Patent Publication Number: US-9846389-B2

Title: Developer supply control in electrophotographic type image forming apparatus

Description:
INCORPORATION BY REFERENCE 
     This application is based upon and claims the benefit of priority from the corresponding Japanese Patent Application No. 2015-208041 filed on Oct. 22, 2015, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     The present disclosure relates to an electrophotographic type image forming apparatus and an image forming method. 
     An image forming apparatus, such as a printer, which is capable of forming an image by electrophotography includes a developing portion. The developing portion develops an electrostatic latent image formed on an image carrier such as a photosensitive drum. For example, the developing portion has a circulation-conveyance passage through which a developer is conveyed so as to be circulated, a conveying member that conveys the developer in the circulation-conveyance passage, and an opening portion that is provided in a wall surface at the upper side of the circulation-conveyance passage. A developer supply portion that supplies the developer to the interior of the developing portion is connected to the opening portion of the developing portion. 
     Here, an image forming apparatus is known in which an accumulation portion for the developer is formed in the circulation-conveyance passage and below the opening portion, so that the amount of the developer supplied from the developer supply portion can be caused to follow the amount of the developer consumed at the developing portion. In this image forming apparatus, when printing with a high coverage rate is consecutively executed, the amount of the developer supplied from the developer supply portion may not follow the amount of the developer consumed in the developing portion, so that the amount of the developer held in the developing portion may decrease. In this case, the amount of the developer supplied to the image carrier may become insufficient, so that a defect called void may occur in an image formed by the image forming apparatus. On the other hand, an image forming apparatus is known which is capable of executing a developer supply mode in which, when printing with a high coverage rate is consecutively executed, the developer within the circulation-conveyance passage is circulated by using the conveying member to continue supply of the developer from the developer supply portion to the developing portion, while execution of a printing process is suspended. 
     SUMMARY 
     An image forming apparatus according to one aspect of the present disclosure includes an image carrier, a developing portion, a developer supply portion, a first rotation control portion, and a second rotation control portion. The developing portion has a circulation-conveyance passage through which a developer is conveyed so as to be circulated, a conveying member configured to convey the developer within the circulation-conveyance passage in a predetermined direction, an opening portion formed at an upper side of the circulation-conveyance passage and configured to guide the developer supplied from outside, to the circulation-conveyance passage, and a reducing portion provided in the conveying member and configured to reduce an ability to convey the developer. The developing portion is configured to develop an electrostatic latent image formed on the image carrier, by using the developer conveyed in the circulation-conveyance passage. The developer supply portion is connected to the opening portion and configured to supply the developer to the developing portion. The first rotation control portion is configured to rotate the conveying member at a predetermined first speed when a printing process of forming an image on the basis of image data is executed. The second rotation control portion is configured to suspend execution of the printing process and rotate the conveying member at a predetermined second speed that is lower than the first speed, when an average coverage rate per page of a printed matter having a predetermined reference number of pages that are consecutively printed by the printing process exceeds a predetermined threshold. 
     An image forming method according to another aspect of the present disclosure is executed by an image forming apparatus including: an image carrier; a developing portion having a circulation-conveyance passage through which a developer is conveyed so as to be circulated, a conveying member configured to convey the developer within the circulation-conveyance passage in a predetermined direction, an opening portion formed at an upper side of the circulation-conveyance passage and configured to guide the developer supplied from outside, to the circulation-conveyance passage, and a reducing portion provided in the conveying member and configured to reduce an ability to convey the developer, the developing portion being configured to develop an electrostatic latent image formed on the image carrier, by using the developer conveyed in the circulation-conveyance passage; and a developer supply portion connected to the opening portion and configured to supply the developer to the developing portion, and includes a first step and a second step. In the first step, when a printing process of forming an image on the basis of image data is executed, the conveying member is rotated at a predetermined first speed. In the second step, when an average coverage rate per page of a printed matter having a predetermined reference number of pages that are consecutively printed by the printing process exceeds a predetermined threshold, execution of the printing process is suspended and the conveying member is rotated at a predetermined second speed that is lower than the first speed. 
     This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description with reference where appropriate to the accompanying drawings. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. Furthermore, the claimed subject matter is not limited to implementations that solve any or all disadvantages noted in any part of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing the configuration of an image forming apparatus according to an embodiment of the present disclosure. 
         FIG. 2  is a block diagram showing the system configuration of the image forming apparatus according to the embodiment of the present disclosure. 
         FIG. 3  is a diagram showing the configuration of a developing portion of the image forming apparatus according to the embodiment of the present disclosure. 
         FIG. 4  is a diagram showing the configuration of a part of the developing portion of the image forming apparatus according to the embodiment of the present disclosure. 
         FIG. 5  is a diagram showing the configuration of a developer supply portion of the image forming apparatus according to the embodiment of the present disclosure. 
         FIG. 6  is a flowchart showing an example of a rotation control process executed by the image forming apparatus according to the embodiment of the present disclosure. 
