Patent Publication Number: US-10310408-B2

Title: Image forming apparatus and method for determining usable period of cleaner used for image forming operations

Description:
The entire disclosure of Japanese Patent Application No. 2017-052651 filed on Mar. 17, 2017 is incorporated herein by reference in its entirety. 
     BACKGROUND 
     Technological Field 
     The present disclosure relates to image forming apparatuses, and more specifically relates to a technique for determining the replacement timing of a component of an image forming apparatus. 
     Description of the Related Art 
     Generally, the usable period of a product or a component of a product is estimated to be shorter in order to guarantee the performance of the product even under harsh environments. Moreover, a technique for preventing replacement of a product or a component of a product that is actually still usable, i.e., a technique for improving the precision in predicting the replacement timing of a product or a component of a product, has been in demand. For image forming apparatuses, such a technique has been in strong demand. 
     Regarding the technique for improving the precision in predicting the replacement timing of a component of an image forming apparatus, Japanese Laid-Open Patent Publication No. 2005-178058 for example discloses an image forming apparatus “acquiring, from a wireless IC tag attached to a process cartridge, information about the date of manufacture, the date of installation, the deterioration factor, and the threshold limit value, calculating the pre-installation degree of deterioration representing deterioration in the packaged state, the pre-operation degree of deterioration representing deterioration in the non-operating state after installation, and the in-operation degree of deterioration representing deterioration in the operating state, and comparing the total degree of deterioration that is the sum of these degrees of deterioration with the threshold limit value to thereby calculate the shelf life” (see “Abstract”). 
     Japanese Laid-Open Patent Publication No. 2004-004544 discloses an image forming apparatus “performing control such that the cleaner-abutting pressure in a second operation is higher than that in a first operation, where the first operation removes developer remaining on a first image bearing member upon transfer of a developer image to a transfer material, and the second operation removes the developer image on the first image bearing member that has not been transferred to a second image bearing member” (see “Abstract”). 
     Japanese Laid-Open Patent Publication No. 2009-288481 relates to a technique for preventing a toner image from slipping through a cleaning blade, and discloses an image forming apparatus in which “toner with older date of manufacture, which has deteriorated chargeability and thus deteriorated electrostatic suction to an intermediate transfer belt, is used in a larger amount to form a discharged-toner image so that the discharged-toner image is small in charge amount and less likely to slip through the cleaning blade” (see “Abstract”). 
     Japanese Laid-Open Patent Publication No. 2006-047330 discloses a technique for setting a different usable period (number of printable pages) for each member or device that is a component of an image forming apparatus. 
     SUMMARY 
     A variety of techniques have been proposed for improving the precision in predicting the replacement timing of a component of an image forming apparatus as described above. Recent environmental issues and the like, however, lead to demand for further improvement of the precision in prediction. Essentially Japanese Laid-Open Patent Publication No. 2009-288481 does not aim to improve the precision in predicting the timing of replacing a component. 
     The present disclosure is given for providing solutions to the above problem, and an object in an aspect is to provide a technique for improving the precision in predicting the replacement timing of a component of an image forming apparatus, relative to the conventional image forming apparatus. 
     To achieve at least one of the abovementioned objects, according to an aspect of the present invention, an image forming apparatus reflecting one aspect of the present invention includes: a rotatable body; a cleaner to remove developer adhered to the rotatable body; and a controller configured to determine that a usable period of the cleaner is longer as a charge amount of the developer is smaller. 
     Preferably, the image forming apparatus further includes: a developing device to develop an electrostatic latent image formed on a photoconductor by the developer; and a density sensor to detect a density of a patch image developed by the developing device. The controller is configured to determine that the charge amount of the developer is smaller as the density of the patch image detected by the density sensor is higher. 
     Preferably, the image forming apparatus further includes: a developing device to develop an electrostatic latent image formed on a photoconductor by the developer; and a density sensor to detect a density of a patch image developed by the developing device. The controller is configured to determine that the charge amount of the developer is smaller as a developing bias applied to the developing device is smaller. The developing bias is set based on the density of the patch image detected by the density sensor. 
     Preferably, the controller is configured to determine that the usable period of the cleaner has expired, when the charge amount of the developer is larger than a predetermined value. 
     Preferably, the controller is configured to determine the usable period of the cleaner after a count value reaches a predetermined value. The count value is incremented as printing is performed. The count value is initialized when the cleaner is replaced. 
     More preferably, the image forming apparatus further includes: a developing device to develop an electrostatic latent image formed on a photoconductor by the developer; and a density sensor to detect a density of a patch image developed by the developing device. The controller is configured to determine that the usable period of the cleaner has expired, when the density of the patch image detected by the density sensor is a predetermined value or less. 
     More preferably, the image forming apparatus further includes: a developing device to develop an electrostatic latent image formed on a photoconductor by the developer; and a density sensor to detect a density of a patch image developed by the developing device. The controller is configured to determine that the usable period of the cleaner has expired, when a developing bias applied to the developing device is a predetermined value or more. The developing bias is set based on the density of the patch image detected by the density sensor. 
     Preferably, the image forming apparatus further includes a developing device to develop an electrostatic latent image formed on a photoconductor by the developer. The cleaner is replaceable independently of the developing device. The controller is configured to determine that the usable period of the cleaner is longer as the number of the cleaners replaced for the same developing device is larger. 
     More preferably, the controller is configured to set a pressure smaller as the number of the cleaners replaced for the same developing device is larger. The pressure is applied to press the cleaner against the rotatable body. 
     Preferably, the controller is configured to determine that the usable period of the cleaner has expired, when a count value reaches a predetermined value. The count value is incremented as printing is performed. The predetermined value is set larger as the number of the cleaners replaced for the same developing device is larger. The count value is initialized when the cleaner is replaced. 
     More preferably, the controller is configured to set the predetermined value larger as a coverage rate is higher. 
     More preferably, the controller is configured to set the predetermined value larger as an average value of the number of pages printed per print job is larger. 
     Preferably, the controller is configured to determine that the usable period of the cleaner has expired, when the count value reaches the predetermined value and the controller determines that a usable period of the developing device has expired. 
     Preferably, the rotatable body includes at least one of a photoconductor, an intermediate transfer belt, or a secondary transfer roller. 
     According to another aspect, there is provided an image forming system including an image forming apparatus and a server apparatus. The image forming apparatus includes: a rotatable body; a cleaner to remove developer adhered to the rotatable body; an acquisition unit to acquire a charge amount of the developer; and a communication interface to transmit a result of acquisition by the acquisition unit to the server apparatus. The server apparatus is configured to determine that a usable period of the cleaner is longer as the charge amount of the developer is smaller, based on the result of acquisition. 
     According to a further aspect, there is provided a method for determining a usable period of a cleaner used for an image forming apparatus. The method includes: acquiring a charge amount of developer used for the image forming apparatus; and determining that the usable period of the cleaner is longer as the acquired charge amount of the developer is smaller. 
