an image carrier, a charging roller  35  that electrically charges the surface of the photosensitive drum  33 , a light emitting diode (LED) head  37  that selectively illuminates the photosensitive drum  33  to form a latent electrostatic image thereon, and a developer roller  39  that develops the latent electrostatic image by applying toner of the appropriate color. The transfer roller  31  below each photosensitive drum  33  attracts the toner from the surface of the photosensitive drum  33  onto the paper. As the paper travels past the four image forming units  27 Bk,  27 Y,  27 M,  27 C, a full-color image is built up. Toner that fails to be transferred from the photosensitive drum  33  to the paper is recovered from the photosensitive drum  33  by a cleaning blade (shown later). 
     The paper next enters a fuser  41  comprising a heat roller  43  and a pressure roller  45 , which fuse the toner image onto the paper by a combination of heat and pressure. A pair of delivery rollers  47  then eject the paper onto the stacker  9 . The photosensitive drum  33 , the delivery rollers  47 , and the other rollers mentioned above are driven by motors (not shown). 
       FIG. 2  shows the black image forming unit  27 Bk in more detail. The other image forming units  27 Y,  27 M,  27 C have similar structures. 
     As shown in  FIG. 2 , each image forming unit comprises a main image forming unit  49  and a detachable developing agent container or toner cartridge  51 . The toner cartridge  51  includes a toner reservoir  53  that stores a supply of unused toner (developing agent)  55 , and a toner recovery chamber  57 . At the bottom of the toner reservoir  53  is an outlet  59  through which the toner  55  drops into a toner supply chamber  61  in the main image forming unit  49 . The toner supply chamber  61  includes the developer roller  39 , a toner supply roller  63  with a foam plastic or foam rubber surface that supplies toner  55  to the developer roller  39 , a doctor blade  65  located just above the developer roller  39 , and a film  67  that prevents unwanted toner  55  from entering the space between the developer roller  39  and photosensitive drum  33 . The cleaning blade  69  is located on the far side of the photosensitive drum  33  from the developer roller  39 . As the developer roller  39  turns counterclockwise in the drawing, toner  55  is transferred to the developer roller  39  from the toner supply roller  63  in an amount regulated by the doctor blade  65 , forming a thin layer of toner on the surface of the developer roller  39 . The toner  55  is transferred electrostatically from the developer roller  39  to exposed parts of the photosensitive drum  33  to develop the latent electrostatic image. 
     Located just above the toner supply roller  63  is an agitator  71  that turns clockwise, periodically making contact with the surface of the toner supply roller  63 . If the printer operates at a high speed, the agitator  71  turns at a correspondingly high speed, which can lead to the problem described in the background discussion: the agitator  71  carves out a hollow space, outside which the toner  55  becomes compacted against the walls of the toner supply chamber  61 ; inadequate toner reaches the surface of the toner supply roller  63 ; the supply of toner  55  to the developer roller  39  also becomes inadequate; and printing becomes faint. 
     To prevent this problem, the present invention provides the shaking mechanism  29  shown partly in  FIG. 1 , and more completely in  FIGS. 3 to 6 , to shake the image forming units  27 Bk,  27 Y,  27 M,  27 C. 
     As seen in the top plan view in  FIG. 3 , the shaking mechanism  29  includes a pair of horizontal members  73 L,  73 R disposed on opposite sides of the printer, terminating in respective racks  75 L,  75 R that mesh with respective pinion gears  77 . The pinion gears  77  are driven by a pair of transfer gears  79 , which are connected by a connecting shaft  81  so that they turn in unison. The top edge of the main part  83  of each of the horizontal members  73 L,  73 R includes four grooves  85   a ,  85   b ,  85   c ,  85   d  interspersed with flat sections  87   a ,  87   b ,  87   c ,  87   d , as best seen in the side views in  FIGS. 4 and 5 . 
     As shown in  FIGS. 4 and 5 , the transfer gears  79  are driven by a lifting motor  89  through a reducing gear train  91 , thereby moving the horizontal members  73 L,  73 R back and forth parallel to the direction in which paper is transported past the image forming units  27 Bk,  27 Y,  27 M,  27 C. The reducing gear train  91  includes a small gear  93  mounted on the shaft of the lifting motor  89 , a first double gear  95  comprising a large gear  95   a  and a small gear  95   b  that turn in unison, a second double gear  97  comprising a large gear  97   a  and a small gear  97   b  that turn in unison, and a further transfer gear  99 . Small gear  93  meshes with large gear  95   a , small gear  95   b  meshes with large gear  97   a , and small gear  97   b  meshes with transfer gear  99 , which meshes with one of the two transfer gears  79  that drive the pinion gears  77 . Since these two transfer gears  79  are linked by the connecting shaft  81  and turn in unison, the two horizontal members  73 L,  73 R move back and forth together. 
     When the lifting motor  89  is driven in its forward direction, small gear  93  turns in the direction of arrow A in  FIG. 4 , causing the pinion gears  77  to turn in the direction of arrow B, so that the horizontal members  73 L,  73 R move backward, in the direction of arrow C, opposite to the direction of paper transport. When the lifting motor  89  is driven in its reverse direction, small gear  93  turns in the direction of arrow D in  FIG. 5 , causing the pinion gears  77  to turn in the direction of arrow E, so that the horizontal members  73 L,  73 R move forward, in the direction of arrow F, the direction of paper transport. 
     As shown in  FIG. 5 , when the horizontal members  73 L,  73 R are driven fully forward, the ends of the drum shafts  33   a  of the photosensitive drums of the image forming units  55 Bk,  27 Y,  27 M,  27 C rest at the bottoms of the grooves  85   a ,  85   b ,  85   c ,  85   d.    
     As shown in  FIG. 4 , when the horizontal members  73 L,  73 R are driven fully backward, the ends of the drum shafts  33   a  climb onto the flat sections  87   a ,  87   b ,  87   c ,  87   d  of the horizontal members  73 L,  73 R, thereby lifting the image forming units  55 Bk,  27 Y,  27 M,  27 C upward. 
     Referring to  FIG. 6 , the lengths w 1 -w 4  of the grooves  85   a - 85   d  and the lengths m 1 -m 3  of the flat sections  87   a - 87   c  satisfy the following conditions:
 