         FIG. 7  is a diagram showing results of experiments using the image forming apparatus according to the embodiment of the present disclosure. 
         FIG. 8  is a diagram showing results of the experiments using the image forming apparatus according to the embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, an embodiment of the present disclosure will be described with reference to the accompanying drawings in order to allow understanding of the present disclosure. It should be noted that the following embodiment is an example embodying the present disclosure and does not limit the technical scope of the present disclosure. 
     [Schematic Configuration of Image Forming Apparatus  10 ] 
     First, a schematic configuration of an image forming apparatus  10  according to the embodiment of the present disclosure will be described with reference to  FIGS. 1 and 2 . Here,  FIG. 1  is a cross-sectional schematic diagram showing the configuration of the image forming apparatus  10 . 
     The image forming apparatus  10  is a multifunction peripheral having a plurality of functions such as a scanning function to read image data from a document and a printing function to form an image on the basis of image data, as well as a facsimile function, a copy function, etc. The present disclosure is applicable to image forming apparatuses such as a printer apparatus, a facsimile apparatus, and a copy machine. 
     As shown in  FIG. 1 , the image forming apparatus  10  includes an automatic document feeder (ADF)  1 , an image reading portion  2 , an image forming portion  3 , a sheet feed portion  4 , an operation display portion  6 , and a control portion  7 . 
     The ADF  1  includes a document set portion, a plurality of conveying rollers, a document holder, and a sheet discharge portion. At the ADF  1 , a document P set on the document set portion is conveyed by the plurality of conveying rollers to the sheet discharge portion via an image reading position where an image is read by the image reading portion  2 . 
     The image reading portion  2  includes a document table, a light source, a plurality of mirrors, an optical lens, and a CCD. At the image reading portion  2 , image data is read from a document P placed on the document table or from a document P conveyed by the ADF  1 . Specifically, in the image reading portion  2 , light emitted from the light source to the document P and reflected by the document P is inputted via the plurality of mirrors and the optical lens into the CCD, whereby the image data on the document P is read. 
     The operation display portion  6  includes: a display portion, such as a liquid crystal display, which displays various kinds of information in accordance with control instructions from the control portion  7 ; and an operation portion, such as an operation key or a touch panel, which inputs various kinds of information to the control portion  7  in accordance with operations of a user. 
     The control portion  7  includes control devices such as a CPU, a ROM, a RAM, and an EEPROM that are not shown. The CPU is a processor that executes various calculation processes. The ROM is a non-volatile storage portion in which information such as a control program for causing the CPU to execute various processes is stored in advance. The RAM is a volatile storage portion, and the EEPROM is a non-volatile storage portion. The RAM and the EEPROM are used as temporary storage memories (working areas) for various processes executed by the CPU. At the control portion  7 , various control programs stored in advance in the ROM are executed by the CPU. Accordingly, overall control of the image forming apparatus  10  is performed by the control portion  7 . The control portion  7  may be composed of an electronic circuit such as an integrated circuit (ASIC) or may be a control portion provided independently of a main control portion that performs overall control of the image forming apparatus  10 . 
     The image forming portion  3  is capable of executing a printing process of forming an image by electrophotography on the basis of image data read by the image reading portion  2 . In addition, the image forming portion  3  is also capable of executing the printing process on the basis of image data inputted from an information processing apparatus such as an external personal computer. As shown in  FIG. 1 , the image forming portion  3  includes a photosensitive drum  31 , a charging device  32 , a laser scanning unit (LSU)  33 , a developing portion  5 , a developer supply portion  34 , a transfer roller  35 , a cleaning device  36 , a fixing roller  37 , a pressure roller  38 , and a sheet discharge tray  39 . 
     The sheet feed portion  4  feeds a sheet S on which an image is to be formed at the image forming portion  3 . As shown in  FIG. 1 , the sheet feed portion  4  includes a sheet feed cassette  41  that contains sheets S. At the sheet feed portion  4 , the sheets S contained in the sheet feed cassette  41  are taken out one by one, and are fed to an image forming position where an image is formed by the image forming portion  3 . 
     At the image forming portion  3 , an image is formed on the sheet S fed from the sheet feed portion  4 , by the following procedure, and the sheet S having the image formed thereon is discharged to the sheet discharge tray  39 . The sheets S are sheet materials such as paper, coated paper, postcards, envelopes, OHP sheets, or the like. 
     First, the surface of the photosensitive drum  31  is uniformly charged at a predetermined potential by the charging device  32 . Next, light based on image data is applied by the laser scanning unit  33  to the surface of the photosensitive drum  31 . Accordingly, an electrostatic latent image corresponding to the image data is formed on the surface of the photosensitive drum  31 . Then, the electrostatic latent image on the photosensitive drum  31  is developed (visualized) as a toner image by the developing portion  5 . A developer is supplied to the developing portion  5  from the developer supply portion  34  that is mountable to and detachable from the image forming portion  3 . Here, the photosensitive drum  31  is an example of an image carrier in the present disclosure. 
