Patent Publication Number: US-11392083-B2

Title: Image forming apparatus including drum cartridge having photosensitive drum and toner cartridge having developing roller

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 16/828,198, filed Mar. 24, 2020, which claims priority from Japanese Patent Applications No. 2019-062588 filed Mar. 28, 2019 and No. 2019-062594 filed Mar. 28, 2019. The entire contents of the priority applications are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to an image forming apparatus to which a drum cartridge and a toner cartridge are detachably attachable. 
     BACKGROUND 
     There has been conventionally known an image forming apparatus to which a drum cartridge and a toner cartridge are detachably attachable. In the conventional image forming apparatus, the drum cartridge is mounted on the image forming apparatus after the toner cartridge is attached to the drum cartridge. 
     Generally, the lifetime of a drum cartridge is longer than the lifetime of a toner cartridge. Accordingly, a plurality of toner cartridges are used in succession with respect to a single drum cartridge such that when toner runs out in one toner cartridge, for example, the toner cartridge is replaced with the next toner cartridge. 
     SUMMARY 
     However, if the initial toner cartridge and the subsequent toner cartridges are used under the same conditions, the amount of toner supplied from a toner cartridge to the drum cartridge may vary among the respective toner cartridges, failing to stabilize print density. 
     The disclosure has been made in view of the above-described problem and an object thereof is to stabilize the print density of an image forming apparatus. 
     According to one aspect, the disclosure provides an image forming apparatus including an apparatus body, a controller, a drum cartridge and a toner cartridge. The drum cartridge is detachably attachable to the apparatus body. The drum cartridge includes a photosensitive drum and a drum memory. The drum memory stores data of a cumulative number of drum rotations of the photosensitive drum. The toner cartridge is configured to be used to perform image formation together with the drum cartridge. A first toner cartridge is used as the toner cartridge, before a second toner cartridge is used as the toner cartridge. The toner cartridge includes a developing roller and a toner memory. The developing roller is configured to be applied with a developing bias. A first developing bias is the developing bias applied to the developing roller of the first toner cartridge. A second developing bias is the developing bias applied to the second toner cartridge. The toner memory stores data of a cumulative dot count. An initial developing bias is the developing bias that is applied to the developing roller of the toner cartridge when the cumulative dot count stored in the toner memory of the toner cartridge is equal to zero. A first initial developing bias is the initial developing bias for the first toner cartridge. A second initial developing bias is the initial developing bias for the second toner cartridge. The controller is configured to perform determining a value of the first initial developing bias based on the cumulative number of drum rotations that is stored in the drum memory at the time when the cumulative dot count stored in the toner memory of the first toner cartridge is equal to zero. After determining the first initial developing bias, the controller is configured to perform determining a value of the second initial developing bias based on the cumulative number of drum rotations that is stored in the drum memory at the time when the cumulative dot count stored in the toner memory of the second toner cartridge is equal to zero. The value of the second initial developing bias is different from the value of the first initial developing bias. 
     According to another aspect, the disclosure provides an image forming apparatus including an apparatus body, a controller, a drum cartridge and a toner cartridge. The drum cartridge is detachably attachable to the main body. The drum cartridge includes a photosensitive drum, at least one of a transfer roller and a cleaning roller and a drum memory. Each of the transfer roller and the cleaning roller faces the photosensitive drum. The transfer roller is configured to be applied with a transfer current. The cleaning roller is configured to be applied with a cleaning bias. The drum memory stores data of a cumulative number of drum rotations of the photosensitive drum. The toner cartridge is configured to be used to perform image formation together with the drum cartridge. A first toner cartridge is used as the toner cartridge before a second toner cartridge is used as the toner cartridge. A first transfer current is the transfer current applied to the transfer roller when the first toner cartridge is used. A second transfer current is the transfer current applied to the transfer roller when the second toner cartridge is used. A first cleaning bias is the cleaning bias applied to the cleaning roller when the first toner cartridge is used. A second cleaning bias is the cleaning bias applied to the cleaning roller when the second toner cartridge is used. The toner cartridge includes a cartridge housing, a developing roller and a toner memory. The cartridge housing accommodates toner therein. The toner memory stores data of a cumulative dot count. An initial transfer current is the transfer current that is applied to the transfer roller when the cumulative dot count stored in the toner memory of the toner cartridge is equal to zero. A first initial transfer current is the initial transfer current that is applied to the transfer roller when the first toner cartridge is used. A second initial transfer current is the initial transfer current that is applied to the transfer roller when the second toner cartridge is used. An initial cleaning bias is the cleaning bias that is applied to the cleaning roller of the drum cartridge when the cumulative dot count stored in the toner memory of the toner cartridge is equal to zero. A first initial cleaning bias is the initial cleaning bias when the first toner cartridge is used. A second initial cleaning bias is the initial cleaning bias when the second toner cartridge is used. In a case where the drum cartridge includes the transfer roller, the controller is configured to perform determining a value of the first initial transfer current based on the cumulative number of drum rotations that is stored in the drum memory at the time when the cumulative dot count stored in the toner memory of the first toner cartridge is equal to zero. After determining the first initial transfer current, the controller is configured to perform determining a value of the second initial transfer current based on the cumulative number of drum rotations that is stored in the drum memory at the time when the cumulative dot count stored in the toner memory of the second toner cartridge is equal to zero. The value of the second initial transfer current is different from the value of the first initial transfer current. In a case where the drum cartridge includes the cleaning roller, the controller is configured to perform determining a value of the initial cleaning bias based on the cumulative number of drum rotations that is stored in the drum memory at the time when the cumulative dot count stored in the toner memory of the first toner cartridge is equal to zero. After determining the first initial cleaning bias, the controller is configured to perform determining a value of the second initial cleaning bias based on the cumulative number of drum rotations that is stored in the drum memory at the time when the cumulative dot count stored in the toner memory of the second toner cartridge is equal to zero. The value of the second initial cleaning bias is different from the value of the first initial cleaning bias. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The particular features and advantages of the disclosure will become apparent from the following description taken in connection with the accompanying drawings, in which: 
         FIG. 1  is a cross sectional view of an image forming apparatus according to a first embodiment of the present disclosure; 
         FIG. 2  is a conceptual diagram illustrating connection between a charging bias application circuit, a developing bias application circuit, a blade bias application circuit, a supply bias application circuit, and motors, according to the first embodiment; 
         FIG. 3  is a graph showing a relationship between a cumulative number of drum rotations and a charging bias according to the first embodiment; 
         FIG. 4A  is a graph showing a relationship between a cumulative dot count and a developing bias reference value according to the first embodiment; 
         FIG. 4B  is a graph showing a relationship between the cumulative dot count and a blade bias reference value according to the first embodiment; 
         FIG. 4C  is a graph showing a relationship between the cumulative dot count and a supply bias reference value according to the first embodiment; 
         FIG. 4D  is a graph showing a relationship between the cumulative dot count and a circumferential velocity difference value according to the first embodiment; 
         FIG. 5A  is a graph showing a relationship between the cumulative number of drum rotations and a developing bias correction amount according to the first embodiment; 
         FIG. 5B  is a graph showing a relationship between the cumulative dot count (the cumulative number of drum rotations) and a developing bias in a case where a plurality of toner cartridges are used in succession for one drum cartridge according to the first embodiment; 
         FIG. 6A  is a graph showing a relationship between the cumulative number of drum rotations and a blade bias correction amount according to the first embodiment; 
         FIG. 6B  is a graph showing a relationship between the cumulative dot count (cumulative number of drum rotations) and a blade bias in the case where a plurality of toner cartridges are used in succession for one drum cartridge according to the first embodiment; 
         FIG. 7A  is a graph showing a relationship between the cumulative number of drum rotations and a supply bias correction amount according to the first embodiment; 
         FIG. 7B  is a graph showing a relationship between the cumulative dot count (the cumulative number of drum rotations) and a supply bias in the case where a plurality of toner cartridges are used in succession for one drum cartridge in the interchanging manner according to the first embodiment; 
         FIG. 8A  is a graph showing a relationship between the cumulative number of drum rotations and a circumferential velocity difference correction amount according to the first embodiment; 
         FIG. 8B  is a graph showing a relationship between the cumulative dot count (the cumulative number of drum rotations) and a circumferential velocity difference in the case where a plurality of toner cartridges are used in succession for one drum cartridge according to the first embodiment; 
         FIG. 9  is a conceptual diagram illustrating connection between a toner cartridge and a charging bias application circuit, a transfer current application circuit and a cleaning bias application circuit according to a second embodiment; 
         FIG. 10A  is a graph showing a relationship between a cumulative dot count and a transfer current reference value according to the second embodiment; 
         FIG. 10B  is a graph showing a relationship between the cumulative dot count and a cleaning bias reference value according to the second embodiment; 
         FIG. 11A  is a graph showing a relationship between a cumulative number of drum rotations and a transfer current correction amount according to the second embodiment; 
         FIG. 11B  is a graph showing a relationship between the cumulative dot count (the cumulative number of drum rotations) and a transfer current in a case where a plurality of toner cartridges are used for one drum cartridge; 
         FIG. 12A  is a graph showing a relationship between the cumulative number of drum rotations and a cleaning bias correction amount according to the second embodiment; and 
         FIG. 12B  is a graph showing a relationship between the cumulative dot count (the cumulative number of drum rotations) and a cleaning bias in the case where a plurality of toner cartridges are used for one drum cartridge. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, a first embodiment of the present disclosure will be described in detail with reference to  FIGS. 1 through 8B  as necessary. 
