Patent Publication Number: US-2009232556-A1

Title: Counting method of mixing time of developer

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from provisional U.S. Application 61/036,567 filed on Mar. 14, 2008, the entire contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a measuring apparatus of mixing time of developer, which manages whether a developing device used in an image forming apparatus, such as a copy machine or a printer, reaches a replacement timing. 
     BACKGROUND 
     Among developing devices used in an image forming apparatus such as a copy machine or a printer, there is a device which is used while a toner cartridge filled with toner as a consumable is replaced. In such a developing device, in addition to the management of the replacement timing of the toner cartridge, it is necessary to manage the replacement timing of the developer or the replacement timing of the developing device. Hitherto, there is a device in which in order to manage the replacement timing of the developer or the developing device, the driving time of the developing device is counted, and when the accumulated count of the driving time reaches a predetermined specified time, it is judged that the developer or the developing roller reaches its end of life. 
     However, when an image forming apparatus has plural image forming speeds, or when the operation of a developing roller is changed with respect to the driving of a mixer, the degree of deterioration of the developer varies according to the image formation speed or the operation mode of the developing roller. When the image forming apparatus has the plural image forming speeds or the operation of the developing roller is changed, even if the accumulated count of the driving time of the developing device reaches the specified time, there is a possibility that the developer actually does not reach its end of life. There is a fear that even if the accumulated count obtained by simply accumulating the driving time of the developing device is compared with the specified time, the life of the developer can not be accurately obtained. 
     Even when the image forming apparatus has the plural image forming speeds, or even when the operation of the developing roller is changed with respect to the driving of the mixer, it is desired that the life of the developer is accurately judged. 
     SUMMARY 
     In an aspect of the invention, judges the life of a developer accurately, prevents to replace a usable developer wastefully, improves the economic efficiency, and reduces the maintenance. 
     According to an aspect, a measuring apparatus of mixing time of developer includes a mixing member to mix the developer in a developing container, a developing member that has plural driving speeds and supplies the developer in the developing container to an image carrier, and an arithmetic member to multiply a driving time of the mixing member by a coefficient which is set correspondingly to an operation of the developing member in a period when the mixing member is driven, and to accumulate a multiplication count obtained by multiplication of the coefficient. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a whole structural view showing an image forming apparatus of a first embodiment; 
         FIG. 2  is a schematic structural view showing a developing device of the first embodiment and a driving system of the developing device; 
         FIG. 3  is a graph showing a relation between driving time count and background fogging degree of the first embodiment; 
         FIG. 4  is an explanatory view showing a driving time count of a reference example of the first embodiment; 
         FIG. 5  is a flowchart showing measurement of the life of a developer of the first embodiment; 
         FIG. 6  is an explanatory view showing a calculation example of a driving time count of the first embodiment; 
         FIG. 7  is an explanatory view showing a calculation example of a driving time count of a comparative example of the first embodiment; and 
         FIG. 8  is an explanatory view showing a calculation example of a driving time count of a second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, a first embodiment will be described.  FIG. 1  is a schematic structural view of a color printer  1  as an image forming apparatus of the first embodiment. The color printer  1  is of a train-of-four tandem type. The color printer  1  performs printing at plural printing speeds. For example, when printing is performed on standard paper with a weight per unit area of about 64 to 80 g/m 2 , the color printer  1  sets the printing speed at 150 mm/sec. For example, when printing is performed on thick paper with a weight per unit area heavier than 80 g/m 2 , the color printer  1  sets the printing speed at 75 mm/sec. 
     The color printer  1  includes a paper eject portion  3  at an upper part. The color printer  1  includes four sets of image forming stations  11 Y,  11 M,  11 C and  11 K disposed 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 respectively include photoconductive drums  12 Y,  12 M,  12 C and  12 K as image carriers. The respective image forming stations  11 Y,  11 M,  11 C and  11 K form toner images of yellow (Y), magenta (M), cyan (C) and black (K) on the respective photoconductive drums  12 Y,  12 M,  12 C and  12 K. 