     Subsequently, the toner image formed on the photosensitive drum  31  is transferred onto the sheet S by the transfer roller  35 . Thereafter, the toner image transferred onto the sheet S is heated by the fixing roller  37  to be melted and fixed on the sheet S, when the sheet S passes through between the fixing roller  37  and the pressure roller  38 . Toner remaining on the surface of the photosensitive drum  31  is removed by the cleaning device  36 . 
     [Configurations of Developing Portion  5  and Developer Supply Portion  34 ] 
     Next, the developing portion  5  and the developer supply portion  34  will be described in detail with reference to  FIGS. 3 to 5 . Here,  FIG. 3  is a cross-sectional schematic diagram showing the configuration of the developing portion  5 .  FIG. 4  is a plan schematic diagram showing a first conveying member  52  and a second conveying member  53  of the developing portion  5 .  FIG. 5  is a cross-sectional schematic diagram showing the configuration of the developer supply portion  34 . 
     The developing portion  5  develops an electrostatic latent image on the photosensitive drum  31  by using toner contained in the developer. For example, the developer is a single-component developer that contains magnetic toner as a principal component. The developer may be a single-component developer that contains non-magnetic toner as a principal component. 
     As shown in  FIGS. 3 and 4 , the developing portion  5  includes a housing  51 , the first conveying member  52 , the second conveying member  53 , a developing roller  54 , and a restricting member  55 . 
     The housing  51  contains each component of the developing portion  5  and also contains the developer. The housing  51  is long along the right-left direction of the image forming apparatus  10  (the depth direction of the sheet surface of  FIG. 3 ). As shown in  FIGS. 3 and 4 , the interior of the housing  51  is divided into a first conveyance passage  51 B and a second conveyance passage  51 C by a partition wall  51 A. The partition wall  51 A is provided so as to be long along the longitudinal direction of the housing  51 . Communication passages  51 F and  51 G are provided in both end portions, in the longitudinal direction, of the partition wall  51 A so as to provide communication between the first conveyance passage  51 B and the second conveyance passage  51 C. 
     As shown in  FIG. 3 , an opening portion  51 D is provided in an upper wall surface of the housing  51  and at the first conveyance passage  51 B. The opening portion  51 D is a through hole that guides the developer supplied from the developer supply portion  34 , to the first conveyance passage  51 B. An opening portion  51 E is provided in a rear end portion of the housing  51  and at the second conveyance passage  51 C. In the image forming apparatus  10 , the developing portion  5  is disposed at such a position that the opening portion  51 E is opposed to the photosensitive drum  31 . 
     As shown in  FIG. 5 , the developer supply portion  34  is connected to the opening portion  51 D of the developing portion  5 . The developer supply portion  34  contains the developer to be supplied to the developing portion  5 . The developer supply portion  34  is long along the right-left direction of the image forming apparatus  10 , and is configured to be mountable to and detachable from the image forming portion  3 . In a state where the developer supply portion  34  is mounted on the image forming portion  3 , it is made possible to supply the developer from the developer supply portion  34  to the opening portion  51 D. 
     A conveying member  341  is provided within the developer supply portion  34 . The conveying member  341  continuously supplies the developer contained in the developer supply portion  34 , from the developer supply portion  34  to the opening portion  51 D. The conveying member  341  is rotatably supported at wall surfaces of both ends, in the longitudinal direction, of the developer supply portion  34 . For example, the conveying member  341  is a conveyance screw having a helical blade formed along a rotation shaft thereof. In  FIG. 5 , the conveying member  341  is shown by a dotted line, since the conveying member  341  is located within the developer contained in the developer supply portion  34 . A driving force is supplied from a driving portion  3 A (see  FIG. 2 ) such as a motor to the conveying member  341 . Accordingly, the conveying member  341  is rotated, so that the developer contained in the developer supply portion  34  is conveyed in the axial direction of the rotation shaft of the conveying member  341  by the helical blade. 
     In the developer supply portion  34 , an exhaust port  34 A is formed below the rotation shaft of the conveying member  341  so as to lead to the opening portion  51 D. When the conveying member  341  is rotationally driven, the developer within the developer supply portion  34  is supplied through the exhaust port  34 A to the opening portion  51 D. Accordingly, the developer within the developer supply portion  34  moves to the opening portion  51 D and is supplied from the opening portion  51 D to the first conveyance passage  51 B of the housing  51 . 
     The first conveying member  52  is rotatably provided in the first conveyance passage  51 B of the housing  51 . For example, as shown in  FIGS. 3 and 4 , the first conveying member  52  is a conveyance screw having a helical blade  52 B formed along a rotation shaft  52 A thereof. A driving force is supplied from the driving portion  3 A to the first conveying member  52 . Accordingly, the first conveying member  52  is rotated in a rotation direction  52 C, so that the developer contained in the first conveyance passage  51 B is conveyed along a conveyance direction  52 D, while being agitated, by the helical blade  52 B. In addition, the developer becomes triboelectrically charged due to the agitation by the helical blade  52 B. Here, the first conveying member  52  is an example of a conveying member in the present disclosure. In addition, the conveyance direction  52 D is an example of a predetermined direction in the present disclosure. 
