Abstract:
An image forming apparatus includes a fixing member, first and second image forming portions, and a processor. In the first and second image forming portions, first and second blades are respectively in contact with first and second photosensitive members. A distance between the first image forming portion and the fixing member is shorter than a distance between the second image forming portion and the fixing member. The processor controls a first image forming portion to perform a toner supply operation in which a first supply member supplies toner amounting to a first quantity per a prescribed rotation amount of the first photosensitive member, and controls the second image forming portion to perform a toner supply operation in which the second supply member supplies toner amounting to a second quantity per the prescribed rotation amount of the second photosensitive member. The second quantity is smaller than the first quantity.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2015-071010 filed Mar. 31, 2015. The entire content of the priority application is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to an image forming apparatus in which toner carried on a photosensitive member is collected by a blade, a method for controlling the image forming apparatus, and a storage medium storing a program for controlling the image forming apparatus. More specifically, the disclosure relates to a technology for reducing frictional resistance between the photosensitive member and the blade. 
     BACKGROUND 
     According to a conventional electro-photographic type image forming apparatus, a cleaning blade is in contact with a photosensitive member to scrape off residual toner remaining on the photosensitive member after image transfer. Here, temperature of the cleaning blade is increased and the blade will be softened in accordance with an increase in surface temperature of the photosensitive member. Thus, so called “stick-slip phenomenon” occurs such that an end portion of the blade in contact with the photosensitive member is compressed and shrinks in a rotational direction of the photosensitive body in accordance with the rotation thereof, and the blade returns its original shape because of resiliency upon shrinkage to some extent, and such repeated shrinking and returning causes vibration. Abnormal noise or chatter may occur if some amounts of toner do not remain between the photosensitive member and the blade. 
     There is known a technique for restraining chattering. For example, in Japanese Patent Application Publication No. 2001-356642, a detection device is provided for detecting ambient temperature of a photosensitive member. A toner image is formed at a non-imaging surface portion of the photosensitive member in accordance with the detected temperature. Thus, toner is supplied to the surface portion in contact with the cleaning blade. 
     SUMMARY 
     However, the following problems are recognized by the inventor. That is, in an internal space of the image forming apparatus where the photosensitive member is accommodated, a spot ambient to a fixing device as a heat source becomes high temperature, and a spot far from the fixing device may not receive the heat from the fixing device. Therefore, assuming that a plurality of photosensitive members are provided in the internal space of the image forming apparatus, the surface temperature of the photosensitive member positioned close to the fixing device is higher than that of the photosensitive member positioned remote from the fixing device. Accordingly, reduction in frictional resistance may not be uniform with respect to the all photosensitive members if amount of toner to be supplied to each photosensitive member is uniform. More specifically, chattering of the blade cannot be restrained with respect to the photosensitive member close to the fixing device, and excessive amount of toner may be supplied to the photosensitive member positioned remote from the fixing device. 
     It is an object of the present disclosure to overcome the above-described problems, and to provide an image forming apparatus provided with a plurality of photosensitive members and a plurality of blades each in contact with corresponding photosensitive member, the apparatus being capable of reducing frictional resistance between each photosensitive member and corresponding blade, and restraining variation in frictional resistance reduction effect irrespective of positions of photosensitive members. 
     Another object of the present disclosure is to provide a method for controlling the image forming apparatus to reduce frictional resistance between each photosensitive member and each blade, and to restrain variation in frictional resistance reduction effect irrespective of positions of photosensitive members. 
     Still another object of the present disclosure is to provide a storage medium storing a program capable of executing a control the image forming apparatus to reduce frictional resistance between each photosensitive member and each blade, and to restrain variation in frictional resistance reduction effect irrespective of positions of photosensitive members. 
