Patent Publication Number: US-2020301338-A1

Title: Image forming apparatus

Description:
The entire disclosure of Japanese patent Application No. 2019-049784, filed on Mar. 18, 2019, is incorporated herein by reference in its entirety. 
     BACKGROUND 
     Technological Field 
     The present disclosure relates to an image forming apparatus, and more specifically, to lifetime prediction of a photoreceptor. 
     Description of the Related Art 
     In a photoreceptor of an image forming apparatus, image noise due to wear of a surface of the photoreceptor is a bottleneck that reduces the printable number of sheets of the photoreceptor. In recent years, in order to increase the printable number of sheets of the photoreceptor, a super-hard photoreceptor provided with an overcoat layer (hereinafter referred to as “OCL”) on an outermost surface of the photoreceptor is often employed. 
     By providing the OCL on the photoreceptor, it is possible to suppress wear of a surface layer of the photoreceptor and to increase the printable number of sheets several times that with the conventional photoreceptor. 
     On the other hand, when the OCL becomes thick, reproducibility of a thin line of a toner image might be deteriorated or a line width might increase. This is because, when the surface of the photoreceptor is negatively charged, positive charges inside the photoreceptor flow in a lateral direction. The thicker the OCL, the easier the positive charges flow laterally. In order to suppress a phenomenon that the positive charges flow laterally, the OCL needs to be thinner than the surface layer of the conventional photoreceptor. 
     The thin OCL layer means that the super-hard photoreceptor provided with the OCL has a smaller wear margin on the surface of the photoreceptor than that of the conventional photoreceptor. If the wear allowance is small, conventional film thickness measurement based on charging current detection cannot be applied. This is because the change in the charging current is too small because the wear of the surface layer is small. However, in order to estimate the remaining printable number of sheets of the photoreceptor, it is necessary to measure the film thickness of the surface layer of the photoreceptor. Therefore, a technology of estimating the film thickness of the super-hard photoreceptor provided with the OCL is required. 
     Regarding the estimation of the film thickness of the photoreceptor, for example, JP 2017-049278 A (JP 2017-049278 A) discloses an image forming apparatus in which “a controller includes an arithmetic device which stores relationship information regarding a relationship between a film thickness of a first photoreceptor and a charging current by using an initial film thickness of the first photoreceptor stored in a memory, a change in film thickness of the first photoreceptor calculated by a calculator, and a change in charging current detected by a charging current detecting circuit, and calculates a film thickness of a second photoreceptor based on the relationship information when the first photoreceptor is detached from a main body and the second photoreceptor other than the first photoreceptor is mounted on the main body” (refer to “Abstract”). 
     Other technologies relating to estimation of a degree of deterioration and film thickness of the photoreceptor are disclosed in, for example, JP 2007-187734 A, JP 2005-0 17970 A, JP 2017-207618 A, JP 2013-120261 A, JP 02-235073 A, and JP 2004-354485 A. 
     According to the technologies disclosed in JP 2017-049278 A, JP 2007-187734 A, JP 2005-017970 A, JP 2017-207618 A, JP 2013-120261 A, JP 02-235073 A, and JP 2004-354485 A, it is not possible to appropriately estimate the film thickness of a super-hard photoreceptor in which the thickness of the OCL vanes at the time of manufacture. Therefore, there is a need for a technology for appropriately estimating the film thickness of the super-hard photoreceptor that causes variation in OCL thickness at the time of manufacture. 
     SUMMARY 
     The present disclosure is achieved in view of the background as described above, and an object in one aspect thereof is to provide a technology for appropriately estimating the film thickness and life of the super-hard photoreceptor in which variation occur in the thickness of the OCL at the time of manufacture. 
     To achieve the abovementioned object, according to an aspect of the present invention, an image forming apparatus reflecting one aspect of the present invention comprises: a photoreceptor that forms a toner image on a surface of the photoreceptor, a charger that charges the surface of the photoreceptor, an exposure device that exposes the surface of the photoreceptor; a developer that supplies toner to the surface of the photoreceptor, an intermediate transfer belt for transferring the toner image formed on the surface of the photoreceptor, a density sensor that detects a density of the toner image on the intermediate transfer belt; and a controller that controls the image forming apparatus, wherein the controller changes a light amount of the exposure device a plurality of times to expose a plurality of parts on the surface of the photoreceptor with different light amounts, allows the developer to form toner images for inspection for respective parts on the surface of the photoreceptor exposed with the different light amounts, detects densities of the toner images for inspection transferred to the intermediate transfer belt based on an output from the density sensor, and compares the densities of the respective toner images for inspection to estimate a life of the photoreceptor based on a change amount of the densities of the toner images for inspection caused by a change in light amount of the exposure device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The objects, advantages, aspects, 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 winch 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 view illustrating a configuration example of an image forming apparatus according to one embodiment; 
         FIG. 2  is a view illustrating an example of a relationship between an OCL film thickness and a line width of a toner image; 
         FIG. 3  is a view illustrating an example of a flow in a lateral direction of positive charges of a photoreceptor; 
         FIG. 4  is a view illustrating an example of a method of estimating a life of a conventional photoreceptor; 
         FIG. 5  is a view illustrating an example of characteristics of the conventional photoreceptor and a super-hard OCL photoreceptor provided with the OCL; 
         FIG. 6  is a view illustrating an example of life prediction using an intermediate transfer belt; 
         FIG. 7  is a view illustrating an example of a change amount of a density of the toner images of the photoreceptor when a PH light amount is changed; 
         FIG. 8  is a view illustrating an example of a relationship between the OCL film thickness, the change amount of the density of the toner images when the PH light amount is changed, and the life of the photoreceptor; 
         FIG. 9  is a view illustrating an example of an estimated life of the photoreceptor; 
         FIG. 10  is a view illustrating an example of a relationship between a rotational speed of the photoreceptor and an inclination (change amount of the density of the toner images when the PH light amount is changed); 
         FIG. 11  is a view illustrating an example of a process of the life prediction of the photoreceptor; 
         FIG. 12  is a view illustrating an example of a state in which the photoreceptor charged with different voltages is exposed; 
         FIG. 13  is a view illustrating an example of the change amount of the density of the toner images on the photoreceptor when the charging potential of the photoreceptor is changed; 
         FIG. 14  is a view illustrating an example of a relationship between the OCL film thickness, the change amount of the density of the toner images when the charging potential of the photoreceptor is changed, and the life of the photoreceptor; 
         FIG. 15  is a view illustrating an example of the process of the life prediction of the photoreceptor; 
         FIG. 16  is a view illustrating an example of an effect of the change in volume resistance of the OCL for each environment; 
         FIG. 17  is a view illustrating an example of a correlation among the OCL film thickness, the inclination for each environment, and the life of the photoreceptor; and 
         FIG. 18  is a view illustrating an example of a method of estimating the life of the photoreceptor according to one 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 description, the same components are assigned with the same reference numerals. Their names and functions are also the same. Therefore, detailed description thereof is not repeated. 
     First Embodiment 
     First, an application configuration of an image forming apparatus  100  according to this embodiment is described. Hereinafter, as a typical example, the image forming apparatus  100  mounted as a multi-functional peripheral (MFP) is described. The image forming apparatus  100  is, for example, a color image forming apparatus, but an application target of the technological thought according to this embodiment is not limited to the color image forming apparatus, and the technological thought is also applicable to a monochrome image forming apparatus. 
     In the following description, a remaining film thickness of a surface layer of a photoreceptor or the remaining printable number of sheets of the photoreceptor is collectively referred to as a “life”. Estimation of the film thickness of the photoreceptor or estimation of the remaining printable number of sheets of the photoreceptor is collectively referred to as “life prediction”. 