     The developer conveyed by the first conveying member  52  flows through the communication passage  51 F provided at the downstream side, in the conveyance direction  52 D, of the first conveyance passage  51 B and is contained in the second conveyance passage  51 C. 
     The second conveying member  53  is rotatably provided in the second conveyance passage  51 C of the housing  51 . For example, as shown in  FIGS. 3 and 4 , the second conveying member  53  is a conveyance screw having a helical blade  53 B formed along a rotation shaft  53 A thereof. A driving force is supplied from the driving portion  3 A to the second conveying member  53 . Accordingly, the second conveying member  53  is rotated in a rotation direction  53 C, so that the developer contained in the second conveyance passage  51 C is conveyed along a conveyance direction  53 D, while being agitated, by the helical blade  53 B. In addition, the developer becomes triboelectrically charged due to the agitation by the helical blade  53 B. 
     A part of the developer conveyed by the second conveying member  53  is supplied to the developing roller  54 . The developing portion  5  develops an electrostatic latent image formed on the photosensitive drum  31 , by using the developer supplied from the second conveying member  53  to the developing roller  54 . 
     In addition, the remaining part of the developer conveyed by the second conveying member  53  flows through the communication passage  51 G provided at the downstream side, in the conveyance direction  53 D, of the second conveyance passage  51 C and is contained in the first conveyance passage  51 B. That is, at the developing portion  5 , the first conveyance passage  51 B, the communication passage  51 F, the second conveyance passage  51 C, and the communication passage  51 G form a circulation-conveyance passage through which the developer is conveyed so as to be circulated. In the developing portion  5 , the developer contained in the housing  51  is conveyed so as to be circulated through the circulation-conveyance passage by the first conveying member  52  and the second conveying member  53 . 
     Here, in the image forming apparatus  10 , as shown in  FIG. 4 , a reducing portion  52 E is provided in the first conveying member  52 . The reducing portion  52 E reduces the ability to convey the developer by the first conveying member  52 . For example, at the reducing portion  52 E, the helical blade  52 B is cut such that the outer diameter thereof is smaller than that at the other portion. Accordingly, in a region of the first conveyance passage  51 B where the reducing portion  52 E is provided, the amount of the conveyed developer is reduced, and an accumulation portion T (see  FIG. 5 ) for the developer is formed. At the reducing portion  52 E, the pitch of the helical blade  52 B in the axial direction of the rotation shaft  52 A may be set so as to be shorter than that at the other portion. In addition, at the reducing portion  52 E, a cutout may be provided in the helical blade  52 B. Furthermore, at the reducing portion  52 E, rib members may be disposed on the peripheral edge portion of the helical blade  52 B at equal intervals so as to be parallel to the rotation shaft  52 A. 
     In the image forming apparatus  10 , as shown in  FIG. 4 , the opening portion  51 D is formed at the upper side of the reducing portion  52 E. For example, the opening portion  51 D is formed at the upper side of the downstream side, in the conveyance direction  52 D, of the reducing portion  52 E. Thus, supply of the developer from the developer supply portion  34  is restricted by the accumulation portion T, for the developer, which is formed below the opening portion  51 D by the reducing portion  52 E. Specifically, in the image forming apparatus  10 , when the printing process is executed so that the amount of the developer held in the circulation-conveyance passage decreases, a force, by the accumulation portion T, pushing the developer brought in from the opening portion  51 D, back to the developer supply portion  34  decreases, so that the developer is supplied from the developer supply portion  34 . On the other hand, in the image forming apparatus  10 , when the amount of the developer held in the circulation-conveyance passage increases due to the supply of the developer from the developer supply portion  34 , the force of pushing the developer brought in from the opening portion  51 D, back to the developer supply portion  34  by the accumulation portion T increases, so that the supply of the developer from the developer supply portion  34  is stopped. That is, in the image forming apparatus  10 , the amount of the developer supplied from the developer supply portion  34  follows the amount of the developer consumed at the developing portion  5 . 
     The developing roller  54  is rotatably provided in the housing  51 . As shown in  FIG. 3 , the developing roller  54  is provided at a position, in the second conveyance passage  51 C, rearward of the second conveying member  53  in the image forming apparatus  10  so as to be opposed to the second conveying member  53 . A part of a circumferential surface  54 B of the developing roller  54  is exposed from the opening portion  51 E to the outside of the housing  51 . The part of the circumferential surface  54 B exposed to the outside of the housing  51  is opposed to the photosensitive drum  31 . The developing roller  54  holds the developer contained in the housing  51 , on the circumferential surface  54 B. 
