Patent Publication Number: US-9846395-B2

Title: Image forming apparatus and image forming method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of application Ser. No. 14/856,649 filed on Sep. 17, 2015, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD 
     Embodiments described herein relate generally to an image forming apparatus and an image forming method. 
     BACKGROUND 
     There is an image forming apparatus that forms a multicolor toner image. The image forming apparatus is arranged with multiple image forming units that form different color toner images on the peripheral portion of an intermediate transfer belt. For example, each image forming unit includes a photoconductive drum. A charger, a light exposure unit, a developing device, a cleaning unit, a static electricity discharger, and the like are arranged around the photoconductive drum. 
     In most cases, the image forming apparatus includes four color image forming units for yellow, magenta, cyan, and black colors. However, the image forming apparatus may include an image forming unit for fifth color other than yellow, magenta, cyan, and black. For example, the fifth color image forming unit may form a toner image with a gold toner or a silver toner, which may not be reproduced with the four color toners. For example, the fifth color image forming unit may form a toner image with an erasable toner that causes the image to be erasable. 
     A fifth color image may be an image in the fifth color only. A fifth color image may overlay a full color image formed with the four colors. 
     The image forming apparatus including five color image forming units has a different frequency of use for each image forming unit. For example, the black image forming unit forms a monochrome image in addition to a full color image. Thus, the frequency of use of the black image forming unit may be high. The frequency of use of the fifth color image forming unit, for example, may be low in comparison with the other four colors. 
     Inmost cases, the photoconductive drum of an image forming unit having a low frequency of use is brought into contact with the intermediate transfer belt without a toner therebetween. Thus, the photoconductive drum is likely to be degraded. 
     Separating the photoconductive drum from the intermediate transfer belt is also considered when images are not formed. However, a problem arises in that a complicated mechanism is required to separate each photoconductive drum from the intermediate transfer belt individually. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic sectional view illustrating an example of the entire configuration of an image forming apparatus in an embodiment. 
         FIG. 2  is a schematic sectional view illustrating a configuration example of main portions of the image forming apparatus in the embodiment. 
         FIG. 3  is a block diagram illustrating a functional configuration example of the image forming apparatus in the embodiment. 
         FIG. 4  is a schematic sectional view illustrating a second state of abutting in the image forming apparatus of the embodiment. 
         FIG. 5  is a schematic sectional view illustrating a third state of abutting in the image forming apparatus of the embodiment. 
         FIG. 6  is a schematic sectional view illustrating a fourth state of abutting in the image forming apparatus of the embodiment. 
         FIG. 7  is a flowchart illustrating the operation of the image forming apparatus in the embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, an image forming apparatus includes an intermediate transfer belt, multiple first type photoconductive drums, and multiple first type transfer rollers. The image forming apparatus further includes a second type photoconductive drum, a second type transfer roller, a roller, and a roller moving unit. The intermediate transfer belt rotates and carries a toner image that is to be primarily transferred. The toner image on the intermediate transfer belt is secondarily transferred onto a sheet at a secondary transfer position. The multiple first type photoconductive drums are arranged to have a common tangent plane to drum surfaces on the outer peripheral side of the intermediate transfer belt. The multiple first type transfer rollers are arranged at positions that face the multiple first type photoconductive drums on the inner peripheral side of the intermediate transfer belt. The multiple first type transfer rollers transfer toner images that are formed on the multiple first type photoconductive drums onto the intermediate transfer belt. The second type photoconductive drum is arranged on the outer peripheral side of the intermediate transfer belt. The second type photoconductive drum is arranged at a position that is farther separated from the secondary transfer position than the multiple first type photoconductive drums and at a position where the drum surface of the second type photoconductive drum is not tangent to the common tangent plane from the same direction as the multiple first type photoconductive drums. The second type transfer roller is arranged on the inner peripheral side of the intermediate transfer belt. The second type transfer roller is arranged at a position that faces the second type photoconductive drum. The second type transfer roller transfers a toner image that is formed on the second type photoconductive drum onto the intermediate transfer belt. The roller is arranged on the inner peripheral side of the intermediate transfer belt. The roller is arranged between the second type transfer roller and one transfer roller of the multiple first type transfer rollers that is the closest to the second type transfer roller. The roller moving unit presses and moves the roller to the inner peripheral face of the intermediate transfer belt. The roller moving unit changes the state of abutting of the multiple first type photoconductive drums and the second type photoconductive drum on the intermediate transfer belt. 