     According to a still further aspect, there is provided a non-transitory computer-readable recording medium storing a determination program to be executed by a computer of an image forming apparatus for determining a usable period of a cleaner used for the image forming apparatus. The determination program causes the computer to perform: acquiring a charge amount of developer used for the image forming apparatus; and determining that the usable period of the cleaner is longer as the acquired charge amount of the developer is smaller. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The advantages and features provided by one or more embodiments of the invention will become more fully understood from the detailed description given hereinbelow and the appended drawings which are given by way of illustration only, and thus are not intended as a definition of the limits of the present invention. 
         FIG. 1  is a diagram illustrating how toner remaining on a surface of a photoconductor is collected. 
         FIG. 2  shows a relation between the number of printed pages and the cleaning blade&#39;s ability for each of a small toner charge amount and a large toner charge amount. 
         FIG. 3  is a diagram illustrating an example of an external configuration of an image forming apparatus according to a first embodiment. 
         FIG. 4  is a diagram illustrating an example of an electrical configuration of the image forming apparatus according to the first embodiment. 
         FIG. 5  shows an example of a functional configuration of a CPU. 
         FIG. 6  is a diagram for illustrating a process for determining whether the usable period of a cleaning blade has expired, based on a developing bias voltage value regarded as a toner charge amount. 
         FIG. 7  is a flowchart for illustrating an example of the timing for a determination unit to perform a determination process. 
         FIG. 8  is a flowchart showing the determination process in step S 720  in  FIG. 7 . 
         FIG. 9  is a flowchart showing the determination process in step S 720  in  FIG. 7  that is performed using the density of a patch image. 
         FIG. 10  is a flowchart showing the determination process in step S 720  in  FIG. 7 , in the case where an upper limit of the usable period of a member is defined. 
         FIG. 11  is a flowchart showing the determination process in step S 720  in  FIG. 7 , in the case where notification of the remaining usable period of a member is made. 
         FIG. 12  is a diagram for illustrating a process for replacing a photoconductor unit in an image forming apparatus according to a related art. 
         FIG. 13  is a diagram for illustrating a process for replacing a photoconductor unit in an image forming apparatus according to a second embodiment. 
         FIG. 14  is a flowchart showing a process for replacing a photoconductor unit including a cleaning blade. 
         FIG. 15A  is a diagram showing usable periods of photoconductor units in the case where the average coverage rate is low. 
         FIG. 15B  is a diagram showing usable periods of photoconductor units in the case where the average coverage rate is high. 
         FIG. 16  is a diagram for illustrating a process for setting the pressing force for a cleaning blade in the image forming apparatus according to the second embodiment. 
         FIG. 17  is a diagram for illustrating an example of a configuration of an image forming apparatus according to a third embodiment. 
         FIG. 18  shows an example of a functional configuration of a CPU according to the third embodiment. 
         FIG. 19  shows an example of a configuration of an image forming system according to a fourth embodiment. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, one or more embodiments of the present invention will be described with reference to the drawings. However, the scope of the invention is not limited to the disclosed embodiments. 
     In the following, embodiments of the technological concept are described in detail with reference to the drawings. In the following description, the same parts are denoted by the same reference characters. They are named identically and function identically. The detailed description thereof is therefore not repeated. Embodiments and modifications described below may be selectively combined as appropriate. 
     Technological Concept 
     With reference to  FIGS. 1 and 2 , a description is given of an overview of control for an image forming apparatus according to embodiments.  FIG. 1  illustrates how toner remaining on the surface of a photoconductor  110  is collected. With reference to  FIG. 1 , the image forming apparatus based on electrophotography includes photoconductor  110 , an intermediate transfer belt  120 , a primary transfer roller  130 , and a cleaning blade  140 . 
     Photoconductor  110  is supplied with toner  122  which is developer from a developing device (not shown). A toner image formed on photoconductor  110  is transferred to intermediate transfer belt  120  by the action of electric field formed between photoconductor  110  and primary transfer roller  130 . 
     All the toner  122  formed on the surface of photoconductor  110  is not transferred to intermediate transfer belt  120 . Toner remaining on the surface of photoconductor  110  is a factor causing an image defect (stripes in the image, for example). Cleaning blade  140  therefore collects the remaining toner from the surface of photoconductor  110 . 
     Generally, cleaning blade  140  is made of rubber, and the blade&#39;s ability to collect remaining toner from the surface of photoconductor  110  deteriorates due to wear resultant from contact with the photoconductor and/or any change of the material. As a result of this, the amount of slip-through toner  122 L that has not been collected by cleaning blade  140  increases. When the amount of slip-through toner  122 L exceeds a predetermined amount, an image defect that is visually recognizable by a user is generated. 
       FIG. 2  shows a relation between the number of printed pages and the cleaning blade&#39;s ability for each of a small toner charge amount and a large toner charge amount. The ability of cleaning blade  140  is represented by the ratio of the toner amount collectable by cleaning blade  140  to the amount of toner remaining on the surface of photoconductor  110 . As described above, the ability of cleaning blade  140  lowers gradually with increase of the total number of printed pages. When the ability of cleaning blade  140  becomes equal to or lower than a threshold ability Cth, an image defect visually recognizable by a user is generated. Cleaning blade  140  whose ability becomes equal to or less than threshold ability Cth is therefore replaced. 
     The ability of cleaning blade  140  is higher as the charge amount of toner  122  is smaller. This is because of smaller electrostatic interaction between toner  122  and photoconductor  110 . Therefore, the number of printed pages N 2  at which the ability of cleaning blade  140  reaches threshold ability Cth for the smaller toner charge amount is larger than the number of printed pages N 1  at which the ability of cleaning blade  140  reaches threshold ability Cth for the larger toner charge amount. 
     The image forming apparatus according to the embodiments makes use of this characteristic to acquire the toner charge amount directly or indirectly and, as the toner charge amount is smaller, the image forming apparatus determines that the period for which the cleaning blade is usable (hereinafter also referred to as “usable period”) is longer. For example, when the acquired toner charge amount is less than a predetermined charge amount, the image forming apparatus determines that the cleaning blade is still usable. The image forming apparatus according to the embodiments configured in the above-described manner enables increase of the precision in predicting the replacement timing of the cleaning blade, as compared with the conventional image forming apparatus. Accordingly, the cleaning blade is usable for a longer period than the conventional one. In the case where the cleaning blade forms a unit together with another component(s) (photoconductor, for example), the image forming apparatus according to the embodiments enables increase of the usable period of the unit including the cleaning blade. In the following, a description is given of a specific configuration of and control for the image forming apparatus according to the embodiments. 