     As illustrated in  FIG. 1 , an image forming apparatus  1  is a monochrome laser printer. The image forming apparatus  1  has an apparatus body  2 , a feeder part  3 , an image forming part  4 , and a controller  100 . 
     The apparatus body  2  is a case formed into a hollow shape. The apparatus body  2  has a pair of left and right side walls  21  and a front wall  22  connecting the side walls  21 . The front wall  22  has an opening  22 A. The front wall  22  is provided with a front cover  23  for opening/closing the opening  22 A. 
     The feeder part  3  has a feed tray  31  and a feed mechanism  32 . The feed tray  31  is detachably attached to a lower portion of the apparatus body  2 . The feed mechanism  32  feeds a sheet S stored in the feed tray  31  toward the image forming part  4 . 
     The image forming part  4  has a scanner unit  5 , a fixing device  7 , a drum cartridge  8 , and a toner cartridge  9 . 
     The scanner unit  5  is provided at an upper part of the apparatus body  2  and includes a laser emitting part, a polygon mirror, a lens, and a reflecting mirror, which are not illustrated. The scanner unit  5  irradiates the surface of a photosensitive drum  81  (described later) with laser beam in a high-speed scanning motion. 
     The controller  100  has, for example, a CPU, a RAM, a ROM, and an input/output circuit. The controller  100  performs arithmetic processing based on information related to the attached cartridge or program/data stored in the ROM to thereby execute print control. As described later, data shown in  FIGS. 3, 4A-4D, 5A, 6A, 7A and 8A  is stored in the ROM of the controller  100 . 
     The drum cartridge  8  is detachably attached to the apparatus body  2  of the image forming apparatus  1 . Specifically, the drum cartridge  8  is detachably attached to the apparatus body  2  through the opening  22 A opened by the front cover  23  of the apparatus body  2 . The drum cartridge  8  is disposed at a position between the feeder part  3  and the scanner unit  5  when the drum cartridge  8  is attached to the apparatus body  2 . The toner cartridge  9  is detachably attached to the drum cartridge  8 . The toner cartridge  9  is detached from or attached to the apparatus body  2  in a state where the toner cartridge  9  has been assembled to the drum cartridge  8 . That is, when the toner cartridge  9  is replaced with a next one, the drum cartridge  8  and the toner cartridge  9  are integrally removed from and attached to the image forming apparatus  1 . 
     The lifetime of the drum cartridge  8  is longer than the lifetime of the toner cartridge  9 . Here, the lifetime is determined from the total number of prints or the total number of printable dots. Thus, while the same, single drum cartridge  8  is used, a plurality of different toner cartridges  9  are used in succession such that when the lifetime of a toner cartridge  9  currently attached to the drum cartridge  8  has been reached, the toner cartridge  9  is replaced with a new one. For example, three to five toner cartridges  9  may be used in succession for one drum cartridge  8 . This means that the lifetime of the drum cartridge  8  is approximately three to five times as long as the lifetime of the toner cartridge  9 . 
     In the present embodiment, while one drum cartridge  8  is used in the image forming apparatus  1 , a plurality of toner cartridges  9  are used in succession. A toner cartridge  9  that is used first among the plurality of toner cartridges  9  will be referred to as a first toner cartridge  9 . A toner cartridge  9  that is used second among the plurality of toner cartridge  9  will be referred to as a second toner cartridge  9 . A toner cartridge  9  that is used third among the plurality of toner cartridges  9  will be referred to as a third toner cartridge  9 . A toner cartridge  9  that is used fourth among the plurality of toner cartridges  9  will be referred to as a fourth toner cartridge  9 . A toner cartridge  9  that is used fifth among the plurality of toner cartridges  9  will be referred to as a fifth toner cartridge  9 . To summarize, a toner cartridge  9  that is used X-th in the series of the toner cartridge  9  (where X is an integer greater than zero (0)) will be referred to as an X-th toner cartridge  9 . 
     The drum cartridge  8  has a frame  80 , a photosensitive drum  81 , a transfer roller  82 , a charger  83 , and a drum memory  85 . The toner cartridge  9  can be detachably attached to the frame  80 . The photosensitive drum  81  is rotatably supported by the frame  80 . 
     The toner cartridge  9  has a housing  90 , a developing roller  91 , a supply roller  92 , a blade  93 , and a toner memory  95 . The housing  90  stores toner therein. The supply roller  92  supplies toner stored in the housing  90  to the developing roller  91 . The developing roller  91  supplies toner to the photosensitive drum  81 . The blade  93  restricts the layer thickness of the toner supplied to the developing roller  91 . 
     In the drum cartridge  8 , while the photosensitive drum  81  is rotating, the surface of the photosensitive drum  81  is uniformly charged by the charger  83  and then exposed by the high-speed scanning of the laser beam from the scanner unit  5 . As a result, the potential of the exposed portion decreases and an electrostatic latent image based on image data is formed on the surface of the photosensitive drum  81 . 
     Subsequently, the toner stored in the toner cartridge  9  is supplied to the electrostatic latent image on the photosensitive drum  81  by the rotationally driven developing roller  91 , whereby a toner image is formed on the surface of the photosensitive drum  81 . Thereafter, a sheet S is conveyed to a position between the photosensitive drum  81  and the transfer roller  82 , and the toner image carried on the surface of the photosensitive drum  81  is transferred onto the sheet S. 
     The fixing device  7  has a heating roller  71  and a pressing roller  72 . The pressing roller  72  is positioned so as to face the heating roller  71 . The pressing roller  72  presses the heating roller  71 . The fixing device  7  thermally fixes the toner image transferred onto the sheet S while the sheet S is passing between the heating roller  71  and the pressing roller  72 . 
     The sheet S onto which the toner image has been thermally fixed by the fixing device  7  is conveyed to a sheet discharge roller  24  provided downstream from the fixing device  7  and then fed onto a sheet discharge tray  25  by the sheet discharge roller  24 . 
     The drum memory  85  of the drum cartridge  8  is a medium that stores information of the drum cartridge  8 . The drum memory  83  is, for example, an IC chip, but is not limited to the IC chip. The drum memory  85  stores therein data of a cumulative number of drum rotations “N” of the photosensitive drum  81  that is counted by the controller  100 . Here, the cumulative number of drum rotations “N” indicates how many times the photosensitive drum  81  has been rotated since the drum cartridge  8  was newly attached to the image forming apparatus  1  and while the drum cartridge  8  has been used in the image forming apparatus  1 . In other words, the cumulative number of drum rotations “N” is the total number of drum rotations that have been attained since the drum cartridge  8  was newly attached to the image forming apparatus  1  and while the drum cartridge  8  has been used in the image forming apparatus  1 . Even when the drum cartridge  8  is removed from the image forming apparatus  1  and attached to the image forming apparatus  1  again before the lifetime of the drum cartridge  8  has been reached, the controller  100  continues updating the cumulative number of drum rotations “N” without initializing the cumulative number of drum rotations “N”. As described later, it is noted that charging capability of the photosensitive drum  81  degrades as the cumulative number of drum rotations “N” increases. 
     The toner memory  95  of the toner cartridge  9  is a medium that stores information of the toner cartridge  9 . The toner memory  95  is, for example, an IC chip, but is not limited to the IC chip. The toner memory  95  stores therein, for example, a cumulative number of rotations of the developing roller  91 , a cumulative dot count “n” which is the cumulative number of developed dots counted by the controller  100 , and a toner residual amount. In the present embodiment, the toner memory  95  stores at least the cumulative dot count “n”. Here, the cumulative dot count “n” indicates the accumulated number of dots that have been developed since the toner cartridge  9  was newly mounted in the image forming apparatus  1  and while the toner cartridge  9  has been used. In other words, the cumulative dot count is the total number of dots that have been attained since the toner cartridge  9  was newly mounted in the image forming apparatus  1  and while the toner cartridge  9  has been used. Even when the toner cartridge  9  is removed from the image forming apparatus  1  and attached to the image forming apparatus  1  again before the lifetime of the toner cartridge  9  has been reached, the controller  100  continues updating the cumulative dot count “n” without initializing the cumulative dot count “n”. 
     As illustrated in  FIG. 2 , the image forming apparatus  1  further has a charging bias application circuit  210 , a developing bias application circuit  220 , a blade bias application circuit  230 , a supply bias application circuit  240 , a first motor  250 , and a second motor  260 . 
     In the present embodiment, positively charged type toner is used. Correspondingly, the charging bias application circuit  210 , developing bias application circuit  220 , blade bias application circuit  230 , and supply bias application circuit  240  are each applied with positive bias voltage. 
     The charging bias application circuit  210  is a circuit for applying a charging bias V T  to the charger  83 . The value of the charging bias applied by the charging bias application circuit  210  is controlled by the controller  100 . Specifically, as illustrated in  FIG. 3 , the controller  100  controls the charging bias application circuit  210  such that the value of the charging bias V T  gradually increases as the cumulative number of drum rotations “N” of the photosensitive drum  81  increases. The relationship ( FIG. 3 ) between the cumulative number of drum rotations “N” and the charging bias V T  is determined such that the surface potential (electric potential) of the photosensitive drum will become constant even when the cumulative number of drum rotations “N” of the photosensitive drum  81  increases. The relationship of  FIG. 3  is determined based on experimental data which is previously acquired. The data of the relationship between the cumulative number of drum rotations “N” and the charging bias V T  shown in  FIG. 3  is stored in the ROM of the controller  100 . By controlling the charging bias V T  in accordance with the relationship of  FIG. 3 , even though the charging capability of the photosensitive drum  81  degrades with an increase in the cumulative number of drum rotations “N”, the degradation of the charging capability can be complemented. 