     The respective photoconductive drums  12 Y,  12 M,  12 C and  12 K rotate in an arrow m direction. Charging chargers  13 Y,  13 M,  13 C and  13 K, developing devices  14 Y,  14 M,  14 C and  14 K, and photoreceptor cleaners  16 Y,  16 M,  16 C and  16 K are respectively disposed around the respective photoconductive drums  12 Y,  12 M,  12 C and  12 K along the rotation direction. 
     For the respective image forming stations  11 Y,  11 M,  11 C and  11 K, the photoconductive drums  12 Y,  12 M,  12 C and  12 K, the charging chargers  13 Y,  13 M,  13 C and  13 K, the developing devices  14 Y,  14 M,  14 C and  14 K, and the photoreceptor cleaners  16 Y,  16 M,  16 C and  16 K may be respectively united to form process cartridges. When the process cartridges are formed, the respective process cartridges are independently and integrally attached to and detached from the main body of the color printer  1 . 
     Exposure lights by a laser exposure device  17  are irradiated between the charging chargers  13 Y,  13 M,  13 C and  13 K and the developing devices  14 Y,  14 M,  14 C and  14 K around the photoconductive drums  12 Y,  12 M,  12 C and  12 K. The laser exposure device  17  scans laser beams emitted from a semiconductor laser element in an axial direction of the photoconductive drum  12 , and includes a polygon mirror  17   a , an imaging lens system  17   b , a reflecting mirror  17   c  and the like. Electrostatic latent images are respectively formed on the photoconductive drums  12 Y,  12 M,  12 C and  12 K by the irradiation of the exposure lights from the laser exposure device  17 . The respective charging chargers  13 Y,  13 M,  13 C and  13 K and the laser exposure device  17  constitute a latent image forming portion. 
     The respective developing devices  14 Y,  14 M,  14 C and  14 K supply toners to the electrostatic latent images on the photoconductive drums  12 Y,  12 M,  12 C and  12 K, and visualize the electrostatic latent images. The respective developing devices  14 Y,  14 M,  14 C and  14 K perform development with a two-component developer containing a carrier and a toner of each of yellow (Y), magenta (M), cyan (C) and black (K). 
     The intermediate transfer belt  10  is stretched by a backup roller  21 , a driven roller  20  and first to third tension rollers  22  to  24 , and is rotated in an arrow s direction. The intermediate transfer belt  10  is opposite to the photoconductive drums  12 Y,  12 M,  12 C and  12 K, and contacts therewith. Primary transfer rollers  18 Y,  18 M,  18 C and  18 K are provided at positions where the intermediate transfer belt  10  is opposite to the photoconductive drums  12 Y,  12 M,  12 C and  12 K. The respective primary transfer rollers  18 Y,  18 M,  18 C and  18 K primarily transfer the toner images formed on the photoconductive drums  12 Y,  12 M,  12 C and  12 K to the intermediate transfer belt  10 . The respective photoreceptor cleaners  16 Y,  16 M,  16 C and  16 K remove and collect residual toners on the photoconductive drums  12 Y,  12 M,  12 C and  12 K after the primary transfer. 
     A secondary transfer roller  27  is opposite to the intermediate transfer belt  10  at a secondary transfer portion where the intermediate transfer belt is supported by the backup roller  21 . In the secondary transfer portion, a specified secondary transfer bias is applied to the backup roller  21 . When a sheet passes through between the intermediate transfer belt  10  and the secondary transfer roller  27 , the toner image on the intermediate transfer belt  10  is secondarily transferred onto the sheet. The sheet P is supplied from a paper feed cassette  4   a ,  4   b  or a manual feed mechanism  31 . After the secondary transfer is finished, a belt cleaner  10   a  cleans the intermediate transfer belt  10 . 