w1&gt;w2=w3=w4
 
m1&lt;m2=m3
 
The grooves  85   a - 85   d  have sharply slanted back ends, which are disposed at mutual spacings equal to the spacing between the drum shafts  33   a  of the image forming units  55 Bk,  27 Y,  27 M,  27 C. The front ends of the grooves  85   a - 85   d  slant upward more gradually, and the first groove  85   a  is elongated by a flat level floor between its slanted ends. When the horizontal members  73 L,  73 R are driven backward from the position in  FIG. 5  to the position in  FIG. 4 , first the yellow, magenta, and cyan image forming units  27 Y,  27 M,  27 C are lifted up; then the black image forming unit  27 Bk is lifted up. When the horizontal members  73 L,  73 R are driven forward from the position in  FIG. 4  to the position in  FIG. 5 , the black image forming unit  27 Bk is the first to drop back to its rest position on the floor of groove  85   a , followed by the yellow, magenta, and cyan image forming units  27 Y,  27 M,  27 C.
 
     The horizontal members  73 L,  73 R accordingly have an intermediate position (not illustrated) at which the ends of the drum shaft  33   a  of the black image forming unit  27 Bk rests on the floor of groove  85   a  and the ends of the drum shafts  33   a  of the yellow image forming unit  27 Y, magenta image forming unit  27 M, and cyan image forming unit  27 C rest on flat sections  87   a ,  87   i , and  87   j . This intermediate position can be advantageously used in a black-and-white printing mode in which only the black image forming unit  27 Bk is driven and the color image forming units  27 Y,  27 M,  27 C are left idle to conserve power and avoid needless toner agitation. 
     Referring to  FIG. 7 , the lifting motor  89  is controlled by a controller  101  on the basis of a page count output from a page counter  103 . The page counter  103  counts the number of pages printed by the printer  1 , as a measure of the amount of use of the image forming units  27 Bk,  27 Y,  27 M,  27 C. The controller  101  includes a computing device (not shown) equipped with a central processing unit, memory, and other well-known facilities. The controller  101  is programmed to activate the lifting motor  89  as explained below. 
     The controller  101  is also programmed to process image data received from a host device (not shown) and execute printing operations by controlling the image forming units  27 Bk,  27 Y,  27 M,  27 C, the fuser  41 , and the motors (not shown) that drive the various rollers in the printer  1 . Each time one page is printed, the controller  101  sends the page counter  103  a signal that increments the page count. 
     Image data are received in units referred to as jobs, each job including an arbitrary number of continuously printed pages. The controller  101  is programmed to recognize the end of a job by well-known methods. At the end of a job, the controller  101  checks the page count in the page counter  103 . If the page count is equal to or greater than a predetermined shaking threshold such as, for example, one hundred pages, the controller  101  drives the lifting motor  89  backward and forward at least once, thereby shaking the image forming units  27 Bk,  27 Y,  27 M,  27 C by raising and lowering them at least once. The controller  101  concludes by driving the lifting motor  89  forward to leave the image forming units  27 Bk,  27 Y,  27 M,  27 C in the rest position shown in  FIG. 5 , and clears the page counter  103  to zero. 
     The effect of raising and lowering the image forming units  27 Bk,  27 Y,  27 M,  27 C is shown schematically in  FIG. 8 . The vertical shaking motion is indicated by arrow H. If the toner  55  in the vicinity of the agitator  71  has been compacted by rapid rotation of the agitator  71 , the vertical shaking motion H loosens the compacted toner. If the agitator  71  has hollowed out a space in its radius of motion, the vertical shaking motion H causes the loosened toner to fall into this space as indicated by arrow J, so that the toner supply roller  63  can pick up an adequate amount of toner  55  to deliver to the developer roller  39 . 
     The optimum threshold value depends on the average density of printing on the pages. The higher the density of printing, the more pages can be printed without the problem of toner compaction. 
     Table 1 indicates the results of experiments performed at printing densities from 0.3% to 50%, with the shaking threshold set at values from fifty to three hundred pages. OK indicates that the problems of toner compaction and hollowing out were prevented, X indicates that these problems sometimes occurred, and P indicates that these problems occurred infrequently but were not completely prevented. 
     