     In order to attain the above and other objects, the disclosure provides an image forming apparatus. The image forming apparatus includes a first image forming portion, a second image forming portion, a fixing member, and a processor. The first image forming portion includes a first photosensitive member, a first supply member, and a first blade. The first photosensitive member is configured to rotate. The first supply member is configured to supply toner of first color to the first photosensitive member. The first blade is in contact with the first photosensitive member. The second image forming portion includes a second photosensitive member, a second supply member, and a second blade. The second photosensitive member is configured to rotate. The second supply member is configured to supply toner of second color to the second photosensitive member. The second blade is in contact with the second photosensitive member. The fixing member is configured to thermally fix toner on a sheet. The fixing member, the first image forming portion, and the second image forming portion are arranged in such a positional relation that a distance between the first image forming portion and the fixing member is shorter than a distance between the second image forming portion and the fixing member. The processor is configured to perform: controlling, while the image forming operation by the first image forming portion is not performed, the first image forming portion to perform a toner supply operation in which the first supply member supplies, to the first photosensitive member, toner amounting to a first quantity per a prescribed rotation amount of the first photosensitive member; and controlling, while the image forming operation by the second image forming portion is not performed, the second image forming portion to perform a toner supply operation in which the second supply member supplies, to the second photosensitive member, toner amounting to a second quantity per the prescribed rotation amount of the second photosensitive member, the first quantity being larger than the second quantity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The particular features and advantages of the disclosure as well as other objects will become apparent from the following description taken in connection with the accompanying drawings, in which: 
         FIG. 1  is a schematic cross-sectional view of a printer as an example of an image forming apparatus according to one embodiment; 
         FIG. 2  is a plan view for description of a frame and a temperature sensor in the printer according to the embodiment; 
         FIG. 3  is a block diagram showing an electrical configuration in the printer according to the embodiment; 
         FIG. 4  is a flowchart illustrating a printing process in the printer according to the embodiment; 
         FIG. 5  is a flowchart showing a determination process in the printer according to the embodiment; 
         FIG. 6  is a correction table according to a first example in the printer according to the embodiment; 
         FIG. 7  is a correction table according to a second example in the printer according to the embodiment; 
         FIG. 8  is a correction table according to a third example in the printer according to the embodiment; 
         FIG. 9  is a correction table according to a fourth example in the printer according to the embodiment; and 
         FIG. 10  is a correction table according to a fifth example in the printer according to the embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     A printer  100  as an example of an image forming apparatus according to one embodiment will be described with reference to drawings. 
     The printer  100  is an electro-photographic type color printer, and includes a process portion  5 , an exposure unit  6 , a conveyer belt  7 , and a fixing device  8  as shown in  FIG. 1 . The process portion  5  includes a process unit  50 Y for a color of yellow, a process unit  50 M for a color of magenta, a process unit  50 C for a color of cyan, and a process unit  50 K for a color of black. These process units are arrayed in line with a constant space between neighboring process units. Each of the process units  50 Y,  50 M,  50 C and  50 K is an example of an image forming member. The fixing device  8  is an example of a fixing member. 
     As shown in  FIG. 1 , the yellow process unit  50 Y includes a photosensitive drum  51 , a charger  52  positioned beside a peripheral surface of the photosensitive drum  51 , a developing unit  54 , a transfer device  55 , and a blade  56 . Remaining process units  50 M,  50 C,  50 K are the same as the yellow process unit  50 Y except for colors of toner. For example, these process units have diameters of the photosensitive drums  51  identical to one another. The developing unit  54  is an example of a supply member. 
     The printer  100  further includes a sheet supply tray  91  to accommodate sheets for a subsequent printing operation (for example, a toner transferring operation), and a discharge tray  92  for receiving a sheet on which an image has been formed. A generally S-shaped sheet passage  11  is provided as indicated by two dotted chain line in  FIG. 1 , and a plurality of conveyer rollers are provided along the sheet passage  11 . That is, each one of the sheet is conveyed from the sheet supply tray  91  to the discharge tray  92  along the sheet passage  11  by the conveyer rollers and the conveyer belt  7 . 
     In the image forming operation, the photosensitive drum  51  is charged by the charger  52 , and is exposed to light by the exposure unit  6 . Thus, an electrostatic latent image based on print data is formed on the surface of the photosensitive drum  51 . Then, toner is supplied from the developing unit  54  to the electrostatic latent image, thereby forming a toner image on the photosensitive drum  51 . Further, the toner image is transferred, by the transfer device  55 , from the photosensitive drum  51  to a sheet when the sheet passes between the photosensitive drum  51  and the transfer device  55 . Then, the toner image on the sheet is thermally fixed to the sheet by the fixing device  8 . 
     There remains toner on the photosensitive drum  51  after the toner image is transferred to the sheet. The remaining tonner is scraped off by the blade  56 . The blade  56  is a cleaning blade made of a resin, and is fixed at a position where one side thereof is in contact with a surface of the photosensitive drum  51 . In addition, the blade  56  is in contact with at least an entire toner-supplyable range of the photosensitive drum  51  in an axial direction (or widthwise direction). Here, the tonner within the toner-supplyable range can be supplied to the sheet. With the rotation of the photosensitive drum  51 , the contact part of the blade  56  with the photosensitive drum  51  moves relatively to the photosensitive drum  51 . Some of the toner scraped off by the blade  56  remains in a space between the blade  56  and the photosensitive drum  51 . The toner remaining in the space functions as lubricant. 