     Pickup rollers  2   a  and  2   b , separation rollers  5   a  and  5   b , conveying rollers  6   a  and  6   b , and a register roller pair  36  are provided from the paper feed cassettes  4   a  and  4   b  to the secondary transfer roller  27 . A manual feed pickup roller  31   b , and a manual feed separation roller  31   c  are provided from a manual feed tray  31   a  of the manual feed mechanism  31  to the register roller pair  36 . A fixing device  30  is provided downstream of the secondary transfer portion along a direction of a longitudinal conveying path  34 . The fixing device  30  fixes the toner image transferred to the sheet P by the secondary transfer portion to the sheet P. A gate  33  to distribute the sheet to a direction of a paper eject roller  41  or a direction of a re-conveying unit  32  is provided downstream of the fixing device  30 . The sheet guided to the paper eject roller  41  is ejected to the paper eject portion  3 . The sheet guided to the re-conveying unit  32  is again guided to the direction of the secondary transfer roller  27 . 
     Next, the developing devices  14 Y,  14 M,  14 C and  14 K will be described. Since the developing devices  14 Y,  14 M,  14 C and  14 K have the same structure, common reference numerals are used and a description will be made. As shown in  FIG. 2 , each of the developing devices  14 Y,  14 M,  14 C and  14 K includes a case  50  which is a developing container and contains a two-component developer  51 , a first and a second mixers  56  and  57  as a mixing member, a developing roller  58  as a developing member, and a toner density sensor  60 . 
     The first mixer  56  and the second mixer  57  of the case  50  are partitioned from each other by a partition plate  64 . The first and the second mixers  56  and  57  mix the developer  51 , and circulate and convey the developer  51  in the case  50 . The toner density sensor  60  is disposed at the bottom of the case  50 . The toner density sensor  60  uses, for example, a permeability sensor. When the lowering of the toner density of the developer  51  in the case  50  is detected from the detection result of the toner density sensor  60 , the toner is supplied from, for example, a toner cartridge to the case  50  according to the detection result. By this, the toner density of the developer  51  in the case  50  is kept constant. 
     The second mixer  57  mixes and conveys the developer  51 , and supplies it to the developing roller  58 . The developing roller  58  supplies the toner to the electrostatic latent images on the respective photoconductive drums  12 Y,  12 M,  12 C and  12 K, and forms the toner images on the photoconductive drums  12 Y,  12 M,  12 C and  12 K. The developer  51  passing the developing roller  58  is circulated and conveyed to the first mixer  56  side by the second mixer  57 . 
     A new carrier is supplied to the case  50  from, for example, a carrier cartridge. In the supply of the new carrier, only the carrier may be supplied. Alternatively, the new carrier may be supplied by supplying a two-component developer containing a toner and a carrier. A discharge port  53  as a developer discharging member is formed at the side part of the case  50 . The volume of the developer  51  in the case  50  is increased by the supply of the new carrier, and the excess developer is discharged from the discharge port  53  and is collected. In the case  50 , the amount of the developer  51  is kept constant. In the case  50 , the deteriorated old carrier is replaced little by little by the new carrier. The charge performance of the toner of the developer  51  in the case  50  is kept constant. 
     The first and the second mixers  56  and  57  are driven by a first drive motor  62  as a first driving member at a constant speed of, for example, 400 rpm. The first and the second mixers  56  and  57  are driven at the constant speed, and the height of the surface of the mixed and conveyed developer  51  is made constant. The variation in discharge amount of the excess developer from the discharge port  53  is suppressed, and the amount of the developer  51  in the case  50  is stabilized. 
     The developing roller  58  is driven by a second drive motor  63  as a second driving member. The driving speed of the developing roller  58  is changed according to the printing speed of the color printer  1  (rotation speed of the photoconductive drums  12 Y,  12 M,  12 C and  12 K). When printing is performed on standard paper, the driving speed of the developing roller  58  is 150 mm/sec. When printing is performed on thick paper, the driving speed of the developing roller  58  is 75 mm/sec. When the traveling distance of the developing roller  58  per unit time when printing is performed on the standard paper is made  1 , the traveling distance of the developing roller  58  per unit time when printing is performed on the thick paper is ½. 
