Patent Publication Number: US-9885989-B2

Title: Image forming apparatus for controlling a color density of an image on a continous recording medium

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus and is preferably applied to, for example, an electrophotographic image forming apparatus, such as a printer, a facsimile machine, or a copier, having a density correction function. 
     2. Description of the Related Art 
     There is a conventional image forming apparatus including: an image forming unit for forming an image with developer, such as toner; a transfer unit for transferring the image (also referred to as the developer image) formed by the image forming unit onto a recording medium, such as a sheet of paper; and a conveying unit including a belt for conveying the recording medium. 
     The conventional image forming apparatus performs density correction by forming a test pattern (referred to below as the density correction pattern) for density correction on the conveying unit and reading the density correction pattern by a density correction sensor when the image forming apparatus is started or between print jobs (i.e., when no printing is performed). 
     Japanese Patent Application Publication No. 2006-227336 discloses an image forming apparatus including a conveying member for conveying a recording medium in a conveying direction, the conveying member having a width (a length in a direction across the conveying direction) greater than that of the recording medium; an image forming unit for forming an image on the recording medium; an image forming processor for causing the image forming unit to form a density detection image outside the recording medium on the conveying member; a sensor for detecting the density of the density detection image; and a density correction processor for performing density correction based on the detected density. With this configuration, when the image forming apparatus prints on a long recording medium, such as a recording medium web from a roll of recording medium or a paper web from a roll of paper, it can perform density correction during the printing. 
     However, the above image forming apparatus forms the density detection image outside the recording medium in the width direction of the recording medium to perform density correction during printing. Thus, the conveying member and image forming unit need to have widths sufficiently greater than the width of the recording medium. This increases the size of the image forming apparatus. 
     SUMMARY OF THE INVENTION 
     An aspect of the present invention is intended to provide a small image forming apparatus capable of forming a test pattern during printing. 
     According to an aspect of the present invention, there is provided an image forming apparatus, including: an image forming unit that forms a plurality of successive images and a test pattern image with developer on a primary transfer member, the plurality of successive images including a first image and at least one second image subsequent to the first image; a conveying unit that conveys a continuous recording medium to bring the continuous recording medium into contact with the primary transfer member; a secondary transfer unit that transfers the plurality of successive images formed on the primary transfer member onto the continuous recording medium in contact with the primary transfer member; a detector that detects the test pattern image formed on the primary transfer member; a controller that causes the image forming unit to form the plurality of successive images in accordance with image data, the controller causing the image forming unit to form the test pattern image between the first image and the at least one second image, the controller causing the image forming unit to form the at least one second image on the primary transfer member based on the detection of the test pattern image by the detector; and a transfer separation unit that separates the primary transfer member and the continuous recording medium from each other and brings the primary transfer member and the continuous recording medium into contact with each other, the transfer separation unit separating the primary transfer member and the continuous recording medium from each other to prevent the test pattern image from being transferred onto the continuous recording medium. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the attached drawings: 
         FIG. 1  is a block diagram illustrating a configuration of a printing system; 
         FIG. 2  is a side view illustrating a configuration of a printing mechanism of an image forming apparatus; 
         FIG. 3  is a view illustrating operation of a transfer separation mechanism and a fixing separation mechanism; 
         FIG. 4  is a diagram illustrating multiple elementary image data items obtained by dividing long image data; 
         FIG. 5  is a diagram illustrating a configuration of image data of a density correction pattern; 
         FIG. 6  is a diagram illustrating a configuration of processed image data; 
         FIG. 7  is a flowchart illustrating a procedure of a process for preparing print data; 
         FIG. 8  is a diagram for explaining a distance between a pattern image and a preceding elementary image and a distance between a pattern image and a subsequent elementary image; 
         FIG. 9  is a flowchart illustrating a procedure of a printing process; 
         FIG. 10  is a diagram illustrating a configuration of processed image data of a modification; and 
         FIG. 11  is a diagram illustrating a configuration of a label web of another modification. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An embodiment of the present invention will now be described with reference to the attached drawings. 
     &lt;1. Configuration of Printing System&gt; 
       FIG. 1  illustrates a functional configuration of a printing system  1  of this embodiment. The printing system  1  includes a client personal computer (PC)  2  and an image forming apparatus  3  connected to the client PC  2  via a network or the like. The client PC 2  has an application  10  for generating image data and a driver  11  for giving a print instruction to the image forming apparatus  3 , which are installed in the client PC  2 . 
     The driver  11  generates a print job including image data generated by the application  10  and print settings. The print settings include, for example, designation of a sheet size, designation of monochrome/color printing, or the like. The print job is sent from the client PC  2  to the image forming apparatus  3 . 
     The image forming apparatus  3  includes a data processor  20 , a print processor  21 , and a printing mechanism  36 . 
       FIG. 2  schematically illustrates the printing mechanism  36  of the image forming apparatus  3 . In  FIG. 2 , the printing mechanism  36  includes a developing section  40 , a conveying unit  41 , an intermediate transfer unit  43 , and a fixing unit  44 , which are arranged in a housing  60  of the image forming apparatus  3 . The developing section  40  includes multiple (e.g., four) developing units  40 A,  40 B,  40 C, and  40 D arranged in an upper portion of the housing  60 . The four developing units  40 A,  40 B,  40 C, and  40 D are units corresponding to, for example, cyan, magenta, yellow, and black, respectively. Each of the developing units  40 A,  40 B,  40 C, and  40 D forms a developer image using a developer of the corresponding color and transfers the developer image onto an intermediate transfer belt  42  of the intermediate transfer unit  43 . Specifically, each of the developing units  40 A,  40 B,  40 C, and  40 D includes a photosensitive drum  81  and a transfer  82 , forms a developer image on the photosensitive drum  81  and transfers the developer image by the transfer roller  82  from the photosensitive drum  81  to the intermediate transfer belt  42 . The developing units  40 A,  40 B,  40 C, and  40 D sequentially transfer developer images onto the intermediate transfer belt  42  in a superposed manner to generate a color image, which is a developer image consisting of the four-color developer images. 
     The intermediate transfer unit  43  is disposed below the developing section  40 . The intermediate transfer unit  43  includes the intermediate transfer belt  42 , an intermediate transfer roller  61 , a backup roller  62 , belt rollers  63 A,  63 B, and  63 C, and a belt cleaner  64 . 