     Embodiment 
     Hereinafter, an image forming apparatus  100  of an embodiment will be described with reference to drawings. In each drawing, the same configuration is given the same reference sign unless otherwise specified. 
       FIG. 1  is a schematic sectional view illustrating an example of the entire configuration of the image forming apparatus  100  in the embodiment.  FIG. 2  is a schematic sectional view illustrating a configuration example of main portions of the image forming apparatus  100  in the embodiment.  FIG. 3  is a block diagram illustrating a functional configuration example of the image forming apparatus  100  in the embodiment.  FIG. 4  is a schematic sectional view illustrating a second abutted state in the image forming apparatus  100  of the embodiment.  FIG. 5  is a schematic sectional view illustrating a third abutted state in the image forming apparatus  100  of the embodiment.  FIG. 6  is a schematic sectional view illustrating a fourth abutted state in the image forming apparatus  100  of the embodiment. 
     The image forming apparatus  100  of the embodiment, as illustrated in  FIG. 1 , includes a control panel  1 , a scanner unit  2 , a printer unit  3 , a sheet feeding unit  4 , a carrying unit  5 , and a control unit  6 . 
     The control panel  1  is a part of an input unit on which an operator inputs information so as to operate the image forming apparatus  100 . The control panel  1  includes a touch panel and various hard keys. The control panel  1  includes a start key that initiates image formation. 
     The scanner unit  2  reads image information about a subject as brightness and darkness of light. The scanner unit  2  outputs the read image information to the printer unit  3 . 
     The printer unit  3  forms an output image (hereinafter, referred to as a toner image) with a developer that includes toner and the like on the basis of the image information read by the scanner unit  2  or an image signal from the outside. 
     The printer unit  3  transfers the toner image onto the surface of a sheet S (paper). The printer unit  3  fixes the toner image to the sheet S by applying heat and pressure to the toner image on the surface of the sheet S. 
     The sheet feeding unit  4  feeds the sheet S one by one to the printer unit  3  according to the timing of the printer unit  3  forming the toner image. 
     The sheet feeding unit  4  includes multiple paper feeding cassettes  20 A,  20 B, and  20 C. Each of the paper feeding cassettes  20 A,  20 B, and  20 C accommodates the sheet S having a preset size and a preset type. The paper feeding cassettes  20 A,  20 B, and  20 C respectively include pick-up rollers  21 A,  21 B, and  21 C. Each of the pick-up rollers  21 A,  21 B, and  21 C withdraws one sheet S from each of the paper feeding cassettes  20 A,  20 B, and  20 C. The pick-up rollers  21 A,  21 B, and  21 C feed the withdrawn sheet S to the carrying unit  5 . 
     The carrying unit  5  includes a carrying roller  23  and a resist roller  24 . The carrying unit  5  carries the sheet S fed by the pick-up rollers  21 A,  21 B, and  21 C to the resist roller  24 . The resist roller  24  carries the sheet S according to the timing of the printer unit  3  transferring the toner image onto the sheet S. 
     By the carrying roller  23 , the tip end of the sheet S in the direction of carrying abuts a nip N of the resist roller  24 . The carrying roller  23  aligns the tip end of the sheet S in the direction of carrying by bending the sheet S. 
     The resist roller  24  aligns the tip end of the sheet S to the nip N. Furthermore, the resist roller  24  carries the sheet S toward a later-described transfer unit  28 . 
     Next, a detailed configuration of the printer unit  3  will be described. 
     The printer unit  3  includes image forming units  40 K,  40 C,  40 M,  40 Y, and  40 S, a light exposure unit  26 , an intermediate transfer belt  27 , the transfer unit  28 , a fixer  29 , and a transfer belt cleaning unit  31 . 
     Each of the image forming units  40 K,  40 C,  40 M,  40 Y, and  40 S forms the toner image according to the image signal from the scanner unit  2  or the outside. 
     The image forming units  40 K,  40 C,  40 M,  40 Y, and  40 S, as illustrated in  FIG. 2 , respectively include photoconductive drums  41   k ,  41   c ,  41   m ,  41   y , and  41   s . The photoconductive drums  41   k ,  41   c ,  41   m ,  41   y , and  41   s  in this order are first, second, third, fourth, and fifth photoconductive drums. 