       FIG. 1  is a view illustrating a configuration example of the image forming apparatus  100  according to this embodiment. With reference to  FIG. 1 , the image forming apparatus  100  includes a print engine  110 , a document reader  120 , and a discharge tray  130 . 
     The print engine  110  includes imaging units  10 C,  10 M,  10 Y, and  10 K (hereinafter, sometimes collectively referred to as the “imaging units  10 ”) that form toner images of cyan (C), magenta (M), yellow (Y), and key plate (K), an intermediate transfer belt  12 , intermediate transfer body driving rollers  14  and  16 , a belt cleaner  18 , transfer rollers  20  and  21 , a fixer  22 , a paper feeder  30 , a delivery roller  32 , conveyance rollers  34  and  36 , a controller  50 , a storage  51 , and a density sensor  55 . The imaging unit  10  includes a photoreceptor  1 , a charger  2 , an exposure device  3 , and a developer  4  ( 4 C,  4 M,  4 Y, or  4 K corresponding to a color of the toner image formed by the corresponding imaging unit  10 ), a cleaner  5 , and an intermediate transfer body contact roller  6 . The document reader  120  includes an image scanner  122 , a document feed table  124 , an automatic document feeder  126 , and a document discharge table  128 . 
     The print engine  110  performs a printing process on a medium  40  in the paper feeder  30 . The delivery roller  32  conveys the medium  40  from the paper feeder  30 . Furthermore, the conveyance rollers  34  and  36  convey the medium  40  to the transfer rollers  20  and  21 . The transfer rollers  20  and  21  transfer the toner image to the medium  40 . The fixer  22  performs a fixing process on the medium  40 . Finally, the medium  40  is discharged to the discharge tray  130 . 
     Each imaging unit  10  and the intermediate transfer belt  12  form the toner image to be transferred to the medium  40 . The charger  2  uniformly charges a surface of the photoreceptor  1 . The exposure device  3  exposes the surface of the photoreceptor  1  according to a designated image pattern, thereby forming an electrostatic latent image on the surface by laser writing or the like. The developer  4  develops the electrostatic latent image formed on the photoreceptor  1  being an image carrier as the toner image. Note that, the surface layer of the photoreceptor  1  is provided with an OCL. 
     The toner image formed on the surface of the photoreceptor  1  is transferred to the intermediate transfer belt  12  by the intermediate transfer body contact roller  6 . On the intermediate transfer belt  12 , the toner images are sequentially transferred from the respective photoreceptors  1 , and the toner images of four colors are superimposed. The superimposed toner image is transferred from the intermediate transfer belt  12  to the medium  40  by the transfer rollers  20  and  21 . The density sensor  55  detects a density of the toner image on the intermediate transfer belt  12 . In one aspect, an image density control (IDC) sensor may be used as the density sensor  55 . In one aspect, the density sensor  55  may detect the density of the toner image on the surface of each photoreceptor  1  of the imaging unit  10 . 
     The document reader  120  reads a document and outputs a read result as an input image to the print engine  110 . The image scanner  122  scans the document arranged on a platen glass. The automatic document feeder  126  continuously scans the documents arranged on the document feed table  124 . The documents arranged on the document feed table  124  are fed one by one by a delivery roller (not illustrated), and sequentially scanned by the image scanner  122  or an image sensor arranged in the automatic document feeder  126 . The scanned document is discharged to the document discharge table  128 . 
     The controller  50  controls an entire image forming apparatus  100 . The storage  51  stores firmware and various settings of the image forming apparatus  100 . The controller  50  refers to necessary data and programs from the storage  51 . 
       FIG. 2  is a view illustrating an example of a relationship between an OCL film thickness and a line width of the toner image. A graph  201  is a graph illustrating the relationship between the OCL film thickness and the line width of the toner image. As is understood from the graph  201 , as the OCL film thickness increases, the line width of the toner image increases. 
     The OCL is provided on the surface layer of photoreceptors  1 A,  1 B, and  1 C. Among the photoreceptors  1 A,  1 B, and  1 C, the OCL of the photoreceptor  1 A has a minimum film thickness, the OCL of the photoreceptor  1 B has a second minimum film thickness, and the OCL of the photoreceptor  1 C has a maximum film thickness. The line widths in a case where the same toner image is formed on the photoreceptors  1 A,  1 B, and  1 C are, for example, indicated by points  202 A,  202 B, and  202 C, respectively. 
       FIG. 3  is a view illustrating an example of a flow in a lateral direction of positive charges of the photoreceptor  1 . The photoreceptor  1  includes an OCL  301  in the surface layer, and a charge transport layer (CTL)  302  inside the photoreceptor  1 . The exposure device  3  negatively charges the surface layer of the photoreceptor  1  by PH light  302 . By doing so, negative charges  303  cover the surface layer of the photoreceptor  1 . At that time, positive charges  304  inside the photoreceptor  1  gather on the surface layer, but slightly flow in the lateral direction. An amount of the positive charges  304  that flow laterally is proportional to a thickness of the OCL  301 . 
     In a case where the developer  4  supplies toner to the photoreceptor  1  in a state in which the positive charges  304  flow laterally, a phenomenon occurs that the toner supplied to the photoreceptor  1  is attracted by the positive charges  304  that flow laterally to laterally spread on the photoreceptor  1 , so that the line width of the toner image increases. Therefore, it is desirable that the OCL film thickness is small to some extent. However, there is a case where it becomes difficult to obtain the life of the photoreceptor  1  as the OCL film thickness decreases. 
     Next, with reference to  FIGS. 4 and 5 , a method of estimating the life of a conventional photoreceptor is described, then, a problem in a case where the method of estimating the life of the conventional photoreceptor is applied to the photoreceptor  1  provided with the OCL is described. 
     Note that, in the following description, for comparison, a conventional image forming apparatus is referred to as an “image forming apparatus A” in contrast to the image forming apparatus  100  according to this embodiment. The image forming apparatus  100  according to this embodiment includes a super-hard “photoreceptor  1 ” provided with the OCL, whereas the image forming apparatus A includes a conventional “photoreceptor B” that is not super hard with large wear allowance. 
     As a method of obtaining the life of the conventional photoreceptor B, there is a method of examining a charging current of the photoreceptor B. The charging current of the photoreceptor B changes in proportion to wear of the surface layer of the photoreceptor B. Therefore, the image forming apparatus A estimates a wear amount or the remaining printable number of sheets of the photoreceptor B by detecting the change in the charging current of the photoreceptor B. 
       FIG. 4  is a view illustrating an example of the method of estimating the life of the conventional photoreceptor B. With reference to  FIG. 4 , life prediction using the charging current of the photoreceptor B is described. A graph  401  illustrates life prediction of the photoreceptor B obtained by measuring the charging current each time the image forming apparatus A prints a certain number of media. A graph  402  illustrates life prediction of the conventional photoreceptor B in a case where this is assumed to be continuously used in the most severe situation assumed. 
     Points  403 A and  403 B indicate the wear allowance (film thickness of the surface layer of the photoreceptor B) when the photoreceptor B is new (when the number of printed sheets is 0) in the graphs  401  and  402 , respectively. A difference in the wear allowances between the points  403 A and  403 B is variation in the wear allowance (film thickness) when the photoreceptor B is manufactured. Points  404 A and  404 B indicate a state in which the wear allowance of the photoreceptor B does not remain, that is, the photoreceptor B is used to its limit. 
     In the graph  402 , the photoreceptor B is used in the most severe situation (the same print setting). Therefore, in the graph  402 , the wear allowance decreases linearly as the number of printed sheets increases. On the other hand, in the graph  401 , the photoreceptor B is used under different conditions (paper size, toner usage amount and the like) every time, so that the wear allowance does not decrease linearly in proportion to the number of printed sheets. 