     Referring back to  FIG. 2 , the developing bias application circuit  220  is a circuit for applying a developing bias V G  to the developing roller  91 . The controller  100  determines the value of the developing bias V G  according to both of the cumulative number of drum rotations “N”, which is stored in the drum memory  85  of the drum cartridge  8  currently attached to the image forming apparatus  1 , and the cumulative dot count “n”, which is stored in the toner memory  95  of the toner cartridge  9  currently attached to the drum cartridge  8 , and controls the developing bias application circuit  220  to apply the determined developing bias V G  to the developing roller  91 . 
     Specifically, the controller  100  determines the value of the developing bias V GX  for an X-th toner cartridge  9  by adding a developing bias reference value V G(n) , which varies according to the cumulative dot count “n”, and a developing bias correction amount V MG(N) , which varies according to the cumulative number of drum rotations “N” (V GX(n, N) =V G(n) +V MG(N) ), wherein X is an integer greater than zero (0). 
       FIG. 4A  shows a relationship between the cumulative dot count “n” and the developing bias reference value V G(n)  that should be satisfied while a single toner cartridge  9  is used for image formation, that is, after the toner cartridge  9  is newly attached to the drum cartridge  8  and until the toner cartridge  9  is replaced with a next one. The relationship ( FIG. 4A ) between the cumulative dot count “n” and the developing bias reference value V G(n)  is previously determined to ensure that the amount by which toner is moved from the developing roller  91  to the photosensitive drum  81  will become constant even when the characteristics of the toner degrades with an increase in the dot count. It is noted that data of the relationship of  FIG. 4A  is previously determined based on previously-acquired experimental data, and is previously stored in the ROM of the controller  100 . As illustrated in  FIG. 4A , the developing bias reference value V G(n)  is equal to V G(n=0)  when the cumulative dot count “n” is equal to zero (n=0). The absolute value of the developing bias reference value V G(n)  gradually decreases as the cumulative dot count “n” increases. 
       FIG. 5A  shows the relationship between the cumulative number of drum rotations “N” and the developing bias correction amount V MG(N)  that should be satisfied while a single drum cartridge  8  is used for image formation, that is, after the drum cartridge  8  is newly attached to the image forming apparatus  1  and until the drum cartridge  8  is replaced with a next one. The relationship ( FIG. 5A ) between the cumulative number of drum rotations “N” and the developing bias correction amount V MG(N)  is previously determined to ensure that even when the characteristics of photosensitive drum  81  degrade with an increase in the cumulative number of drum rotations “N”, the amount of toner adhering to the photosensitive drum  81  will become constant. It is noted that data of the relationship of  FIG. 5A  is previously determined based on previously-acquired experimental data, and is previously stored in the ROM of the controller  100 . As illustrated in  FIG. 5A , the developing bias correction amount V MG(N)  is equal to zero (0) when the cumulative number of drum rotations “N” is equal to zero (N=0). The absolute value of the developing bias correction amount V MG(N)  gradually increases as the cumulative number of drum rotations “N” increases. 
     As illustrated in  FIG. 5B , the controller  100  determines an initial developing bias for the X-th toner cartridge  9  based on the cumulative number of drum rotations “N” that is stored at the time when the X-th toner cartridge  9  is newly assembled to the drum cartridge  8 . Here, the initial developing bias for the X-th toner cartridge  9  is a developing bias to be applied to the developing roller  91  at an initial stage of the image formation performed by the X-th toner cartridge  9  together with the drum cartridge  8 . In other words, the initial developing bias for the X-th toner cartridge  9  is a developing bias that is applied to the developing roller  91  when the cumulative dot count “n” stored in the toner memory  95  of the X-th toner cartridge  9  is equal to zero (0). More specifically, when the drum cartridge  8  is also new, both of the cumulative number of drum rotations “N” stored in the drum memory  85  and the cumulative dot count “n” stored in the toner memory  95  are equal to zero (0). At this time, the controller  100  determines the initial developing bias V G1(n=0, N=0)  for the first toner cartridge  9  based on only the developing bias reference value V G(n=0)  with reference to  FIG. 4A  because the developing bias correction amount V MG(N)  is equal to zero (0) (see  FIG. 5A ). In this way, the controller  100  executes a processing of determining the initial developing bias V G1(n=0, N=0)  for the first toner cartridge  9 . 
     When the lifetime of the first toner cartridge  9  is reached, the first toner cartridge  9  is replaced with the second toner cartridge  9 . In the example of  FIG. 5B , at a timing when the lifetime of the first toner cartridge  9  is reached, the cumulative number of drum rotations “N” becomes equal to N 1 . That is, at a timing when the second toner cartridge  9  is newly attached to the drum cartridge  8 , the cumulative number of drum rotations “N” is equal to N 1 , and the cumulative dot counts n is equal to zero (0). At this time, the controller  100  determines the initial developing bias V G2(n=0, N=N1)  for the second toner cartridge  9  based on the developing bias reference value V G(n=0)  and the developing bias correction amount V MG(N=N1) . Specifically, the controller  100  determines the initial developing bias V G2(n=0, N=N1)  for the second toner cartridge  9  by calculating the formula of V G(n=0) +V MG(N=N1) . Thus, the initial developing bias V G2(n=0, N=N1)  of the second toner cartridge  9  differs from the initial developing bias V G1(n=0, N=0)  for the first toner cartridge  9  (see  FIG. 5B ). In the present embodiment, the absolute value of the initial developing bias V G2(n=0, N=N1)  for the second toner cartridge  9  is larger than the absolute value of the initial developing bias V G1(0, 0)  for the first toner cartridge  9 . That is, |V G2(n=0, N=N1) |&gt;|V G1(n=0, N=0) |. 
     Similarly, as the third, fourth, and fifth toner cartridges  9  are attached to the same drum cartridge  8  in this order to perform image formation, the controller  100  execute processings of determining initial developing biases V G3(n=0, N=N2) , V G4(n=0, N=N3) , and V G5(n=0, N=N4)  for the third, fourth and fifth toner cartridges  9  by calculating formulas: V G(n=0) +V MG(N=N2) , V G(n=0) +V MG(N=N3) , and V G(n=0) +V MG(N=N4) , respectively. The controller  100  determines the initial developing bias for the third, fourth and fifth toner cartridges  9  such that the absolute value of the initial developing bias increases according to the cumulative number of drum rotations “N”. That is, |V G5(n=0, N=N4) |&gt;|V G4(n=0, N=N3) |&gt;|V G3(n=0, N=N2) |&gt;|V G2(n=0, N=N1) |. 
     As illustrated in  FIG. 5B , after determining the initial developing bias V GX(n=0)  for the X-th toner cartridge  9 , the controller  100  changes the developing bias V GX(n, N)  for the X-th toner cartridge  9  as both of the cumulative number of drum rotations “N” and the cumulative dot count “n” increase. In other words, the controller  100  determines the developing bias V GX(n, N)  for the X-th toner cartridge  9  such that the absolute value |V GX(n, N) | of the developing bias V G  gradually reduces as the cumulative dot count “n” increases and the cumulative number of drum rotations “N” increases until the X-th toner cartridge  9  is replaced with a next one ((X+1)-th toner cartridge  9 ). 
     Referring back to  FIG. 2 , the blade bias application circuit  230  is a circuit for applying a blade bias V B  to the blade  93 . The controller  100  determines the value of the blade bias V B  according to both of the cumulative number of drum rotations “N” stored in the drum memory  85  of the drum cartridge  8  and the cumulative dot count “n” stored in the toner memory  95  of the toner cartridge  9 , and controls the blade bias application circuit  230  to apply the determined blade bias V B  to the blade  93 . 
     Specifically, the controller  100  determines the value of the blade bias V BX  for the X-th toner cartridge  9  by adding a blade bias reference value V B(n) , which varies according to the cumulative dot count “n”, and a blade bias correction amount V MB(N) , which varies according to the cumulative number of drum rotations “N” (V BX(n, N) =V B(n) +V MB(N) ), wherein X is an integer greater than zero (0). 
       FIG. 4B  shows the relationship between the cumulative dot count “n” and the blade bias reference value V B(n)  that should be satisfied while a single toner cartridge  9  is used for image formation, that is, after the toner cartridge  9  is newly attached to the drum cartridge  8  and until the toner cartridge  9  is replaced with a next one. The relationship ( FIG. 4B ) between the cumulative dot count “n” and the blade bias reference value V B(n)  is previously determined to ensure that the amount of toner adhering to the photosensitive drum  81  will become constant even when the fluidity of the toner decreases due to the degradation of the toner with an increase in the dot count. It is noted that data of the relationship of  FIG. 4B  is previously determined based on previously-acquired experimental data, and is previously stored in the ROM of the controller  100 . As illustrated in  FIG. 4B , the blade bias reference value V B(n)  is equal to V B(0)  when the cumulative dot count “n” is equal to zero (n=0). The absolute value of the blade bias reference value V B(n)  gradually decreases as the cumulative dot count “n” increases. 