       
         
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Density 
                   
               
             
          
           
               
                 Threshold 
                 0.3% 
                 3% 
                 5% 
                 10% 
                 25% 
                 50% 
               
               
                   
               
               
                  50 pages 
                 OK 
                 OK 
                 OK 
                 OK 
                 OK 
                 OK 
               
               
                 100 pages 
                 OK 
                 OK 
                 OK 
                 OK 
                 OK 
                 OK 
               
               
                 150 pages 
                 X 
                 X 
                 P 
                 OK 
                 OK 
                 OK 
               
               
                 200 pages 
                 X 
                 X 
                 X 
                 P 
                 OK 
                 OK 
               
               
                 300 pages 
                 X 
                 X 
                 X 
                 X 
                 P 
                 OK 
               
               
                   
               
             
          
         
       
     
     A shaking threshold of one hundred pages prevented toner compaction and hollowing out at all printing densities. In the present embodiment, which uses a fixed page count as a shaking threshold, a threshold of about one hundred pages is appropriate. 
       FIG. 9  summarizes the operation of the first embodiment with a threshold of one hundred pages. In step S 1 , the printer starts a printing job. In step S 2 , one page is printed. In step S 3 , the page count (n) in the page counter  103  is incremented. In step S 4 , the controller  101  decides whether the printing job has ended, and returns to step S 2  if the job has not ended. 
     When the job has ended, the controller  101  proceeds to step S 5  and check the page count (n). If the page count is greater than or equal to one hundred pages (n≧100), then in step S 6  the controller  101  drives the lifting motor  89  to shake the image forming units as described above, in step S 7  the controller  101  clears the page counter  103  to zero, and the procedure then ends. If the page count is less than one hundred pages (n&lt;100), the procedure ends immediately after step S 5 . 
     By shaking the image forming units from time to time, the controller  101  is able to prevent the problems of toner compaction and hollowing out and the consequent faint printing that occurred in the prior art. 
     Another problem prevented in the first embodiment is the display of an incorrect message on the printer&#39;s message display panel, indicating that the printer is running out of toner, when in fact the toner only needs to be shaken up. 
     Second Embodiment 
     The second embodiment has the same hardware configuration as the first embodiment. The lifting motor  89  is controlled by a controller  101  and a page counter  103  as shown in  FIG. 7 , but in the second embodiment the controller  101  is programmed to shake the image forming units after every hundred printed pages, regardless of whether the current printing job is finished or not. 
     The printer in the second embodiment operates according to the flowchart in  FIG. 10 . In step S 11 , the printer starts a printing job. In step S 12 , one page is printed. In step S 13 , the page count (n) in the page counter  103  is incremented. 
     In step S 14 , the controller  101  checks the page count (n) in the page counter  103 . If the page count is greater than or equal to one hundred pages (n≧100), then in step S 15  the controller  101  drives the lifting motor  89  backward and forward to shake the image forming units as described in the first embodiment, and in step S 16  the controller  101  clears the page counter  103  to zero. If the page count is less than one hundred pages (n&lt;100), then steps S 15  and S 16  are skipped. 
     In step S 17 , the controller  101  decides whether the printing job has ended, and returns to step S 12  if the job has not ended. If the job has ended, the procedure in  FIG. 10  ends. 
     By shaking the image forming units every hundred pages, the second embodiment prevents faint printing even during long printing jobs, lasting more than one hundred pages. 
     Third Embodiment 
     The third embodiment replaces the page counter of the first and second embodiments with a drum revolution counter  105 , shown in  FIG. 11 , that counts revolutions of the photosensitive drums  33  of the image forming units  27 Bk,  27 Y,  27 M,  27 C. 
     In the third embodiment, the controller  101  checks the revolution count in the drum revolution counter  105  at the end of each printed page, and drives the lifting motor  89  to shake the image forming units  27 Bk,  27 Y,  27 M,  27 C if the revolution count has reached a predetermined shaking threshold. After driving the lifting motor  89 , the controller  101  clears the drum revolution counter  105  to zero before printing the next page. 
     The third embodiment is particularly advantageous when the rotation of the agitator  71  in each image forming unit is linked to the rotation of the photosensitive drum  33 , and when the photosensitive drums  33  make different numbers of revolutions per page depending on, for example, the page length. 