     As described above, conventionally, the image forming apparatus A may set a minimum guaranteed value (the number of printed sheets at the point  404 B) of the graph  402  or predict the life of the photoreceptor B by the charging current each time a certain number of sheets are printed, thereby estimating the remaining wear allowance or the remaining printable number of sheets of the photoreceptor B. 
       FIG. 5  is a view illustrating an example of characteristics of the conventional photoreceptor B and the super-hard OCL photoreceptor  1  provided with the OCL. The above-described “minimum guaranteed value” and “measurement using the charging current” are suitable for the life prediction of the conventional photoreceptor B, but are not suitable for the life prediction of the super-hard photoreceptor  1  provided with the OCL. A reason therefor is described with reference to  FIG. 5 . 
     As is understood from a table  500 , the wear allowance of the conventional photoreceptor B is larger than that of the super-hard photoreceptor  1  provided with the OCL. That is, the photoreceptor B has a large change amount of the wear allowance between when the number of printed sheets is 0 (wear allowance=30 μm) and when the maximum printable number of sheets are printed (wear allowance=0 μm). Since the charging current of the photoreceptor B depends on the thickness of the wear allowance, the charging current of the photoreceptor B also changes significantly according to the change in the wear allowance. Therefore, the image forming apparatus A may predict the life of the photoreceptor B by detecting the change in the charging current of the photoreceptor B. 
     On the other hand, the photoreceptor  1  has a small change amount of the wear allowance between when the number of printed sheets is 0 (wear allowance=3 μm) and when the maximum printable number of sheets are printed (wear allowance=0 μm). Therefore, the change in the charging current of the photoreceptor  1  also decreases. Therefore, in the image forming apparatus  100 , even when the change in the charging current of the photoreceptor  1  is detected, the change amount of the charging current is too small, so that the life of the photoreceptor  1  cannot be predicted. 
     In the photoreceptor  1 , as compared with the photoreceptor B, an effect of the variation in the film thickness (wear allowance) at the time of manufacture and a measurement error of the wear allowance are significantly large. The photoreceptor B has the variation in the film thickness at the time of manufacture of “±1 μm”. The life per wear allowance “1 μm” of the photoreceptor B is “150/30=5 (kp/μm)”. Therefore, in the photoreceptor B, a difference of “1*5=±2.5 (kp)” might occur in the life due to individual difference. 
     On the other hand, in the photoreceptor  1 , the variation in the film thickness at the time of manufacture is “±0.5 μm”. The life per wear allowance “1 μm” of the photoreceptor  1  is “450/3=150 (kp/μm)”. Therefore, in the photoreceptor  1 , a difference of “0.5*150=±75 (kp)” might occur in the life due to individual difference. 
     As is understood from the above-described comparison, the life of the photoreceptor  1  varies significantly due to the variation at the time of manufacture. Since the printable number of sheets per wear allowance of “1 μm” is significantly larger than that of the photoreceptor B, the measurement error of the wear allowance due to the charging current has a significant effect. 
     From above, the super-hard photoreceptor  1  provided with the OCL has smaller wear allowance and a larger effect of the measurement error than those of the conventional photoreceptor B. Therefore, the life prediction of the photoreceptor  1  cannot be appropriately performed in the measurement of the wear allowance by the charging current. 
     In this embodiment, the image forming apparatus  100  utilizes a property that the positive charges inside the photoreceptor  1  spread laterally in proportion to the OCL film thickness and the PH light by the exposure device  3  in order to predict the life of the photoreceptor  1 . 
     When a “light amount of the PH light” by the exposure device  3  changes, an “amount of the positive charges that laterally spread inside the photoreceptor  1 ” also changes. A change amount of the “amount of the positive charges that laterally spread inside the photoreceptor  1 ” based on the change in the “light amount of the PH light” varies depending on the “OCL film thickness”. 
     As described with reference to  FIG. 3 , the line width of the toner image increases as the positive charges inside the photoreceptor  1  spread laterally. Therefore, when the “light amount of the PH light” changes, the “amount of the positive charges that laterally spread inside the photoreceptor  1 ” also changes, and further, the “line width of the toner image” also changes. Utilizing this property, the image forming apparatus  100  may estimate the OCL film thickness by detecting the change amount of the “line width of the toner image” due to the change in the “light amount of the PH light”. 
     Next, with reference to  FIGS. 6 to 8 , the life prediction of the photoreceptor  1  utilizing the change in the light amount of the PH light is described.  FIG. 6  is a view illustrating an example of the life prediction using the intermediate transfer belt  12 . 
     In the example illustrated in  FIG. 6 , the image forming apparatus  100  transfers toner images  602 A,  602 B,  602 C, and  602 D (hereinafter, when they are collectively referred to, they are referred to as the “toner images  602 ”) that are toner images for inspection for examining the OCL film thickness on the intermediate transfer belt  12 . The toner image  602  is formed of thin lines with a fixed number of dots. With the toner images  602 A,  602 B,  602 C, and  602 D, the PH light amount at the time of exposure of the photoreceptor  1  is different. 
     First, a procedure of forming the toner image  602  by the image forming apparatus  100  is described. The image forming apparatus  100  forms a plurality of toner images on the photoreceptor  1  that is a target of the life prediction. At that time, the controller  50  changes the PH light amount of the exposure device  3  stepwise to expose different parts on the photoreceptor  1  with different PH light amounts. 
     Next, the controller  50  allows the developer  4  to supply the toner to respective parts exposed with the different PH light amounts. Note that, the controller  50  allows the developer  4  to form the same toner image formed of the thin lines with the fixed number of dots in the respective parts exposed with the different PH light amounts. 
     All the toner images formed in the respective parts exposed with the different PH light amounts are transferred to the intermediate transfer belt  12  to become the toner images  602 . The PH light amount increases in the order of the toner images  602 D,  602 C,  602 B, and  602 A. When the “PH light amount” is large, the “amount of the positive charges that spread laterally inside the photoreceptor  1 ” also increases, and as a result, the “line width of the toner image” also increases. Since the toner image  602  is assembly of the thin lines, when the “line width of the toner image  602 ” increases, the “density of the toner image  602 ” similarly increases. 
     The density sensor  55  detects the densities of the toner images  602  on the intermediate transfer belt  12 . The controller  50  may obtain “the change amount of the density of the toner images when the PH light amount is changed” by comparing the densities of the toner images  602 A,  602 B,  602 C, and  602 D. 
       FIG. 7  is a view illustrating an example of the change amount of the density of the toner images of the photoreceptor  1  when the PH light amount is changed. A graph  701  illustrates “the change amount of the density of the toner images when the PH light amount is changed” of the photoreceptor  1 C having a thin OCL. A graph  702  illustrates “the change amount of the density of the toner images when the PH light amount is changed” of the photoreceptor  1 D laving a thick OCL. 
     Points  703 A,  703 B,  703 C, and  703 D indicate the densities when the toner images  602 A,  602 B,  602 C, and  602 D are detected by the density sensor  55 , respectively. The graphs  701  and  702  are obtained based on the points  703 A,  703 B,  703 C, and  703 D. As is understood when comparing the graphs  701  and  702 , the thicker the OCL, the larger “the change amount of the density of the toner images when the PH light amount is changed”. 
     Note that, although the controller  50  changes the PH light amount in four steps and detects the densities of the toner images for respective PH light amounts in this embodiment, the number of times of detection is not limited to this. When the controller  50  changes the PH light amount in more steps and detects the densities of the toner images for the respective PH light amounts, detection accuracy is improved accordingly. The controller  50  may detect the densities of the toner images for the PH light amounts in at least two steps. 