       FIG. 6A  shows the relationship between the cumulative number of drum rotations “N” and the blade bias correction amount V MB(N)  that should be satisfied while a single drum cartridge  8  is used for image formation, that is, after the drum cartridge  8  is newly attached to the image forming apparatus  1  and until the drum cartridge  8  is replaced with a new one. The relationship ( FIG. 6A ) between the cumulative number of drum rotations “N” and the blade bias correction amount V MB(N)  is previously determined to ensure that the amount of toner adhering to the photosensitive drum  81  will become constant even when the photosensitive drum  81  degrades with an increase in the cumulative number of drum rotations “N”. It is noted that data of the relationship of  FIG. 6A  is previously determined based on previously-acquired experimental data, and is previously stored in the ROM of the controller  100 . As illustrated in  FIG. 6A , the blade bias correction amount V MB(N)  is equal to zero (0) when the cumulative number of drum rotations “N” is equal zero (N=0). The absolute value of blade bias correction amount V MB(N)  gradually increases as the cumulative number of drum rotations “N” increases. 
     As illustrated in  FIG. 6B , the controller  100  determines the initial blade bias for the X-th toner cartridge  9  based on the cumulative number of drum rotations “N” that is stored at the time when the X-th toner cartridge  9  is newly assembled to the drum cartridge  8 . Here, the initial blade bias for the X-th toner cartridge  9  is a bias to be applied to the blade  93  at an initial stage of the image formation performed by the X-th toner cartridge  9  together with the drum cartridge  8 . In other words, the initial blade bias for the X-th toner cartridge  9  is a blade bias that is applied to the blade  93  when the cumulative dot count “n” stored in the toner memory  95  of the X-th toner cartridge  9  is equal to zero (0). More specifically, when the drum cartridge  8  is also new, both of the cumulative number of drum rotations “N” stored in the drum memory  85  and the cumulative dot count “n” stored in the toner memory  95  are equal to zero (0). At this time, the controller  100  determines the initial blade bias V B1(n=0, N=0)  for the first toner cartridge  9  based on only the blade bias reference value V B(n=0)  with reference to  FIG. 4B  because the blade bias correction amount V MG(N)  is equal to zero (see  FIG. 6A ). In this way, the controller  100  executes a processing of determining the initial blade developing bias V B1(0, 0)  for the first toner cartridge  9 . 
     As described above, at the timing when the second toner cartridge  9  is newly attached to the drum cartridge  8 , the cumulative number of drum rotations “N” is equal to N 1  and the cumulative dot counts n is equal to zero (0). At this time, the controller  100  determines the initial blade bias V B2(n=0, N=N1)  for the second toner cartridge  9  based on the blade bias reference value V B(n=0)  and the developing bias correction amount V MB(N=N1) . Specifically, the controller  100  determines the initial blade bias V B2(n=0, N=N1)  for the second toner cartridge  9  by calculating the formula of V B(n=0) +V MB(N=N1) . Thus, the initial blade bias V B2(n=0, N=N1)  of the second toner cartridge  9  differs from the initial blade bias V B1(n=0, N=0)  for the first toner cartridge  9  (see  FIG. 6B ). In the present embodiment, the absolute value of the initial blade bias V B2(n=0, N=N1)  for the second toner cartridge  9  is larger than the absolute value of the initial blade bias V B1(n=0, N=0)  for the first toner cartridge  9 . That is, |V B2(n=0, N=N1) |&gt;|V B1(n=0, N=0) |. 
     Similarly, as the third, fourth, and fifth toner cartridges  9  are attached to the same drum cartridge  8  in this order to perform image formation, the controller  100  executes processings of determining initial blade biases V B3(n=0, N=N2) , V B4(n=0, N=N3) , and V B5(n=0, N=N4)  for the third, fourth and fifth toner cartridges  9  by calculating formulas: V B(n=0) +V MB(N=N2) , V B(n=0) +V MB(N=N3) , and V B(n=0) +V MB(N=N4) , respectively. The controller  100  determines the initial blade bias for the third, fourth and fifth toner cartridges  9  such that the absolute value of the initial blade bias increases according to the cumulative number of drum rotations “N”. That is, |V B5(n=0, N=N4) |&gt;|V B4(n=0, N=N3) |&gt;|V B3(n=0, N=N2) |&gt;|V B2(n=0, N=N1) |. 
     As illustrated in  FIG. 6B , after determining the initial blade bias V BX(n=0)  for the X-th toner cartridge  9 , the controller  100  changes the blade bias V BX(n, N)  for the toner cartridge  9  such that the blade bias V BX(n, N)  changes as both of the cumulative number of drum rotations “N” and cumulative dot count “n” increase. In other words, the controller  100  determines the blade bias V BX(n, N)  such that the absolute value |V BX(n, N) | of the blade bias V BX(n, N)  gradually reduces as the cumulative dot count “n” increases and the cumulative number of drum rotations “N” increases until the X-th toner cartridge  9  is replaced with a next one ((X+1)-th toner cartridge  9 ). 
     Referring back to  FIG. 2 , the supply bias application circuit  240  is a circuit for applying a supply bias V K  to the supply roller  92 . The controller  100  determines the value of the supply bias V K  according to both of the cumulative number of drum rotations “N” stored in the drum memory  85  of the drum cartridge  8  currently attached to the image forming apparatus  1  and the cumulative dot count “n” stored in the toner memory  95  of the toner cartridge  9  currently attached to the drum cartridge  8 , and controls the supply bias application circuit  240  to apply the determined supply bias V K  to the supply roller  92 . 
     Specifically, the controller  100  determines the value of the supply bias V KX  for the X-th toner cartridge  9  by adding a supply bias reference value V K(n) , which varies according to the cumulative dot count “n”, and a supply bias correction amount V MK(N) , which varies according to the cumulative number of drum rotations “N” (V KX(n, N) =V K(n) +V MK(N) ), wherein X is an integer greater than zero (0). 
       FIG. 4C  shows the relationship between the cumulative dot count “n” and the supply bias reference value V K(n)  that should be satisfied while a single toner cartridge  9  is used for image formation, that is, after the toner cartridge  9  is newly attached to the drum cartridge  8  and until the toner cartridge  9  is replaced with a next one. The relationship ( FIG. 4C ) between the supply bias reference value V K(n)  and the cumulative dot count “n” is previously determined to ensure that the amount of the toner adhering to the developing roller  91  will become constant even when the charging performance of the toner degrades due to the degradation of the toner with an increase in the dot count. It is noted that data of the relationship of  FIG. 4C  is previously determined based on previously-acquired experimental data, and is previously stored in the ROM of the controller  100 . As illustrated in  FIG. 4C , the supply bias reference value V K(n)  is equal to V K(0)  when the cumulative dot count “n” is equal to zero (n=0). The absolute value of the supply bias reference value V K(n)  gradually increases as the cumulative dot count “n” increases. 
       FIG. 7A  shows the relationship between the cumulative number of drum rotations “N” and the supply bias correction amount V MK(N)  that should be satisfied while a single drum cartridge  8  is used for image formation, that is, after the drum cartridge  8  is newly attached to the image forming apparatus  1  and until the drum cartridge  8  is replaced with a new one. The relationship ( FIG. 7A ) between the cumulative number of drum rotations “N” and the supply bias correction amount V MK(N)  is previously determined to ensure that the amount of the toner adhering to the photosensitive drum  81  will become constant even when the photosensitive drum  81  degrades with an increase in the cumulative number of drum rotations “N”. It is noted that data of the relationship of  FIG. 7A  is previously determined based on previously-acquired experimental data, and is previously stored in the ROM of the controller  100 . As illustrated in  FIG. 7A , the supply bias correction amount V MK(N)  is equal to zero (0) when the cumulative number of drum rotations “N” is equal to zero (N=0). The absolute value of the supply bias correction amount V MK(N)  gradually increases as the cumulative number of drum rotations “N” increases. 
     As illustrated in  FIG. 7B , the controller  100  determines the initial supply bias for the X-th toner cartridge  9  based on the cumulative number of drum rotations “N” that is stored at the time when the X-th toner cartridge  9  is newly assembled to the drum cartridge  8 . Here, the initial supply bias for the X-th toner cartridge  9  is a bias to be applied to the supply roller  92  at an initial stage of the image formation performed by the X-th toner cartridge  9  together with the drum cartridge  8 . In other words, the initial supply bias for the X-th toner cartridge  9  is a supply bias that is applied to the supply roller  92  when the cumulative dot count “n” stored in the toner memory  95  of the X-th toner cartridge  9  is equal to zero (0). More specifically, when the drum cartridge  8  is also new, both of the cumulative number of drum rotations “N” stored in the drum memory  85  and the cumulative dot count “n” stored in the toner memory  95  are equal to zero (0). At this time, the controller  100  determines the initial supply bias V K1(n=0, N=0)  for the first toner cartridge  9  based on only the developing bias reference value V K(n=0)  with reference to  FIG. 4C  because the supply bias correction amount V MK(N)  is equal to zero. In this way, the controller  100  executes a processing of determining the initial supply bias V K1(n=0, N=0)  for the first toner cartridge  9 . 
     As described above, at the timing when the second toner cartridge  9  is newly attached to the drum cartridge  8 , the cumulative number of drum rotations “N” is equal to N 1  and the cumulative dot counts n is equal to zero (0). At this time, the controller  100  determines the initial supply bias V K2(n=0, N=N1)  for the second toner cartridge  9  based on the supply bias reference value V K(n=0)  and the supply bias correction amount V MK(N=N1) . Specifically, the controller  100  determines the initial supply bias V K2(n=0, N=N1)  for the second toner cartridge  9  by calculating the formula of V K(n=0) +V MK(N=N1) . Thus, the initial supply bias V K2(n=0, N=N1)  for the second toner cartridge  9  differs from the initial supply bias V K1(n=0, N=0)  of the first toner cartridge  9 . In the present embodiment, the absolute value of the initial supply bias V K2(n=0, N=N1)  for the second toner cartridge  9  is larger than the absolute value of the initial supply bias V K1(n=0, N=0)  for the first toner cartridge  9 . That is, |V K2(n=0, N=N1) |&gt;|V K1(n=0, N=0) |. 