     As shown in  FIG. 3 , the developing roller  54  includes a developing sleeve  541  and a magnet  542 . The developing sleeve  541  has a cylindrical shape and is rotatably provided in the housing  51 . For example, the developing sleeve  541  is formed from a non-magnetic material such as aluminum. For example, the circumferential surface (circumferential surface  54 B) of the developing sleeve  541  is subjected to a roughening treatment for adjusting the amount of the developer held thereon and the amount of friction thereof. 
     The magnet  542  is fixedly provided within the developing sleeve  541 . The magnet  542  has a plurality of magnetic poles. For example, the magnet  542  has a restricting pole that generates a peak magnetic force at a position opposing the restricting member  55 , and a developing pole that generates a peak magnetic force at a position opposing the photosensitive drum  31 , etc. 
     At the developing roller  54 , the developer having magnetism is attracted to the circumferential surface  54 B by the magnetic pole of the magnet  542 . Meanwhile, the developer is triboelectrically charged due to the agitation by the first conveying member  52  and the second conveying member  53 , and thus adheres to the circumferential surface  54 B due to electrostatic force. Therefore, a part of the developer conveyed by the second conveying member  53  adheres to the circumferential surface  54 B at a supply position P 1  (see  FIG. 3 ) at which the second conveying member  53  and the developing roller  54  are opposed to each other. Accordingly, the developer is held on the circumferential surface  54 B. 
     A driving force is supplied from the driving portion  3 A to the developing roller  54 . Accordingly, the developing roller  54  is rotated in a rotation direction  54 A, so that the developer held on the circumferential surface  54 B is conveyed from the supply position P 1  to a development position P 2  (see  FIG. 3 ) at which the developing roller  54  is opposed to the photosensitive drum  31 . In the case where a single-component developer that is composed of non-magnetic toner not having magnetism instead of magnetic toner, it is not necessary to provide magnetic poles within the developing sleeve  541 . 
     The restricting member  55  is provided with a gap relative to the circumferential surface  54 B of the developing roller  54 . As shown in  FIG. 3 , the restricting member  55  is provided at the downstream side, in the rotation direction  54 A, of the supply position P 1  and at the upstream side, in the rotation direction  54 A, of the development position P 2 . The restricting member  55  is provided so as to be long along the longitudinal direction of the housing  51 . For example, the restricting member  55  has a blade shape and is formed from a metal having magnetism, or the like. A layer thickness of the developer held on the circumferential surface  54 B of the developing roller  54  is restricted by the restricting member  55 . Accordingly, a thin layer of the developer having a uniform thickness is formed on the circumferential surface  54 B of the developing roller  54 . The gap between the restricting member  55  and the circumferential surface  54 B of the developing roller  54  may be arbitrarily set in accordance with the layer thickness of the developer formed by the restricting member  55 . 
     Meanwhile, in the image forming apparatus  10 , when printing with a high coverage rate is consecutively executed, the amount of the developer supplied from the developer supply portion  34  may not follow the amount of the developer consumed at the developing portion  5 , so that the amount of the developer held in the developing portion  5  may decrease. In this case, the amount of the developer supplied to the photosensitive drum  31  may become insufficient, so that a defect called void may occur in an image formed by the image forming apparatus  10 . On the other hand, an image forming apparatus is known which is capable of executing a developer supply mode in which, when printing with a high coverage rate is consecutively executed, the developer within the circulation-conveyance passage is circulated by using the first conveying member  52  and the second conveying member  53  to continue supply of the developer from the developer supply portion  34  to the developing portion  5 , while execution of the printing process is suspended. 
     Here, in the developer supply mode, the developer supplied from the developer supply portion  34  via the opening portion  51 D circulates through the circulation-conveyance passage without being consumed. Thus, in the developer supply mode, a larger amount of the developer is brought in to the upstream side, in the conveyance direction  52 D, of the accumulation portion T (see  FIG. 5 ), than that during execution of the printing process. Accordingly, the pressure at the accumulation portion T increases, so that supply of the developer from the developer supply portion  34  is hindered. That is, in the developer supply mode, when a certain time elapses, the amount of the developer supplied from the developer supply portion  34  decreases. On the other hand, in the image forming apparatus  10  according to the embodiment of the present disclosure, as described later, it is possible to inhibit a decrease in the amount of the developer supplied from the developer supply portion  34  in the developer supply mode, to shorten a supply time of the developer in the developer supply mode. 
     Specifically, a rotation control program for causing the CPU of the control portion  7  to execute a later-described rotation control process (see a flowchart in  FIG. 6 ) is stored in advance in the ROM of the control portion  7 . The rotation control program may be stored in a computer-readable storage medium such as a flash memory, read from the recording medium, and installed into the EEPROM or the like. 
     As shown in  FIG. 2 , the control portion  7  includes a first rotation control portion  71 , a second rotation control portion  72 , and a restart processing portion  73 . Specifically, the control portion  7  executes the rotation control program stored in the ROM, by using the CPU. Accordingly, the control portion  7  functions as the first rotation control portion  71 , the second rotation control portion  72 , and the restart processing portion  73 . 