       FIG. 8  is a view illustrating an example of a relationship between the OCL film thickness, the change amount of the density of the toner images when the PH light amount is changed, and the life of the photoreceptor  1 . A table  800  includes association information obtained by associating the OCL film thickness, an inclination, and the life of the photoreceptor  1 . The inclination is the inclination of the graphs  701  and  702  in  FIG. 7  and corresponds to “the change amount of the density of the toner images when the PH light amount is changed”. The storage  51  stores the table  800  including the association information with respect to several types of OCL film thicknesses. In the illustration in  FIG. 8 , the table  800  includes the association information with respect to three types of OCL film thicknesses, but may also include the association information with respect to more types of OCL film thicknesses. 
     The controller  50  compares the “change amount of the density of the toner images when the PH light amount is changed (hereinafter referred to as an “actual inclination”)” obtained by measurement by the density sensor  55  with the “inclination” for each OCL film thickness included in the table  800 , thereby predicting the life of the photoreceptor  1 . 
     In one aspect, the controller  50  may compare the measured “actual inclination” with the “inclination” for each OCL film thickness included in the table  800  and determine the “OCL film thickness and life” corresponding to the closest “inclination” as an estimated value of the life. In another aspect, the controller  50  may calculate the life corresponding to the “actual inclination” based on the measured “actual inclination” and the contents of the table  800 . For example, if “actual inclination=0.028”, the controller  50  may obtain an intermediate value between the “OCL film thicknesses” and that between the “lives” corresponding to “inclination=0.030” and “inclination=0.026”. 
     When the photoreceptor  1  is used to some extent, lubricant covers the surface layer of the photoreceptor  1 , so that surface resistance of the photoreceptor  1  might change. At the same time, surface roughness of the photoreceptor  1  might change to change the line width of the toner image. Due to these causes, accuracy of the life prediction of the photoreceptor  1  utilizing the toner image  602  might be deteriorated. Therefore, it is desirable that the controller  50  predicts the life of the photoreceptor  1  when the photoreceptor  1  is replaced (when the photoreceptor  1  is new). 
     In one aspect, the density sensor  55  may also detect the density of the toner image on the surface of the photoreceptor  1 . In this case, the density sensor  55  detects the density of the toner image on the surface of each photoreceptor  1  for each color. 
       FIG. 9  is a view illustrating an example of the estimated life of the photoreceptor  1 . A graph  901  illustrates the life prediction of the photoreceptor  1  in a case where this is assumed to be continuously used in a longest-lasting situation assumed. A graph  902  illustrates the life prediction of the photoreceptor  1  in a case where this is assumed to be continuously used in the most severe situation assumed. A graph  903  illustrates the life prediction of the photoreceptor  1  actually predicted by the method described with reference to  FIGS. 6 to 8 . 
     Points  904 A,  904 B, and  904 C indicate the wear allowance (OCL film thickness) when the photoreceptor  1  is new (when the number of printed sheets is 0) in the graphs  901 ,  902 , and  903 , respectively. The point  904 A indicates a thickness of the thickest OCL assumed, and the point  904 B indicates a thickness of the thinnest OCL assumed. The point  904 C is the OCL film thickness estimated by the measurement. Points  905 A,  905 B, and  905 C indicate a state in which the wear allowance of the photoreceptor  1  does not remain, that is, the photoreceptor  1  is used to its limit. 
     The controller  50  may keep a printing quality by predicting the life of the photoreceptor  1  based on the graph  902 . However, since a difference in the life (difference in the printable number of sheets) due to the difference in the OCL film thickness is large, in many cases, it is expected that the photoreceptor  1  is replaced even though this may still print sufficiently. 
     In order to improve this waste, in this embodiment, the controller  50  predicts the OCL film thickness of the photoreceptor  1  (wear allowance at the point  904 C) and the life of the photoreceptor  1  (estimates the number of printed sheets at the point  905 C) by the method described with reference to  FIGS. 6 to 8 . 
     In this manner, the controller  50  may count the actual number of printed sheets, and display an instruction to replace the photoreceptor  1  on a monitor (not illustrated) of the image forming apparatus  100  at a time point when a count value reaches the printable number of sheets of the photoreceptor  1 . In one aspect, the controller  50  may output the instruction to replace the photoreceptor  1  at a time point when a difference between the actual number of printed sheets and the printable number of sheets of the photoreceptor  1  is equal to or smaller than a predetermined number of sheets. A certain number herein may be 1,000, for example, but is not limited thereto. In one aspect, the controller  50  may notify a computer of a user and the like of the image forming apparatus  100  of the instruction to replace the photoreceptor  1  via a network. 
     In one aspect, in a case where the controller  50  detects that the photoreceptor  1  is replaced, this may display information to encourage the user to predict the life of the newly set photoreceptor  1  on the monitor. In one aspect, in a case where the controller  50  detects that the photoreceptor  1  is replaced, this may automatically start predicting the life of the newly set photoreceptor  1 . 
       FIG. 10  is a view illustrating an example of a relationship between a rotational speed of the photoreceptor  1  and the inclination (change amount of the density of the toner images when the PH light amount is changed). A table  1000  associates the OCL film thickness, an inclination X, an inclination Y, and the life of the photoreceptor  1 . Unlike the table  800 , the table  1000  includes the two inclinations X and Y. 
     The inclination X is the inclination obtained by the controller  50  by rotating the photoreceptor  1  at a rotational speed half as high as that at the time of printing. The inclination Y is the inclination obtained by the controller  50  by rotating the photoreceptor  1  at the sane rotational speed as that at the time of printing. The PH light amount when obtaining the inclinations X and Y is the same. 
     A numerical value  1001  is a difference between a maximum value and a minimum value of the inclination X. A numerical value  1002  is a difference between a maximum value and a minimum value of the inclination Y. As is understood from the numerical values  1001  and  1002 , the lower the rotational speed of the photoreceptor  1 , the larger the change amount of the inclination by the difference in the OCL film thickness. This is because, the lower the rotational speed, the longer a time from when the latent image is formed on the photoreceptor until the toner image is formed. The longer this time, the larger the lateral flow of the positive changes in  FIG. 3 , and the greater the inclination. As a result, the accuracy of the life prediction of the photoreceptor  1  is increased. 
     In a case where the controller  50  changes the rotational speed of the photoreceptor  1  and performs the life prediction of the photoreceptor  1 , the storage  51  must store the table  1000  including correspondence between the inclination and the OCL film thickness for each speed. The controller  50  rotates the photoreceptor  1  at the rotational speed lower than that at the time of printing and refers to the inclination corresponding to the rotational speed from the table  1000 , thereby performing the life prediction of the photoreceptor  1 . 
     Note that, in this embodiment, the controller  50  controls the rotational speed of the photoreceptor  1  to be half the rotational speed at the time of printing when the inclination of the photoreceptor  1  is obtained; however, the rotational speed of the photoreceptor  1  does not need to be half the rotational speed at the time of printing. The controller  50  may also appropriately set the rotational speed of the photoreceptor  1  when obtaining the inclination of the photoreceptor  1 . 
     The OCL film thickness might vary in a longitudinal direction of the photoreceptor  1 . Therefore, in one aspect, the controller  50  may perform the life prediction of the photoreceptor  1  according to this embodiment in a plurality of locations in the longitudinal direction of the photoreceptor  1 , and determine the smallest of the lives estimated at the respective locations as the life of the photoreceptor  1 , thereby improving the accuracy of the life prediction. 
       FIG. 11  is a view illustrating an example of a process of the life prediction of the photoreceptor  1 . In one aspect, the controller  50  may read to execute a program for performing the process in  FIG. 11  from the storage  51 . 