     A first driver  62   a  of the first drive motor  62  and a second driver  63   a  of the second drive motor  63  are respectively connected to a CPU  100  to control the color printer  1 . 
     The CPU  100  includes a memory  110 , a mixer management unit  120  to instruct the first driver  62   a  to drive the first motor  62 , an on and off management unit  130  to instruct the second driver  63   a  to turn on and off the second motor  63 , and a speed management unit  140  to instruct the second driver  63   a  to control the driving speed of the second motor  63 . 
     The CPU  100  includes an arithmetic unit  150  as an arithmetic member to calculate the driving time count of the respective developing devices  14 Y,  14 M,  14 C and  14 K according to the operation of the second motor  63 , and a judgment unit  160  as a judgment member to compare the calculation result of the arithmetic unit  150  with a life threshold in the memory  110  and to judge whether the developer  51  reaches its end of life. The CPU  100  controls a control panel  170  that performs input and output of data of the color printer  1  and displays the judgment result of the judgment unit  160 . 
     The arithmetic unit  150  of the CPU  100  calculate a multiplication count from multiplies the driving time of the first and the second mixers  56  and  57  by a count coefficient. The count coefficient is a coefficient corresponding to the operation of the developing roller  58  in a period when the first and the second mixers  56  and  57  are driven. The arithmetic portion unit  150  accumulates the multiplication count and calculates a driving time count as an accumulated count. 
     The judgment of the life of the developer  51  of the developing devices  14 Y,  14 M,  14 C and  14 K will be described. The memory  110  stores the life threshold used for the judgment as to whether the developer  51  reaches its end of life. The memory  110  stores the driving time count as the calculation result of the arithmetic unit  150 . 
     (Setting of the Life Threshold) 
     When the developer  51  is deteriorated while a print job is being performed, the toner is adhered to a background portion where the toner image is not formed. When the background fogging degree as a toner adhesion degree to the background portion becomes large and the background fogging degree becomes, for example, 2 or more, it is judged that a replacement is necessary because of the developer  51  reaches its end of life. The driving time of the developing devices  14 Y,  14 M,  14 C and  14 K until the developer  51  reaches its end of life is weighted (multiplied by a count coefficient), and a driving time count obtained by accumulating the weighted driving time (multiplication count) is stored as the life threshold into the memory  110 . 
     The setting of the life threshold will be described while using the developing device  14 Y of yellow (Y). The developing device  14 Y of yellow (Y) is replaced by a new one, and next, a print job is performed using JIS A4 size standard paper. In one print job, printing of five sheets is performed. Each time the printing of five sheets is performed, the deterioration degree of the developer  51  is measured. In the print job on the standard paper, the first and the second mixers  56  and  57  are driven at a constant speed, and the developing roller  58  is driven at a driving speed of 150 mm/sec. The count coefficient multiplied to the driving time of the developing device  14 Y of yellow (Y) is set to, for example, 4. The numerical value of the count coefficient is not limited. 
     In  FIG. 3 , the horizontal axis indicates the driving time count of the developing device  14 Y of yellow (Y), and the vertical axis indicates the background fogging degree. When the background fogging degree of the toner image is actually measured while printing is performed on five pieces of A4 size standard paper, a solid line α (reference example) indicated by ▪ in  FIG. 3  is obtained. As shown in  FIG. 4 , the driving time count of the solid line α is obtained from accumulate the multiplication count. The multiplication count is obtained from multiplies the driving time of the first and the second mixers  56  and  57  as the driving time of the developing device  14 Y of yellow (Y) by the count coefficient  4 . 
     The driving time count (400×1000) when the solid line α exceeds the background fogging degree of 2 is set as the life threshold of the developing device  14 Y of yellow (Y) and is stored in the memory  110 . Also with respect to each of the developing devices  14 M,  14 C and  14 K of magenta (M), cyan (C) and black (K), similarly to the developing device  14 Y of yellow (Y), the driving time count as the life threshold is set and is stored in the memory  110 . 