     The intermediate transfer belt  42  is stretched by the backup roller  62  and belt rollers  63 A,  63 B, and  63 C in an inverted triangular shape with its upper side flat and its lower side projecting downward. The intermediate transfer belt  42  has a flat portion on its upper side and a projecting portion on its lower side. The intermediate transfer roller  61  is disposed outside the projecting portion of the intermediate transfer belt  42 . The backup roller  62  is disposed inside the projecting portion of the intermediate transfer belt  42 . The intermediate transfer roller  61  is disposed facing the backup roller  62 . The intermediate transfer roller  61  and backup roller  62  form a nip portion therebetween. The position of the nip portion of the intermediate transfer unit  43  will be referred to as the nip position T 1 . 
     The multiple developing units  40 A,  40 B,  40 C and  40 D transfer (primary-transfer) developer images onto an outer surface of the flat portion of the intermediate transfer belt  42 . The developer images thus transferred onto the intermediate transfer belt  42  are conveyed by the intermediate transfer belt  42  traveling or rotating clockwise in  FIG. 2  and transferred (secondary-transferred) onto a recording medium P when the recording medium P passes through the nip portion. The recording medium P is conveyed by the conveying unit  41  to the nip portion as described later. 
     The belt cleaner  64  is disposed downstream (on the left side in  FIG. 2 ) of the nip portion of the intermediate transfer belt  42  in a conveying direction of the developer images and outside the intermediate transfer belt  42 . The belt cleaner  64  is disposed at a predetermined position (near the belt roller  63 C located at one end of the flat portion) between the nip portion and the flat portion. The belt cleaner  64  removes developer remaining on the intermediate transfer belt  42 . 
     Further, an image position sensor  50  is disposed upstream (on the right side in  FIG. 2 ) of the nip portion of the intermediate transfer belt  42  in the conveying direction of the developer images and outside the intermediate transfer belt  42 . The image position sensor  50  is disposed at a predetermined position (near the belt roller  63 B located at the other end of the flat portion) between the nip portion and the flat portion. Further, a density correction sensor  51  is disposed near and downstream of the image position sensor  50 . 
     The conveying unit  41  is disposed below the intermediate transfer unit  43 . The conveying unit  41  includes a conveying belt  45  and belt rollers  65 A and  65 B. The conveying belt  45  is stretched by the belt rollers  65 A and  65 B in an oval shape with its upper and lower sides flat. The conveying belt  45  travels or rotates counterclockwise in  FIG. 2 , thereby conveying the recording medium P placed on its upper flat portion. Conveying rollers  66 A and  66 B are disposed upstream of the conveying belt  45  in a conveying direction (referred to below as the medium conveying direction) of the recording medium P so as to face each other. A direction across or perpendicular to the medium conveying direction will be referred to as a main-scanning direction. A direction parallel or corresponding to the medium conveying direction will be referred to as a sub-scanning direction. The recording medium P is a paper web from a roll of paper. The recording medium P is drawn from a roll of paper contained in a container (not illustrated) and conveyed to the conveying belt  45  by the conveying rollers  66 A and  66 B or the like. 
     The intermediate transfer roller  61  of the intermediate transfer unit  43  is disposed on the inside of the upper flat portion of the conveying belt  45 . In the nip portion, the intermediate transfer belt  42  and conveying belt  45  are in contact with each other between the intermediate transfer roller  61  and the backup roller  62 . The recording medium P passes through the nip portion while sandwiched between the intermediate transfer belt  42  and the conveying belt  45 . At this time, the developer images transferred on the intermediate transfer belt  42  are transferred onto the recording medium P. 
     The fixing unit  44  is disposed downstream of the nip portion of the intermediate transfer unit  43  in the medium conveying direction. The fixing unit  44  includes a fixing belt  47  heated by a heater and a pressure roller  48 . The fixing belt  47  and pressure roller  48  are disposed in contact with each other. The fixing unit  44  applies heat and pressure to the recording medium P when the recording medium P passes between the fixing belt  47  and the pressure roller  48 , thereby fixing the developer images to the recording medium P. Then, the recording medium P is continuously discharged through an outlet (not illustrated). 
     Further, the image forming apparatus  3  includes a transfer separation mechanism  46  and a fixing separation mechanism  49  in the housing  60 . The transfer separation mechanism  46  includes, for example, a solenoid. The transfer separation mechanism  46  moves the backup roller  62  in a direction away from the intermediate transfer roller  61  (or a direction toward the inside of the intermediate transfer belt  42 ) as illustrated in  FIG. 3 , thereby separating the intermediate transfer belt  42  and conveying belt  45  from each other. 
     If only the backup roller  62  is moved toward the inside of the intermediate transfer belt  42 , the tension of the intermediate transfer belt  42  decreases. Thus, corresponding to the movement of the backup roller  62  in the direction toward the inside of the intermediate transfer belt  42 , the image forming apparatus  3  moves the belt rollers  63 B and  63 C, which stretch the intermediate transfer belt  42 , in directions toward the outside of the intermediate transfer belt  42  by roller slide units  70  and  71 , respectively. The roller slide units  70  and  71  each include, for example, a solenoid. Thereby, the tension of the intermediate transfer belt  42  can be kept constant. Since the belt cleaner  64  is disposed near the belt roller  63 C, the roller slide unit  71  slides or moves the belt cleaner  64  together with the belt roller  63 C, for example. 
     The transfer separation mechanism  46  and roller slide units  70  and  71  respectively return the backup roller  62  and belt rollers  63 B and  63 C to their original positions, thereby bringing the intermediate transfer belt  42  and conveying belt  45  into contact with each other again. 
     The transfer separation mechanism  46  can move the backup roller  62  in the direction away from the intermediate transfer roller  61  when the recording medium P has reached the nip portion, thereby separating the intermediate transfer belt  42  from the recording medium P. 
     The fixing separation mechanism  49  includes, for example, a solenoid. The fixing separation mechanism  49  moves the fixing belt  47  in a direction away from the pressure roller  48 , thereby separating the fixing belt  47  and pressure roller  48  from each other, as illustrated in  FIG. 3 . Also, the fixing separation mechanism  49  returns the fixing belt  47  to its original position, thereby bringing the fixing belt  47  and pressure roller  48  into contact with each other again. The fixing separation mechanism  49  can move the fixing belt  47  in the direction away from the pressure roller  48  when the recording medium P has reached the fixing unit  44 , thereby separating the fixing belt  47  from a print surface, which is a surface on which developer images are transferred, of the recording medium P and stopping application of heat and pressure to the print surface of the recording medium P. 