     The blade  56  abuts against the photosensitive drum  51  in a direction counter to a rotation of the photosensitive drum  51 . For example, in  FIG. 1 , the photosensitive drum  51  rotates in a clockwise direction. At the contact portion, the blade  56  is pressed in a direction that has a component opposite to a moving direction of the surface of the photosensitive drum  51  at the contact position. 
     The fixing device  8  includes a heating roller  81  and a pressure roller  82 , and thermally fixes an unfixed toner to a sheet. As illustrated in  FIG. 1 , the fixing device  8  is disposed downstream of the process portion  5  and the conveyer belt  7  in a conveying direction of the sheet. The black process unit  50 K is at the most downstream side out of the process units  50  for respective colors, and is closest to the fixing device  8 . Therefore, the periphery of the black process unit  50 K tends to be high temperature compared to the process units for other colors. In other words, the photosensitive drum  51  of the process unit  50 K is closest to the fixing device from among the photosensitive drums  51  of the process units  50 K,  50 C,  50 M, and  50 Y, and thus the temperature of an outer surface of photosensitive drum  51  of the process unit  50  K tends to become higher than those of the photosensitive drums  51  of the remaining process units  50 C,  50 M, and  50 Y. 
     While performing printing color image, the printer  100  transfers sequentially toner images of respective colors formed on the photosensitive drums  51  onto the sheet so as to superimpose the toner images on the sheet. On the other hand, while performing printing monochromatic image, only a black toner image is formed and is transferred onto the sheet. However, in the printer  100 , the photosensitive drums  51  of the process units for all colors simultaneously rotate regardless of the types of image (color image or monochromatic image, for example) to be printed. That is, all of the photosensitive drums  51  rotate even at the time of monochrome printing. 
     Furthermore, as illustrated in  FIG. 2 , the printer  100  includes a housing  70  covering the entire printer  100  and a frame  71  supporting the process portion  5 . The housing  70  is formed with an intake port  73  and an exhaust port  74 , and a fan  76  is disposed inside the exhaust port  74  to discharge air in the printer  100  to the outside thereof. The printer  100  includes a temperature sensor  62  provided near the intake port  73  between the housing  70  and the frame  71 . 
     The temperature sensor  62  outputs different signals depending on a temperature of a space between the housing  70  and the frame  71 . Since the temperature sensor  62  is provided near the intake port  73 , it is assumed that when printing is not executed by the printer  100 , a temperature of air at the position of the temperature sensor  62  is closer to that of air outside of the printer  100 . The temperature at any position inside the printer  100  is likely to be equal to or higher than a temperature obtained on the basis of an output signal of the temperature sensor  62 . 
     Subsequently, an electrical configuration of the printer  100  will be described. As illustrated in  FIG. 3 , the printer  100  according to this embodiment has a controller  30  including a CPU  31 , a ROM  32 , a RAM  33 , and an NVRAM (non-volatile RAM)  34 . In addition, the printer  100  includes the process portion  5 , a network interface  37 , a USB interface  38 , an operation panel  40 , and the temperature sensor  62 , and these components are electrically connected to the controller  30 . The controller  30  illustrated in  FIG. 1  is a general term for hardware such as the CPU  31  used for control of the printer  100 , and does not necessarily represent single hardware existing actually in the printer  100 . 
     The ROM  32  stores firmware that is a control program for controlling the printer  100 , various settings, or initial values. The RAM  33  is used as a work area into which various control programs are read, or as a storage area for temporarily storing image data. The CPU  31  executes various processes according to the control programs read from the ROM  32  or signals sent from various sensors. The CPU  31  controls the respective components of the printer  100  while storing the processed results in the RAM  33  or the NVRAM  34 . The CPU  31  is an example of a processor. The controller  30  may be a processor. 
     The network interface  37  is hardware for communicating with a device connected through a network using a LAN cable. The USB interface  38  is hardware for communicating with a device connected through a USB cable. Furthermore, the operation panel  40  includes a liquid crystal display, and buttons such as a start key, a stop key, and a ten key. The operation panel  40  is hardware for displaying notification to a user and for receiving instruction input by the user. 
     Subsequently, sounds to be generated in the contact part of the blade  56  with the photosensitive drum  51  in the printer  100  will be described. The blade  56  has high hardness and is hardly deformed in a low-temperature state, but tends to be gradually softened as a temperature rises. When the blade  56  is softened, a stick-slip easily occurs at the contact part of the blade  56  with the photosensitive drum  51 . That is, due to the rotation of the photosensitive drum  51 , the end of the blade that is in contact with the photosensitive drum  51  is pressed in a rotation direction of the photosensitive drum  51  and the blade  56  is compressed. When the blade  56  is compressed to a certain extent, the blade  54  returns to an original state by an elastic force of the blade  54 . The blade  54  is repeatedly compressed and returns to the original state, whereby there are cases in which a vibration occurs and sounds are generated. 