     The memory  110  stores the count coefficient which is set according to the operation of the developing roller  58 . In the developing devices  14 Y,  14 M,  14 C and  14 K, there is a case where while the first and the second mixers  56  and  57  are being driven at a constant speed, the driving speed of the developing roller  58  is changed, or the driving of the developing roller  58  is stopped. The count coefficient is set according to the operation of the developing roller  58 , and is stored in the memory. 
     (Setting of the Count Coefficient) 
     The driving speed of the developing roller  58  is different between the case where printing is performed on standard paper and the case where printing is performed on thick paper, and the traveling distance per unit time of the developing roller  58  varies. When printing is performed on the thick paper, the traveling distance per unit time of the developing roller  58  is ½ of that of the case where printing is performed on the standard paper. The deterioration degree of the developer  51  is influenced by the traveling distance variation per unit time of the developing roller  58  and is changed. 
     When the traveling distance per unit time of the developing roller  58  is large at the time of driving of the developing devices  14 Y,  14 M,  14 C and  14 K, the load applied to the developer becomes large, and the deterioration of the developer  51  proceeds. When the traveling distance per unit time of the developing roller  58  is small at the time of driving of the developing devices  14 Y,  14 M,  14 C and  14 K, the load applied to the developer  51  becomes small, and the deterioration of the developer  51  becomes slow. 
     The count coefficient multiplied to the driving time of the developing device  14 Y is set to, for example, 4 for the standard paper in which the traveling distance per unit time of the developing roller  58  is large. The count coefficient is set to 2 for the thick paper in which the traveling distance per unit time of the developing roller  58  is small. Besides, the count coefficient is set to 1 when the developing roller  58  is in a stopped state and the print job is not performed although the first and the second mixers  56  and  57  are driven at a constant speed. 
     The count coefficient of the case of the thick paper is made smaller than the count coefficient of the case of the standard paper, and the difference in progress of deterioration of the developer  51  between the case where printing is performed on the standard paper and the case where printing is performed on the thick paper is reflected on the judgment of the life of the developer  51 . The count coefficient of 4 for the standard paper, the count coefficient of 2 for the thick paper, and the count coefficient of 1 for the stop of the developing roller  58  which are set upped are stored in the memory  110 . 
     The measurement of the life of the developer  51  of the developing device  14 Y of yellow (Y) will be described with reference to a flowchart of  FIG. 5 . The measurement of the life is started and the developing device  14 Y of yellow (Y) is replaced by a new one (Act  200 ). The driving time count stored in the memory  110  is reset to 0 (Act  201 ). The driving of the first motor  62  is detected by the mixer management unit  120  (Act  202 ). When the first and the second mixers  56  and  57  are driven by turning-on of the first motor  62 , the on and off management unit  130  detects turning on and off of the second motor  63  (Act  203 ). 
     At Act  203 , when the second motor  63  is off and the developing roller  58  is stopped, the print job is not performed, and accordingly, the count coefficient of 1 stored in the memory  110  is selected (Act  204 ). Next, in the arithmetic unit  150 , the count coefficient of 1 is multiplied to the driving time of the first and the second mixers  56  and  57  and the multiplication count is calculated (Act  206 ). The calculated multiplication count is accumulated to the driving time count in the memory  110 , and the driving time count in the memory  110  is rewritten (Act  207 ). Next, advance is made to Act  214 . 
     When the second motor  63  is on at Act  203 , the driving speed of the developing roller  58  by the second motor  63  is detected by the speed management unit  140  (Act  208 ). At Act  208 , when the print job is for the standard paper, the count coefficient of 4 stored in the memory  110  is selected (Act  210 ). Next, in the arithmetic unit  150 , the count coefficient of 4 is multiplied to the driving time of the first and the second mixers  56  and  57  to calculate the multiplication count (Act  211 ), and advance is made to Act  207 . At Act  207 , the calculated multiplication count is accumulated to the driving time count stored in the memory  110 , and the driving time count in the memory  110  is rewritten. Next, advance is made to Act  214 . 