     The image forming apparatus  3  further includes, in the housing  60 , a medium sensor  72  for detecting the recording medium P. The medium sensor  72  is disposed between the belt roller  65 A of the conveying unit  41  and the conveying roller  66 B, for example. 
     Referring back to  FIG. 1 , the data processor  20  includes a data receiver  22 , a data analyzer  23 , and a data editor  24 . The data receiver  22  receives a print job from the client PC  2 . 
     The print job may include long image data LD as the image data. The long image data LD represent a long image LI. The long image data LD will be referred to as the long image LD. A developer image formed based on the long image LD will be referred to as a long image LT.  FIG. 4  illustrates the long images LD, LI, and LT. The long image LD is to be printed on a continuous recording medium, such as a paper web from a roll of paper, longer in the medium conveying direction than recording media of normal size (e.g., A4 size). The long images LD, LI, and LT are long in the sub-scanning direction as compared to image data or an image to be printed on a recording medium of normal size. The sub-scanning direction is indicated by arrow A in  FIG. 4 . In this example, the long image LI consists of multiple identical elementary images SI 1 , SI 2 , . . . , SIn (n is an integer greater than 1) successively arranged in the sub-scanning direction. The elementary images SI 1 , SI 2 , . . . , SIn will also be referred to as the elementary images SI. The elementary images SI are, for example, label images. 
     The data analyzer  23  analyzes the image data included in the print job received by the data receiver  22 . If the print job includes the long image LD as the image data, the data analyzer  23  detects the boundaries between each adjacent pair of the elementary images SI and divides the long image LD into multiple elementary image data items SD 1 , SD 2 , . . . , SDn. The elementary image data items SD 1 , SD 2 , . . . , SDn respectively represent or correspond to the elementary images SI 1 , SI 2 , . . . , SIn. The elementary image data items SD 1 , SD 2 , . . . , SDn will be referred to as the elementary images SD 1 , SD 2 , . . . , SDn. The elementary images SD 1 , SD 2 , . . . , SDn will also be referred to as the elementary images SD. Developer images formed based on the elementary images SD 1 , SD 2 , . . . , SDn will be referred to as elementary images ST 1 , ST 2 , . . . , STn, respectively. The elementary images ST 1 , ST 2 , . . . , STn will also be referred to as the elementary images ST. 
     Further, the data analyzer  23  calculates an actual size (referred to below as the actual print size) of an image obtained by printing the long image LD on a recording medium, and determines, prior to printing, whether density correction needs to be performed during printing depending on whether the actual print size exceeds a predetermined threshold. The print settings included in the print job may include an instruction to perform density correction. In this case, according to the print settings, the data analyzer  23  determines that density correction needs to be performed. 
     The data editor  24  is a processor that edits data based on the analysis of the image data by the data analyzer  23 . The data editor  24  includes a pattern generator  25  and a pattern combiner  26 . The pattern generator  25  generates image data (or density correction pattern data) PD of a density correction pattern necessary for density correction. The image data PD represents a pattern image PI. The image data PD will be referred to as the pattern image PD. A developer image formed based on the pattern image PD will be referred to as a pattern image PT.  FIG. 5  illustrates the pattern images PD, PI, and PT. The pattern image PI is, for example, an image in which for each developer color, multiple rectangular images with different densities are arranged in the sub-scanning direction. The pattern image PD is image data for performing density correction, and the pattern image PT formed based on the pattern image PD is not to be transferred onto a recording medium. Thus, the image forming apparatus  3  is configured to prevent the pattern image PT from being transferred onto a recording medium. 
     If the data analyzer  23  determines that density correction needs to be performed during printing, the pattern combiner  26  inserts the pattern image PD generated by the pattern generator  25  between adjacent two of the multiple elementary images SD obtained by dividing the long image LD, thereby combining the multiple elementary images SD with the pattern image PD to generate processed image data PGD consisting of the multiple elementary images SD and pattern image PD. The processed image data PGD represent a processed image PGI. The processed image data PGD will be referred to as the processed image PGD. A developer image formed based on the processed image PGD will be referred to as a processed image PGT.  FIG. 6  illustrates the processed images PGD, PGI, and PGT. In the example of  FIG. 6 , the pattern image PD is inserted between elementary images SD 1  and SD 2 . 
     The processed image PGI has a non-transfer region NTI between the elementary images SI 1  and SI 2 . The non-transfer region NTI includes no image to be transferred onto the recording medium P. The pattern image PI is placed in the non-transfer region NTI after the elementary image SI 1  and before the elementary image SI 2 . A region corresponding to the non-transfer region NTI on the intermediate transfer belt  42  will be referred to as a non-transfer region NTB. 
     To make it possible to easily distinguish the pattern image PT from the elementary images ST, in the processed image PGD or PGI, the pattern image PD or PI is spaced a first distance L 1  from the rear end of the elementary image SD 1  or SI 1  and spaced a second distance L 2  from the front end of the elementary image SD 2  or SI 2 . That is, the processed image PGD is generated so that the pattern image PT is spaced the distance L 1  from the rear end of the elementary image ST 1  and spaced the distance L 2  from the front end of the elementary image ST 2 . The non-transfer region NTI includes the region of the pattern image PI and the other region, which is colorless or blank and will be referred to as the blank region BLI. A region corresponding to the blank region BLI on the intermediate transfer belt  42  will be referred to as a blank region BLB. 
     Referring back to  FIG. 1 , the print processor  21  includes a developing controller  27 , a medium conveying controller  28 , a transfer controller  29 , a fixing controller  30 , a transfer separation controller  31 , a fixing separation controller  32 , an image position detector  33 , a pattern detector  34 , and a density corrector  35 . 
     The developing controller  27  controls the developing section  40  ( FIG. 2 ) in accordance with the print job. The medium conveying controller  28  controls the conveying unit  41  ( FIG. 2 ) for conveying the recording medium P. The transfer controller  29  controls the intermediate transfer unit  43  ( FIG. 2 ) for conveying, by the intermediate transfer belt  42 , developer images transferred on the intermediate transfer belt  42  ( FIG. 2 ) by the developing section  40  and transferring the developer images onto the recording medium P. The fixing controller  30  controls the fixing unit  44  ( FIG. 2 ) for applying heat and pressure to the recording medium P with developer images transferred thereon and thereby fixing the developer images to the recording medium P. 