     First Embodiment 
     External Configuration of Image Forming Apparatus 
       FIG. 3  is a diagram illustrating an example of an external configuration of an image forming apparatus  300  according to a first embodiment. Image forming apparatus  300  is an electrophotographic image forming apparatus such as laser printer or LED printer, and forms an image on a medium such as a sheet of paper based on an input image signal. As shown in  FIG. 3 , image forming apparatus  300  includes an intermediate transfer belt  1  as a belt member at a substantially central portion in the image forming apparatus. Under a lower horizontal portion of intermediate transfer belt  1 , four imaging units  2 Y,  2 M,  2 C,  2 K corresponding respectively to the colors: yellow (Y), magenta (M), cyan (C), black (K) are arranged along intermediate transfer belt  1 , and these units have photoconductors  3 Y,  3 M,  3 C,  3 K, respectively. Each of imaging units  2 Y,  2 M,  2 C,  2 K is configured to be replaceable. Photoconductors  3 Y,  3 M,  3 C,  3 K for carrying and transporting a latent image each develop a toner image on a photoconductor film formed on the outer periphery of the photoconductor. The toner image is to be transferred to a medium such as a sheet of paper. 
     Around photoconductors  3 Y,  3 M,  3 C,  3 K, the following components are arranged in order in the rotational direction of the photoconductors. The components are: charging rollers  10 Y,  10 M,  10 C,  10 K, a laser unit  20 , developing devices  22 Y,  22 M,  22 C,  22 K, primary transfer rollers  24 Y,  24 M,  24 C,  24 K that face respective photoconductors  3 Y,  3 M,  3 C,  3 K with intermediate transfer belt  1  interposed between the photoconductor and the primary transfer roller, and cleaning blades  26 Y,  26 M,  26 C,  26 K. Developing devices  22 Y,  22 M,  22 C,  22 K include developing rollers  25 Y,  25 M,  25 C,  25 K, respectively. Cleaning blades  26 Y,  26 M,  26 C,  26 K are pressed against photoconductors  3 Y,  3 M,  3 C,  3 K respectively by a pressing mechanism (not shown). 
     To developing devices  22 Y,  22 M,  22 C,  22 K, toner bottles  23 Y,  23 M,  23 C,  23 K are connected, respectively. A cleaning blade  27  is pressed against intermediate transfer belt  1  by a pressing mechanism (not shown). A density sensor  4  is disposed between imaging unit  2 K and a secondary transfer region on the most downstream side of intermediate transfer belt  1 . 
     A secondary transfer roller  28  is pressed against intermediate transfer belt  1 . In this region, secondary transfer is performed. A cleaning blade  29  is pressed against secondary transfer roller  28  by a pressing mechanism (not shown). A fixing device  30  including a fixing roller  32  and a pressure roller  34  is arranged downstream of a transport path located behind the secondary transfer region. 
     In a lower portion of image forming apparatus  300 , a paper feed cassette  40  is removably disposed. Sheets of paper stacked and contained in paper feed cassette  40  are fed one by one from the top sheet to the transport path by rotation of a transport roller  42   a.  On the transport path, transport roller pairs  42   b,    42   c,    42   d,    42   e,    42   f,    42   g  are arranged. Moreover, in an upper portion of image forming apparatus  300 , an operation panel  44  is disposed. For example, operation panel  44  is made up of a display and buttons for receiving user&#39;s input. 
     While image forming apparatus  300  in the present embodiment uses the tandem intermediate transfer system by way of example, the image forming apparatus is not limited to this. The image forming apparatus may use the cycle system or the direct transfer system by which toner is directly transferred from a developing device to a printing medium. Alternatively, the image forming apparatus may be a multifunction device incorporating functions such as copier, printer, and facsimile functions. 
     General Operation of Image Forming Apparatus 
     Next, a general operation of image forming apparatus  300  configured in the above-described manner is described. Upon input of an image signal from an external device (such as personal computer for example) to image forming apparatus  300 , image forming apparatus  300  generates a digital image signal by color conversion of the input image signal to yellow, magenta, cyan, black. Based on the generated digital image signal, image forming apparatus  300  causes laser unit  20  to emit light so as to perform exposure. 
     Accordingly, an electrostatic latent image formed on each of photoconductors  3 Y,  3 M,  3 C,  3 K is developed by toner supplied from a corresponding one of developing devices  22 Y,  22 M,  22 C,  22 K to generate a toner image of each color. When the amount of toner in each of developing devices  22 Y,  22 M,  22 C,  22 K decreases, toner is supplied from corresponding toner bottles  23 Y,  23 M,  23 C,  23 K. 
     Toner images of respective colors are successively laid on one another on intermediate transfer belt  1  by the action of primary transfer rollers  24 Y,  24 M,  24 C,  24 K. Primary transfer is thus accomplished. After the primary transfer, toner remaining on each photoconductor  3 Y,  3 M,  3 C,  3 K is collected by corresponding cleaning blade  26 Y,  26 M,  26 C,  26 K. 
     The toner images thus formed on intermediate transfer belt  1  undergo secondary transfer all together onto a sheet of paper by the action of secondary transfer roller  28 . Toner remaining on intermediate transfer belt  1  is collected by cleaning blade  27 . 
     The toner image which is secondary-transferred to the sheet of paper reaches fixing device  30 . The toner image is fixed on the sheet of paper by the action of heated fixing roller  32  and pressure roller  34 . Toner remaining on secondary transfer roller  28  is collected by cleaning blade  29 . The paper on which the toner image is fixed is discharged through transport roller pair  42   d  to a copy receiving tray. 
     When images are to be formed on both sides of a sheet of paper, transport roller pair  42   d  is rotated in the opposite direction after the sheet of paper has passed through fixing device  30 , and the sheet of paper is transported again by transport roller pairs  42   e,    42   f,  and  42   g  to the secondary transfer region. In this way, the above-described secondary transfer and fixing for the sheet of paper are performed. After this, the paper is discharged by transport roller pair  42   d  to the copy receiving tray. 
     In the following, the characters: yellow “Y,” magenta “M,” cyan “C,” and black “K” may not be used for reference characters for devices. Such a device represents respective devices for the four colors. For example, photoconductor  3  represents photoconductors  3 Y,  3 M,  3 C,  3 K. 
     Electrical Configuration of Image Forming Apparatus 
       FIG. 4  is a diagram illustrating an example of an electrical configuration of image forming apparatus  300  according to the first embodiment. Image forming apparatus  300  includes a CPU (Central Processing Unit)  410  functioning as a controller for image forming apparatus  300 . CPU  410  is electrically connected to each of a RAM (Random Access Memory)  415 , a ROM (Read Only Memory)  420 , a nonvolatile memory  430 , a power supply apparatus  435 , laser unit  20 , a temperature sensor  440 , a humidity sensor  445 , density sensor  4 , operation panel  44 , and a communication interface (I/F)  450 . CPU  410  reads and executes a control program  422  stored in ROM  420  to thereby control operation of each device connected to CPU  410 . 