     The image forming units  40 K,  40 C,  40 M, and  40 Y respectively form toner images with black, cyan, magenta, and yellow toners on the photoconductive drums  41   k ,  41   c ,  41   m , and  41   y . Black, cyan, magenta, and yellow in this order are a first color, a second color, a third color, and a fourth color. 
     The image forming unit  40 S forms a toner image with a fifth color toner on the photoconductive drum  41   s . Examples of the fifth color toner may include a gold toner and a silver toner. Metallic colors such as gold and silver include photoluminescent pigments so as to give a metallic luster that characterizes metallic colors. 
     Examples of the fifth color toner may include an erasable toner. The erasable toner can be decolorized after the toner image is transferred and fixed to the sheet S. A method for decolorizing the erasable toner is not limited. For example, the erasable toner may be an erasable toner that is decolorized by heat. For example, the erasable toner may be an erasable toner that is decolorized by chemical agents. 
     As an example, the fifth color toner will be described as a gold toner below. The “five colors” mean black, cyan, magenta, yellow, and gold below unless otherwise specified. The “four colors” mean black, cyan, magenta, and yellow. 
     The photoconductive drums  41   s ,  41   y ,  41   m ,  41   c , and  41   k  are arranged parallel to each other at intervals as illustrated in  FIG. 2 . The photoconductive drums  41   s ,  41   y ,  41   m ,  41   c  and  41   k  are lined up in this order. The axial direction of each of the photoconductive drums  41   s ,  41   y ,  41   m ,  41   c , and  41   k  is orthogonal with respect to the direction of rotation of the later-described intermediate transfer belt  27 . The photoconductive drums  41   s ,  41   y ,  41   m ,  41   c , and  41   k  have the same drum radius rd. 
     The photoconductive drums  41   y ,  41   m ,  41   c , and  41   k  are positioned to have a common tangent plane T to the drum surfaces. Each of the photoconductive drums  41   y ,  41   m ,  41   c , and  41   k  is a first type photoconductive drum. 
     Meanwhile, the photoconductive drum  41   s  is arranged at a position to which the common tangent plane T of the photoconductive drums  41   y ,  41   m ,  41   c , and  41   k  is not tangent in the same direction as the photoconductive drums  41   y ,  41   m ,  41   c , and  41   k . The position to which the tangent plane T is not tangent in the same direction includes a position to which the common tangent plane T is not tangent and a position to which the common tangent plane T is tangent in the opposite direction. 
     The photoconductive drum  41   s  is a second type photoconductive drum. 
     In the present embodiment, all of the rotational axis lines of the photoconductive drums  41   y ,  41   m ,  41   c , and  41   k  are positioned a distance rd from the common tangent plane T downward in  FIG. 2 . The rotational axis lines of the photoconductive drums  41   y ,  41   m ,  41   c , and  41   k  are positioned on one plane. This plane will be referred to as a plane A. The rotational axis line of the photoconductive drum  41   s  is separated by a distance h (where h&gt;0) from the plane A. In the case of h=2×rd, the common tangent plane T of the photoconductive drums  41   y ,  41   m ,  41   c , and  41   k  is tangent to the photoconductive drum  41   s . In this case, the tangent plane T is tangent to the photoconductive drum  41   s  in the opposite direction from the photoconductive drums  41   y ,  41   m ,  41   c , and  41   k . Therefore, h=2×rd may be allowed. 
     Although not illustrated in  FIG. 2 , known types of a charger, a developing device, a cleaning unit, and a static electricity discharger are arranged around each of the photoconductive drums  41   s ,  41   y ,  41   m ,  41   c , and  41   k.    
     The chargers of the image forming units  40 S,  40 Y,  40 M,  40 C, and  40 K respectively charge the surfaces of the photoconductive drums  41   s ,  41   y ,  41   m ,  41   c , and  41   k.    
     The developing devices of the image forming units  40 S,  40 Y,  40 M,  40 C, and  40 K respectively accommodate developers including gold, yellow, magenta, cyan, and black toners. Each developing device develops electrostatic latent images on the photoconductive drums  41   s ,  41   y ,  41   m ,  41   c , and  41   k . As a result, toner images are formed with each color toner on the photoconductive drums  41   s ,  41   y ,  41   m ,  41   c , and  41   k.    
     Primary transfer rollers  42   s ,  42   y ,  42   m ,  42   c , and  42   k  face the photoconductive drums  41   s ,  41   y ,  41   m ,  41   c , and  41   k . The later-described intermediate transfer belt  27  is interposed between the primary transfer rollers  42   s ,  42   y ,  42   m ,  42   c , and  42   k  and the photoconductive drums  41   s ,  41   y ,  41   m ,  41   c , and  41   k.    