     The transfer separation controller  31  controls the transfer separation mechanism  46  ( FIG. 2 ) for separating the intermediate transfer belt  42  and conveying belt  45  from each other and bringing the separated intermediate transfer belt  42  and conveying belt  45  into contact with each other again. The fixing separation controller  32  controls the fixing separation mechanism  49  ( FIG. 2 ) for separating the fixing belt  47  and pressure roller  48  of the fixing unit  44  from each other and bringing the separated fixing belt  47  and pressure roller  48  into contact with each other again. 
     The image position detector  33  detects boundaries between the elementary images ST and pattern image PT transferred on the intermediate transfer belt  42  based on output from the image position sensor  50 , which is disposed facing an outer surface of the intermediate transfer belt  42 . In the example of  FIG. 6 , the image position detector  33  detects a boundary T 10  between the elementary image ST 1  and the blank region BLB and a boundary T 11  between the blank region BLB and the elementary image ST 2 . The pattern detector  34  determines, as the pattern image PT, a developer image formed after the boundary T 10  detected by the image position detector  33 , and reads the developer image by the density correction sensor  51 , which is disposed facing the outer surface of the intermediate transfer belt  42 , to generate read pattern data. 
     Based on the read pattern data generated by the pattern detector  34 , for each color, the density corrector  35  determines a density (referred to as the actual density) of the developer image in the pattern image PT transferred on the intermediate transfer belt  42 , compares the determined actual density with an optimum density required for printing, and performs density correction on the developing section  40  so that the actual density approaches the optimum density. 
     The functions of the data processor  20  and print processor  21  may be implemented by one or more circuits, such as hard-wired circuits or programmable processors. For example, the image forming apparatus  3  includes a memory configured to store instructions or program, and a processor configured to execute the instructions or program to perform the functions. 
     &lt;2. Printing Operation of Image Forming Apparatus&gt; 
     The printing operation of the image forming apparatus  3  will be described below. In the printing operation, the image forming apparatus  3  performs a preparation process to prepare image data (or print data) to be printed, and then performs a printing process based on the print data. 
     The preparation process will be first described with reference to the flowchart illustrated in  FIG. 7 . The preparation process is performed by the data processor  20  of the image forming apparatus  3 . 
     First, in step SP 1 , the data receiver  22  of the data processor  20  receives a print job sent from the client PC  2 . The print job includes print settings and image data, as described above. Here, it is assumed that the print job includes the long image LD as the image data. 
     Next, in step SP 2 , the data analyzer  23  of the data processor  20  analyzes the long image LD included in the print job received by the data receiver  22 , divides the long image LD into the elementary images SD, and determines whether density correction needs to be performed during printing, based on the actual print size of the long image LD. 
     If the data analyzer  23  determines that no density correction need to be performed during printing (NO in step SP 2 ), the data processor  20  determines, as the print data, the long image LD included in the print job, ending the preparation process. 
     On the other hand, if the data analyzer  23  determines that density correction needs to be performed during printing (YES in step SP 2 ), the data analyzer  23  obtains, in step SP 3 , the multiple elementary images SD obtained by dividing the long image LD included in the print job, as illustrated in  FIG. 4 . 
     Then, in step SP 4 , the pattern generator  25  of the data processor  20  generates the pattern image PD illustrated in  FIG. 5 . Then, in step SP 5 , the pattern combiner  26  of the data processor  20  combines the multiple elementary images SD with the pattern image PD by inserting the pattern image PD between adjacent two (for example, the elementary images SD 1  and SD 2 ) of the multiple elementary images SD as illustrated in  FIG. 6 , thereby generating the processed image PGD. The position in which the pattern image PD is inserted is determined by the data analyzer  23  based on the actual print size of the long image LD. 
     At this time, the pattern image PD (or PI) is inserted between the elementary images SD 1  and SD 2  (or SI 1  and SI 2 ) so that the pattern image PD (or PI) is spaced the distance L 1  from the preceding elementary image SD 1  (or SI 1 ) and spaced the distance L 2  from the subsequent elementary image SD 2  (or SI 2 ). The distances L 1  and L 2  are set as follows. 
     The distance L 1  is set so that when the rear end of the preceding elementary image ST 1  transferred on and conveyed by the intermediate transfer belt  42  reaches the nip position T 1  of the intermediate transfer unit  43 , transfer of the pattern image PT onto the intermediate transfer belt  42  has not yet been started. That is, the distance L 1  is set to be equal to or greater than a distance corresponding to a distance L 1   x , illustrated in  FIG. 8 , between the nip position T 1  of the intermediate transfer belt  42  and a transfer position T 2  at which a developer image is transferred from the developing unit  40 A, by which the pattern image PT is first transferred, onto the intermediate transfer belt  42 . The pattern combiner  26  generates the processed image PGD so that after completion of transfer of the elementary image ST 1  onto the recording medium P, transfer of the pattern image PT onto the intermediate transfer belt  42  is started. 
     The distance L 2  is set so that when the rear end of the pattern image PT transferred on and conveyed by the intermediate transfer belt  42  reaches a reading position T 3 , illustrated in  FIG. 8 , of the density correction sensor  51 , formation of the subsequent elementary image ST 2  has not yet been started. That is, the distance L 2  is set to be equal to or greater than a distance corresponding to a distance L 2   x , illustrated in  FIG. 8 , between the reading position T 3  of the density correction sensor  51  and the transfer position T 2 . The pattern combiner  26  generates the processed image PGD so that after completion of detection of the pattern image PT by the density correction sensor  51 , formation of the elementary image ST 2  is started. The reason why the distances L 1  and L 2  are set as above will be described later. 
     As above, if the data analyzer  23  determines that density correction needs to be performed during printing (YES in step SP 2 ), the data processor  20  generates the processed image PGD. Then, the data processor  20  sets the generated processed image PGD as the print data, ending the preparation process. 
     In this example, the pattern image PD is inserted in one position between the elementary images SD 1  and SD 2 . However, depending on the actual print size, the pattern image PD may be inserted in two or more positions. 
     Next, the printing process to print the print data prepared in the preparation process will be described with reference to the flowchart illustrated in  FIG. 9 . The printing process is performed by the print processor  21  of the image forming apparatus  3 . 