     RAM  415  functions as a working memory for CPU  410  to execute control program  422 . Nonvolatile memory  430  stores a count value  432 , an average coverage rate  434 , an average number of printed pages  436 , and an average humidity  438 . 
     Count value  432  is a value incremented as printing is performed. CPU  410  stores, in nonvolatile memory  430 , count value  432  for each of the colors Y, M, C, and K as well as count value  432  for cleaning blades  27  and  29 . For example, count value  432  for yellow includes the total number of printed pages printed with yellow toner, the travel distance of photoconductor  3 Y, and the number of rotations of photoconductor  3 Y. For example, count value  432  of cleaning blade  27  includes the total number of printed pages for which cleaning blade  27  is used, the travel distance of intermediate transfer belt  1 , and the number of rotations of intermediate transfer belt  1 . When CPU  410  detects replacement of any cleaning blade or any unit including a cleaning blade, CPU  410  initializes the corresponding count value (sets the count value to zero). 
     Based on an input image signal, CPU  410  calculates the coverage rate. The coverage rate is the rate of a printed area to a printable area. Average coverage rate  434  is an average value of the coverage rate. Average number of printed pages  436  is an average value of the number of printed pages included in a print job. Average humidity  438  is an average value of the relative humidity detected by humidity sensor  445 . In an aspect, average humidity  438  is an average value of the relative humidity detected by humidity sensor  445  each time a print job is input. In another aspect, average humidity  438  is an average value of the absolute humidity calculated from the temperature detected by temperature sensor  440  and the relative humidity detected by humidity sensor  445 . 
     CPU  410  stores, in nonvolatile memory  430 , average coverage rate  434 , average number of printed pages  436 , and average humidity  438  for each of the colors Y, M, C, and K. When CPU  410  detects replacement of a cleaning blade or a unit including a cleaning blade, CPU  410  initializes (sets to zero) average coverage rate  434 , average number of printed pages  436 , and average humidity  438  of a corresponding color. 
     Power supply apparatus  435  applies a negative developing bias voltage to developing roller  25  based on an instruction from CPU  410 . Toner carried and transported by developing roller  25  is adhered to an exposed portion of photoconductor  3  based on a potential difference between developing roller  25  and exposed photoconductor  3 . 
     Temperature sensor  440  measures the temperature in image forming apparatus  300  and outputs the result of the measurement to CPU  410 . Temperature sensor  440  may be of contact type or non-contact type. For example, temperature sensor  440  is implemented by a thermocouple. 
     Humidity sensor  445  measures the humidity in image forming apparatus  300  and outputs the result of the measurement to CPU  410 . Humidity sensor  445  may be polymer capacitive humidity sensor or polymer resistive humidity sensor. 
     Density sensor  4  detects the density of a toner image formed on intermediate transfer belt  1 . For example, density sensor  4  includes a light emitting device (not shown) emitting light, and a light receiving device (not shown) receiving the light emitted from the light emitting device and reflected. As intermediate transfer belt  1  is irradiated with light emitted from the light emitting device, the light receiving device detects the light reflected from the toner image on intermediate transfer belt  1 . Density sensor  4  outputs, to CPU  410 , the amount of photocurrent (detected voltage) generated at the light receiving device. 
     Communication I/F  450  is implemented by a wireless LAN (Local Area Network) card, for example. Image forming apparatus  300  is configured to be capable of communicating with external devices (such as personal computer, smart phone, tablet, and server) connected to a LAN or WAN (Wide Area Network) via communication interface  450 . 
     In connection with the first to fourth embodiments described below, a description is given of a process for determining whether the usable period of cleaning blade  26 Y has expired or not, for the sake of facilitating understanding. 
     Control Structure 
       FIG. 5  shows an example of a functional configuration of CPU  410 . CPU  410  reads and executes control program  422  to thereby function as an acquisition unit  510 , a determination unit  520 , an update unit  530 , and a developing bias determination unit  540 . 
     Acquisition unit  510  indirectly acquires the charge amount of toner stored in developing device  22 , and outputs the result of the acquisition to determination unit  520 . Based on a value that is input from acquisition unit  510  to determination unit  520 , determination unit  520  determines whether the usable period of cleaning blade  26 Y has expired or not. 
     When determination unit  520  determines that the usable period of cleaning blade  26 Y has expired, determination unit  520  causes operation panel  44  to display an instruction to replace cleaning blade  26 Y. Recognizing this instruction, a user replaces cleaning blade  26 Y. 
     In another aspect, unit-by-unit replacement may be performed for replacing a unit including cleaning blade  26 Y and another component(s) (photoconductor  3 Y for example) with a new unit. In this case, determination unit  520  causes operation panel  44  to display an instruction to replace this unit. 
     Based on an input print job (image data), update unit  530  updates count value  432 , average coverage rate  434 , average number of printed pages  436 , and average humidity  438 . For example, it is supposed that a monochrome print job is input to image forming apparatus  300 . In this case, when the print job is completed, update unit  530  updates count value  432 , average coverage rate  434 , average number of printed pages  436 , and average humidity  438  for black. 
     Developing bias determination unit  540  calculates a developing bias voltage value based on the result of detection by density sensor  4  for a patch image having a predetermined shape and tone value. Developing bias determination unit  540  outputs information indicating the calculated developing bias voltage value to power supply apparatus  435 . Based on the input information, power supply apparatus  435  applies the developing bias voltage to developing roller  25 . 
     In an aspect, acquisition unit  510  acquires the developing bias voltage value calculated by developing bias determination unit  540  for using the developing bias voltage as a toner charge amount. This is for the reason that there is a proportional relation between the developing bias voltage value and the toner charge amount as described below. 
     It is supposed that a potential difference between a surface potential of exposed photoconductor  3 Y and a surface potential of developing roller  25 Y (developing bias voltage) is ΔV, and the toner charge amount (charge amount of toner stored in developing device  22 Y) is Vt. In this case, density C of a toner image is calculated by determining: potential difference ΔV/toner charge amount Vt. Developing bias determination unit  540  sets the developing bias voltage value so that density C is constant. Thus, a proportional relation holds between potential difference ΔV and toner charge amount Vt. Usually, the surface potential of exposed photoconductor  3 Y is controlled so that the surface potential is constant. A proportional relation therefore holds between the developing bias voltage value and the toner charge amount. 
     Developing Bias Voltage 
       FIG. 6  is a diagram for illustrating a process for determining whether the usable period of cleaning blade  26 Y has expired or not, based on a developing bias voltage regarded as a toner charge amount. The horizontal axis of  FIG. 6  represents the count value (total number of printed pages, for example), the left vertical axis represents the absolute value of the developing bias voltage value, and the right vertical axis represents the toner charge amount. 
     A curve  610  indicated by a solid line represents the developing bias voltage value assumed based on count value  432  for yellow. In an aspect, it is supposed that a relational expression representing curve  610  is stored in nonvolatile memory  430 . As count value  432  increases, the developing bias voltage value decreases. This is for the reason that the toner charge amount having a proportional relation with the developing bias voltage value decreases gradually with time, relative to the toner charge amount at the time of shipment from a factory. 