     When the printing process is executed by the image forming portion  3 , the first rotation control portion  71  rotates the first conveying member  52  and the second conveying member  53  at a predetermined first speed. For example, the first rotation control portion  71  transmits a control signal to the driving portion  3 A to cause the driving portion  3 A to rotate the first conveying member  52  and the second conveying member  53  at the first speed. The first speed is set to an appropriate value in accordance with a printing speed of the image forming apparatus  10 . 
     When a predetermined mode shift condition is satisfied, the second rotation control portion  72  suspends execution of the printing process and rotates the first conveying member  52  and the second conveying member  53  at a predetermined second speed that is lower than the first speed. That is, in the image forming apparatus  10 , a state where the first conveying member  52  and the second conveying member  53  are rotated at the second speed by the second rotation control portion  72  is the developer supply mode. 
     Specifically, the mode shift condition is that an average coverage rate per page of a printed matter having a predetermined reference number of pages that are consecutively printed by the printing process exceeds a predetermined threshold. For example, in the image forming apparatus  10 , the reference number of pages is set to 250 pages, and the threshold is set to 50%. The reference number of pages and the threshold may be arbitrarily set by an operation of the user on the operation display portion  6 . 
     For example, each time a printed matter of one page is outputted by the printing process, the second rotation control portion  72  obtains the coverage rate of the printed matter of one page on the basis of image data. Then, when the cumulative total number of pages of a printed matter printed by the printing process reaches the reference number of pages, the second rotation control portion  72  calculates the average coverage rate per page of the printed matter having the reference number of pages. In addition, after the cumulative total number of pages exceeds the reference number of pages, each time a printed matter of one page is outputted, the second rotation control portion  72  calculates the average coverage rate per page of a printed matter having the reference number of pages traced back from this outputted printed matter. 
     When the sum of the cumulative total number of pages in the printing process executed previously and the cumulative total number of pages in the printing process executed next reaches the reference number of pages, the second rotation control portion  72  may calculate the average coverage rate per page of the printed matter having the reference number of pages. 
     For example, when the mode shift condition is satisfied, the second rotation control portion  72  stops input of image data from the control portion  7  to the laser scanning unit  33  and also stops application of a voltage to the charging device  32  and the transfer roller  35  by a power supply, which is not shown, to suspend execution of the printing process. 
     Then, the second rotation control portion  72  transmits a control signal to the driving portion  3 A to cause the driving portion  3 A to rotate the first conveying member  52  and the second conveying member  53  at the second speed. For example, the second speed is half the first speed. The second rotation control portion  72  may not decrease the rotation speed of the second conveying member  53  and may rotate only the first conveying member  52  at the second speed. 
     After a predetermined supply time elapses, the restart processing portion  73  restarts execution of the printing process. For example, in the image forming apparatus  10 , the supply time is set to three minutes. The supply time may be arbitrarily set by an operation of the user on the operation display portion  6 . 
     For example, after the supply time elapses, the restart processing portion  73  restarts input of image data from the control portion  7  to the laser scanning unit  33  and also restarts application of a voltage to the charging device  32  and the transfer roller  35  by the power supply, which is not shown, to restart execution of the printing process. 
     When the number of remaining pages to be printed at the time of suspension of the printing process is less than a predetermined specific number of pages, the restart processing portion  73  shortens the supply time. For example, in the image forming apparatus  10 , the specific number of pages is set to 10 pages. The specific number of pages may be arbitrarily set by an operation of the user on the operation display portion  6 . 
     For example, when the number of remaining pages to be printed at the time of suspension of the printing process is less than the specific number of pages, the restart processing portion  73  shortens the supply time to a time that is half the supply time. 
     When the supply time is shortened by the restart processing portion  73 , the second rotation control portion  72  may rotate the first conveying member  52  and the second conveying member  53  at the second speed for the shortened time after end of the printing process. 
     [Rotation Control Process] 
     Hereinafter, together with an example of the procedure of the rotation control process executed by the control portion  7  in the image forming apparatus  10 , an image forming method of the present disclosure will be described with reference to  FIG. 6 . Here, S 1 , S 2  . . . represent numbers of process procedures (steps) executed by the control portion  7 . When the printing process is executed in the image forming apparatus  10 , the rotation control process is executed together with the printing process. 
     &lt;Step S 1 &gt; 
     First, in step S 1 , the control portion  7  rotates the first conveying member  52  and the second conveying member  53  at the first speed. For example, the control portion  7  transmits a control signal to the driving portion  3 A to cause the driving portion  3 A to rotate the first conveying member  52  and the second conveying member  53  at the first speed. Here, the process in step S 1  is an example of a first step in the present disclosure, and is executed by the first rotation control portion  71  of the control portion  7 . 
     &lt;Step S 2 &gt; 
     In step S 2 , the control portion  7  determines whether the printing process has ended. 