     First, in step SP 20 , the print processor  21  performs printing based on the print data. Specifically, the developing controller  27 , medium conveying controller  28 , transfer controller  29 , and fixing controller  30  of the print processor  21  control the printing mechanism  36  to print one of the multiple elementary images SD included in the long image LD or processed image PGD so that the multiple elementary images SD are printed in order. 
     At this time, the developing section  40  forms developer images of the respective colors based on the elementary image SD and transfers the developer images onto the intermediate transfer belt  42 , thereby forming an elementary image ST on the intermediate transfer belt  42 . The elementary image ST is conveyed by the intermediate transfer belt  42  to the nip position T 1 , and transferred onto the recording medium P conveyed by the conveying belt  45  at the nip position T 1 . The recording medium P passes through the fixing unit  44 , so that the transferred elementary image ST is fixed to the recording medium P. As such, the image based on the elementary image SD is printed on the recording medium P. 
     Next, in step SP 21 , the print processor  21  determines whether printing of all the elementary images SD included in the long image LD or processed image PGD has been completed. If printing of all the elementary images SD has been completed (YES in step SP 21 ), the print processor  21  ends the print process. 
     On the other hand, if printing of all the elementary images SD has not been completed (NO in step SP 21 ), the print processor  21  proceeds to step SP 22 . In step SP 22 , the image position detector  33  of the print processor  21  determines whether it has detected a boundary between an elementary image ST and the pattern image PT. If no boundary has been detected (NO in step SP 22 ), the print processor  21  returns to step SP 20  and prints the subsequent elementary image SD. 
     On the other hand, if a boundary (e.g., the boundary T 10  between the elementary image ST 1  and the pattern image PT illustrated in  FIG. 6 ) has been detected (YES in step SP 22 ), the print processor  21  proceeds to step SP 23 . In step SP 23 , when the rear end of the elementary image ST 1  formed before the boundary T 10  reaches the nip position T 1  of the intermediate transfer unit  43 , the developing controller  27 , medium conveying controller  28 , and transfer controller  29  stop operation of the developing section  40 , conveyance of the recording medium P by the conveying belt  45 , and movement of the intermediate transfer belt  42 . At this time, transfer of the elementary image ST 1  onto the recording medium P has been completed. Since the pattern image PD or PI is spaced the distance L 1  from the rear end of the elementary image SD 1  or SI 1  as described above, no pattern image PT has been transferred onto the intermediate transfer belt  42 . 
     Next, in step SP 24 , the transfer separation controller  31  drives the transfer separation mechanism  46  to separate the intermediate transfer belt  42  from the recording medium P, as illustrated in  FIG. 3 . This is to prevent the pattern image PT subsequent to the elementary image ST 1  from being transferred onto the recording medium P. Further, at this time, the fixing separation controller  32  drives the fixing separation mechanism  49  to separate the fixing belt  47  from the recording medium P, as illustrated in  FIG. 3 . As such, in this embodiment, the print processor  21  separates, from the recording medium P, not only the intermediate transfer belt  42  but also the fixing belt  47 . This is because if the fixing belt  47  is left in contact with the recording medium P while conveyance of the recording medium P is stopped, the recording medium P may be deformed or damaged by heat. 
     Next, in step SP 25 , the developing controller  27  and transfer controller  29  cause the developing section  40  and intermediate transfer belt  42  to operate so that the developing section  40  forms a pattern image PT and transfers the pattern image PT onto the intermediate transfer belt  42 . The pattern detector  34  detects and reads the pattern image PT conveyed by the intermediate transfer belt  42  to generate read pattern data. When the rear end of the pattern image PT reaches the reading position T 3  of the density correction sensor  51 , the reading of the pattern image PT is completed. At this time, since the elementary image SD 2  or SI 2  subsequent to the pattern image PD or PI is spaced the distance L 2  from the rear end of the pattern image PD or PI as described above, no developer image based on the elementary image SD 2  has been formed. 
     Next, in step SP 26 , based on the read pattern data generated by the pattern detector  34 , for each color, the density corrector  35  performs density correction by calculating, as a density correction amount (or value), a difference between the actual density and the optimum density, and applies the correction result (i.e., density correction amount) to the density of a developer image formed by the developing section  40  by notifying the developing controller  27  of the correction result (i.e., density correction amount), so that the density of a developer image formed by the developing section  40  is corrected to the optimum density. 
     After the density correction, the developing section  40  forms a developer image (elementary image ST 2 ) based on the elementary image SD 2  subsequent to the pattern image PD, and transfers the elementary image ST 2  onto the intermediate transfer belt  42 . Thus, the result of the density correction (or the density correction amount for each color) is applied to or reflected in the elementary image ST 2  and subsequent developer images. 
     Next, in step SP 27 , when the front end of the elementary image ST 2  subsequent to the pattern image PT reaches the nip position T 1  of the intermediate transfer belt  42 , the developing controller  27  and transfer controller  29  stop operation of the developing section  40  and movement of the intermediate transfer belt  42 . At this time, the pattern image PT transferred on the intermediate transfer belt  42  has passed through the nip position T 1  without being transferred onto the recording medium P. 
     The transfer separation controller  31  drives the transfer separation mechanism  46  to bring the intermediate transfer belt  42  into contact with the recording medium P again, and the fixing separation controller  32  drives the fixing separation mechanism  49  to bring the fixing belt  47  into contact with the recording medium P again. Then, the print processor  21  returns to step SP 20  and restarts the printing of the elementary images SD. Thus, the elementary images SD 2 , SD 3 , . . . are printed on the recording medium P following the elementary image SD 1 . Thus, the processed image PGI illustrated in  FIG. 6  is printed on the recording medium P except for the non-transfer region NTI including the pattern image PI between the boundaries T 10  and T 11 . That is, only the elementary images SI of the processed image PGI are printed on the recording medium P. 
     As such, when the print data include the pattern image PD (i.e., the print data are the processed image PGD), the print processor  21  reads the pattern image PT to perform density correction during the printing, and separates the intermediate transfer belt  42  from the recording medium P to prevent the pattern image PT from being printed on the recording medium P. 
     &lt;3. Advantages&gt; 
     As described above, when the image forming apparatus  3  prints, on the recording medium P, an image based on the long image LD requiring density correction during printing, it divides the long image LD into the multiple elementary images SD and inserts the pattern image PD between adjacent two of the multiple elementary images SD, thereby generating the processed image PGD. 