     In a variation of the third embodiment, the controller  101  checks the drum revolution counter  105  only at the end of each printing job, as in the first embodiment, instead of at the end of each page. 
     Fourth Embodiment 
     The fourth embodiment adds a dot counter  107 , shown in  FIG. 12 , to the page counter  103  of the first and second embodiments. The controller  101  controls the lifting motor  89  according to both the number of pages printed and the number of dots printed on the pages, and shortens the intervals at which the image forming units are shaken when the printing density is low. The intervals between shakings are thus varied according to the rate of consumption of the toner. 
     The operation of the fourth embodiment will be described with reference to the flowchart in  FIG. 13 . 
     In step S 21 , the printer starts a printing job. In step S 22 , one page is printed. During this step, the dot counter  107  is incremented by one for each printed dot. The dot counter  107  may be incremented when, for example, the dot data are read into the LED heads  37 . 
     In step S 23 , the page count (n) in the page counter  103  is incremented. In step S 24 , the controller  101  decides whether the printing job has ended, and returns to step S 22  if the job has not ended. 
     When the job has ended, the controller  101  proceeds to step S 25  and checks the page count (n). If the page count is less than fifty pages (n&lt;50), the procedure ends. 
     If the page count is greater than or equal to fifty pages (n≧50), then in steps S 26  and S 27  the controller  101  reads the printed dot count from the dot counter  107 , divides the printed dot count by the page count do determine the average number of dots printed per page, and compares this average number with a predetermined value to decide whether or not the printing density is greater than 5%. 
     If the printing density is greater than 5% in step S 27 , the controller  101  waits for the next printing job to start. When the next printing job starts, the controller  101  prints a page in step S 28 , increments the page counter  103  in step S 29 , decides whether the job has ended in step S 30 , and returns to step S 28  if the job has not ended. When the job ends, the controller  101  proceeds to step S 31  and decides whether the page count in the page counter  103  is greater than or equal to one hundred. If the page count is less than one hundred (n&lt;100), the procedure ends. 
     If the page count is equal to or greater than one hundred (n≧100) in step S 31 , or if the average printing density is equal to or less than 5% in step S 27 , the controller  101  proceeds to step S 32  and drives the lifting motor  89  to shake the image forming units as described in the first embodiment, then clears the page counter  103  and dot counter  107  to zero in step S 33 , after which the procedure ends. 
     In the fourth embodiment, when the printing density is less than 5%, the image forming units are shaken at intervals of fifty pages or so. When the printing density is 5% or higher, the image forming units are shaken at longer intervals of one hundred pages or so. In view of the data in Table 1 above, these intervals between shakings can be expected to provide adequate protection from toner compaction and faint printing. 
     In a variation of the fourth embodiment, the controller  101  checks the page count at the end of each printed page, instead of the end of each job, as in the second embodiment, to ensure that the intervals between shakings are exactly fifty pages for low-density printing and one hundred pages for higher-density printing. 
     In another variation of the fourth embodiment, the page counter  103  is replaced by a drum revolution counter as in the third embodiment. 
     Fifth Embodiment 
     The fifth embodiment adds an ambient temperature sensor  109  and an ambient humidity sensor  111 , shown in  FIG. 14 , to the page counter  103  of the first and second embodiments, and controls the intervals between shakings according to environmental conditions as well as the number of pages printed. The ambient temperature sensor  109  senses the ambient temperature τ and sends the controller  101  a signal indicating the temperature. The ambient humidity sensor ill senses the ambient humidity f and sends the controller  101  a signal indicating the humidity. The controller  101  controls the lifting motor  89  according to the ambient temperature, ambient humidity, and printed page count. 
     The problems of toner compaction, hollowing out, and faint printing are most likely to occur under conditions of high temperature and high humidity. Table 2 indicates the results of high-speed printing trials made under various temperature and humidity conditions. OK indicates that that the above problems did not occur, X indicates that these problems sometimes occurred, P indicates that these problems occurred infrequently but were not completely prevented, and dashes indicate combinations of conditions that were not tested. All problems observed occurred at temperatures of 20° C. or higher and in almost all cases at a humidity of 40% or higher. In the fourth embodiment, accordingly, the image forming units are shaken only when the temperature is above 20° C. and the humidity is above 40%. Under these conditions, the shaking interval is fifty pages. 
     