     Similarly, as the third, fourth, and fifth toner cartridges  9  are attached to the same drum cartridge  8  in this order to perform image formation, the controller  100  executes processings of determining initial supply biases V K3(n=0, N=N2) , V K4(n=0, N=N3) , and V K5(n=0, N=N4)  for the third, fourth and fifth toner cartridges  9  by calculating formulas: V K(n=0) +V MK(N=N2) , V K(n=0) +V MK(N=N3) , and V K(n=0) +V MK(N=N4) , respectively. The controller  100  determines the initial supply bias for the third, fourth and fifth toner cartridges  9  such that the absolute value of the initial supply biases increases according to the cumulative number of drum rotations “N”. That is, |V K5(n=0, N=N4) |&gt;|V K4(n=0, N=N3) |&gt;V K3(n=0, N=N2) |&gt;|V K2(n=0, N=N1) |. 
     As illustrated in  FIG. 7B , after determining the initial supply bias V KX(n=0)  for the X-th toner cartridge  9 , the controller  100  changes the supply bias V KX(n, N)  for the X-th toner cartridge  9  such that the supply bias V KX(n, N)  changes as both of the cumulative number of drum rotations “N” and cumulative dot count “n” increase. In other words, the controller  100  determines the supply bias V KX(n, N)  for the X-th toner cartridge  9  such that the absolute value |V KX(n, N) | of the supply bias V KX(n, N)  gradually increases as the cumulative dot count “n” increases and the cumulative number of drum rotations “N” increases until the X-th toner cartridge  9  is replaced with a next one ((X+1)-th toner cartridge  9 ). 
     Referring back to  FIG. 2 , the first motor  250  is a drive source for supplying drive force to the photosensitive drum  81 . In the present embodiment, the photosensitive drum  81  is rotated by the first motor  250  in a clockwise direction in  FIG. 2 . The velocity of the first motor  250  is controlled by the controller  100 . The second motor  260  is a drive source for supplying drive force to the developing roller  91 . In the present embodiment, the developing roller  91  is rotated by the second motor  260  in a counterclockwise direction in  FIG. 2 . The velocity of the second motor  260  is controlled by the controller  100 . The controller  100  determines a circumferential velocity difference ΔV between a circumferential velocity V E  of the developing roller  91  and a circumferential velocity V D  of the photosensitive drum  81  according to the increases in the cumulative dot count and cumulative number of drum rotations. 
     The motors  250  and  260  for driving the photosensitive drum  81  and developing roller  91  are independent from one another. Accordingly, when necessary, the controller  100  is able to change the difference between the circumferential velocity V D  of the photosensitive drum  81  and the circumferential velocity V E  of the developing roller  91  (i.e. the circumferential velocity difference ΔV). In the present embodiment, the circumferential velocity V D  of the photosensitive drum  81  is made constant, and the circumferential velocity V E  of the developing roller  91  is changed. The circumferential velocity V E  of the developing roller  91  is equal to or higher than the circumferential velocity V D  of the photosensitive drum  81  (V E  V D ). Hereinafter, the circumferential velocity difference V E −V D  between the circumferential velocity V E  of the developing roller  91  and the circumferential velocity V D  of the photosensitive drum  81  is referred to merely as “circumferential velocity difference ΔV”. 
     Specifically, the controller  100  determines the value of the circumferential velocity difference ΔV X  for an X-th toner cartridge  9  by adding a circumferential-velocity-difference reference value ΔV (n) , which varies according to the cumulative dot count “n”, and a circumferential-velocity-difference correction amount ΔV M(N) , which varies according to the cumulative number of drum rotations “N” (ΔV X(n, N) =ΔV (n) +ΔV M(N) ), wherein X is an integer greater than zero (0). 
       FIG. 4D  shows the relationship between the cumulative dot count “n” and the circumferential-velocity-difference reference value ΔV (n)  that should be satisfied while a single toner cartridge  9  is used for image formation, that is, after the toner cartridge  9  is newly attached to the drum cartridge  8  and until the toner cartridge  9  is replaced with a next one. The relationship ( FIG. 4D ) between the cumulative dot count “n” and the circumferential-velocity-difference reference value ΔV (n)  is previously determined to ensure that the amount of the toner moving from the developing roller  91  to the photosensitive drum  81  will become constant even when toner degrades with an increase in the cumulative dot count “n”. It is noted that data of the relationship of  FIG. 4D  is previously determined based on previously-acquired experimental data, and is previously stored in the ROM of the controller  100 . As illustrated in  FIG. 4D , the circumferential-velocity-difference reference value ΔV (n)  is equal to ΔV 0  when the cumulative dot count is equal to zero (n=0). The circumferential-velocity-difference reference value ΔV (n)  gradually decreases as the cumulative dot count “n” increases. 
       FIG. 8A  shows the relationship between the cumulative number of drum rotations “N” and the circumferential-velocity-difference correction amount ΔV M(N)  that should be satisfied while a single drum cartridge  8  is used for image formation, that is, after the drum cartridge  8  is newly attached to the image forming apparatus  1  and until the drum cartridge  8  is replaced with a next one. The relationship ( FIG. 8A ) between the cumulative number of drum rotations “N” and the circumferential-velocity-difference correction amount ΔV M(N)  is previously determined to ensure that the amount of toner adhering to the photosensitive drum  81  will become constant even when the photosensitive drum  81  degrades with an increase in the cumulative number of drum rotations “N”. It is noted that data of the relationship of  FIG. 8A  is previously determined based on previously-acquired experimental data, and is previously stored in the ROM of the controller  100 . As illustrated in  FIG. 8A , the circumferential-velocity-difference correction amount ΔV M(N)  is equal to zero (0) when the cumulative number of drum rotations is equal to zero (N=0). The circumferential-velocity-difference correction amount ΔV M(N)  gradually increases as the cumulative number of drum rotations “N” increases. 
     As illustrated in  FIG. 8B , the controller  100  determines the initial circumferential velocity difference ΔV X(n=0)  for the X-th toner cartridge  9  based on the cumulative number of drum rotations “N” that is stored at the time when the X-th toner cartridge  9  is newly assembled to the drum cartridge  8 . Here, the initial circumferential velocity difference for the X-th toner cartridge  9  is a circumferential velocity difference to be attained at an initial stage of the image formation performed by the X-th toner cartridge  9  together with the drum cartridge  8 . In other words, the initial circumferential velocity difference for the X-th toner cartridge  9  is a circumferential velocity difference that is attained when the cumulative dot count “n” stored in the toner memory  95  of the X-th toner cartridge  9  is equal to zero (0). More specifically, when the drum cartridge  8  is also new, both of the cumulative number of drum rotations “N” stored in the drum memory  85  and the cumulative dot count “n” stored in the toner memory  95  are equal to zero (0). At this time, the controller  100  determines the initial circumferential velocity difference ΔV 1(n=0, N=0)  based on only the circumferential velocity difference reference value ΔV (n=0)  with reference to  FIG. 4D  because the circumferential velocity difference correction amount ΔV M(N)  is equal to zero (see  FIG. 8A ). In this way, the controller  100  executes a processing of determining the initial circumferential velocity difference ΔV 1(0, 0)  for the first toner cartridge  9 . 
     As described above, at the timing when the second toner cartridge  9  is newly attached to the drum cartridge  8 , the cumulative number of drum rotations “N” is equal to N 1 , and the cumulative dot counts n is equal to zero (0). At this time, the controller  100  determines the initial circumferential velocity difference ΔV 2(n=0, N=N1)  for the second toner cartridge  9  based on the circumferential velocity difference reference value ΔV (n=0)  and the circumferential velocity difference correction amount ΔV M(N=N1) . Specifically, the controller  100  determines the initial circumferential velocity difference ΔV 2(n=0, N=N1)  of the second toner cartridge  9  by calculating the formula of ΔV (n=0) +ΔV M(N=N1) . Thus, the initial circumferential velocity difference ΔV 2(n=0, N=N1)  for the second toner cartridge  9  differs from the initial circumferential velocity difference ΔV 1(n=0, N=0)  of the first toner cartridge  9 . In the present embodiment, the initial circumferential velocity difference ΔV 2(n=0, N=N1)  for the second toner cartridge  9  is larger than the initial circumferential velocity difference ΔV 1(n=0, N=0)  for the first toner cartridge  9 . That is, ΔV 2(n=0, N=N1) &gt;ΔV 1(n=0, N=0) . 
     Similarly, as the third, fourth, and fifth toner cartridges  9  are attached to the drum cartridge  8  in this order to perform image formation, the controller  100  executes processings of determining initial circumferential velocity differences ΔV 3(n=0, N=N2) , ΔV 4(n=0, N=N3) , and ΔV 5(n=0, N=N4)  for the third, fourth and fifth toner cartridges  9  by calculating formulas: ΔV (n=0) +ΔV M(N=N2) , ΔV (n=0) +ΔV M(N=N3) , and ΔV (n=0) +ΔV M(N=N4) , respectively. The controller  100  determines the initial circumferential velocity difference for the third, fourth and fifth toner cartridges  9  such that the initial circumferential velocity differences increases according to the cumulative number of drum rotations “N”. That is, ΔV 5(n=0, N=N4) &gt;ΔV 4(n=0, N=N3) &gt;ΔV 3(n=0, N=N2) &gt;ΔV 2(n=0, N=N1) . 