     Then, while the image forming apparatus  3  prints an image based on the processed image PGD on the recording medium P, it transfers the pattern image PT based on the pattern image PD included in the processed image PGD onto the intermediate transfer belt  42  and performs density correction by reading the pattern image PT by the density correction sensor  51 . The image forming apparatus  3  (or print processor  21 ) may correct density of at least one of the elementary images ST subsequent to the elementary image ST 1  based on the detection of the pattern image PT by the density correction sensor  51 . Further, to prevent the pattern image PT transferred on the intermediate transfer belt  42  from being transferred onto the recording medium P, the image forming apparatus  3  causes the pattern image PT to pass through the nip position T 1  of the intermediate transfer unit  43  while the intermediate transfer belt  42  and recording medium P are separated from each other. 
     The image forming apparatus  3  forms the pattern image PT in a portion facing the recording medium P on the intermediate transfer belt  42  and prevents the pattern image PT from being transferred onto the recording medium P by separating the intermediate transfer belt  42  and the recording medium P from each other when the pattern image PT passes through the nip position T 1  of the intermediate transfer unit  43 . 
     Thus, compared to an image forming apparatus that includes an intermediate transfer belt having a width greater than that of a recording medium and forms a pattern image on the intermediate transfer belt  42  in a portion that does not face the recording medium, the image forming apparatus  3  can perform density correction during printing with the intermediate transfer belt  42  having a narrow width. Thus, it is possible to provide an image forming apparatus capable of performing density correction (or forming a density correction pattern image or test pattern image) during printing and small in size as compared to the conventional image forming apparatus. 
     The image forming apparatus  3  places the pattern image PD (or PI) between the preceding elementary image SD 1  (or SI 1 ) and the subsequent elementary image SD 2  (or SI 2 ) so that the pattern image PD (or PI) is spaced the distance L 1  from the elementary image SD 1  (or SI 1 ) and spaced the distance L 2  from the elementary image SD 2  (or SI 2 ). The distance L 1  is set so that when the rear end of the preceding elementary image ST 1  reaches the nip position T 1  of the intermediate transfer unit  43 , transfer of the pattern image PT onto the intermediate transfer belt  42  has not yet been started. 
     This makes it possible to transfer the pattern image PT onto the intermediate transfer belt  42  after the rear end of the elementary image ST 1  reaches the nip position T 1  of the intermediate transfer unit  43  and the intermediate transfer belt  42  is separated from the recording medium P. In the image forming apparatus  3 , when the intermediate transfer belt  42  is separated from the recording medium P, although the intermediate transfer belt  42  is stopped, movement of the backup roller  62  and belt rollers  63 B and  63 C may swing the intermediate transfer belt  42  or move the intermediate transfer belt  42  in the conveying direction. Thus, if the intermediate transfer belt  42  and recording medium P are separated from each other in the middle of the transfer of the pattern image PT onto the intermediate transfer belt  42 , the pattern image PT may be transferred inaccurately. Inaccurate transfer of the pattern image PT may lead to inaccurate density correction. 
     The image forming apparatus  3  transfers the pattern image PT onto the intermediate transfer belt  42  after separating the intermediate transfer belt  42  from the recording medium P, so that the image forming apparatus  3  can accurately transfer the pattern image PT onto the intermediate transfer belt  42 , resulting in accurate density correction. 
     The distance L 2  is set so that when the rear end of the pattern image PT transferred on and conveyed by the intermediate transfer belt  42  reaches the reading position T 3  of the density correction sensor  51 , formation of the subsequent elementary image ST 2  has not yet been started. This makes it possible to perform density correction based on the read pattern data and apply the result of the density correction to the elementary image ST 2  immediately after the pattern image PT. 
     &lt;4. Modifications&gt; 
     &lt;4-1. First Modification&gt; 
     In the above embodiment, the image forming apparatus  3  applies the result of the density correction to one or more elementary images ST subsequent to the pattern image PT. This is not mandatory. The image forming apparatus  3  (or print processor  21 ) may gradually correct density of a plurality of the at least one elementary image ST subsequent to the elementary image ST 1  based on the detection of the pattern image PT by the density correction sensor  51 . Specifically, instead of simply applying the density correction amount, which is the difference between the actual density and the optimum density, to the elementary images ST subsequent to the pattern image PT, the image forming apparatus  3  may apply a density correction amount to the elementary images ST subsequent to the pattern image PT while increasing the density correction amount by a given amount (e.g., 5%) per elementary image ST so that the density correction amount finally reaches a target density correction amount, which is the difference between the actual density and the optimum density. Specifically, the image forming apparatus  3  may apply a density correction amount to the elementary images ST subsequent to the pattern image PT while increasing the density correction amount by a given amount in such a manner as to apply 5% of the target density correction amount to the elementary image ST 2  subsequent to the pattern image PT, 10% of the target density correction amount to the subsequent elementary image ST 3 , 15% of the target density correction amount to the subsequent elementary image ST 4 , . . . . 
     This can prevent a situation where the density of the elementary image ST 1  immediately before the density correction is greatly different from the density of the elementary image ST 2  immediately after the density correction, resulting in uncomfortable print images. Also, the image forming apparatus  3  may apply a density correction amount to the elementary images ST subsequent to the pattern image PT while increasing the density correction amount by a given amount only if the target density correction amount, which is the difference between the actual density and the optimum density, exceeds a predetermined threshold (i.e., only if the target density correction amount is great). 
     &lt;4-2. Second Modification&gt; 
     In the above embodiment, the pattern image PD (or PI) is placed between the preceding elementary image SD 1  (or SI 1 ) and the subsequent elementary image SD 2  (or ST 2 ) so that the pattern image PD (or PI) is spaced the distance L 1  from the elementary image SD 1  (or SI 1 ) and spaced the distance L 2  from the elementary image SD 2  (or ST 2 ). However, the distances L 1  and L 2  may be decreased as compared to the above embodiment. Merely to allow the pattern image PT to be distinguished from the preceding and subsequent elementary images ST 1  and ST 2 , it is sufficient to provide blank portions for distinguishing them. 
     For example, as illustrated in  FIG. 10 , the distance L 1  between the pattern image PD (or PI) and the preceding elementary image SD 1  (or SI 1 ) may be set to a minimum distance required to distinguish the pattern image PT and the preceding elementary image ST 1 ; the distance L 2  between the pattern image PD (or PI) and the subsequent elementary image SD 2  (or SI 2 ) may be set to a minimum distance required to distinguish the pattern image PT and the subsequent elementary image ST 2 . This can reduce the length of the processed image PGD (or PGI) in the sub-scanning direction (or medium conveying direction) indicated by arrow A in  FIG. 10 , thereby reducing print time as compared to the above embodiment. 