     The primary transfer rollers  42   s ,  42   y ,  42   m ,  42   c , and  42   k  are arranged on the inner peripheral side of the later-described intermediate transfer belt  27 . 
     The photoconductive drums  41   s ,  41   y ,  41   m ,  41   c , and  41   k  are arranged on the outer peripheral side of the later-described intermediate transfer belt  27 . 
     A transfer bias is applied to the primary transfer rollers  42   s ,  42   y ,  42   m ,  42   c , and  42   k  by the later-described control unit  6 . The primary transfer rollers  42   s ,  42   y ,  42   m ,  42   c , and  42   k  primarily transfer the respective toner images formed by the image forming units  40 S,  40 Y,  40 M,  40 C, and  40 K onto the intermediate transfer belt  27 . 
     The primary transfer rollers  42   k ,  42   c ,  42   m ,  42   y , and  42   s  in this order are first, second, third, fourth, and fifth transfer rollers. The primary transfer rollers  42   y ,  42   m ,  42   c , and  42   k  are first type transfer rollers. The primary transfer roller  42   s  is a second type transfer roller. 
     Each cleaning unit of the image forming units  40 S,  40 Y,  40 M,  40 C, and  40 K removes non-transferred toner on the surface of each photoconductive drum by scraping or the like after the primary transfer. 
     Each static electricity discharger of the image forming units  40 S,  40 Y,  40 M,  40 C, and  40 K irradiates the surface of the photoconductive drum that passes through the cleaning unit with light. Each static electricity discharger discharges the photoconductive drums  41   s ,  41   y ,  41   m ,  41   c , and  41   k.    
     The later-described light exposure unit  26  is positioned below the chargers and the developing devices as illustrated in  FIG. 1 . 
     The light exposure unit  26  irradiates the surfaces of the photoconductive drums  41   s ,  41   y ,  41   m ,  41   c , and  41   k  with exposure light rays L 0 , L 1 , L 2 , L 3 , and L 4 . The exposure light rays L 0 , L 1 , L 2 , L 3 , and L 4  are modulated on the basis of the image signal transmitted from the scanner unit  2  or the outside. The image signal transmitted from the scanner unit  2  or the outside to the light exposure unit  26  corresponds to an image formed on the sheet S. 
     The light exposure unit  26  forms electrostatic latent images on the photoconductive drums  41   s ,  41   y ,  41   m ,  41   c , and  41   k  according to the image signal. The position of irradiation of each exposure light ray is between the charger and the developing device in each photoconductive drum. 
     A configuration of scanning a laser beam can be used as an example of the light exposure unit  26 . A configuration of performing a solid-state scan by using an LED light-emitting element can also be used as an example of the light exposure unit  26 . 
     The intermediate transfer belt  27  is configured of an endless belt as illustrated in  FIG. 1 . Multiple rollers abut the inner peripheral face of the intermediate transfer belt  27 . The multiple rollers apply a tensile force to the intermediate transfer belt  27 . The multiple rollers tension the intermediate transfer belt  27  flat. 
     The intermediate transfer belt  27  is positioned above the photoconductive drums  41   s ,  41   y ,  41   m ,  41   c , and  41   k . The intermediate transfer belt  27  is tensioned along the direction of the photoconductive drums  41   s ,  41   y ,  41   m ,  41   c , and  41   k  being lined up. 
     A support roller  28   a  abuts the photoconductive drum  41   k  side end portion of the inner peripheral face of the intermediate transfer belt  27  in the direction of tension. A transfer belt roller  32  (second fixed roller) abuts the photoconductive drum  41   s  side end portion of the inner peripheral face of the intermediate transfer belt  27  in the direction of tension. 
     The positions of the support roller  28   a  and the transfer belt roller  32  are fixed with respect to the intermediate transfer belt  27 . 
     The support roller  28   a  constitutes a part of the later-described transfer unit  28 . The support roller  28   a  guides the intermediate transfer belt  27  to a later-described secondary transfer position. 
     The transfer belt roller  32  guides the intermediate transfer belt  27  to a cleaning position. 
     The support roller  28   a  and the transfer belt roller  32  are connected to an intermediate transfer belt drive unit  39  (refer to  FIG. 3 ). 