     Here, when the control portion  7  determines that the printing process has ended (Yes in S 2 ), the control portion  7  ends the rotation control process. When the printing process has not ended (No in S 2 ), the control portion  7  shifts the process to step S 3 . 
     &lt;Step S 3 &gt; 
     In step S 3 , the control portion  7  determines whether the mode shift condition has been satisfied. 
     For example, each time a printed matter of one page is outputted by the printing process, the control portion  7  obtains the coverage rate of the printed matter of one page on the basis of image data. Subsequently, when the cumulative total number of pages of a printed matter printed by the printing process reaches the reference number of pages, the control portion  7  calculates the average coverage rate per page of the printed matter having the reference number of pages. In addition, after the cumulative total number of pages exceeds the reference number of pages, each time a printed matter of one page is outputted, the control portion  7  calculates the average coverage rate per page of a printed matter having the reference number of pages traced back from this outputted printed matter. Then, when the calculated average coverage rate exceeds the threshold, the control portion  7  determines that the mode shift condition has been satisfied. 
     Here, when the control portion  7  determines that the mode shift condition has been satisfied (Yes in S 3 ), the control portion  7  shifts the process to step S 4 . When the mode shift condition has not been satisfied (No in S 3 ), the control portion  7  shifts the process to step S 2  and waits for end of the printing process or satisfaction of the mode shift condition. 
     &lt;Step S 4 &gt; 
     In step S 4 , the control portion  7  suspends execution of the printing process. For example, the control portion  7  stops input of image data to the laser scanning unit  33  and also stops application of a voltage to the charging device  32  and the transfer roller  35  by the power supply, which is not shown, to suspend execution of the printing process. 
     &lt;Step S 5 &gt; 
     In step S 5 , the control portion  7  rotates the first conveying member  52  and the second conveying member  53  at the second speed. For example, the control portion  7  transmits a control signal to the driving portion  3 A to cause the driving portion  3 A to rotate the first conveying member  52  and the second conveying member  53  at the second speed. Here, the processes in steps S 3  to S 5  are an example of a second step in the present disclosure, and are executed by the second rotation control portion  72  of the control portion  7 . 
     When the rotation speed of each of the first conveying member  52  and the second conveying member  53  is decreased to the second speed, the time taken until the developer supplied from the opening portion  51 D during execution of the developer supply mode reaches the upstream side, in the conveyance direction  52 D, of the accumulation portion T (see  FIG. 5 ), becomes long. Accordingly, timing at which supply of the developer from the developer supply portion  34  is restricted by the accumulation portion T in the developer supply mode is delayed. Therefore, a decrease in the amount of the developer supplied from the developer supply portion  34  in the developer supply mode is inhibited. 
     At the downstream end, in the conveyance direction  52 D, of the accumulation portion T, accumulation of the developer is collapsed by the pressure of the developer conveyed by the reducing portion  52 E of the first conveying member  52  and the pressure of the developer supplied from the opening portion  51 D, and the developer collapsed from the accumulation of the developer is conveyed by the first conveying member  52 . Here, when the rotation speed of the first conveying member  52  is decreased from the first speed to the second speed, the conveyance speed at the reducing portion  52 E of the first conveying member  52  decreases. However, the amount of the developer conveyed at the reducing portion  52 E is smaller than that at the other portion. Thus, even when the rotation speed of the first conveying member  52  is decreased, the ability to convey the developer at the reducing portion  52 E almost does not decrease. Therefore, even when the rotation speed of the first conveying member  52  is decreased, influence on the amount of the accumulation of the developer collapsed at the downstream end, in the conveyance direction  52 D, of the accumulation portion T (the amount of the developer conveyed from the accumulation portion T) is small, so that the amount of the developer supplied from the developer supply portion  34  almost does not decrease. 
     &lt;Step S 6 &gt; 
     In step S 6 , the control portion  7  determines whether the number of remaining pages to be printed at the time of suspension of the printing process is less than the specific number of pages. 
     Here, when the control portion  7  determines that the number of remaining pages to be printed at the time of suspension of the printing process is less than the specific number of pages (Yes in S 6 ), the control portion  7  shifts the process to the step S 7 . When the number of remaining pages to be printed at the time of suspension of the printing process is not less than the specific number of pages (No in S 6 ), the control portion  7  shifts the process to step S 8 . 
     &lt;Step S 7 &gt; 
     In step S 7 , the control portion  7  shortens the supply time. For example, the control portion  7  shortens the supply time to a time that is half the supply time. 
     &lt;Step S 8 &gt; 
     In step S 8 , the control portion  7  determines whether the supply time has elapsed from the time of suspension of the printing process in step S 4 . When the supply time is shortened in step S 7 , the control portion  7  determines whether the supply time shortened in step S 7  has elapsed from the time of suspension of the printing process in step S 4 . 