       
         
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
             
             
               
                   
                   
               
               
                   
                 Humidity 
                   
               
             
          
           
               
                 Temperature 
                 20% 
                 40% 
                 60% 
                 80% 
               
               
                   
               
               
                 10° C. 
                 OK 
                 — 
                 — 
                 — 
               
               
                 15° C. 
                 OK 
                 — 
                 — 
                 — 
               
               
                 20° C. 
                 — 
                 P 
                 X 
                 — 
               
               
                 25° C. 
                 — 
                 P 
                 X 
                 — 
               
               
                 30° C. 
                 P 
                 — 
                 — 
                 X 
               
               
                   
               
             
          
         
       
     
     The printer in the fifth embodiment operates according to the flowchart in  FIG. 15 . In step S 41 , the printer starts a printing job. In step S 42 , one page is printed. In step S 43 , the page count (n) in the page counter  103  is incremented. 
     In step S 44 , the controller  101  checks the page count (n) in the page counter  103  and returns to step S 42  if the page count is less than fifty pages (n&lt;50). If the page count is greater than or equal to fifty pages (n≧50), then in steps S 45  and S 46  the controller  101  senses the ambient temperature by checking the ambient temperature sensor  109  and decides whether the temperature τ is greater than 20° C. If the temperature τ is greater than 20° C., then in steps S 47  and S 48  the controller  101  checks the ambient humidity sensor  111  and decides whether the humidity f is greater than 40%. If the humidity f is greater than 40%, then in step S 49  the controller  101  drives the lifting motor  89  backward and forward to shake the image forming units as described in the first embodiment, and in step S 50  the controller  101  clears the page counter  103  to zero. Next, in step S 51 , the controller  101  decides whether the printing job has ended, and returns to step S 42  if the job has not ended. 
     If the temperature is less than or equal to 20° C., the controller  101  skips steps S 47  to S 50  and proceeds directly from step S 46  to step S 51 . If the humidity is less than or equal to 40%, the controller  101  skips steps S 49  and S 50  and proceeds directly from step S 48  to step S 51 . In either of these two cases the image forming units are not shaken. 
     By shaking the image forming units at intervals of fifty pages, but only under conditions of comparatively high temperature and humidity, the second embodiment prevents toner compaction and hollowing out under conditions that produce these problem, and avoids needlessly shaking the image forming units under conditions in which toner compaction is unlikely to occur. 
     In a variation of the fifth embodiment, the page counter  103  is replaced by a drum revolution counter as in the third embodiment. 
     In another variation of the fifth embodiment, the image forming units are shaken at intervals that decrease with increasing temperature and humidity. 
     In yet another variation of the fifth embodiment, only a temperature sensor, or only a humidity sensor, is provided. 
     The invention is not limited to the fifty-page and hundred-page thresholds shown in the embodiments above. Other threshold values may be used. In the fifth embodiment, the threshold page count may be varied according to the ambient temperature and humidity, so that the intervals between shakings become shorter with increasing temperature and humidity. 
     The invention is not limited to image forming units of the type shown in  FIGS. 1 and 2 . For example, the image carrier may be a photosensitive belt instead of a photosensitive drum. 
     Those skilled in the art will recognize that further variations are possible within the scope of the invention, which is defined in the appended claims.