     At step S 1110 , the controller  50  changes the rotational speed of the photoreceptor  1  to be lower than that at the time of printing before forming the toner image for inspection  602  on the photoreceptor  1 . Note that, if the “actual inclination” is sufficiently large even in a case where the photoreceptor  1  rotates at the rotational speed at the time of printing, the controller  50  does not need to perform the process at step S 1110 . 
     At step S 1120 , the controller  50  allows the exposure device  3  to expose a plurality of parts on the surface of the photoreceptor  1  with different light amounts. Note that the charger  2  charges the surface of the photoreceptor  1  before the exposure. 
     At step S 1130 , the controller  50  allows the developer  4  to form the toner images for inspection  602  for the respective parts of the surface of the photoreceptor  1  exposed with the different light amounts. Next, the toner images for inspection  602  formed on the photoreceptor  1  are transferred to the intermediate transfer belt  12 . 
     At step S 1140 , the controller  50  obtains an output from the density sensor  55 , and detects the densities of the respective toner images for inspection  602  on the intermediate transfer belt  12 . 
     At step S 1150 , the controller  50  compares the densities of the toner images for inspection  602  to obtain the “actual inclination”. 
     At step S 1160 , the controller  50  refers to the table  800  or the table  1000 . The controller  50  refers to the table  1000  in a case where the rotational speed of the photoreceptor  1  is set to be lower than the rotational speed at the time of printing. Otherwise, the controller  50  refers to the table  800 . 
     At step S 1170 , the controller  50  estimates the life of the photoreceptor  1  based on the change amount “actual inclination” of the density of the toner images for inspection caused by the change in the light amount of the exposure device  3 . In one aspect, the controller  50  may compare the measured “actual inclination” with the “inclination” for each OCL film thickness included in the table  800  or  1000  and make the “OCL film thickness and life” corresponding to the closest “inclination” as an estimated value of the life. In another aspect, the controller  50  may calculate the life corresponding to the “actual inclination” based on the measured “actual inclination” and the contents of the table  800  or the table  1000 . For example, if “actual inclination=0.028”, the controller  50  may obtain an intermediate value between the “OCL film thicknesses” and that between the “lives” corresponding to “inclination=0.030” and “inclination=0.026” in the table  800 . 
     As described above, according to the image forming apparatus  100  according to this embodiment, the life of the photoreceptor  1  with variation in the OCL film thickness at the time of manufacture may be predicted based on the change amount of the density of the toner images caused by clanging the PH light of the exposure device  3 . As a result, the image forming apparatus  100  may output an alert for replacement after printing an appropriate number of sheets in accordance with the life of the photoreceptor  1 , and a situation where the photoreceptor  1  is replaced even though this is still usable may be avoided. 
     Second Embodiment 
     Next, a second embodiment is described. A hardware configuration of an image forming apparatus  100  according to the second embodiment is the same as that of the image forming apparatus  100  described in the first embodiment. Therefore, the description of the same configuration is not repeated. The image forming apparatus  100  according to this embodiment is different from that of the first embodiment in that a charging potential of a photoreceptor  1  is changed in place of PH light when a life of the photoreceptor  1  is predicted. 
       FIG. 12  is a view illustrating an example of a state in which the photoreceptor  1  charged with different voltages is exposed. In the controller  50 , a charger  2  charges a surface of the photoreceptor  1 . In the example illustrated in  FIG. 12 , a controller  50  charges the surface of the photoreceptor  1  so as to have two different types of charging potentials. 
     A potential  1200  is a reference potential V dc  of the surface of the photoreceptor  1 . A potential  1201 A is a potential when the charger  2  charges the surface of the photoreceptor  1  with a voltage V a . A potential  1201 B is a potential when the charger  2  charges the surface of the photoreceptor  1  with a voltage V b . The potential  1201 B has a higher absolute value of the potential than the potential  1201 A. 
     Next, the controller  50  allows an exposure device  3  to expose a part of the surface of the photoreceptor  1 . A part  1202 A is a portion in which the surface of the photoreceptor  1  having the potential  1201 A is exposed. A part  1202 B is a portion in which the surface of the photoreceptor  1  having the potential  1201 B is exposed. The potentials of the parts  1202 A and  1202 B are both V i . 
     When the surface of the charged photoreceptor  1  is exposed, the potential becomes V i . However, the potential of the exposed part does not fall vertically but falls with a slight inclination. Therefore, in a case where this is exposed from a higher potential, an area of an exposed portion becomes smaller. Therefore, the area of the part  1202 B is smaller than the area of the part  1202 A. Therefore, in a case where the controller  50  tries to form a toner image formed of thin lines on the surface of the photoreceptor  1 , if the number of dots is fixed, the higher the absolute value of the charging potential of the photoreceptor  1 , the smaller the area of the exposed portion, so that the narrower the line width of the thin line. 
     A change amount of the line width of the toner image accompanying the change in the charging potential of the photoreceptor  1  increases as a film thickness of an OCL increases. Therefore, as in  FIGS. 6 to 8 , the controller  50  may form toner images for inspection  602  while changing the charging potential of the photoreceptor  1  in place of the PH light amount, and examine a change amount of density, thereby predicting the life of the photoreceptor  1 . 
       FIG. 13  is a view illustrating an example of the change amount of the density of the toner images on the photoreceptor  1  when the charging potential of the photoreceptor  1  is changed. A graph  1301  illustrates “the change amount of the density of the toner images when the charging potential of the photoreceptor  1  is changed” of a photoreceptor  1 E having a thin OCL. A graph  1302  illustrates “the change amount of the density of the toner images when the charging potential of the photoreceptor  1  is changed” of a photoreceptor  1 F having a thick OCL. 
     Points  1303 A,  1303 B,  1303 C, and  1303 D indicate the densities when toner images  602 A,  602 B,  602 C, and  602 D are detected by a density sensor  55 , respectively. The graphs  1301  and  1302  are obtained based on the points  1303 A,  1303 B,  1303 C, and  1303 D. As is understood when comparing the graphs  1301  and  1302 , the thicker the OCL, the larger “the change amount of the density of the toner images when the charging potential of the photoreceptor  1  is changed”. 
     Note that, although the controller  50  changes the charging potential of the photoreceptor  1  in four steps and detects the densities of the toner images for the respective charging potentials in this embodiment, the number of times of detection is not limited to this. When the controller  50  changes the charging potential of the photoreceptor  1  in more steps and detects the densities of the toner images for the charging potentials of the photoreceptor  1 , detection accuracy is improved accordingly. The controller  50  may detect the densities of the toner images for the charging potentials of the photoreceptor  1  in at least two steps. 
       FIG. 14  is a view illustrating an example of a relationship between the film thickness of the OCL, the change amount of the density of the toner images when the charging potential of the photoreceptor  1  is changed, and the life of the photoreceptor  1 . A table  1400  includes association information obtained by associating the film thickness of the OCL, an inclination, and the life of the photoreceptor  1 . The inclination is the inclination of the graphs  1301 ,  1302  and the like in  FIG. 13  and corresponds to “the change amount of the density of the toner images when the charging potential of the photoreceptor  1  is changed”. A storage  51  stores the table  1400  including the association information for several types of OCL film thicknesses. In the example of  FIG. 14 , the table  1400  includes the association information for three types of OCL film thicknesses, but may also include the association information for more types of OCL film thicknesses. 
     The controller  50  compares the change amount “actual inclination” of the density of the toner images when the charging potential of the photoreceptor  1  obtained by the measurement by the density sensor  55  is changed with the “inclination” of the table  1400  in the storage  51 , thereby predicting the life of the photoreceptor  1 . 