     The intermediate transfer belt drive unit  39  rotates the support roller  28   a  and the transfer belt roller  32  counterclockwise in  FIG. 1 . The intermediate transfer belt drive unit  39  is controlled by the later-described control unit  6 . 
     Other rollers that tension the intermediate transfer belt  27  are a passive roller  37  (first fixed roller), an abutting switch roller  34  (roller), and a tension roller  35  as illustrated in  FIG. 2 . 
     The passive roller  37  and the abutting switch roller  34  abut the inner peripheral face of the intermediate transfer belt  27  on the lower side between the support roller  28   a  and the transfer belt roller  32 . The passive roller  37  is positioned between the photoconductive drum  41   k  and the support roller  28   a  along the intermediate transfer belt  27 . The abutting switch roller  34  is positioned between the photoconductive drums  41   s  and  41   y  along the intermediate transfer belt  27 . 
     The height of the rotational axis line of the passive roller  37  from the plane A is h 37 . The height h 37  has dimensions that allow the intermediate transfer belt  27  abutting the passive roller  37  to be arranged at a position separated by the distance rd from the plane A. Given that the radius of the passive roller  37  is r 37 , and the thickness of the intermediate transfer belt  27  is t, for example, h 37 =rd+r 37 +t can be established. 
     The position of the passive roller  37  is fixed with respect to the intermediate transfer belt  27 . 
     The part of the intermediate transfer belt  27  being tensioned between the support roller  28   a  and the passive roller  37  will be referred to as a first tensioned area  27   d . The first tensioned area  27   d  is inclined in a certain direction. 
     The abutting switch roller  34  is movably supported by an abutting switch roller moving unit  36  (roller moving unit). 
     The abutting switch roller moving unit  36  moves the abutting switch roller  34 . The abutting switch roller moving unit  36  changes the position of pressing the abutting switch roller  34  to the inner peripheral face of the intermediate transfer belt  27 . When the position of pressing the abutting switch roller  34  is changed, the tensioned shape of the intermediate transfer belt  27  is changed. 
     The part of the intermediate transfer belt  27  being tensioned between the passive roller  37  and the abutting switch roller  34  will be referred to as a second tensioned area  27   a  below. The part of the intermediate transfer belt  27  being tensioned between the passive roller  37  and the transfer belt roller  32  will be referred to as a third tensioned area  27   b . The part of the intermediate transfer belt  27  being tensioned between the transfer belt roller  32  and the support roller  28   a  will be referred to as a fourth tensioned area  27   c.    
     The state of abutting between the intermediate transfer belt  27  and the photoconductive drums  41   s ,  41   y ,  41   m ,  41   c , and  41   k  is changed when the position of pressing the abutting switch roller  34  is changed. 
     For example, a configuration of the abutting switch roller moving unit  36  includes a motor that is a drive source, a gear that transmits the driving force of the motor, and a cam that interlocks with the gear. The cam moves the position of the abutting switch roller  34  to a predetermined position. 
     Hereinafter, the state of abutting between the intermediate transfer belt  27  and the photoconductive drums  41   s ,  41   y ,  41   m ,  41   c , and  41   k  may be shortly referred to as a “belt abutting state”. 
     The abutting switch roller moving unit  36  in the embodiment selectively switches the belt abutting state among first, second, third, and fourth states of abutting. 
     The first state of abutting is a belt abutting state in which all of the multiple first type photoconductive drums abut the intermediate transfer belt, and the second type photoconductive drum is separated from the intermediate transfer belt. 
     In the first state of abutting, as illustrated in  FIG. 2 , the photoconductive drums  41   y ,  41   m ,  41   c , and  41   k  abut the intermediate transfer belt  27  in the second tensioned area  27   a  (refer to positions P 4 , P 3 , P 2 , and P 1 ). Furthermore, in the first state of abutting, the third tensioned area  27   b  facing the photoconductive drum  41   s  is separated from the photoconductive drum  41   s . Thus, the photoconductive drum  41   s  is separated from the intermediate transfer belt  27 . 
     Since  FIG. 2  is schematically illustrated, the second tensioned area  27   a  in the first state of abutting is tensioned as a plane conforming to the tangent plane T of the photoconductive drums  41   y ,  41   m ,  41   c , and  41   k . However, the second tensioned area  27   a  is not necessarily a plane when the intermediate transfer belt  27  is pressed to each photoconductive drum by the primary transfer rollers  42   y ,  42   m ,  42   c , and  42   k . The passive roller  37  and the abutting switch roller  34  may be arranged at positions that cause the intermediate transfer belt  27  in the vicinity of the passive roller  37  and the abutting switch roller  34  to be shifted from the tangent plane T. This is applied in the same manner to the later-described second state of abutting. 