     At Act  208 , when the print job is for the thick paper, the count coefficient of 2 stored in the memory  110  is selected (Act  212 ). Next, in the arithmetic unit  150 , the count coefficient of 2 is multiplied to the driving time of the first and the second mixers  56  and  57  to calculate the multiplication count (Act  213 ), and advance is made to Act  207 . At Act  207 , the calculated multiplication count is accumulated to the driving time count stored in the memory  110 , and the driving time count in the memory  110  is rewritten. Next, advance is made to Act  214 . 
     At Act  214 , the judgment unit  160  compares the driving time count with the life threshold (400×1000) in the memory  110 . At Act  214 , when the driving time count stored in the memory  110  does not reach the life threshold, return to Act  202 , and the measurement of the life of the developer  51  is continued. 
     At Act  214 , when the driving time count reaches the life threshold, it is judged that the developer  51  reaches its end of life (Act  216 ). At Act  216 , the control panel  170  displays that the developer  51  reaches its end of life, and the measurement of the life of the developer  51  is ended. The user replaces the developing device  14 Y of yellow (Y), in which the developer  51  reaches its end of life, by a new one. 
       FIG. 6  shows a calculation example of actual driving time count in accordance with the flowchart of  FIG. 5 . FIG.  6  shows the calculation example of the driving time count including a case where a print job is performed while standard paper and thick paper are changed at random and a case which the print job is not performed in which only the first and the second mixers  56  and  57  are driven. With respect to the print job on the standard paper, the count coefficient of 4 is multiplied to the driving time. With respect to the print job on the thick paper, the count coefficient of 2 is multiplied to the driving time. With respect to the case which the print job is not performed, the count coefficient of 1 is multiplied to the driving time. The driving time count obtained by accumulating the multiplication count becomes 11. 
     Besides, in accordance with the flowchart of  FIG. 5  of this embodiment, the count coefficient is changed according to the operation of the developing roller  58 , the driving time count is calculated, and the actual life test of the developing device  14 Y of yellow (Y) is performed. As a result, a dotted line β indicated by Δ in  FIG. 3  is obtained. In the life test, after the developing device  14 Y of yellow (Y) is replaced by a new one, JIS A4 size standard paper and thick paper are alternately used, and a print job is performed. In one print job, printing of five sheets is performed, and each time the printing of five sheets is performed, the background fogging degree of a toner image is measured. 
     The dotted line β indicates almost the same transit as the solid line α of the reference example. In the dotted line β, when the driving time count reaches (400×1000) of the life threshold, the state is such that the background fogging degree of the obtained toner image reaches 2. Even if the operation of the developing roller  58  in the print job is changed, the measurement accuracy of the life of the developer  51  is hardly shifted from the reference example, and the excellent life measurement accuracy can be obtained. 
     Incidentally, as a comparative example, in the life test, the count coefficient is made constant irrespective of the operation of the developing roller  58 , the driving time count is calculated, and the test is performed.  FIG. 7  shows the driving time count of the comparative example. In  FIG. 7 , the count coefficient of 4 is multiplied to the driving time in all cases including a case of a print job for standard paper, a case of a print job for thick paper, and a case which the print job is not performed. The driving time count obtained by accumulating the multiplication count is 16. 
     In the life test of the comparative example, a dotted line γ indicated by  in  FIG. 3  is obtained. In the dotted line γ of the comparative example, although the driving time count reaches (400×1000) of the life threshold, the background fogging degree of a toner image keeps almost 1, and actually, the developer does not reach its end of life. In the dotted line γ of the comparative example, in the driving of the first and the second mixers  56  and  57 , the count coefficient is made constant although the operation of the developing roller  58  is changed. Accordingly, the measurement accuracy of the life of the developer is significantly shifted from the reference example. In the comparative example, the measurement accuracy of the life of the developer is reduced. 