     In one aspect, the controller  50  may compare the measured “actual inclination” with the “inclination” in the table  1400  and determine the “OCL film thickness and life” corresponding to the closest “inclination” as an estimated value of the life. In one aspect, the controller  50  may calculate the life corresponding to the “actual inclination” based on the measured “actual inclination” and the contents of the table  1400 . For example, if “actual inclination=0.028”, the controller  50  may obtain an intermediate value between the “OCL film thicknesses” and that between the “lives” corresponding to “inclination=0.030” and “inclination=0.026”. 
     Note that, it is desirable that the controller  50  predicts the life of the photoreceptor  1  when the photoreceptor  1  is replaced (when the photoreceptor  1  is new). When the photoreceptor  1  is used to some extent, a surface layer of the photoreceptor  1  is covered with lubricant (change in surface resistance), and surface roughness of the photoreceptor  1  is changed, so that a cause to change the line width of the toner image other than the OCL film thickness might occur. As a result, accuracy of the life prediction of the photoreceptor  1  using the toner image  602  is lowered. 
     In one aspect, the density sensor  55  may also detect the density of the toner image on the surface of the photoreceptor  1 . In this case, the density sensor  55  detects the density of the toner image on the surface of each photoreceptor  1  for each toner color. 
     In one aspect, in a case where the controller  50  lowers a rotational speed of the photoreceptor  1  and estimates the OCL film thickness, the storage  51  must store the table including correspondence between the inclination for each speed and the OCL film thickness. The controller  50  rotates the photoreceptor  1  at the rotational speed lower than that at the time of printing and refers to the inclination corresponding to the rotational speed from the table, thereby performing the life prediction of the photoreceptor  1 . 
     In one aspect, the controller  50  may obtain the change amount of the density of the toner images when both the PH light amount and the charging potential of the photoreceptor  1  are changed. In this case, the inclination of the table  1400  means the change amount of the density of the toner image when both the PH light amount and the charging potential of the photoreceptor  1  are changed. 
     As described above, the controller  50  may count the actual number of printed sheets, and display an instruction to replace the photoreceptor  1  on a monitor of the image forming apparatus  100  at a time point when a count value reaches the printable number of sheets of the photoreceptor  1 . In one aspect, the controller  50  may output the instruction to replace the photoreceptor  1  at a time point when a difference between the actual number of printed sheets and the printable number of sheets of the photoreceptor  1  is equal to or smaller than a predetermined number of sheets. A certain number herein may be 1,000, for example, but is not limited thereto. In one aspect, the controller  50  may notify a computer of a user and the like of the image forming apparatus  100  of the instruction to replace the photoreceptor  1  via a network. 
     In one aspect, in a case where the controller  50  detects that the photoreceptor  1  is replaced, this may display information to encourage the user to estimate the life of the newly set photoreceptor  1  on the monitor. In one aspect, in a case where the controller  50  detects that the photoreceptor  1  is replaced, this may automatically start estimating the life of the newly set photoreceptor  1 . 
     The OCL film thickness might vary in a longitudinal direction of the photoreceptor. Therefore, in one aspect, the controller  50  may perform the life prediction of the photoreceptor  1  according to this embodiment in a plurality of locations in the longitudinal direction, and determine the smallest of the lives estimated at the respective locations as the life of the photoreceptor  1 , thereby improving the accuracy of the life prediction. 
       FIG. 15  is a view illustrating an example of a process of the life prediction of the photoreceptor  1 . In one aspect, the controller  50  may read to execute a program for performing the process in  FIG. 15  from the storage  51 . 
     At step S 1510 , the controller  50  changes the rotational speed of the photoreceptor  1  to be lower than that at the time of printing before forming the toner image for inspection  602  on the photoreceptor  1 . Note that, if the “actual inclination” is sufficiently large even in a case where the photoreceptor  1  rotates at the rotational speed at the time of printing, the controller  50  does not need to perform the process at step S 1510 . 
     At step S 1520 , the controller  50  allows the charger  2  to charge different portions of the surface of the photoreceptor  1  while changing the charging potential a plurality of times. 
     At step S 1530 , the controller  50  allows the exposure device  3  to expose the portions having different charging potentials on the surface of the photoreceptor  1 . 
     At step S 1540 , the controller  50  allows a developer  4  to form the toner images for inspection  602  for the respective exposed parts of the surface of the photoreceptor  1 . Next, the toner images for inspection  602  formed on the photoreceptor  1  are transferred to the intermediate transfer belt  12 . 
     At step S 1550 , the controller  50  obtains an output from the density sensor  55 , and detects the densities of the respective toner images for inspection  602  on the intermediate transfer belt  12 . 
     At step S 1560 , the controller  50  compares the densities of the toner images for inspection  602  to obtain the “actual inclination”. 
     At step S 1570 , the controller  50  refers to the table  1400 . Note that, in a case where the controller  50  sets the rotational speed of the photoreceptor  1  to be lower than the rotational speed at the time of printing, the table  1400  needs to include the “OCL film thickness”, “inclination”, and “life” for each speed. 
     At step S 1580 , the controller  50  estimates the life of the photoreceptor  1  based on the change amount “actual inclination” of the density of the toner images for inspection caused by the change in the clanging potential of the photoreceptor  1 . In one aspect, the controller  50  may compare the measured “actual inclination” with the “inclination” for each OCL film thickness included in the table  1400  and determine the “OCL film thickness and life” corresponding to the closest “inclination” as an estimated value of the life. In another aspect, the controller  50  may calculate the life corresponding to the “actual inclination” based on the measured “actual inclination” and the contents of the table  1400 . 
     As described above, according to the image forming apparatus  100  according to this embodiment, the life of the photoreceptor  1  with variation in the OCL film thickness at the time of manufacture may be predicted based on the change amount of the density of the toner images caused by changing the charging potential of the photoreceptor  1 . As a result, the image forming apparatus  100  may output an alert for replacement after printing an appropriate number of sheets in accordance with the life of the photoreceptor  1 , and a situation where the photoreceptor  1  is replaced even though this is still usable may be avoided. 
     Third Embodiment 
     Next, a third embodiment is described. A hardware configuration of an image forming apparatus  100  according to the third embodiment is the same as that of the image forming apparatus  100  described in the above-described embodiments. Therefore, the description of the same configuration is not repeated. The image forming apparatus  100  according to this embodiment differs from that of the above-described embodiment in that a life of a photoreceptor  1  is predicted under a plurality of environments. 
     As described above, a toner image formed by the photoreceptor  1  provided with an OCL has a line width thicker than that of the toner image formed by the conventional photoreceptor. A reason that the line width becomes thick is that volume resistance of the OCL is smaller than that of a surface of the conventional photoreceptor. The volume resistance of the OCL changes with temperature and humidity. In particular, the volume resistance of the OCL is susceptible to humidity. In a case where the humidity is high, a surface layer of the OCL absorbs moisture and the resistance decreases, so that the line width becomes thicker. 
     When the volume resistance of the OCL changes, the “inclination (the change amount of the density of the toner images when the PH light amount is changed)” illustrated in  FIG. 8  and the “inclination (the change amount of the density of the toner image when the charging potential of the photoreceptor  1  is changed)” illustrated in  FIG. 14  change. 
       FIG. 16  is a view illustrating an example of an effect of the change in the volume resistance of the OCL for each environment. A graph  1601  illustrates life prediction of the photoreceptor  1  in an environment of “low temperature and low humidity”. A graph  1602  illustrates life prediction of the photoreceptor  1  in an environment of “middle temperature and middle humidity”. A graph  1603  illustrates life prediction of the photoreceptor  1  in an environment of “high temperature and high humidity”. An OCL film thickness of the photoreceptors  1  in the graphs  1601 ,  1602 , and  1603  is the same. 