     Loudness of the sound generated from the contact part of the blade  56  with the photosensitive drum  51  changes depending on softness of the blade  56  and slipperiness between the photosensitive drum  51  and the blade  56 . For example, as the temperature of the blade  56  or the photosensitive drum  51  is higher, the blade  56  is softened and the loudness of the sound tends to increase. For example, as the amount of the toner remaining in the contact part of the blade  56  with the photosensitive drum  51  becomes larger, the photosensitive drum  51  easily slips on the blade  56 , and thus the loudness of the sound tends to be smaller. That is, the remaining toner functions as lubricant. 
     Since the fixing device  8  has a heat source, the black process unit  50 K disposed near the fixing device  8  easily becomes high temperature compared to the process units  50 Y,  50 M, and  50 C for other colors. In the printer  100 , a large amount of toner is supplied to the black process unit  50 K than the process units for other colors. When a large amount of toner is supplied to the black process unit  50 K than the process units for other colors, the difference in loudness of the sound generated from the contact part of the blade  56  with the photosensitive drum  51  becomes smaller between the black process unit  50 K and each of the process units  50 Y,  50 M, and  50 C for other colors. 
     In order to supply a large amount of toner to the black process unit  50 K than the process units for other colors, the CPU  31  executes a toner supply operation for supplying toner to a developing unit  54  at the timing other than the timing when the image is formed. In the toner supply operation, for example, the CPU  31  forms a toner image of a solid image having a proper width over the entire toner-supplyable range, in the axial direction of the photosensitive drum  51 . In the printer  100 , the photosensitive drum  51  rotates without performing the transfer process by the transfer device  55 , and the formed toner image is scraped by the blade  56 . When the toner image is formed, and the formed toner image is scraped off by the blade  56 , a toner functioning as lubricant is supplied to the contact part of the blade  56  with the photosensitive drum  51 . 
     Some of the toner remaining in the space between the blade  56  and the photosensitive drum  51  pass through the space in association with the rotation of the photosensitive drum  51  or are discharged together with residual toner collected by the blade  56  in the previous transfer process. The amount of toner remaining in the space tends to decrease in association with the rotation of the photosensitive drum  51 . Therefore, the printer  100  repeats the toner supply operation with appropriate frequency to appropriately maintain the amount of toner. 
     Then, the toner supply operation in the black process unit  50 K is executed to supply a large amount of toner per a prescribed rotation number (or a prescribed rotation amount) of the photosensitive drum  51  compared to the toner supply operation in the process units  50 Y,  50 M, and  50 C for other colors. The prescribed rotation amount may be a total angle of the rotations by the photosensitive drum  51  for example. For example, when the prescribed rotation number is 10, the total angle of the rotation is 3600 degrees. Specifically, in the printer  100 , the frequency of the toner supply operation with respect to the black process unit  50 K is higher than that of the toner supply operation with respect to the process units  50 Y,  50 M, and  50 C for other colors. Alternatively, the amount of toner to be supplied once to the black process unit  50 K in the toner supply operation is larger than the amount of toner to be supplied once to the process units  50 Y,  50 M, and  50 C for other colors in the toner supply operation. Alternatively, both of the frequency and the amount of toner to be supplied once for the black process unit  50 K may be larger than those of the process units  50 Y,  50 M, and  50 C. 
     In order to realize the toner supply operation described above, a print process in the printer  100  will be described below with reference to a flowchart illustrated in  FIG. 4 . The print process is executed by the CPU  31  in response to a printing job. 
     The NVRAM  34  stores a sheet number counter for each color, and a continuation counter used in the print process. Each sheet number counter is a counter for storing the number of printed sheets after the previous toner supply operation of the corresponding color. The sheet number counter is reset at the time when the toner supply operation of the corresponding color is started, and is counted up every time execution of printing. Since the toner supply operation may be executed at different timings for respective colors, the sheet number counter is provided for each color. The continuation counter is a counter for storing the number of sheets to be continuously printed. The continuation counter is reset when there is no next job to be continuously executed at the end of a job, and is counted up every time execution of printing. 