     In the first embodiment, the count coefficient multiplied to the driving time of the first and the second mixers  56  and  57  is changed according to the operation of the developing roller  58 , and the driving time count is calculated. When the count coefficient is changed and the driving time count is calculated, regardless of irrespective of the difference in progress of deterioration of the developer  51  due to the difference in the operation of the developing roller  58 . The replacement timing caused by the life of the developer  51  can be accurately judged. The developer which does not reach its end of life and can be used is not wastefully replaced. 
     Next, a second embodiment will be described. The second embodiment is different from the first embodiment in a count coefficient corresponding to an operation of a developing roller. The others are the same as the first embodiment. In the second embodiment, the same structure as that explained in the first embodiment is denoted by the same reference numerals and signs, and its detailed description will be omitted. 
     In this embodiment, with respect to the case of a state which the print job is not performed in which a developing roller  58  is stopped at the time of driving of a first and a second mixers  56  and  57 , a dedicated count coefficient is not set. A count coefficient for standard paper or a count coefficient for thick paper is used also as the count coefficient for the case of the state which the print job is not performed. 
     As the state which the print job is not performed, for example, there is a case where toner supply is performed after the print job is finished. However, the frequency thereof is not high, and an influence is hardly exerted on the measurement of the life of the developer. From this, as the count coefficient for the case which the print job is not performed, the count coefficient for the standard paper or the count coefficient for the thick paper is used. Accordingly, the count coefficient of 4 for the standard paper and the count coefficient of 2 for the thick paper are stored in the memory  110 . 
     For example, when the count coefficient of 2 for the thick paper is used also as the count coefficient for the case which the print job is not performed, the count coefficient of 2 is selected at Act  204  of  FIG. 5 . At Act  206 , the count coefficient of 2 is multiplied to the driving time of the first and the second mixers  56  and  57 , and the multiplication count for the case which the print job is not performed is calculated. 
       FIG. 8  shows a calculation example of an actual driving time count. Similarly to  FIG. 6  of the first embodiment,  FIG. 8  shows the calculation example of the driving time count including the case where the print job is performed while standard paper and thick paper are changed at random, and the case which the print job is not performed in which only the first and the second mixers  56  and  57  are driven. With respect to the print job on the standard paper, the count coefficient of 4 is multiplied to the driving time. With respect to the print job on the thick paper and the case which the print job is not performed, the count coefficient of 2 is multiplied to the driving time. The driving time count obtained by accumulating the multiplication count becomes 12. 
     As compared with the first embodiment, the multiplication count for the case which the print job is not performed is increased by one per unit time. However, the rate of the increase hardly influences the life threshold (400×1000). Even if the count coefficient of 2 for the thick paper is used also as the count coefficient for the case which the print job is not performed, the excellent life measurement accuracy almost equal to the first embodiment can be obtained. 
     Incidentally, in the second embodiment, the count coefficient of 4 for the standard paper may be used also as the count coefficient for the case which the print job is not performed. 
     The invention is not limited to the above embodiments, but various modifications can be made within the scope of the invention. For example, the developing device may be mounted in a monochrome image forming apparatus. The coefficient multiplied to the driving time of the developing device is not limited. It is sufficient if the coefficient is such a value that the difference in deterioration of the developer due to the operation of the developing member can be reflected on the actual life of the developer. The image forming speed of the image forming apparatus is not limited. The image forming apparatus may be an apparatus in which the image forming speed is changed to, for example, three speeds. When the image forming speed is changed to the three speeds of a standard speed, a high speed two times higher than the standard speed, and a low speed of half the standard speed, the coefficient for the case of the high speed may be 6, the coefficient for the case of the standard speed may be 4, and the coefficient for the case of the low speed may be 2. The image forming apparatus is not limited to the printer, but may be a copy machine or a facsimile. The structure of the image forming apparatus may be such that a toner image formed on an image carrier is directly transferred to a sheet.