     However, if the distance L 1  is too small, the pattern image PI is located shortly after the preceding elementary image SI 1 , so the intermediate transfer belt  42  may be separated from the recording medium P in a state where the pattern image PT is halfway transferred onto the intermediate transfer belt  42 , for example. Thus, when accuracy of density correction is given more priority than reduction in print time, the distance L 1  is desirably set to the distance described in the above embodiment. The length of the pattern image PD or PI in the sub-scanning direction (or medium conveying direction) may be set so that when the rear end of the preceding elementary image ST 1  reaches the nip position T 1  of the intermediate transfer unit  43 , transfer of the pattern image PT onto the intermediate transfer belt  42  has been completed. This can prevent a situation where the intermediate transfer belt  42  is separated from the recording medium P in a state where the pattern image PT is halfway transferred onto the intermediate transfer belt  42 , and can reduce print time while ensuring accuracy of density correction. 
     If the distance L 2  is too small, the subsequent elementary image SI 2  is located shortly after the pattern image PI, so the result of the density correction cannot be applied to the elementary image SI 2  subsequent to the pattern image PT and can be applied only to elementary images ST formed after completion of reading of the pattern image PT. Thus, if the result of the density correction should be applied to the elementary image ST 2  immediately after the pattern image PT, the distance L 2  is desirably set to the distance described in the above embodiment. Also in this case, it is possible to apply a density correction amount to the elementary images ST while increasing the density correction amount by a given amount per image. 
     &lt;4-3. Third Modification&gt; 
     In the above embodiment, the image forming apparatus  3  separates the intermediate transfer belt  42  from the recording medium P by moving the backup roller  62  in the direction toward the inside of the intermediate transfer belt  42  and moving the belt rollers  63 B and  63 C in the directions toward the outside of the intermediate transfer belt  42 . However, how to separate the intermediate transfer belt  42  from the recording medium P is not limited to this. For example, the image forming apparatus  3  may move the backup roller  62  in a direction toward the inside of the intermediate transfer belt  42  and move one of the belt rollers  63 B and  63 C in a direction toward the outside of the intermediate transfer belt  42 . 
     Further, for example, the image forming apparatus  3  may include a transfer separation unit (not illustrated) for moving the entire intermediate transfer unit  43  except for the intermediate transfer roller  61  in a direction (upward direction) away from the conveying belt  45  and intermediate transfer roller  61 , and move the entire intermediate transfer unit  43  except for the intermediate transfer roller  61  by the transfer separation unit to separate the intermediate transfer belt  42  from the recording medium P. Since the developing section  40  is disposed above the intermediate transfer unit  43 , the transfer separation unit desirably moves the developing section  40  together with the intermediate transfer unit  43  in the same direction. 
     The image forming apparatus  3  may include a transfer separation unit for moving the conveying unit  41  and intermediate transfer roller  61  in a direction (downward direction) away from the intermediate transfer belt  42  and backup roller  62  without moving the intermediate transfer belt  42 , and separate the recording medium P from the intermediate transfer belt  42  by means of this transfer separation unit. 
     In the above embodiment, the image forming apparatus  3  separates the fixing belt  47  from the recording medium P by moving the fixing belt  47  of the fixing unit  44  in the direction (upward direction) away from the pressure roller  48 . However, how to separate the fixing belt  47  from the recording medium P is not limited to this. The image forming apparatus  3  may include a fixing separation unit for moving both the fixing belt  47  and pressure roller  48  away from each other and separate the fixing belt  47  from the recording medium P by means of this fixing separation unit. 
     Further, for example, when the image forming apparatus  3  includes the transfer separation unit for moving the conveying unit  41  and intermediate transfer roller  61  in the direction (downward direction) away from the intermediate transfer belt  42  and backup roller  62 , as described above, the image forming apparatus  3  may include a fixing separation unit for moving the pressure roller  48  in a direction (downward direction) away from the fixing belt  47 . 
     &lt;4-4. Fourth Modification&gt; 
     In the above embodiment, when the image forming apparatus  3  detects the boundary between the elementary image ST 1  and the pattern image PT, it waits until the rear end of the elementary image ST 1  preceding the pattern image PT reaches the nip position T 1  of the intermediate transfer unit  43 ; when the rear end reaches the nip position T 1 , the image forming apparatus  3  stops the printing, separates the intermediate transfer belt  42  from the recording medium P, and separates the fixing belt  47  from the recording medium P. 
     However, this is not mandatory, and for example, when the rear end of the elementary image ST 1  reaches the nip position T 1  of the intermediate transfer unit  43 , the image forming apparatus  3  may separate the intermediate transfer belt  42  and recording medium P from each other while leaving the fixing belt  47  and recording medium P in contact with each other. At this time, a developer image (referred to as the non-fixed developer image) that has not been fixed exists on the recording medium P between the fixing unit  44  and the nip position T 1  of the intermediate transfer unit  43 . 
     After separating the intermediate transfer belt  42  and recording medium P from each other, the image forming apparatus  3  may operate as follows. The image forming apparatus  3  conveys again the recording medium P to fix the non-fixed developer image on the recording medium P by the fixing unit  44 . Upon completion of the fixing of the non-fixed developer image, the image forming apparatus  3  stops the conveyance of the recording medium P and separates the fixing belt  47  and recording medium P from each other. After that, the image forming apparatus  3  conveys the recording medium P in the reverse direction back to the former position (i.e., the position where the rear end of the elementary image ST 1  transferred on the recording medium P is just below the nip position T 1  of the intermediate transfer unit  43 ) and stops the conveyance of the recording medium P again. 
     While performing density correction with the intermediate transfer belt  42  and recording medium P separated from each other, the image forming apparatus  3  may fix the non-fixed developer image on the recording medium P by the fixing unit  44 . Immediately after the fixing belt  47  is brought into contact with the recording medium P again, the fixing unit  44  may be unable to sufficiently fix developer due to insufficient heating of the recording medium P. By fixing the non-fixed developer image on the recording medium P by the fixing unit  44  before separating the fixing belt  47  from the recording medium P, as described above, the non-fixed developer image on the recording medium P can be fixed appropriately. A developer image transferred onto the recording medium P after the fixing belt  47  is brought into contact with the recording medium P again takes time to reach the fixing unit  44  and thus can be fixed sufficiently. 