     As illustrated in  FIG. 8B , after determining the initial circumferential velocity difference ΔV X(n=0)  for the X-th toner cartridge  9 , the controller  100  changes the circumferential velocity difference ΔV X(n, N)  for the X-th toner cartridge  9  such that the circumferential velocity difference ΔV X(n, N)  changes as the cumulative number of drum rotations “N” and cumulative dot count “n” increase. In other words, the controller  100  determines the circumferential velocity difference ΔV X(n, N)  for the X-th toner cartridge  9  such that of the circumferential velocity difference ΔV X(n, N)  gradually decreases as the cumulative dot count “n” increases and the cumulative number of drum rotations “N” increases until the X-th toner cartridge  9  is replaced with a next one ((X+1)-th toner cartridge  9 ). 
     According to the above-described image forming apparatus  1 , the following advantages are achieved. 
     According to the image forming apparatus  1 , by making the initial developing bias V G1(n=0, N=0)  and the initial developing bias V G2(n=0, N=N1)  different (specifically, by making |V G2(n=0, N=N1) | larger than |V G1(n=0, N=0) |), the amount of toner adhering to the photosensitive drum  81  when the second toner cartridge is used can be brought close to the amount of toner adhering to the photosensitive drum  81  when the first toner cartridge is used. This allows the print density of the image forming apparatus  1  to be stabilized while a plurality of toner cartridges are used in succession for one drum cartridge. 
     Further, the controller  100  changes the developing bias V GX(n, N)  to be applied to the developing roller  91  according to an increase in the cumulative number of drum rotations “N” and to an increase in the cumulative dot count “n”, so that the developing bias can be changed according to both degradation of the photosensitive drum  81  and degradation of toner. 
     For example, the photosensitive drum  81  degrades in charging capability in accordance with degradation of the photosensitive drum  81 . Accordingly, the developing bias should be gradually increased so as to compensate for the degradation in the charging capability. On the other hand, toner increases in adhesion force in accordance with degradation of toner. Accordingly, the developing bias should be gradually reduced so as to prevent excessive adhesion. By changing the developing bias according to both degradation in charging capability of the photosensitive drum  81  and degradation of toner, the amount of toner adhering to the photosensitive drum  81  can be made constant. 
     Further, by making the initial blade bias V B1(n=0, N=0)  and the initial blade bias V B2(n=0, N=N1)  different from each other, the amount of toner adhering to the developing roller  91  of the second toner cartridge can be brought close to the amount of toner adhering to the developing roller  91  of the first toner cartridge. This stabilizes the amount of toner supplied from the developing roller  91  to the photosensitive drum  81 . As a result, the print density of the image forming apparatus  1  can be stabilized. 
     Further, the controller  100  changes the blade bias V BX(n, N)  to be applied to the developing roller  91  according to an increase in the cumulative number of drum rotations “N” and to an increase in the cumulative dot count “n”, so that the developing bias can be changed according to both degradation of the photosensitive drum  81  and degradation of toner. This allows the amount of toner adhering to the photosensitive drum  81  to be made constant. 
     Further, by making the initial supply bias V K1(n=0, N=0)  and the initial supply bias V K2(n=0, N=N1)  different from each other, the amount of toner adhering to the developing roller  91  of the second toner cartridge can be brought close to the amount of toner adhering to the developing roller  91  of the first toner cartridge. This stabilizes the amount of toner supplied from the developing roller  91  to the photosensitive drum  81 . As a result, the print density of the image forming apparatus  1  can be stabilized. 
     Further, the controller  100  changes the supply bias V KX(n,N)  to be applied to the supply roller  92  according to an increase in the cumulative number of drum rotations and to an increase in the cumulative dot count, so that the supply bias can be changed according to both degradation of the photosensitive drum  81  and degradation of toner. This allows the amount of toner to be supplied to the photosensitive drum  81  to be made constant. 
     Further, by making the initial circumference velocity difference ΔV 1(n=0, N=0)  and the initial circumference velocity difference ΔV 2(n=0, N=N1)  different from each other, the amount of toner adhering to the photosensitive drum  81  when the second toner cartridge is used can be brought close to the amount of toner adhering to the photosensitive drum  81  when the first toner cartridge is used. This allows the amount of toner to be supplied from the developing roller  91  to the photosensitive drum  81  to be constant. As a result, the print density of the image forming apparatus  1  can be stabilized. 
     Further, the controller  100  changes the circumferential velocity difference ΔV X(n, N)  between the developing roller  91  and the photosensitive drum  81  according to increases in the cumulative number of drum rotations and the cumulative dot count, so that the circumferential velocity difference can be changed according to both degradation of the photosensitive drum  81  and degradation of toner. This allows the amount of toner to be supplied to the photosensitive drum  81  to be made constant. 
     Next, a second embodiment of the present disclosure will be described in detail with reference to  FIGS. 9 through 12B . Since an image forming apparatus  1 , a drum cartridge  8  and a toner cartridge  9  according to the second embodiment have basically the same configurations as the configurations of the image forming apparatus  1 , the drum cartridge  8  and the toner cartridge  9  according to the first embodiment, only those configurations that are different from the first embodiment will be described below. Similarly to the first embodiment, a plurality of different toner cartridge  9  are used in succession together with the same, single drum cartridge  8 , and each toner cartridge  9  is referred to also as an X-th toner cartridge  9  that is used X-th in the series of toner cartridge, where X is an integer greater than zero (0). 
     As illustrated in  FIG. 9 , the drum cartridge  8  of the second embodiment has a cleaning roller  84 . The cleaning roller  84  is disposed at a position different from the position of the transfer roller  82  in the frame  80  of the drum cartridge  8 . The cleaning roller  84  is configured to remove a residual toner and foreign substances from the photosensitive drum  81 . 
     The image forming apparatus  1  of the second embodiment has the charging bias application circuit  210 , a transfer current application circuit  270  and a cleaning bias application circuit  280 . Similarly to the first embodiment, the controller  100  controls the charging bias application circuit  210  such that the value of the charging bias V T  gradually increases as the cumulative number of drum rotations “N” of the photosensitive drum  81  increases as shown in  FIG. 3 . As described below, the controller  100  further stores data shown in  FIGS. 10A, 10B, 11A and 12A  in the ROM. 
     As illustrated in  FIG. 9 , the transfer current application circuit  270  is a circuit for applying a transfer current I to the transfer roller  82 . The controller  100  determines the value of the transfer current I according to both of the cumulative number of drum rotations “N” that is stored in the drum memory  85  of the drum cartridge  8  currently attached to the image forming apparatus  1  and the cumulative dot count “n” that is stored in the toner memory  95  of the toner cartridge  9  currently attached to the drum cartridge  8 , and controls the transfer current application circuit  270  to apply the determined transfer current I to the transfer roller  82 . 
     Specifically, the controller  100  determines the value of the transfer current I X  for an X-th toner cartridge  9  by adding a transfer current reference value I (n) , which varies according to the cumulative dot count “n”, and a transfer current correction amount I (N) , which varies according to the cumulative number of drum rotations (I X(n, N) =I (n) +I (N) ), wherein X is an integer greater than zero (0). 
       FIG. 10A  shows the relationship between the cumulative dot count “n” and the transfer current reference value I (n)  that should be satisfied while a single toner cartridge  9  is used for image formation, that is, after the toner cartridge  9  is newly attached to the drum cartridge  8  and until the toner cartridge  9  is replaced with a next one. The relationship ( FIG. 10A ) between the cumulative dot count “n” and the transfer current reference value I (n)  is previously determined to ensure that the transfer amount of toner moved from the photosensitive drum  81  to a sheet S will become constant even when the toner degrades with an increase in the dot count. It is noted that data of the relationship of  FIG. 10A  is previously determined based on previously-acquired experimental data, and is previously stored in the ROM of the controller  100 . As illustrated in  FIG. 10A , the transfer current reference value I (n)  is equal to I 0  when the cumulative dot count is equal to zero (n=0). The transfer current reference value I (n)  gradually increases as the cumulative dot count increases. 
       FIG. 11A  shows the relationship between the cumulative number of drum rotations “N” and the transfer current correction amount I (N)  that should be satisfied while a single drum cartridge  8  is used for image formation, that is, after the drum cartridge  8  is newly attached to the image forming apparatus  1  and until the drum cartridge  8  is replaced with a new one. The relationship ( FIG. 11A ) between the cumulative number of drum rotations “N” and the transfer current correction amount I (N)  is previously determined to ensure that the transfer amount of toner moved from the photosensitive drum  81  to a sheet S will become constant even when the photosensitive drum  81  degrades with an increase in the cumulative number of drum rotations “N”. It is noted that data of the relationship of  FIG. 11A  is determined based on previously-acquired experimental data, and is previously stored in the ROM of the controller  100 . As illustrated in  FIG. 11A , the transfer current correction amount I (N)  is equal to zero (0) when the cumulative number of drum rotations “N” is equal to zero (N=0). The transfer current correction amount I (N)  gradually increases as the cumulative number of drum rotations “N” increases. 