     Points  1604 A,  1604 B,  1604 C, and  1604 D indicate the densities when the toner images  602 A,  602 B,  602 C, and  602 D are detected by the density sensor  55 , respectively. The graphs  1601 ,  1602 , and  1603  are obtained based on the points  1604 A,  1604 B,  1604 C, and  1604 D. As is understood by comparing the graphs  1601 ,  1602 , and  1603 , the “inclination” increases as the temperature and humidity increase. 
     In one aspect, the controller  50  may obtain the change amount of the density of the toner images when the PH light amount is changed, or obtain the change amount of the density of the toner image when the charging potential of the photoreceptor  1  is changed, or combine both of them. 
     Note that, although a controller  50  changes the PH light amount or the charging potential of the photoreceptor  1  in four steps and detects the densities of the toner images for the respective PH light amounts or the respective charging potentials of the photoreceptor  1  in this embodiment, the number of times of detection is not limited to this. When the controller  50  changes the density of the toner images in more steps and detects the changed densities of the toner images, detection accuracy is improved accordingly. The controller  50  may detect the densities of the toner images for the PH light amounts or the charging potentials of the photoreceptor  1  in at least two steps. 
     As described above, as environmental information “temperature and humidity” changes, the “inclination” also changes. Therefore, in order to appropriately obtain the life of the photoreceptor  1  even in a case where the temperature and humidity change, the controller  50  desirably refers to a table of a correlation between the “inclination” for each environmental information such as “low temperature and low humidity” and “high temperature and high humidity” and the “OCL film thickness and life”. 
       FIG. 17  is a view illustrating an example of the correlation among the OCL film thickness, the inclination for each environment, and the life of the photoreceptor  1 . A table  1700  includes association information obtained by associating the OCL film thickness, the inclination for each environment, and the life of the photoreceptor  1 . The inclination is the inclination of the graphs  1601 ,  1602 , and  1603  in  FIG. 16  and corresponds to “the change amount of the density of the toner images when the PH light amount is changed”. 
     An “inclination LL” is an inclination under a “low temperature and low humidity” environment. An “inclination NN” is an inclination under a “medium temperature and medium humidity” environment. An “inclination HH” is an inclination under a “high temperature and high humidity” environment. For example, the inclination of the photoreceptor  1  having the OCL film thickness of “3.5 μm” under the “low temperature and low humidity” environment is “−0.0380”. 
     Note that, in this embodiment, for example, “low temperature and low humidity” means “temperature=10 degrees and humidity=15%”, “medium temperature and medium humidity” means “temperature=23 degrees and humidity=65%”, and “high temperature and high humidity” means “temperature=30 degrees and humidity=85%”; however, setting of the temperature and humidity is not limited thereto. 
     A storage  51  stores the table  1700  including the association information for several types of OCL film thicknesses. In the example of  FIG. 17 , the table  1700  includes the association information for three types of OCL film thicknesses, but may also include the association information for more types of OCL film thicknesses. 
     The controller  50  may compare the “change amount of the density of the toner images when the PH light amount is changed or the change amount (actual inclination) of the density of the toner images when the charging potential of the photoreceptor  1  is changed” obtained by the measurement by a density sensor  55  with the “inclination” under a certain environment of the table  1700  of the storage  51 , thereby predicting the life of the photoreceptor  1 . The controller  50  selects an inclination to be compared from the table  1700  based on a detection result of an environmental sensor (not illustrated) provided in the image forming apparatus  100 . The environmental sensor includes, for example, a temperature sensor and a humidity sensor. 
     In one aspect, the controller  50  may compare the measured “actual inclination” with the “inclination” in the table  1700  and determine the “OCL film thickness and life” corresponding to the closest “inclination” as an estimated value of the life. In one aspect, the controller  50  may also calculate the “OCL film thickness” and “life” corresponding to the “actual inclination” based on the measured “actual inclination” and the contents of the table  1700 . 
     Note that, it is desirable that the controller  50  predicts the life of the photoreceptor  1  when the photoreceptor  1  is replaced (when the photoreceptor  1  is new). When the photoreceptor  1  is used to some extent, a surface layer of the photoreceptor  1  is covered with lubricant (change in surface resistance), and surface roughness of the photoreceptor  1  is changed, so that a cause to change the line width of the toner image other than the OCL film thickness might occur. As a result, accuracy of the life prediction of the photoreceptor  1  using the toner image  602  is lowered. 
     In one aspect, the density sensor  55  may also detect the density of the toner image on the surface of the photoreceptor  1 . In this case, the density sensor  55  detects the density of the toner image on the surface of each photoreceptor  1  for each toner color. 
     As described above, the controller  50  may count the actual number of printed sheets, and display an instruction to replace the photoreceptor  1  on a monitor of the image forming apparatus  100  at a time point when a count value reaches the printable number of sheets of the photoreceptor  1 . In one aspect, the controller  50  may output the instruction to replace the photoreceptor  1  at a time point when a difference between the actual number of printed sheets and the printable number of sheets of the photoreceptor  1  is equal to or smaller than a predetermined number of sheets. A certain number herein may be 1,000, for example, but is not limited thereto. In one aspect, the controller  50  may notify a computer of a user and the like of the image forming apparatus  100  of the instruction to replace the photoreceptor  1  via a network. 
     In one aspect, in a case where the controller  50  detects that the photoreceptor  1  is replaced, this may display information to encourage the user to estimate the life of the newly set photoreceptor  1  on the monitor. In one aspect, in a case where the controller  50  detects that the photoreceptor  1  is replaced, this may automatically start estimating the life of the newly set photoreceptor  1 . 
     The OCL film thickness might vary in a longitudinal direction of the photoreceptor. Therefore, in one aspect, the controller  50  may perform the life prediction of the photoreceptor  1  according to this embodiment in a plurality of locations in the longitudinal direction, and determine the smallest of the lives estimated at the respective locations as the life of the photoreceptor  1 , thereby improving the accuracy of the life prediction. 
     As described above, according to the image forming apparatus  100  according to this embodiment, the life of the photoreceptor  1  with variation in the OCL film thickness at the time of manufacture may be predicted based on the change amount of the density of the toner images caused by changing the PH light amount or the charging potential of the photoreceptor  1  even under different environments. As a result, the image forming apparatus  100  may output an alert for replacement after printing an appropriate number of sheets in accordance with the life of the photoreceptor  1 , and a situation where the photoreceptor  1  is replaced even though this is still usable may be avoided. 
     Fourth Embodiment 
     Next, a fourth embodiment is described. A hardware configuration of an image forming apparatus  100  according to the fourth embodiment is the same as that of the image forming apparatus  100  described in the above-described embodiments. Therefore, the description of the same configuration is not repeated. The image forming apparatus  100  according to this embodiment differs from that of the above-described embodiment in that a life of a photoreceptor  1  is predicted each time a certain number of sheets are printed. 
     As described above, it is desirable that a controller  50  predicts the life of the photoreceptor  1  when the photoreceptor  1  is replaced (when the photoreceptor  1  is new). When the photoreceptor  1  is used to some extent, a surface layer of the photoreceptor  1  is covered with lubricant (change in surface resistance), and surface roughness of the photoreceptor  1  is changed, so that a cause to change the line width of the toner image other than the OCL film thickness might occur. As a result, accuracy of the life prediction of the photoreceptor  1  using the toner image  602  is lowered. 
     However, if the image forming apparatus  100  prints only up to about half the life (printable number of sleets) of the photoreceptor  1 , an effect of a factor that changes the line width of the toner image is small, and the controller  50  may predict the life of the photoreceptor  1 . 