     &lt;4-5. Fifth Modification&gt; 
     In the above embodiment, the image forming apparatus  3  prints the long image LT on the paper web from a roll of paper as a continuous recording medium. However, the continuous recording medium is not limited to this. The image forming apparatus  3  may continuously or successively convey multiple recording media of normal size (e.g., A4 size) and continuously or successively print images of multiple pages on the multiple recording media. Also, the image forming apparatus  3  may continuously or successively print the same image on a label web (e.g., label paper web) as a continuous recording medium. 
     An exemplary case where the image forming apparatus  3  prints on a label web  100  as illustrated in  FIG. 11  will be briefly described. The label web (e.g., label paper web)  100  consists of a liner (or backing sheet)  101  and multiple labels (or stickers)  102  arranged on the liner  101  in the medium conveying direction at predetermined intervals. The labels  102  have a predetermined shape (e.g., a rectangular shape). In this case, each of the labels  102  is a print region in which an image is to be printed. For example, the elementary images ST are printed on the labels  102  on a one-to-one basis. 
     In this case, the image forming apparatus  3  controls the transfer separation mechanism  46  and conveying unit  41  to separate the label web  100  and intermediate transfer belt  42  from each other and bring them into contact with each other again when the nip position T 1  of the intermediate transfer unit  43  is located near the middle of a portion (i.e., a non-print region extending in a direction across the medium conveying direction) on the liner  101  between adjacent two labels  102 . This makes it possible to prevent an excessive load from being applied to the labels  102  when the intermediate transfer belt  42  and label web  100  are separated from each other and brought into contact with each other again, thereby preventing defects, such as separation of the labels  102 . To locate a non-print region between adjacent two labels  102  just below the nip position T 1 , after detecting a non-print region between adjacent two labels  102  by the medium sensor  72  illustrated in  FIG. 2 , the image forming apparatus  3  may convey the recording medium P by a predetermined amount (or the distance from the detection position of the medium sensor  72  to the nip position T 1 ). 
     &lt;4-6. Sixth Modification&gt; 
     In the above embodiment, the image forming apparatus  3  performs density correction during printing by transferring the pattern image PT, which is an example of a test pattern, onto the intermediate transfer belt  42 . However, the image forming apparatus  3  may perform other types of corrections during printing. For example, the image forming apparatus  3  may transfer, onto the intermediate transfer belt  42 , position correction patterns for correcting displacement of images transferred onto the intermediate transfer belt  42  and read the position correction patterns to perform displacement correction during printing. To correct the displacement, the image forming apparatus  3  may correct positions of developer images formed on the photosensitive drums  81  of the developing section  40 . 
     &lt;4-7. Seventh Modification&gt; 
     In the above embodiment, if the actual print size of the long image LD exceeds the predetermined threshold, the image forming apparatus  3  performs density correction during printing of the long image LD. However, the condition for performing density correction during printing is not limited to this, and may be another condition or a combination of multiple conditions. 
     &lt;4-8. Eighth Modification&gt; 
     In the above embodiment, the present invention is applied to the image forming apparatus  3  having the configuration illustrated in  FIGS. 1 and 2 . However, the present invention is applicable to other image forming apparatuses using a secondary transfer system that transfers an image onto an intermediate transfer belt or member and then transfers the image onto a recording medium. Specifically, the present invention is applicable to various image forming apparatuses, such as printers, facsimile machines, or multi-function products (MFPs). 
     &lt;4-9. Ninth Modification&gt; 
     In the above embodiment, the image forming apparatus  3  has the intermediate transfer belt  42  as a primary transfer member. However, the primary transfer member is not limited to this, and may be other members on which developer images can be formed (or transferred). Further, in the above embodiment, the image forming apparatus  3  has the developing section  40  as an image forming unit for forming multiple successive developer images on the primary transfer member. However, the image forming unit is not limited to this, and may be other units capable of forming multiple successive developer images on the primary transfer member. Further, in the above embodiment, the image forming apparatus  3  has the developing controller  27  and density corrector  35  as a controller for controlling the developing section  40  as the image forming unit in accordance with the long image LD or processed image PGD as image data and performing density correction. However, the controller is not limited to this, and may be other units capable of controlling the image forming unit and performing density correction. 
     Further, in the above embodiment, the image forming apparatus  3  has the pattern detector  34  and density correction sensor  51  as a detector for detecting a developer image formed on the primary transfer member. However, the detector is not limited to this, and may be other detectors capable of detecting a developer image formed on the primary transfer member. Further, in the above embodiment, the image forming apparatus  3  has the intermediate transfer roller  61  and backup roller  62  as a secondary transfer unit for transferring a developer image formed on the primary transfer member onto a recording medium. However, the secondary transfer unit is not limited to this, and may be other units capable of transferring a developer image formed on the primary transfer member onto a recording medium. 
     Further, in the above embodiment, the image forming apparatus  3  has the transfer separation controller  31  and transfer separation mechanism  46  as a transfer separation unit for separating the primary transfer member and the recording medium from each other and bring the primary transfer member and the recording medium into contact with each other. However, the transfer separation unit is not limited to this, and may be other units capable of separating the primary transfer member and the recording medium from each other and bring the primary transfer member and the recording medium into contact with each other. For example, the transfer separation mechanism  46  may use a mechanism other than the solenoid. Further, in the above embodiment, the image forming apparatus  3  has the fixing belt  47  as a heating unit for heating a developer image transferred on a recording medium. However, the heating unit is not limited to this, and may be other units capable of heating a developer image transferred on a recording medium. Further, in the above embodiment, the image forming apparatus  3  has the fixing separation controller  32  and fixing separation mechanism  49  as a fixing separation unit for separating the heating unit and the recording medium from each other and bringing the heating unit and the recording medium into contact with each other. However, the fixing separation unit is not limited to this, and may be other units capable of separating the heating unit and the recording medium from each other and bringing the heating unit and the recording medium into contact with each other. For example, the fixing separation mechanism  49  may use a mechanism other than the solenoid. 
     &lt;4-10. Tenth Modification&gt; 
     The present invention is not limited to the above embodiment or modifications. The present invention also covers all possible combinations or subsets of features of the above embodiment and modifications. The present invention can be practiced in various other aspects without departing from the inventive scope. 
     The present invention can be widely applied to image forming apparatuses having density correction functions.