     The second state of abutting is a belt abutting state in which all of the multiple first type photoconductive drums and the second type photoconductive drum abut the intermediate transfer belt. 
     In the second state of abutting, as illustrated in  FIG. 4 , the photoconductive drums  41   y ,  41   m ,  41   c , and  41   k  abut the intermediate transfer belt  27  in the second tensioned area  27   a  (refer to the positions P 4 , P 3 , P 2 , and P 1 ). Furthermore, in the second state of abutting, the photoconductive drum  41   s  abuts the intermediate transfer belt  27  in the third tensioned area  27   b  (refer to a position P 5 ). 
     The third state of abutting is a belt abutting state in which only the first type photoconductive drum that is closest to the secondary transfer position among the multiple first type photoconductive drums abuts the intermediate transfer belt, and the other first type photoconductive drums among the multiple first type photoconductive drums and the second type photoconductive drum are separated from the intermediate transfer belt. 
     In the third state of abutting, as illustrated in  FIG. 5 , the photoconductive drum  41   k  abuts the intermediate transfer belt  27  in the second tensioned area  27   a  (refer to the position P 1 ). Furthermore, in the third state of abutting, the photoconductive drums  41   y ,  41   m ,  41   c , and  41   s  are separated from the intermediate transfer belt  27 . 
     The fourth state of abutting is a belt abutting state in which the second type photoconductive drum abuts the intermediate transfer belt, and all of the multiple first type photoconductive drums are separated from the intermediate transfer belt. 
     In the fourth state of abutting, as illustrated in  FIG. 6 , the photoconductive drums  41   y ,  41   m ,  41   c , and  41   k  are separated from the intermediate transfer belt  27 . Furthermore, in the fourth state of abutting, the photoconductive drum  41   s  abuts the intermediate transfer belt  27  in the third tensioned area  27   b  (refer to the position P 5 ). 
     There are four types of the positions of pressing the abutting switch roller  34  in the image forming apparatus  100 . The four types of the pressing positions change a belt abutting state to the above first, the second, the third, and the fourth states of abutting. A relationship between the position of pressing the abutting switch roller  34  and a belt abutting state will be described later in an operational description. 
     The tension roller  35  abuts the inner peripheral face of the intermediate transfer belt  27  in the fourth tensioned area  27   c  as illustrated in  FIG. 2 . The tension roller  35  presses the intermediate transfer belt  27  outward. The tension roller  35  constantly maintains the tensile force of the intermediate transfer belt  27  even when the abutting switch roller  34  moves. 
     For example, the tension roller  35  is caused to elastically press the intermediate transfer belt  27  by an unillustrated pressure-applying spring. 
     The primary transfer roller  42   s  is positioned between the transfer belt roller  32  and the abutting switch roller  34  inside the intermediate transfer belt  27 . The primary transfer roller  42   s  faces the photoconductive drum  41   s  as described above. The primary transfer roller  42   s  is supported by a transfer roller pressing unit  43   s.    
     The transfer roller pressing unit  43   s  causes the primary transfer roller  42   s  to abut the inner peripheral face of the third tensioned area  27   b . When the tensioned shape of the intermediate transfer belt  27  is changed by a movement of the abutting switch roller  34 , the transfer roller pressing unit  43   s  follows a change in the third tensioned area  27   b.    
     The primary transfer rollers  42   y ,  42   m ,  42   c , and  42   k  are positioned between the abutting switch roller  34  and the passive roller  37  inside the intermediate transfer belt  27 . The primary transfer rollers  42   y ,  42   m ,  42   c , and  42   k  face the photoconductive drums  41   y ,  41   m ,  41   c , and  41   k  as described above. The primary transfer rollers  42   y ,  42   m ,  42   c , and  42   k  are respectively supported by transfer roller pressing units  43   y ,  43   m ,  43   c , and  43   k.    
     The transfer roller pressing units  43   y ,  43   m ,  43   c , and  43   k  respectively cause the primary transfer rollers  42   y ,  42   m ,  42   c , and  42   k  to abut the inner peripheral face of the second tensioned area  27   a . When the tensioned shape of the intermediate transfer belt  27  is changed by a movement of the abutting switch roller  34 , the transfer roller pressing units  43   y ,  43   m ,  43   c , and  43   k  follow a change in the second tensioned area  27   a.    