     In an aspect, when count value  432  reaches a predetermined value Nth, developing bias determination unit  540  calculates the developing bias voltage based on an output from density sensor  4 . When count value  432  is the total number of printed pages, predetermined value Nth may be set to an assumed number of printable pages for which cleaning blade  26 Y is used. 
     Determination unit  520  acquires, through acquisition unit  510 , the result of the calculation (developing bias voltage value) of developing bias determination unit  540 . Determination unit  520  further acquires assumed developing bias voltage Vth for predetermined value Nth. Developing bias voltage value Vth may be stored in nonvolatile memory  430  or calculated from the relational expression represented by curve  610 . 
     When the developing bias voltage value calculated by developing bias determination unit  540  is Vm 1  that is equal to or larger than voltage value Vth, determination unit  520  determines that the usable period of cleaning blade  26 Y has expired. In other words, because the toner charge amount is an assumed predetermined value or more (because remaining toner adheres to photoconductor  3 Y more firmly than assumed), determination unit  520  determines that the remaining toner cannot be collected sufficiently by cleaning blade  26 Y. 
     In contrast, when the developing bias voltage value calculated by developing bias determination unit  540  is Vm 2  that is less than voltage value Vth, determination unit  520  determines that cleaning blade  26 Y is still usable. In other words, because the toner charge amount is less than the assumed predetermined value (because remaining toner adheres to photoconductor  3 Y more weakly than assumed), determination unit  520  determines that the remaining toner can be collected sufficiently by cleaning blade  26 Y. Next, with reference to  FIGS. 7 and 8 , a description is given of a flow of a process performed by CPU  410 . 
       FIG. 7  is a flowchart for illustrating an example of the timing of performing a determination process by determination unit  520 . The processes shown in  FIG. 7  and  FIGS. 8 to 11, 15A, and 15B  described later herein are implemented by CPU  410  reading and executing control program  422 . 
     In step S 705 , CPU  410  determines whether a timing for an image stabilization process has been reached or not. The timing for the image stabilization process is set for example to the timing when image forming apparatus  300  is powered or the total number of printed pages reaches a predetermined number of pages ( 1000  pages for each case, for example). When the timing for the image stabilization process is reached (YES in step S 705 ), CPU  410  proceeds to step S 710 . In contrast, when the timing for the image stabilization process is not reached (NO in step S 705 ), CPU  410  returns to step S 705 . 
     The image stabilization process is a process for setting the developing bias voltage value so that the density of the toner image is constant. Specifically, under predetermined printing conditions, a predetermined patch image (halftone image, for example) is formed. Based on the density of the patch image detected by density sensor  4 , developing bias determination unit  540  sets the developing bias voltage value. 
     In step S 710 , CPU  410  determines whether count value  432  is predetermined value Nth or more. When count value  432  is predetermined value Nth or more (YES in step S 710 ), CPU  410  proceeds to step S 720 . When count value  432  is less than predetermined value Nth (NO in step S 710 ), CPU  410  returns to step S 705 . 
     In step S 720 , CPU  410  performs a process for determining whether the usable period of cleaning blade  26 Y has expired or not based on the toner charge amount (hereinafter also referred to as “determination process”). 
       FIG. 8  is a flowchart illustrating the determination process in step S 720  in  FIG. 7 . In step S 810 , CPU  410  performs the image stabilization process. In this process, CPU  410  functions as acquisition unit  510  to acquire the developing bias voltage value proportional to the toner charge amount. 
     In step S 820 , CPU  410  functions as determination unit  520  to determine whether the acquired developing bias voltage value is predetermined value Vth or more. When the acquired developing bias voltage value is predetermined value Vth or more (YES in step S 820 ), CPU  410  proceeds to step S 830 . When the acquired developing bias voltage value is less than predetermined value Vth (NO in S 820 ), CPU  410  ends the series of steps. 
     In an aspect, predetermined value Vth in step S 820  is set to a predetermined voltage value. In another aspect, predetermined value Vth is set to a voltage value that is calculated from the relational expression represented by curve  610  and corresponds to count value  432 . 
     In step S 830 , CPU  410  causes operation panel  44  to display an instruction to replace a photoconductor unit including cleaning blade  26 Y. Recognizing this instruction, a user can replace cleaning blade  26 Y. 
     In another aspect, CPU  410  may transmit the instruction to replace cleaning blade  26 Y to an external apparatus owned by a management company of image forming apparatus  300 , through communication I/F  450 . Then, a serviceperson can immediately replace cleaning blade  26 Y. 
     As described above, when the developing bias voltage value is less than a predetermined value, image forming apparatus  300  in the first embodiment determines that the cleaning blade is still usable. Image forming apparatus  300  may therefore determine that the cleaning blade is still usable even after the assumed number of printable pages (predetermined value Nth) for which the cleaning blade is used is reached. In other words, image forming apparatus  300  can improve the precision in predicting the timing for replacing the cleaning blade or the unit including the cleaning blade, as compared with the conventional image forming apparatus. As a result of this, these components of the image forming apparatus can be used for a longer period as compared with the conventional image forming apparatus. 
     Toner Density 
     In the above-described example, determination unit  520  is configured to perform the determination process based on a developing bias voltage value regarded as a toner charge amount. In another aspect, determination unit  520  may perform the determination process based on the density of a patch image detected by density sensor  4 . 
     As described above, density C of the patch image is calculated by determining: potential difference ΔV/toner charge amount Vt. In the image stabilization process, the developing bias voltage value is fixed, and therefore, potential difference ΔV is constant. In this case, density C of the toner image is inversely proportional to toner charge amount Vt. Determination unit  520  therefore determines that toner charge amount Vt is smaller as the density of the patch image detected by density sensor  4  in the image stabilization process is higher. 
       FIG. 9  is a flowchart showing the determination process in step S 720  in  FIG. 7  that is performed using the density of the patch image. In  FIG. 9 , the same step as the step described with reference to  FIG. 8  is denoted by the same reference character, and the description of the step is not repeated. 
     In step S 910 , CPU  410  functions as determination unit  520  to determine whether the density of the patch image acquired from density sensor  4  is a predetermined density or less. When the density of the patch image is the predetermined density or less (YES in step S 910 ), CPU  410  causes operation panel  44  to display an instruction to replace cleaning blade  26 Y (S 830 ). In contrast, when the density of the patch image is higher than the predetermined density (NO in step S 910 ), CPU  410  ends the series of steps. 
     The predetermined density may be set to a density determined in advance, or set to a density that is calculated from the relational expression of the density and the toner charge amount and corresponds to count value  432 . 