     In the print process, in S 101  the CPU  31  reads the sheet number counter and the continuation counter from the NVRAM  34 . In S 102  the CPU  31  acquires a temperature based on the output signal of the temperature sensor  62 , and determines whether the acquired temperature is lower than 40° C. When the acquired temperature is lower than 40° C., it is estimated that the temperature of air outside of the printer  100  is lower than 40° C. The temperature of 40° C. is an example of a prescribed temperature. The prescribed temperature will be described below. When the acquired temperature is lower than 40° C. (S 102 : YES), in S 103  the CPU  31  executes a determination process to determine a threshold sheet number and an image length for each color. 
     The threshold sheet number is the number of printed sheets used for determining the execution frequency of the toner supply operation. The CPU  31  compares the threshold sheet number with the sheet number counter described above. At this time, when the number of printed sheets after the previous toner supply operation exceeds the threshold sheet number, the CPU  31  executes again the toner supply operation. That is, as the threshold sheet number is small, the frequency of the toner supply operation is high. Each of the threshold sheet number and the sheet number counter according to this embodiment is number obtained by converting the rotation number (or, the rotation amount) of the photosensitive drum  51  into the number of printed sheets onto a prescribe size of sheet, such as, an A4-sized sheet, for example. A rotation number itself of the photosensitive drum  51  may be counted instead of counting printed sheet. In this case, both of the threshold sheet number and the sheet number counter are set to values indicating the rotation number of the photosensitive drum  51 . Both of the threshold sheet number and the sheet number counter may be set to values indicating the total angle of the rotation by the photosensitive drum  51 . 
     The image length indicate a length in a sub-scanning direction of the toner image formed on the photosensitive drum  51  by a single toner supply operation. In the toner supply operation, the printer  100  forms, on the photosensitive drum  51 , the toner image of solid image whose length in a circumferential direction of the photosensitive drum  51  is the image length and whose width in the axial direction of the photosensitive drum  51  is a maximum width within which the toner can be supplied (a length of the toner-supplyable range). The solid image has a uniform density of 100% density, for example. That is, as the image length becomes longer, the amount of toner to be supplied by a single toner supply operation becomes larger. According to the embodiment, the image length in the circumferential direction (sub-scanning direction) indicates an amount of toner used when the solid image, which has the image length in the circumferential direction (sub-scanning direction) and the maximum width in the axial direction and has 100% density, is printed. In other words, the image length is a value obtained by converting the amount of toner into a length of the solid image having 100% density in the sub-scanning direction. When the density of the toner image to be formed is not 100%, the image length is a value corresponding to the amount of toner to be supplied based on the density of the image. The image length is an example of a length in the sub-scanning direction while the density of the toner to be supplied is fixed. 
     Next, the determination process executed by the printer  100  will be described with reference to a flowchart illustrated in  FIG. 5 . In the determination process, in S 201  the CPU  31  reads a reference value of the threshold sheet number and a reference value of the image length from the ROM  32 . These reference values are used in common for all colors. The reference value of the threshold sheet number is, for example, 100 sheets, and the reference value of the image length is, for example, 5 mm. 
     Subsequently, in S 202  the CPU  31  reads a correction table for correcting the reference value. The correction table is a table in which correction values are set for respective colors in advance, and is stored in the ROM  32 . The correction table may be stored in the NVRAM  34 . In the correction table, each numerical value may be a design value or may be a value individually determined for an individual device (the printer  100 ) on the basis of a test before shipment. 
     Examples of the correction table read in S 202  are illustrated in  FIGS. 6 to 10 . In each correction table, values are arranged from top to bottom for respective colors of the process units. An order of the values from top to bottom correspond to an order of the process units from upstream to downstream in the conveying direction. That is, as the value is positioned lower in the table, the process unit corresponding to the value is closer to the fixing device  8 . 
     A correction table  21  illustrated in  FIG. 6  and a correction table  22  illustrated in  FIG. 7  are examples of correction tables that store correction values for respective colors. Each of the correction values is added to the image length (a reference value) that is common for all colors while the threshold sheet numbers of all the colors are set to the reference value. Specifically, the correction value of the image length stored in the correction tables  21  and  22  is greater as the process unit is closer to the fixing device  8 . Accordingly, since the image length after the correction is greater as the process unit is closer to the fixing device  8 , the larger the amount of toner to be supplied is in a single toner supply operation, the closer the process unit is to the fixing device  8 . When the threshold sheet number is common to the respective colors, since the toner supply operations are performed simultaneously for all the colors, the number of the execution timings of the toner supply operations is small. For example, in the case where the printing operation is stopped during the toner supply operation, the number of stops of the printing operation is small. 