     Here, when the control portion  7  determines that the supply time has elapsed (Yes in S 8 ), the control portion  7  shifts the process to step S 9 . When the supply time has not elapsed (No in S 8 ), the control portion  7  waits for elapse of the supply time in step S 8 . 
     &lt;Step S 9 &gt; 
     In step S 9 , the control portion  7  restarts execution of the printing process suspended in step S 4 . For example, the control portion  7  restarts input of image data to the laser scanning unit  33  and also restarts application of a voltage to the charging device  32  and the transfer roller  35  by the power supply, which is not shown, to restart execution of the printing process. Here, the processes in steps S 6  to S 9  are executed by the restart processing portion  73  of the control portion  7 . 
     When it is determined in step S 2  that the printing process has ended after the supply time is shortened in step S 7 , the control portion  7  may rotate the first conveying member  52  and the second conveying member  53  at the second speed for a time by which the supply time is shortened. 
     EXAMPLE 
     An experiment was conducted for investigating transition of the amount of the developer held in the developing portion  5  in the developer supply mode in the image forming apparatus  10  according to the aforementioned embodiment.  FIG. 7  shows results of the experiment. In addition,  FIG. 8  shows a distribution of the developer at each portion of the developing portion  5  at a time point when three minutes elapsed from start of the developer supply mode in the experiment. 
     The experiment was conducted by a method in which, after the image forming apparatus  10  is caused to consecutively perform printing with a coverage rate of 50% on 250 sheets, the image forming apparatus  10  is caused to shift to the developer supply mode, and the amount of the developer held in the developing portion  5  during the developer supply mode is measured. In addition, in the experiment, the first speed was set to 23.4 rpm, and the second speed was set to 11.7 rpm, which is half the first speed. Moreover, the specifications of the image forming apparatus  10  during the experiment are as follows. 
     Drum linear velocity: 183.53 mm/sec 
     Printing speed: 35 ppm (page/minute) 
     Diameter of rotation shaft  52 A of first conveying member  52 : 10.0 mm 
     Pitch of helical blade  52 B of first conveying member  52 : 20.0 mm 
     Diameter of helical blade  52 B of first conveying member  52 : 22.5 mm 
     Reducing portion  52 E: formed by cutting a 2.5-turn portion of the helical blade  52 B in the axial direction of the rotation shaft  52 A 
     Diameter of rotation shaft  53 A of second conveying member  53 : 10.0 mm 
     Pitch of helical blade  53 B of second conveying member  53 : 20.0 mm 
     Diameter of helical blade  53 B of second conveying member  53 : 22.5 mm 
     Opening portion  51 D: 21.0 mm×8.0 mm 
     Comparative Example 
     An experiment similar to that as described above was conducted in which the second speed was set to the same speed as the first speed in the image forming apparatus  10 .  FIG. 7  shows results of the experiment. In addition,  FIG. 8  shows a distribution of the developer at each portion of the developing portion  5  at a time point when three minutes elapsed from start of the developer supply mode in the experiment. The method of the experiment is the same as in the example. 
     According to the experiment results shown in  FIG. 7 , in the image forming apparatus  10  in the comparative example, the amount of the developer supplied from the developer supply portion  34  decreased after two minutes elapsed from start of the developer supply mode. On the other hand, in the image forming apparatus  10  in the example, the amount of the developer supplied from the developer supply portion  34  did not decrease even after two minutes elapsed from start of the developer supply mode, and the amount of the developer supplied decreased after three minutes elapsed. This is guessed to be due to the rotation speeds of the first conveying member  52  and the second conveying member  53  being decreased, resulting in the timing at which supply of the developer from the developer supply portion  34  is restricted by the accumulation portion T being delayed so that a decrease in the amount of the developer supplied from the developer supply portion  34  is inhibited. 
     In addition, according to  FIG. 8 , at the second conveying member  53  and the portion of the first conveying member  52  other than the reducing portion  52 E, the amount of the developer held is larger in the image forming apparatus  10  in the example than in the image forming apparatus  10  in the comparative example. This is also guessed to be due to the rotation speeds of the first conveying member  52  and the second conveying member  53  being decreased, resulting in the timing at which supply of the developer from the developer supply portion  34  is restricted by the accumulation portion T being delayed so that a decrease in the amount of the developer supplied from the developer supply portion  34  is inhibited. 
     As described above, in the image forming apparatus  10 , when the mode shift condition is satisfied in the printing process, execution of the printing process is suspended, and the first conveying member  52  and the second conveying member  53  are rotated at the second speed, which is lower than the first speed during execution of the printing process. Accordingly, a decrease in the amount of the developer supplied from the developer supply portion  34  in the developer supply mode is inhibited, so that the supply time of the developer in the developer supply mode is shortened. 
     It is to be understood that the embodiments herein are illustrative and not restrictive, since the scope of the disclosure is defined by the appended claims rather than by the description preceding them, and all changes that fall within metes and bounds of the claims, or equivalence of such metes and bounds thereof are therefore intended to be embraced by the claims.