     Upper Limit of Usable Period 
     In the above-described example, it is determined that cleaning blade  26 Y is usable as long as the developing bias voltage value is not a predetermined value or more, or as long as the density of the patch image is not a predetermined density or less. In the case, however, where unit-by-unit replacement of replacing a unit including cleaning blade  26 Y and another component(s) (photoconductor  3 Y for example) with a new unit is performed and the usable period of the other component has expired, cleaning blade  26 Y should also be replaced together with the other component. Moreover, because cleaning blade  26 Y is degraded due to wear resultant from friction against photoconductor  3 Y for example, cleaning blade  26 Y is not permanently usable. In view of this, according to an aspect, when the count value reaches its upper limit value at which cleaning blade  26 Y should be replaced, CPU  410  informs a user of an instruction to replace cleaning blade  26 Y or a unit including cleaning blade  26 Y. 
       FIG. 10  is a flowchart showing the determination process in step S 720  in  FIG. 7  in the case where an upper limit of the usable period of a component is defined. In  FIG. 10 , the same step as the step described with reference to  FIG. 8  is denoted by the same reference character, and the description of the step is not repeated. 
     In step S 820 , when the acquired developing bias voltage value is less than predetermined value Vth (NO in step S 820 ), CPU  410  proceeds to step S 1010 . 
     In step S 1010 , CPU  410  functions as determination unit  520  to determine whether count value  432  for yellow is the upper limit value or more. When count value  432  is the upper limit value or more (YES in step S 1010 ), CPU  410  causes operation panel  44  to display an instruction to replace cleaning blade  26 Y or a unit including cleaning blade  26 Y (S 830 ). In contrast, when count value  432  is less than the upper limit value (NO in step S 1010 ), CPU  410  ends the series of steps. 
     The upper limit value may be set to a predetermined value, or set so that the upper limit value varies depending on the condition of any component other than cleaning blade  26 Y included in the unit. 
     Inform Remaining Usable Period 
     In  FIGS. 8 to 10 , CPU  410  is configured to end the series of steps when it determines that cleaning blade  26 Y is still usable. In another aspect, CPU  410  may estimate the remaining usable period of cleaning blade  26 Y and inform a user of the estimated remaining usable period. 
       FIG. 11  is a flowchart showing the determination process in step S 720  in  FIG. 7  in the case where a user is informed of the remaining usable period of a component. In  FIG. 11 , the same step as the step described with reference to  FIG. 8  is denoted by the same reference character, and the description of the step is not repeated. 
     In step S 1010 , when count value  432  for yellow is less than the upper limit value (NO in step S 1010 ), CPU  410  proceeds to step S 1110 . 
     In step S 1110 , CPU  410  calculates the difference between the acquired developing bias voltage value and predetermined value Vth. CPU  410  further calculates the remaining usable period of cleaning blade  26 Y that is determined depending on the difference. More specifically, CPU  410  calculates the remaining usable period so that the remaining usable period is longer as the calculated difference is larger. 
     In step S 820 , CPU  410  determines that the developing bias voltage value is less than predetermined value Vth. In other words, in step S 1110 , CPU  410  determines that the usable period of cleaning blade  26 Y is longer as the developing bias voltage value (toner charge amount proportional to the developing bias voltage value) is smaller. 
     For example, CPU  410  calculates the remaining usable period based on the relation between the difference and the remaining usable period that is stored in nonvolatile memory  430 . CPU  410  causes operation panel  44  to display the calculated remaining usable period. 
     According to the foregoing, image forming apparatus  300  informs a user of the remaining usable period of cleaning blade  26 Y or a unit including cleaning blade  26 Y. The user can then recognize the remaining usable period and prepare cleaning blade Y or a unit to be replaced with the current cleaning blade or unit, and can therefore replace it smoothly. 
     CPU  410  may transmit the calculated remaining usable period to an external apparatus owned by a management company of image forming apparatus  300 , through communication I/F  450 . A serviceperson can thus perform efficient replacement. 
     Second Embodiment 
     In an image forming apparatus  300  according to a second embodiment, developing device  22  and cleaning blade  26  are configured to be replaceable independently of each other. For example, in image forming apparatus  300 , cleaning blade  26  may be configured to be separately replaceable or a photoconductor unit including cleaning blade  26  and photoconductor  3  may be replaceable with another photoconductor unit. 
     Related Art 
       FIG. 12  is a diagram for illustrating a process for replacing a photoconductor unit in an image forming apparatus according to a related art. As shown in  FIG. 12 , the usable period of the developing device is longer than the usable period of the photoconductor unit. A plurality of photoconductor units are therefore used while one developing device is used. The image forming apparatus according to the related art sets respective usable periods (total number of printed pages, for example) of a plurality of photoconductor units to the same period. 
     As shown in  FIG. 12 , however, the toner charge amount gradually decreases with passage of time. As described above, the ability of the cleaning blade is higher as the toner charge amount is smaller. Therefore, the usable period of a photoconductor unit to be replaced later should be set longer than the usable period of a photoconductor unit to be replaced earlier. In view of this, image forming apparatus  300  according to the second embodiment sets the usable period of the photoconductor unit in the manner shown in  FIG. 13 . 
     Overview 
       FIG. 13  is a diagram for illustrating a process for replacing the photoconductor unit in image forming apparatus  300  according to the second embodiment. With reference to  FIG. 13 , image forming apparatus  300  according to the second embodiment sets the usable period (total number of printed pages) depending on the number of photoconductor units replaced for the same developing device. More specifically, image forming apparatus  300  sets the usable period longer as the number of photoconductor units replaced for the same developing device is larger. 
     According to the foregoing, the photoconductor unit of image forming apparatus  300  according to the second embodiment is usable for a longer period than the photoconductor unit of the image forming apparatus according to the related art. 
     In the case where cleaning blade  26 Y is configured to be replaceable independently of developing device  22 Y and photoconductor  3 Y, image forming apparatus  300  sets the usable period of cleaning blade  26 Y longer as the number of cleaning blades  26 Y replaced for the same developing device  22 Y is larger. 
     For cleaning blade  27  and cleaning blade  29  as well, image forming apparatus  300  sets the usable period of these cleaning blades longer as the number of cleaning blades replaced for the same developing device  22  is larger. 
     Process Flow 
       FIG. 14  is a flowchart showing a process for replacing a photoconductor unit including cleaning blade  26 Y. In step S 1405 , CPU  410  detects that a photoconductor unit including developing device  22 Y and cleaning blade  26 Y has been replaced. For example, based on information that is input from operation panel  44  operated by a user, CPU  410  detects that these components have been replaced. In step S 1410 , in response to the replacement of the photoconductor unit (cleaning blade  26 Y included in the photoconductor unit), CPU  410  functions as update unit  530  to initialize count value  432  for yellow. 
     In step S 1415 , CPU  410  functions as determination unit  520  to determine whether count value  432  has reached a first threshold value or not. When count value  432  has reached the first threshold value (YES in step S 1415 ), CPU  410  proceeds to step S 1420 . In contrast, when count value  432  has not reached the first threshold value (NO in step S 1415 ), CPU  410  returns to step S 1415 . 