     The transfer unit  28  is positioned at a position adjacent to the image forming unit  40 K on the intermediate transfer belt  27 . 
     The transfer unit  28  includes the above support roller  28   a  and a secondary transfer roller  28   b . The intermediate transfer belt  27  is interposed between the support roller  28   a  and the secondary transfer roller  28   b . The position where the secondary transfer roller  28   b  and the intermediate transfer belt  27  abut each other is the secondary transfer position. 
     The photoconductive drum  41   k  is the closest to the secondary transfer position among the first type photoconductive drums. The photoconductive drum  41   y  is the furthest from the secondary transfer position among the first type photoconductive drums. 
     The photoconductive drum  41   s  which is the second type photoconductive drum is farther separated from the secondary transfer position than any other first type photoconductive drums. 
     The transfer unit  28  transfers the toner image, which is primarily transferred onto the intermediate transfer belt  27 , onto the surface of the sheet S at the secondary transfer position. The transfer unit  28  applies a transfer bias to the secondary transfer position. The transfer unit  28  transfers the toner image on the intermediate transfer belt  27  onto the sheet S with the transfer bias. 
     The fixer  29  applies heat and pressure to the sheet S. The fixer  29  fixes the toner image transferred onto the sheet S with heat and pressure. 
     The transfer belt cleaning unit  31  faces the transfer belt roller  32 . The intermediate transfer belt  27  is interposed in the transfer belt cleaning unit  31 . The transfer belt cleaning unit  31  scrapes toner on the surface of the intermediate transfer belt  27 . The transfer belt cleaning unit  31  collects the scraped toner in a waste toner tank. 
     The printer unit  3  further includes an inverting unit  30 . The inverting unit  30  inverts the sheet S discharged from the fixer  29  with a switchback. The inverting unit  30  carries the inverted sheet S back into a carrying guide in front of the resist roller  24 . The inverting unit  30  inverts the sheet S so that an image can be formed on the rear side of the sheet S. 
     The control unit  6 , as illustrated in  FIG. 3 , is communicably connected with an input unit  200 , the light exposure unit  26 , the image forming units  40 Y,  40 M,  40 C,  40 K, and  40 S, the carrying unit  5 , the sheet feeding unit  4 , the intermediate transfer belt drive unit  39 , and the abutting switch roller moving unit  36 . The control unit  6  controls an image forming operation on the basis of an instruction that is input from the input unit  200 . Furthermore, the control unit  6  controls switching of the belt abutting state. 
     The input unit  200  includes a printer interface  201  and the above control panel  1  and the scanner unit  2 . 
     The printer interface  201  is an interface that is used when the image forming apparatus  100  is used as a printer. The printer interface  201  is connected to a communication line. 
     A control value that is used in the control of the control unit  6  is stored in advance on a storage unit of the control unit  6 . The control value used in the control of the control unit  6  is input from the control panel  1  when necessary. 
     The control unit  6  performs control depending on an operational mode of the image forming apparatus  100 . 
     The image forming apparatus  100  includes at least a four-color mode, a five-color mode, a first color mode, and a fifth color mode as the operational mode. 
     The four-color mode is an operational mode in which the image forming apparatus  100  forms an image that includes a toner image in at least one color of yellow, magenta, cyan, and black toners. The belt abutting state is the first state of abutting in the four-color mode. 
     The five-color mode is an operational mode in which the image forming apparatus  100  forms an image that includes a toner image in at least one color of yellow, magenta, cyan, black, and gold toners. The belt abutting state is the second state of abutting in the five-color mode. 
     The first color mode is an operational mode in which the image forming apparatus  100  forms an image with a toner image only in black which is the first color. The belt abutting state is the third state of abutting in the first color mode. 
     The fifth color mode is an operational mode in which the image forming apparatus  100  forms an image with a toner image only in gold which is the fifth color. The belt abutting state is the fourth state of abutting in the fifth color mode. 
     Details of the control of the control unit  6  will be described later along with the operation of the image forming apparatus  100 . 
     An apparatus configuration of the control unit  6  is configured of appropriate pieces of hardware and a computer that includes a CPU, a memory, an input-output interface, an external storage device, and the like. The memory and the external storage device constitute the storage unit of the control unit  6 . The above control function of the control unit  6  is realized by a computer executing a control program. Alternatively, the above control function of the control unit  6  is realized by the operation of the appropriate pieces of hardware. 