     As illustrated in  FIG. 11B , the controller  100  determines the initial transfer current for the X-th toner cartridge  9  based on the cumulative number of drum rotations “N” that is stored at the time when the X-th toner cartridge  9  is newly assembled to the drum cartridge  8 . Here, the initial transfer current for the X-th toner cartridge  9  is a current to be applied to the transfer roller  82  at an initial stage of the image formation performed by the X-th toner cartridge  9  together with the drum cartridge  8 . In other words, the initial transfer current for the X-th toner cartridge  9  is a transfer current that is applied to the transfer roller  82  when the cumulative dot count “n” stored in the toner memory  95  in the X-th toner cartridge  9  is equal to zero (0). When the drum cartridge  8  is also new, both of the cumulative number of drum rotations “N” stored in the drum memory  85  and the cumulative dot count “n” stored in the toner memory  95  are equal to zero (0). At this time, the controller  100  determines the initial transfer current I 1(n=0, N=0)  based on only the transfer current reference value I 1(n=0)  with reference to  FIG. 10A  because the transfer current correction amount I (N)  is equal to zero (see  FIG. 11A ). In this way, the controller  100  executes a processing of determining the initial transfer current I 1(0, 0)  for the first toner cartridge  9 . 
     As described above, at the timing when the second toner cartridge  9  is newly attached to the drum cartridge  8 , the cumulative number of drum rotations “N” is equal to N 1 , and the cumulative dot counts n is equal to zero (0). At this time, the controller  100  determines the initial transfer current I 2(n=0, N=N1)  for the second toner cartridge  9  based on the transfer current reference value I (n=0)  and the transfer current correction amount I (N=N1) . Specifically, the controller  100  determines the initial transfer current I 2(n=0, N=N1)  for the second toner cartridge  9  by calculating the formula of I (n=0) +I (N=N1) . Thus, the initial transfer current I 2(n=0, N=N1)  of the second toner cartridge  9  differs from the initial transfer current I 1(n=0, N=0)  of the first toner cartridge  9 . In the present embodiment, the initial transfer current I 2(n=0, N=N1)  for the second toner cartridge  9  is larger than the initial transfer current I 1(n=0, N=0)  for the first toner cartridge  9 . That is, I 2(n=0, N=N1) &gt;I 1(n=0, N=0) . 
     Similarly, as the third, fourth, and fifth toner cartridges  9  are attached to the drum cartridge  8  in this order to perform image formation, the controller  100  executes processings of determining initial transfer currents I 3(n=0, N=N2) , I 4(n=0, N=N3) , and I 5(n=0, N=N4)  for the third, fourth and fifth toner cartridges  9  by calculating formulas: I (n=0) +I (N=N2) , I (n=0) +I (N=N3) , and I (n=0) +I (N=N4) , respectively. The controller  100  determines the initial transfer current for the third, fourth and fifth toner cartridges  9  such that the initial transfer current increases according to the cumulative number of drum rotations “N”. That is, I 5(n=0, N=N4) &gt;I 4(n=0, N=N3) &gt;I 3(n=0, N=N2) &gt;I 2(n0, N=N1) . 
     As illustrated in  FIG. 11B , after determining the initial transfer current I X(n=0)  for the X-th toner cartridge  9 , the controller  100  changes the transfer current I X(n, N)  for the X-th toner cartridge  9  such that the transfer current I X(n, N)  changes as both of the cumulative number of drum rotations “N” and the cumulative dot count “n” increase. In other words, the controller  100  determines the transfer current I X(n, N)  for the X-th toner cartridge  9  such that the transfer current IX(n, N) gradually increases as the cumulative dot count “n” increases and the cumulative number of drum rotations “N” increases until the X-th toner cartridge  9  is replaced with a next one ((X+1)-th toner cartridge  9 ). 
     Referring back to  FIG. 9 , the cleaning bias application circuit  280  is a circuit for applying a cleaning bias V C  to the cleaning roller  84 . The controller  100  determines the value of the cleaning bias V C  according to both of the cumulative number of drum rotations “N” that is stored in the drum memory  85  of the drum cartridge  8  currently attached to the image forming apparatus  1  and the cumulative dot count “n” that is stored in the toner memory  95  of the toner cartridge  9  currently attached to the drum cartridge  8 , and controls the cleaning bias application circuit  280  to apply the determined cleaning bias V C  to the cleaning roller  84 . 
     Specifically, the controller determines the value of the cleaning bias V CX  for the X-th toner cartridge  9  by adding a cleaning bias reference value V C(n) , which varies according to the cumulative dot count “n”, and a cleaning bias correction amount V C(N) , which varies according to the cumulative number of drum rotations (V CX(n, N) =V C(n) +V C(N) ) wherein X is an integer greater than zero (0). 
       FIG. 10B  shows the relationship between the cumulative dot count “n” and the cleaning bias reference value V C(n)  that should be satisfied while a single toner cartridge  9  is used for image formation, that is, after the toner cartridge  9  is newly attached to the drum cartridge  8  and until the toner cartridge  9  is replaced with a next one. The relationship ( FIG. 10B ) between the cumulative dot count “n” and the cleaning bias reference value V C(n)  is previously determined to ensure that the amount of the toner removed from the photosensitive drum  81  will become constant even when the toner degrades with an increase in the dot count. It is noted that data of the relationship of  FIG. 10B  is previously determined based on previously-acquired experimental data, and is previously stored in the ROM of the controller  100 . As illustrated in  FIG. 10B , the cleaning bias reference value V C(n)  is equal to V C(n=0)  when the cumulative dot count “n” is equal to zero (n=0). The absolute value of the cleaning bias reference value V C(n)  gradually increases as the cumulative dot count “n” increases. 
       FIG. 12A  shows the relationship between the cumulative number of drum rotations “N” and the cleaning bias correction amount V C(N)  that should be satisfied while a single drum cartridge  8  is used for image formation, that is, after the drum cartridge  8  is newly attached to the image forming apparatus  1  and until the drum cartridge  8  is replaced with a next one. The relationship ( FIG. 12A ) between the cumulative number of drum rotations “N” and the cleaning bias correction amount V C(N)  is previously determined to ensure that the amount of the toner removed from the photosensitive drum  81  will become constant even when the photosensitive drum  81  degrades with an increase in the cumulative number of drum rotations “N”. It is noted that data of the relationship of  FIG. 12A  is previously determined based on previously-acquired data, and is previously stored in the ROM of the controller  100 . As illustrated in  FIG. 12A , the cleaning bias correction amount V C(N)  is equal to zero (0) when the cumulative number of drum rotations “N” is equal to (N=0). The absolute value of the cleaning bias correction amount V C(N)  gradually increases as the cumulative number of drum rotations “N” increases. 
     As illustrated in  FIG. 12B , the controller  100  determines the initial cleaning bias for the X-th toner cartridge  9  based on the cumulative number of drum rotations “N” that is stored at the time when the X-th toner cartridge  9  is newly assembled to the drum cartridge  8 . Here, the initial cleaning bias for the X-th toner cartridge  9  is a cleaning bias that is to be applied to the cleaning roller  84  at an initial stage of the image formation performed by the X-th toner cartridge  9  together with the drum cartridge  8 . In other words, the initial cleaning bias for the X-th toner cartridge is a cleaning bias that is applied to the cleaning roller  84  when the cumulative dot count “n” stored in the toner memory  95  of the X-th toner cartridge  9  is equal to zero (0). More specifically, when the drum cartridge  8  is also new, both of the cumulative number of drum rotations “N” stored in the drum memory  85  and the cumulative dot count “n” stored in the toner memory  95  are equal to zero (0). At this time, the controller  100  determines the initial cleaning bias V C1(n=0, N=0)  based on only the cleaning bias reference value V C(n=0)  with reference to  FIG. 10B  because the cleaning bias correction amount V C(N)  is equal to zero (see  FIG. 12A ). In this way, the controller  100  executes a processing of determining the initial cleaning bias V C1(n=0, N=0)  for the first toner cartridge  9 . 
     As described above, at the timing when the second toner cartridge  9  is newly attached to the drum cartridge  8 , the cumulative number of drum rotations “N” is equal to N 1 , and the cumulative dot counts n is equal to zero (0). At this time, the controller  100  determines the initial cleaning bias V C2(n=0, N=N1)  for the second toner cartridge  9  based on the cleaning bias reference value V C(n=0)  and the cleaning bias correction amount V C(N=N1) . Specifically, the controller  100  determines the initial cleaning bias V C2(n=0, N=N1)  of the second toner cartridge  9  by calculating the formula of V C(n=0) +V C(N=N1) . Thus, the initial cleaning bias V C2(n=0, N=N1)  of the second toner cartridge  9  differs from the initial cleaning bias V C1(n=0, N=0)  of the first toner cartridge  9  (see  FIG. 12B ). In the present embodiment, the absolute value of the initial cleaning bias V C2(n=0, N=N1)  for the second toner cartridge  9  is larger than the absolute value of the initial cleaning bias V C1(n=0, N=0)  for the first toner cartridge  9 . That is, |V C2(n=0, N=N1) |&gt;|V C1(n=0, N=0) |. 
     Similarly, as the third, fourth, and fifth toner cartridges  9  are attached to the drum cartridge  8  in this order to perform image formation, the controller  100  executes processings of determining initial cleaning biases V C3(n=0, N=N2) , V C4(n=0, N=N3) , and V C5(n=0, N=N4)  for the third, fourth and fifth toner cartridges  9  by calculating formulas: V C(n=0) +V C(N=N2) , V C(n=0) +V C(N=N3) , and V C(n=0) +V C(N=N4) , respectively. The controller  100  determines the initial cleaning bias for the third, fourth and fifth toner cartridges  9  such that the absolute value of the initial cleaning bias increases according to the cumulative number of drum rotations “N”. That is, |V C5(n=0, N=N4) |&gt;|V C4(n=0, N=N3) |&gt;|V C3(n=0, N=N2) |&gt;|V C2(n=0, N=N1) |. 