     In step S 1420 , CPU  410  causes operation panel  44  to display an instruction to replace the photoconductor unit. Following the instruction, a user replaces the photoconductor unit and inputs, to operation panel  44 , the fact that the replacement is completed. Based on the information that is input from operation panel  44 , CPU  410  detects that the photoconductor unit has been replaced. In step S 1425 , CPU  410  initializes count value  432 , in response to the replacement of the photoconductor unit. 
     In step S 1430 , CPU  410  functions as determination unit  520  to determine whether count value  432  has reached a second threshold value or not. The second threshold value is set to a value larger than the first threshold value. When count value  432  has reached the second threshold value (YES in step S 1430 ), CPU  410  proceeds to step S 1435 . In contrast, when count value  432  has not reached the second threshold value (NO in step S 1430 ), CPU  410  returns to step S 1430 . 
     In step S 1435 , CPU  410  detects that the photoconductor unit has been replaced. In step S 1440 , in response to the replacement of the photoconductor unit, CPU  410  initializes count value  432 . 
     In step S 1445 , CPU  410  functions as determination unit  520  to determine whether count value  432  has reached a third threshold value or not. The third threshold value is set to a value larger than the second threshold value. When count value  432  has reached the third threshold value (YES in step S 1445 ), CPU  410  proceeds to step S 1450 . When count value  432  has not reached the third threshold value (NO in step S 1445 ), CPU  410  returns to step S 1445 . 
     In step S 1450 , CPU  410  determines whether the time for replacement of developing device  22 Y has been reached or not. For example, when the total number of printed pages by means of developing device  22 Y has reached a predetermined number of pages, CPU  410  determines that the time for replacement of developing device  22 Y has been reached. When the time for replacement of developing device  22 Y has been reached (YES in step S 1450 ), CPU  410  returns to step S 1405 . In contrast, when the time for replacement of developing device  22 Y has not been reached (NO in step S 1450 ), CPU  410  returns to step S 1445 . 
     In the example shown in  FIG. 14 , three cleaning blades  26 Y are to be used for one developing device  22 Y. The number of cleaning blades, however, is not limited to three, as long as a plurality of cleaning blades  26  may be used for one developing device  22 . 
     According to the foregoing, image forming apparatus  300  in the second embodiment sets the usable period of the cleaning blade longer as the number of cleaning blades replaced for the same developing device is larger. Image forming apparatus  300  in the second embodiment can therefore set the usable period of the cleaning blade based on the toner charge amount with higher accuracy than the conventional image forming apparatus. Therefore, the cleaning blade and a unit including the cleaning blade in this image forming apparatus  300  are usable for a longer period as compared with those in the conventional image forming apparatus. 
     Image forming apparatus  300  is also configured so that the last photoconductor unit (cleaning blade included in the last photoconductor unit) for the same developing device is replaced together with the developing device (S 1445 , S 1450 ). Thus, this image forming apparatus  300  can set the usable period of the cleaning blade through a simple process. Moreover, image forming apparatus  300  can prevent the toner charge amount to vary significantly while the same cleaning blade  26  is used. As a result of this, this image forming apparatus  300  can suppress generation of image defects due to significant deterioration of the ability of cleaning blade  26  in operation. 
     Determine Usable Period in Consideration of Average Coverage Rate 
       FIG. 15A  shows usable periods of photoconductor units in the case where the average coverage rate is low.  FIG. 15B  shows usable periods of photoconductor units in the case where the average coverage rate is high. 
     In the following, a description is given of a process for setting the usable period of cleaning blade  26  in consideration of the average coverage rate. When average coverage rate  434  of yellow is low, the speed at which the toner stored in developing device  22 Y is consumed is slow. In other words, the time for which the toner stays in developing device  22 Y is long. 
     Developing device  22 Y contains carriers of a magnetic material such as ferrite. The toner is electrically charged through friction against carriers while the toner stays in developing device  22 Y. When average coverage rate  434  is low, the rate at which the toner charge amount decreases with increase of the count value is low. 
     In contrast, when average coverage rate  434  of yellow is high, the speed at which the toner stored in developing device  22 Y is consumed is high. In other words, the time for which the toner stays in developing device  22 Y is short. Therefore, when average coverage rate  434  is high, the rate at which the toner charge amount decreases with increase of the count value is high. 
     Therefore, as average coverage rate  434  of yellow is higher, image forming apparatus  300  may set the usable period of cleaning blade  26 Y (photoconductor unit including cleaning blade  26 Y and photoconductor  3 Y) longer. For example, image forming apparatus  300  sets the first, second, and third threshold values described in connection with  FIG. 14  to larger values as average coverage rate  434  of yellow is higher. 
     According to the foregoing, image forming apparatus  300  can further improve the precision in predicting the timing for replacing the cleaning blade, in consideration of the average coverage rate that influences the toner charge amount. As a result of this, the cleaning blade or the unit including the cleaning blade is usable for a longer period as compared with the conventional image forming apparatus. 
     Determine Usable Period in Consideration of Average Number of Printed Pages 
     In another aspect, image forming apparatus  300  may be configured to set the usable period of the cleaning blade based on average number of printed pages  436 . 
     When average number of printed pages  436  for yellow is small, the speed at which the toner stored in developing device  22 Y is consumed is slow. In other words, the time for which the toner stays in developing device  22 Y is long. In contrast, when average number of printed pages  436  for yellow is large, the speed at which the toner stored in developing device  22 Y is consumed is high. In other words, the time for which the toner stays in developing device  22 Y is short. 
     Therefore, image forming apparatus  300  may set the first, second, and third threshold values in  FIG. 14  so that the usable period of cleaning blade  26 Y (photoconductor unit including cleaning blade  26 Y and photoconductor unit  3 Y) is longer as average number of printed pages  436  for yellow is larger. 
     Image forming apparatus  300  configured in this manner can further improve the precision in predicting the timing for replacing the cleaning blade, in consideration of the average number of printed pages that influences the toner charge amount. As a result of this, the cleaning blade or a unit including the cleaning blade is usable for a longer period as compared with the conventional image forming apparatus. 
     Determine Usable Period in Consideration of Average Humidity 
     The toner charge amount varies depending on the humidity. More specifically, as the humidity is higher, the amount by which the toner charge amount decreases with passage of time increases. In view of this, in an aspect, image forming apparatus  300  may set the first, second, and third threshold values so that the usable period of the cleaning blade is longer as average humidity  438  is higher. 
     Image forming apparatus  300  configured in this manner can further improve the precision in predicting the timing for replacing the cleaning blade, in consideration of the humidity that influences the toner charge amount. As a result of this, the cleaning blade or a unit including the cleaning blade is usable for a longer period as compared with the conventional image forming apparatus. 