     In addition, a correction table  23  illustrated in  FIG. 8  and a correction table  24  illustrated in  FIG. 9  are examples of correction tables that store correction values for respective colors. Each of the correction values is added to the threshold sheet number (a reference value) that is common for all colors while the image lengths of all the colors are set to a reference value. Specifically, the correction value stored in the correction tables  23  and  24  is smaller as the process unit is closer to the fixing device  8 . Accordingly, since threshold sheet number after the correction is smaller as the process unit is closer to the fixing device  8 , the execution frequency of the toner supply operation is higher as the process unit is closer to the fixing device  8 . When the image length is common to the respective colors, since the amount of toner supplied in a single toner supply operation can be an optimum amount as a lubricant of the blade  56 , waste of the toner is small. 
     The correction table  21  illustrated in  FIG. 6  and the correction table  23  illustrated in  FIG. 8  are examples in which correction values are different from each other for respective colors and depend on the distance between the fixing device  8  and the respective process units. When influence of heat reach a range from the black process unit  50 K to the cyan process unit  50 C or the magenta process unit  50 M, and thus the process units for the respective colors have different temperatures, the optimum toner supply operation can be performed for the respective colors by using the correction table  21  or the correction table  23 . 
     On the other hand, the correction table  22  illustrated in  FIG. 7  and the correction table  24  illustrated in  FIG. 9  are examples in which only a value for the black process unit  50 K closest to the fixing device  8  is corrected so as to supply a large amount of toner compared to other colors, and values of the process units for colors other than the black are maintained to a reference value. When only the black process unit  50 K is greatly influenced by heat and other colors are little influenced by the heat, the processes can be facilitated using the correction table  22  or the correction table  24 . 
     In addition, a correction table  25  illustrated in  FIG. 10  may be used. The correction table  25  is an example of a correction table that stores correction values for respective colors added to both of the threshold sheet number and the image length. That is, both of the threshold sheet number and the image length may be corrected, and thus amounts of toner supplied per the prescribed rotation number of the photosensitive drum  51  may be different for respective colors. 
     For example, when the prescribed rotation number of the photosensitive drum  51  corresponds to 100 printed-sheets, the amounts of toner for respective colors supplied per the prescribed rotation number can be compared on the basis of conversion values of the image lengths. Each of the conversion values is obtained so that the image length is divided by the threshold sheet number and the divided value is multiplied by 100. A conversion value is 5 for the reference threshold sheet number of 100 sheets and the reference image length of 5 mm. When the correction is performed using the correction table  25  shown in  FIG. 10 , in the order of the process units from the upstream in the conveying direction, the conversion value for yellow is 5; the conversion value for magenta is about 5.6; the conversion value for cyan is about 6.3; and the conversion value for black is 7. That is, even when the correction is performed using the correction table  25 , the amount of toner to be supplied per the prescribed rotation number of the photosensitive drum  51  can be larger as the process unit is closer to the fixing device  8 . 
     Then, in S 203  the CPU  31  corrects the reference value read in S 201  for respective colors by using the correction table read in S 202 . Specifically, for each color, the CPU  31  adds the value(s) of the correction table to the reference value(s). That is, for each color, the CPU  31  adds, to one of the reference threshold sheet number and the reference image length, the corresponding correction value in the correction table, or adds, to both the reference threshold sheet number and the reference image length, the respective values in the correction table, and thus obtains the threshold sheet number and the image length. For example, in the printer  100  using the correction table  21  shown in  FIG. 6 , the threshold sheet number is 100 sheets for all the colors, and the image length for yellow is 5 mm; the image length for magenta is 5.3 mm; the image length for cyan is 5.5 mm; and the image length for black is 7 mm. In S 204 , the CPU  31  stores, in the RAM  33 , the threshold sheet number and the image lengths for respective colors after the correction, and ends the determination process. 
     Returning to the print process of  FIG. 4 , when the CPU  31  judges that the temperature is not lower than 40° C. on the basis of the output signal of the temperature sensor  62  (S 102 : NO), in S 105  the CPU  31  determines the threshold sheet number and the image length to fixed values. When the temperature based on the output signal of the temperature sensor  62  is equal to or higher than 40° C., for example, the temperature becomes high in the whole printer  100 . That is, when the temperature based on the output signal of the temperature sensor  62  is equal to or higher than a prescribed temperature, heat from the fixing device  8  hardly makes the temperature differences between the process units. For this reason, in S 105 , the CPU  31  determines both of the threshold sheet number and the image length to the fixed values common to all the colors. 