       FIG. 18  is a view illustrating an example of a method of estimating the life of the photoreceptor  1  according to this embodiment. The controller  50  predicts the life of the photoreceptor  1  by the method illustrated in the first or second embodiment when the photoreceptor  1  is replaced. Next, each time a predetermined number of sheets are printed, the controller  50  may perform the life prediction of the photoreceptor  1  by the method illustrated in the first or second embodiment to correct the previous life prediction. A graph  1801  is obtained by correcting the life prediction of the graph  903  in  FIG. 9 . 
     In one aspect, since the life prediction after printing a certain number of sheets might include an error, the controller  50  may estimate a value of the predicted life to be smaller by multiplying a coefficient by the life of the photoreceptor  1  predicted by the method illustrated in the first and second embodiments. 
     In one aspect, the controller  50  may obtain the change amount of the density of the toner images when the PH light amount is changed, or obtain the change amount of the density of the toner image when the charging potential of the photoreceptor  1  is changed, or combine both of them. In one aspect, the controller  50  may refer to a table in which the OCL film thickness, the inclination for each environment, and the life of the photoreceptor  1  are associated in the life prediction of the photoreceptor  1 . 
     As described above, the controller  50  may count the actual number of printed sheets, and display an instruction to replace the photoreceptor  1  on a monitor of the image forming apparatus  100  at a time point when a count value reaches the printable number of sheets of the photoreceptor  1 . In one aspect, the controller  50  may output the instruction to replace the photoreceptor  1  at a time point when a difference between the actual number of printed sheets and the printable number of sheets of the photoreceptor  1  is equal to or smaller than a predetermined number of sheets. A certain number herein may be 1,000, for example, but is not limited thereto. In one aspect, the controller  50  may notify a computer of a user and the like of the image forming apparatus  100  of the instruction to replace the photoreceptor  1  via a network. 
     In one aspect, in a case where the controller  50  detects that the photoreceptor  1  is replaced, this may display information to encourage the user to estimate the life of the newly set photoreceptor  1  on a monitor of the image forming apparatus  100 . In one aspect, in a case where the controller  50  detects that the photoreceptor  1  is replaced, this may automatically start estimating the life of the newly set photoreceptor  1 . 
     The OCL film thickness might vary in a longitudinal direction of the photoreceptor. Therefore, in one aspect, the controller  50  may perform the life prediction of the photoreceptor  1  according to this embodiment in a plurality of locations in the longitudinal direction, and determine the smallest of the lives estimated at the respective locations as the life of the photoreceptor  1 , thereby improving the accuracy of the life prediction. 
     As described above, according to the image forming apparatus  100  according to this embodiment, the life prediction is corrected while the number of printed sheets is small and the effect of the factor that changes the line width of the toner image is small. As a result, the image forming apparatus  100  may output an alert for replacement after printing an appropriate number of sheets in accordance with the life of the photoreceptor  1  estimated in further detail, and a situation where the photoreceptor  1  is replaced even though this is still usable may be avoided. 
     In still another aspect, the disclosed technical features may be summarized as follows, for example. 
     [Configuration 1] 
     A method of estimating a life of a photoreceptor of an image forming apparatus, including a step of changing a light amount of an exposure device a plurality of times to expose a plurality of parts on a surface of the photoreceptor with different light amounts, a step of allowing a developer to form toner images for inspection for respective parts on the surface of the photoreceptor exposed with the different light amounts, a step of detecting densities of the toner images for inspection transferred to an intermediate transfer belt based on an output from the density sensor, and a step of comparing the densities of the respective toner images for inspection to estimate a life of the photoreceptor based on a change amount of the densities of the toner images for inspection caused by a change in light amount of the exposure device. 
     [Configuration 2] 
     The method according to the configuration 1, further including a step of storing in advance association information in which the change amount of the densities of the toner images for inspection by the change in the light amount of the exposure device and the life of the photoreceptor are associated with each other, and a step of estimating the life of the photoreceptor by comparing the change amount of the densities obtained by comparing the densities of the respective toner images for inspection with the association information. 
     [Configuration 3] 
     A method including a step of allowing a charger to change a charging potential a plurality of tines to charge a surface of a photoreceptor, a step of allowing an exposure device to expose respective portions on a surface of the photoreceptor with different charging potentials, a step of allowing a developer to form toner images for inspection for respective exposed parts on the surface of the photoreceptor, a step of detecting densities of the toner images for inspection transferred to an intermediate transfer belt based on an output from the density sensor, and a step of comparing the densities of the respective toner images for inspection to estimate a life of the photoreceptor based on a change amount of the densities of the toner images for inspection caused by a change in the charging potential of the photoreceptor. 
     [Configuration 4] 
     The method according to the configuration 3, further including a step of storing association information in which the change amount of the densities of the toner images for inspection by the change in the charging potential of the photoreceptor and the life of the photoreceptor are associated with each other, and a step of estimating 
     the life of the photoreceptor by comparing the change amount of the densities of the toner images for inspection with the association information. 
     [Configuration 5] 
     The method according to the configuration 2 or 4, further including a step of storing a plurality of pieces of association information associated with a plurality of pieces of environmental information, and a step of obtaining the stored association information for comparing with the change amount of the densities of the toner images for inspection based on the environmental information obtained from the environmental sensor. 
     [Configuration 6] 
     The association information further includes information of a rotational speed of the photoreceptor. The method according to the configuration 2 or 4, further including a step of referring to the association information based on forming each toner image for inspection and rotating the photoreceptor at a speed included in the association information. 
     [Configuration 7] 
     The method according to the configuration 6, further including a step of setting the rotational speed of the photoreceptor to be lower than the rotational speed at the time of printing based on forming the toner images for inspection on the surface of the photoreceptor. 
     [Configuration 8] 
     The method according to any one of the configurations 1 to 7, further including a step of comparing three or more toner images for inspection in order to obtain the change amount of the densities of the toner images for inspection. 
     [Configuration 9] 
     The method according to any one of the configurations 1 to 8, further including a step of estimating the life of the photoreceptor based on the change amount of the densities of the toner images for inspection for each of a plurality of different sections in a longitudinal direction of the surface of the photoreceptor, and a step of determining a shortest life out of lives estimated for the plurality of different sections in the longitudinal direction of the surface of the photoreceptor as the life of the photoreceptor. 
     [Configuration 10] 
     The method according to any one of the configurations 1 to 9, further including a step of calculating an estimated value of a remaining printable number of media using the photoreceptor based on the life of the photoreceptor estimated based on the change amount of the densities of the toner images for inspection and the number of printed media before estimating the life of the photoreceptor. 
     [Configuration 11] 
     The life of the photoreceptor is an estimated value of the printable number of media using the photoreceptor. The method according to any one of the configurations 1 to 10, further including a step of comparing the number of printed sheets with the estimated value, and a step of displaying an instruction to replace the photoreceptor on a monitor based on a fact that a difference between the printed number of sheets and the estimated value is equal to or smaller than a predetermined number. 
     [Configuration 12] 
     The method according to the configuration 11, further including a step of displaying information that encourages a user to estimate the life of the photoreceptor on the monitor based on detection of replacement of the photoreceptor. 
     [Configuration 13] 
     The method according to the configuration 11, further including a step of estimating the life of the photoreceptor based on detection of replacement of the photoreceptor. 
     Although embodiments of the present invention have been described and illustrated in detail, the disclosed embodiments are made for purposes of illustration and example only and not limitation. The scope of the present invention should be interpreted by terms of the appended claims, and it is intended that equivalents of the scope of claims and all modifications within the scope are included.