     Pressing Force for Cleaning Blade 
       FIG. 16  is a diagram for illustrating the process for setting the pressing force for cleaning blade  26  in image forming apparatus  300  according to the second embodiment. In  FIG. 16 , the horizontal axis represents the count value, the left vertical axis represents the toner charge amount, and the right vertical axis represents the pressure (pressing force) applied by a pressing mechanism (not shown) for pressing cleaning blade  26  against photoconductor  3 . 
     As the toner charge amount is larger, the toner adheres to photoconductor  3  more strongly. Generally, the pressing force for cleaning blade  26  is set strong so that toner can be collected sufficiently even when the toner charge amount is large. The pressing force for cleaning blade  26  is set to the same force even when the number of cleaning blades  26  replaced for the same developing device increases. However, the toner charge amount decreases gradually with passage of time. In view of this, image forming apparatus  300  controls the pressing mechanism so that the pressing force for cleaning blade  26  is smaller as the number of photoconductor units (cleaning blades  26  included in the photoconductor units) replaced for the same developing device is larger. 
     Image forming apparatus  300  configured in this manner can suppress deterioration of cleaning blade  26  due to wear, by making the pressing force for cleaning blade  26  smaller. This image forming apparatus  300  can therefore further increase the usable period of cleaning blade  26 . 
     Image forming apparatus  300  may also control the pressing mechanism so that the pressing force for cleaning blade  27  and cleaning blade  29  is smaller as the number of cleaning blades replaced for the same developing device is larger. 
     Third Embodiment 
     In the foregoing example, image forming apparatus  300  is configured to indirectly acquire the toner charge amount by acquiring the developing bias voltage value or the patch image density, for example. An image forming apparatus  1700  according to a third embodiment directly acquires this toner charge amount. 
       FIG. 17  is a diagram for illustrating an example of a configuration of image forming apparatus  1700  according to the third embodiment. Image forming apparatus  1700  differs from image forming apparatus  300  described above with reference to  FIG. 3  in that the former includes a charge amount measurement device  1710 . 
     Charge amount measurement device  1710  is connected to each of developing devices  22 Y,  22 M,  22 C,  22 K and configured to be capable of measuring the charge amount of toner stored in each of developing devices  22 Y,  22 M,  22 C,  22 K. Charge amount measurement device  1710  measures the charge amount of toner based on the blow-off method, the suction method, the DC field method, or another known method for measuring the charge amount of powder. Charge amount measurement device  1710  measures the toner charge amount of each color and outputs the result of the measurement to CPU  410 . 
       FIG. 18  shows an example of a functional configuration of CPU  410  according to the third embodiment. As shown in  FIG. 18 , acquisition unit  510  acquires the toner charge amount of yellow from charge amount measurement device  1710 , and outputs the result of the acquisition to determination unit  520 . When the acquired toner charge amount is a predetermined charge amount or more, determination unit  520  determines that the usable period of cleaning blade  26  has expired. The predetermined charge amount may be stored in nonvolatile memory  430 , or calculated from a relational expression representing the relation between count value  432  and the toner charge amount. 
     Image forming apparatus  1700  in the third embodiment configured in the above-described manner can acquire the toner charge amount accurately. In other words, image forming apparatus  1700  can determine the timing for replacing the cleaning blade more accurately. Therefore, the cleaning blade of image forming apparatus  1700  is usable for a still longer period. Moreover, image forming apparatus  1700  can suppress occurrence of image defects due to the fact that an estimated toner charge amount is larger than the actual charge amount. 
     Fourth Embodiment 
     In the above-described example, the image forming apparatus is configured to determine whether the usable period of cleaning blade  26 Y has expired or not. In another aspect, a server apparatus configured to be capable of communicating with the image forming apparatus may be configured to make this determination 
       FIG. 19  shows an example of a configuration of an image forming system  1900  according to a fourth embodiment. Image forming system  1900  includes an image forming apparatus  300  and a server apparatus  1910 . Image forming apparatus  300  and server apparatus  1910  are configured to be capable of communicating with each other. Server apparatus  1910  may be an apparatus owned by a manager of image forming apparatus  300 . 
     Image forming apparatus  300  transmits the result of acquisition of the toner charge amount acquired by acquisition unit  510  to server apparatus  1910  through communication I/F  450 . The result of acquisition of the toner charge amount includes the developing bias voltage value for developing device  22 Y, the density of a yellow patch image, count value  432  of yellow, average coverage rate  434 , average number of printed pages  436 , and average humidity  438 , for example 
     Based on the result of acquisition regarding the toner charge amount acquired from image forming apparatus  300 , server apparatus  1910  transmits, to image forming apparatus  300 , information such as the result of the determination as to whether the usable period of cleaning blade  26 Y or a unit including cleaning blade  26 Y has expired or not, and the remaining usable period of cleaning blade  26 Y, for example. Image forming apparatus  300  causes operation panel  44  to display the information received from server apparatus  1910 . 
     Image forming system  1900  configured in this manner can more accurately determine the timing for replacing cleaning blade  26 Y based on the yellow toner charge amount. Accordingly, cleaning blade  26 Y or a unit including cleaning blade  26 Y used for image forming system  1900  is usable for a longer period than the one in the conventional image forming system. 
     Regarding the first to fourth embodiments, the above description is given of the usable period of cleaning blade  26 Y for the sake of facilitating understanding of the description. The processes for determining the usable periods of cleaning blades  26  for other colors (magenta, cyan, black) are also performed in a similar manner 
     For both cleaning blades  27  and  29  as well, the process for determining the usable period of the cleaning blade is performed using the toner charge amount acquired indirectly or directly by acquisition unit  510 , similarly to the process for cleaning blade  26 . It should be noted that CPU  410  performs the process for determining the usable period of cleaning blades  27  and  29  using the largest toner charge amount among respective toner charge amounts of the colors, in order to suppress occurrence of image defects. 
     According to the above description, the processes are implemented by CPU  410  or server apparatus  1910 . The processes, however, are not limited to this. The processes may be implemented by at least one semiconductor integrated circuit such as processor, at least one ASIC (Application Specific Integrated Circuit), at least one DSP (Digital Signal Processor), at lest one FPGA (Field Programmable Gate Array), and/or another circuit having an arithmetic operation capability. 
     These circuits may perform the above-described processes by reading one or more instructions from at least one tangible readable medium. 
     Such a medium may take any form such as magnetic medium (hard disk, for example), optical medium (compact disc (CD), DVD, for example), any type of memory such as volatile memory or nonvolatile memory, for example. The medium, however, is not limited to these forms. 
     The volatile memory may include DRAM (Dynamic Random Access Memory) and SRAM (Static Random Access Memory). The nonvolatile memory may include ROM and NVRAM. The semiconductor memory may form, together with at least one processor, a part of a semiconductor circuit. 
     Although embodiments of the present invention have been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and not limitation, the scope of the present invention should be interpreted by terms of the appended claims.