     In particular, when the temperature is equal to or higher than 40° C., each blade  56  is softened, and it is desired to increase the amount of toner supplied in the toner supply operation. Thus, in S 105  the fixed values are determined so that the conversion value based on the fixed values is larger than any conversion value based on a reference value and/or a value corrected by using any correction table. In the embodiment, the conversion value based on the fixed values may be 8-9. For example, the threshold sheet number is set to 100 sheets and the image length is set to 9 mm, as the fixed values. Alternatively, the threshold sheet number may be set to 60 sheets and the image length may be set to 5 mm, as the fixed values, for example. Even in the case where the conversion value based on the fixed values is larger than or equal to 18, lubrication performance is little improved, and the toner may be wasted or may cause contamination in the printer  100 . Accordingly, the fixed values may be set that the conversion value of the fixed values is smaller than a prescribed value, for example, 18. 
     In S 107  the CPU  31  judges whether each value of the sheet number counters exceeds the threshold sheet number determined in S 103  or S 105 . As described above, the sheet number counters are used for counting the number of printed sheets after the respective previous toner supply operations and the sheet number counters are provided for all colors. In S 107 , the CPU  31  compares the value of the sheet number counter for each color with the threshold sheet number of the corresponding color. When at least one value of the sheet number counters exceeds the corresponding threshold sheet number, the CPU  31  judges to be “YES” in S 107 . 
     When the CPU  31  judges that at least one value of the sheet number counters exceeds the corresponding threshold sheet number (S 107 : YES), in S 109  the CPU  31  executes the toner supply operation for each color whose value of the sheet number counter exceeds the corresponding threshold sheet number. For example, the CPU  31  temporarily stops the printing operation, the CPU  31  controls the process unit of the color whose value of the sheet number counter exceeds the corresponding threshold sheet number to form the toner image corresponding to the image length determined in advance so that the toner image is scraped off by the blade  56 . Alternatively, the CPU  31  may not stop the printing operation and increase the distance between the sheets so that the toner supply operation can be executed at a timing when there is no sheet at the process portion, that is, a timing after a preceding sheet has passed the process portion  5  and before a subsequent sheet does not reach the process portion  5 . Alternatively, after the end of a print job, the toner supply operation may be executed. S 109  is an example of a supply process. 
     In S 110  the CPU  31  resets, to “0”, the sheet number counter for each color for which the toner supply operation is executed in S 109 . After S 110  or when the CPU  31  judges that none of the values of the sheet number counters for all colors exceeds the respective threshold sheet numbers (S 107 : NO), in S 113  the CPU  31  performs a printing operation on one sheet. In S 114  the printer  100  increments each value of the sheet number counters and the continuation counter one by one. The CPU  31  rotates the photosensitive drums  51  for all the colors during the printing operation, and thus increments the values of the counters for all the colors every time one sheet is printed. 
     In S 116  the CPU  31  judges whether the printing operation of the current print job is completed. When the CPU  31  judges that the print operation is not completed (S 116 : NO), in S 118  the CPU  31  judges whether the continuation counter exceeds a prescribed number. The continuation counter is a counter for counting the number of continuously printed sheets. When a plurality of jobs are continuously executed, the CPU  31  increments the continuation counter by a cumulative value of sheets which is number of sheets that has been printed on the basis of the plurality of continuously executed print jobs. The value of the continuation counter is the number of printed sheets corresponding to the continuous rotation number of the photosensitive drum  51 . 
     When the CPU  31  judges that the continuation counter exceeds the prescribed number (S 118 : YES), in S 119  the CPU  31  determines the threshold sheet number and the image length to fixed values. The threshold sheet number and the image length determined in S 119  are common values for all the colors, and may be the same values as the values determined in S 105 . The prescribed number is, for example, 1000 sheets, and is stored in the ROM  32 . 
     When the number of printed sheets exceeds the prescribed number or when the continuous rotation number of the photosensitive drum  51  exceeds a prescribed rotation number, the temperature becomes high in the whole printer  100 . That is, since this is the same state as a case where the determination is “NO” in S 102 , a large amount of toner is preferably supplied in the toner supply operation. Therefore, in S 119 , the threshold sheet number and the image length are set to common values for each color so that the conversion value based on the set threshold sheet number and the set image length is large. 
     After S 119  or when the CPU  31  judges that the continuation counter does not exceed the prescribed number (S 118 : NO), the procedure returns to S 107 , and the printer  100  compares each of the sheet number counters with the corresponding threshold sheet number. 
     On the other hand, when the CPU  31  judges that that the print job is completed (S 116 : YES), in S 121  the CPU  31  judges whether there is a next job to be continuously executed. When the CPU  31  judges that there is no next job (S 121 : NO), in S 122  the CPU  31  resets the continuation counter to “0”. When there is no next job, the CPU  31  temporarily stops the rotation of the photosensitive drum  51 , and thus the continuous printing operation is interrupted. 