     As illustrated in  FIG. 12B , after determining the initial cleaning bias V CX(n=0)  for the X-th toner cartridge  9 , the controller  100  changes the cleaning bias V CX(n, N)  for the X-th toner cartridge  9  such that the cleaning bias V CX(n, N)  changes as both of the cumulative number of drum rotations “N” and the cumulative dot count “n” increase. In other words, the controller  100  determines the cleaning bias V CX(n, N)  fort the X-th toner cartridge  9  such that the absolute value |V CX(n, N) | of the cleaning bias V CX(n, N)  gradually increases as the cumulative dot count “n” increases and the cumulative number of drum rotations “N” increases until the X-th toner cartridge  9  is replaced with a next one ((X+1)-th toner cartridge  9 ). 
     According to the above-described image forming apparatus  1  of the second embodiment, the following advantages are achieved. 
     After determining the initial transfer current I 1(n=0, N=0)  for the first toner cartridge  9 , the controller  100  determines, based on the cumulative number of drum rotations “N”, the initial transfer current I 2(n=0, N=N1)  for the second toner cartridge  9  such that the initial transfer current I 2(n=0, N=N1)  is different from the initial transfer current I 1(n=0, N=0)  In this way, even when the photosensitive drum  81  degrades with an increase in the cumulative number of drum rotations “N”, the transfer amount of toner moved from the photosensitive drum  81  to a sheet S is made constant. Similarly, after determining the initial cleaning bias V C1(n=0, N=0)  for the first toner cartridge  9 , the controller  100  determines, based on the cumulative number of drum rotations “N”, the initial cleaning bias V C2(n=0, N=N1)  for the second toner cartridge  9  such that the initial cleaning bias V C2(n=0, N=N1)  is different from the initial cleaning bias V C1(n=0, N=0) . In this way, even when the photosensitive drum  81  degrades with the increase in the cumulative number of drum rotations “N”, the amount of toner removed from the photosensitive drum  81  is made constant. This allows the print density of the image forming apparatus  1  to be stabilized while a plurality of toner cartridges are used in succession for one drum cartridge. 
     Further, the controller  100  changes the transfer current I X(n, N)  to be applied to the transfer roller  82  according to an increase in the cumulative number of drum rotations “N” and to an increase in the cumulative dot count “n”, so that the transfer current I X(n, N)  will be changed according to both degradation of the photosensitive drum  81  and degradation of toner. This allows the transfer amount of the toner moved from the photosensitive drum  81  to the sheet S is made constant. Similarly, the controller  100  changes the cleaning bias V CX(n, N)  to be applied to the cleaning roller  84  according to the increase in the cumulative number of drum rotations and to the increase in the cumulative dot count, so that the cleaning bias V CX(n, N)  will be changed according to both degradation of the photosensitive drum  81  and degradation of toner. This allows the cleaning amount of toner removed from the photosensitive drum  81  to be constant. 
     Further, the controller  100  gradually increases the transfer current I X(n, N)  to be applied to the transfer roller  82  as the cumulative dot count “n” increases. In this way, even when the toner stored in the toner cartridge  9  degrades, the transfer amount of the toner moved from the photosensitive drum  81  to a sheet S is made constant. More specifically, even though charging performance of toner deteriorates as toner degrades, a resultant deterioration of the toner transfer is suppressed by increasing the transfer current I X(n, N) . Similarly, the controller  100  gradually increases the absolute value of the cleaning bias V CX(n, N)  as the cumulative dot count “n” increases. In this way, even when the toner stored in the toner cartridge  9  degrades, the cleaning amount of the toner is made constant. More specifically, even though charging performance of toner deteriorates as toner degrades, a resultant deterioration of the cleaning amount can be suppressed by increasing the absolute value of the cleaning bias V CX(n, N) . 
     Further, the controller  100  gradually increases the transfer current I X(n, N)  as the cumulative number of drum rotations “N” increases. In this way, even when the photosensitive drum  81  degrades, the transfer amount of the toner moved from the photosensitive drum  81  to a sheet S is made constant. More specifically, as the photosensitive drum  81  deteriorates, the surface of the photosensitive drum  81  becomes rough and the adhesion of toner increases. However, a resultant decrement of the toner transfer amount can be suppressed by increasing the transfer current I X(n, N) . Similarly, the controller  100  gradually increases the cleaning bias V CX(n,N)  as the cumulative number of drum rotations “N” increases. In this way, even when the photosensitive drum  81  degrades, the toner cleaning amount is made constant. More specifically, as the photosensitive drum  81  deteriorates, the surface of the photosensitive drum  81  becomes rough and the adhesion of toner increases. However, a resultant decrement of the toner cleaning amount can be suppressed by increasing the cleaning bias V CX(n,N) . 
     It is noted that the drum cartridge has already deteriorated when the second and subsequent toner cartridges are installed. Accordingly, by making the initial transfer current I 2(n=0, N=N1)  larger than the initial transfer current I 1(0, 0) , the amount of toner transferring from the photosensitive drum  81  to a sheet S when the second toner cartridge is used can be brought close to the amount of toner transferring from the photosensitive drum to a sheet S when the first toner cartridge is used. This allows the print density of the image forming apparatus  1  to be stabilized while a plurality of toner cartridges are used in succession for one drum cartridge such that when toner runs out in one toner cartridge, the toner cartridge is replaced with the next toner cartridge. Similarly, by making the absolute value of the initial cleaning bias |V C2(n=0, N=N1)  larger than |V C1(n=0, N=0) , the cleaning amount of toner removed from the photosensitive drum  81  when the second toner cartridge is used can be brought close to the cleaning amount of toner removed from the photosensitive drum  81  when the first toner cartridge is used. This allows the print density of the image forming apparatus  1  to be stabilized while a plurality of toner cartridges are used in succession for one drum cartridge. 
     The present disclosure is not limited to the above-described embodiments and can be applied to various forms as exemplified below. 
     While the positively charged type toner is used in the first embodiment, negatively charged type toner may be used. In this case, the charging bias application circuit  210 , developing bias application circuit  220 , blade bias application circuit  230 , and supply bias application circuit  240  may each be applied with negative bias voltage. 
     While the positively charged type toner is used in the second embodiment, negatively charged type tone may be used. In this case, the charging bias application circuit  210  and the cleaning bias application circuit may each be applied with negative bias voltage. 
     While the controller  100  reduces the absolute value |V GX(n, N) | of the developing bias V GX(n, N)  as the cumulative dot count of the toner cartridge  9  increases in the first embodiment, the developing bias may be determined according to the cumulative number of drum rotations or the residual amount of toner stored in the toner cartridge  9 , in place of the cumulative dot count. Similarly, the circumferential velocity difference ΔV X(n, N)  between the photosensitive drum and the developing roller may be determined according to the cumulative number of drum rotations or the residual amount of toner stored in the toner cartridge  9 , in place of the cumulative dot count. 
     While the circumferential velocity of the photosensitive drum  81  is constant in the first embodiment, the circumferential velocity of the photosensitive drum  81  may not necessarily be constant but may be changed. Further, while the circumferential velocity of the developing roller  91  is higher than the circumferential velocity of the photosensitive drum  81  in the first embodiment, the circumferential velocity of the developing roller  91  may be equal to or lower than the circumferential velocity of the photosensitive drum  81 . 
     While the photosensitive drum  81  and developing roller  91  are driven by separate motors (i.e. the first motor  250  and the second motor  260 ) in the first embodiment, the photosensitive drum  81  and developing roller  91  may be driven by a single common motor. In this case, the developing roller and motor may be connected together through a transmission mechanism that is capable of changing a transmission ratio. 
     In the second embodiment, the drum cartridge  8  includes both of the transfer roller  82  and the cleaning roller  84 . After determining both of the initial transfer current I 1(n=0, N=0)  and the initial cleaning bias V C1(n=0, N=0)  for the first toner cartridge  9 , the controller  100  determines for the second toner cartridge  9  the initial transfer current I 2(n=0, N=N1)  different from the initial transfer current I 1(n=0, N=0)  and the initial cleaning bias V C2(n=0, N=N1)  different from the initial cleaning bias V C1(n=0, N=0)  based on the cumulative number of the drum rotations N. However, the drum cartridge  8  may include at least one of the transfer roller  82  and the cleaning roller  84 . If the drum cartridge  8  includes the transfer roller  82  but does not include the cleaning roller  84 , after determining the initial transfer current I 1(n=0, N=0) , the controller  100  determines the initial transfer current I 2(n=0, N=N1)  different from the initial transfer current I 1(n=0, N=0)  based on the cumulative number of drum rotations “N. If the drum cartridge  8  includes the cleaning roller  84  but does not include the transfer roller  82 , after determining the initial cleaning bias V C1(n=0, N=0) , the controller  100  determines the initial cleaning bias V C2(n=0, N=N1)  different from the initial cleaning bias V C1(n=0, N=0)  based on the cumulative number of drum rotations “N”. 
     While the controller  100  increases the transfer current I as the cumulative dot count “n” of the toner cartridge  9  increases in the second embodiment, the current value may be determined according to the cumulative number of drum rotations “N” or according to the residual amount of toner stored in the toner cartridge  9 , in place of the cumulative dot count “n”. 
     While the monochrome laser printer is exemplified as the image forming apparatus in each of the first and second embodiments, the image forming apparatus to be used in the present disclosure may be a color laser printer and, a copying machine, or a multifunction machine, or the like. 
     The components described in the first and second embodiments and modifications may arbitrarily be combined. 
     While the description has been made in detail with reference to the embodiments thereof, it would be apparent to those skilled in the art that many modifications and variations may be made therein without departing from the spirit of the disclosure.