     After S 122  or when the CPU  31  judges that there is the next job (S 121 : YES), in S 124  the CPU  31  stores the value of the sheet number counter and the value of the continuation counter in NVRAM  34 , and ends the print process. Subsequently, the CPU  31  executes the print process for the next print job. 
     As described above in detail, the printer  100  according to the embodiment includes the plurality of process units  50 Y,  50 M,  50 C, and  50 K for respective colors and the fixing device  8 . Each of the process units includes the photosensitive drum  51  and the blade  56  which come in contact with each other. The printer  100  executes the toner supply operation while the printer  100  does not execute the printing operation. In the toner supply operation, the larger amount of toner per the prescribed rotation number of the photosensitive drum  51  is supplied, the closer to the fixing device  8  the process unit is. A larger amount of toner is supplied as the lubricant for the process unit that is closer to the fixing device  8  and whose temperature becomes higher. Accordingly, generation of the sound becomes uniform for all the process unit. Thus, frictional resistance between the photosensitive drum  51  and the blade  56  can be reduced. And, fluctuations of reduction in frictional resistance due to the positional differences of the photosensitive drums  51  can be reduced. So, chattering of each blade can be suppressed. 
     While the disclosure has been described in detail with reference to the above embodiments thereof, it would be apparent to those skilled in the art that various changes and modifications may be made therein. For example, the disclosure is applicable to not only the printer but also a apparatus having a function for forming a color image with an electrophotographic method, such as, a copying machine, a multifunction peripheral, and a facsimile machine. 
     For example, the arrangement order of the process units for respective colors is not limited to the example shown in  FIG. 1 . In the printer  100 , however, since the black process unit  50 K is disposed closest to the fixing device  8  according to the embodiment, the occurrence of the sound can be effectively suppressed by the toner supply operation. This is because there is a high possibility for supplying the black toner at the color printing and the monochrome printing, and a larger amount of residual toner of black is also high than other colors after the image transfer. 
     Furthermore, the temperature sensor  62  may be disposed outside of the frame  71 . Moreover, the temperature sensor  62  may be attached to the frame  71  (outer surface of the frame  71 , for example) or may be attached to the housing  70 . 
     The judgement based on the temperature sensor  62  (S 102 ) or the judgement based on the number of sheets continuously printed (S 118 ) may be omitted. That is, the determination process in S 103  may be executed regardless of the temperature. Furthermore, the threshold sheet number or the image length determined in the determination process in S 103  may be used regardless of the number of sheets continuously printed. In S 102 , the temperature of 40° C. used for comparison is an example, and is not limited thereto. 
     The solid image is formed with 100% density in the embodiment, but is not limited to 100% density. For example, the amount of toner can be differently supplied as follows. That is, the densities may be varied among the colors while fixing, to a constant value, lengths of the solid image in the sub-scanning direction for all the colors, instead of varying the image lengths of the solid images of 100% density for the colors. For example, the toner image of 100% density may be formed in the toner supply operation of the black, and the toner image of 50% density may be formed in the toner supply operation of other colors. That is, as the process unit is closer to the fixing device  8 , the higher density of the toner is supplied and thus a larger amount of toner may be supplied in a single toner supply operation. 
     All the numerical values of each correction table described in the embodiment are merely examples and are not limited thereto. In the examples using the correction tables  23  and  24 , values of the sheet number counters are compared with the respective threshold sheet numbers that are different from each other for respective colors. However, the CPU  31  may calculate reference values by adding different correction values to the values of the sheet number counters for respective colors, and compare the obtained reference values with a prescribed threshold value. In this case, the closer the process unit is to the fixing device  8 , the larger the correction value is. 
     Part of the configuration of the invention implemented in hardware in the embodiment described above may be replaced by software and, conversely, part of the configuration of the invention implemented in software may be replaced by hardware. 
     The processes in the embodiment are performed by a single CPU, a plurality of CPUs, or a hardware such as ASIC, or any combination thereof. The processes are performed on the basis of the recording medium storing the programs for executing the processes and/or methods. 
     When all or part of the functions of the present disclosure are implemented with computer programs, the programs can be stored on a computer-readable storage medium (a non-temporary storage medium, for example). The programs may be used on the same storage medium on which they are supplied, or may be transferred to a different storage medium (computer-readable storage medium). The “computer-readable storage medium” may be a portable storage medium, such as a memory card or a CD-ROM; an internal storage device built into the computer, such as any of various ROM or the like; or an external storage device, such as a hard disk drive, connected to the computer.