     The operation of the image forming apparatus  100  that includes the above configuration will be described with a main focus on an operation of switching the belt abutting state. 
       FIG. 7  is a flowchart illustrating the operation of the image forming apparatus  100  in the embodiment. 
     The image forming apparatus  100  realizes the belt abutting state according to the operational mode after the operational mode is set. 
     The operational mode can be set by the operator operating the control panel  1 . The control panel  1  transmits a control signal that specifies the operational mode (hereinafter, referred to as an operational mode specifying signal) to the control unit  6  on the basis of the operational input. The operational mode can also be set by the printer interface  201  transmitting the operational mode specifying signal to the control unit  6 . 
     The assumption is made that a signal initiating printing (hereinafter, referred to as a printing initiation signal) occurs with the operational mode not set. In this case, the control unit  6  initiates operation on the basis of a default operational mode. The default operational mode of the image forming apparatus  100  for the belt abutting state is the four-color mode. The image forming apparatus  100  sets the belt abutting state to the first state of abutting after warming up. 
     The printing initiation signal occurs when the operator operates the start key of the control panel  1 . Alternatively, the printing initiation signal may be transmitted to the control unit  6  through the printer interface  201 . 
     The image forming apparatus  100  is assumed to finish warming up. The control unit  6  waits for an interruption. Hereinafter, simply, the occurrence of the printing initiation signal or the operational mode specifying signal will be regarded as an interruption. 
     The control unit  6  controls the image forming apparatus  100  on the basis of the flow illustrated in  FIG. 7 . 
     In ACT 1, the control unit  6  determines whether the printing initiation signal occurs. 
     ACT 10 is performed when the control unit  6  determines that the printing initiation signal occurs (YES in ACT 1). 
     ACT 2 is performed when the control unit  6  determines that the printing initiation signal does not occur (NO in ACT 1). 
     In ACT 2, the control unit  6  determines whether the operational mode specifying signal for the four-color mode occurs. 
     ACT 3 is performed when the control unit  6  determines that the operational mode specifying signal for the four-color mode occurs (YES in ACT 2). 
     ACT 4 is performed when the control unit  6  determines that the operational mode specifying signal for the four-color mode does not occur (NO in ACT 2). 
     In ACT 3, the control unit  6  causes the abutting switch roller moving unit  36  to move the position of the abutting switch roller  34  to a first roller position Q 1  (refer to  FIG. 2 ). The first roller position Q 1  is a position that sets the belt abutting state to the first state of abutting. The first roller position Q 1  is stored in advance on the storage unit of the control unit  6 . 
     The second tensioned area  27   a  is parallel to the plane A in the first roller position Q 1 . Thus, the intermediate transfer belt  27  abuts the photoconductive drums  41   y ,  41   m ,  41   c , and  41   k  in the first roller position Q 1  (refer to the positions P 4 , P 3 , P 2 , and P 1 ). At this time, the primary transfer rollers  42   y ,  42   m ,  42   c , and  42   k  are pressed from the transfer roller pressing units  43   y ,  43   m ,  43   c , and  43   k  and abut the inner peripheral face of the second tensioned area  27   a . The second tensioned area  27   a  is interposed between the primary transfer rollers  42   y ,  42   m ,  42   c , and  42   k  and the photoconductive drums  41   y ,  41   m ,  41   c , and  41   k.    
     Furthermore, in the first roller position Q 1 , the third tensioned area  27   b  is separated from the photoconductive drum  41   s . At this time, the primary transfer roller  42   s  abuts the inner peripheral face of the third tensioned area  27   b . However, the pressure of the transfer roller pressing unit  43   s  is not great enough to press the third tensioned area  27   b  to the photoconductive drum  41   s.    
     ACT 3 ends, and ACT 1 is performed after ACT 3. 
     In ACT 4, the control unit  6  determines whether the operational mode specifying signal for the five-color mode occurs. 
     ACT 5 is performed when the control unit  6  determines that the operational mode specifying signal for the five-color mode occurs (YES in ACT 4). 
     ACT 6 is performed when the control unit  6  determines that the operational mode specifying signal for the five-color mode does not occur (NO in ACT 4). 
     In ACT 5, the control unit  6  causes the abutting switch roller moving unit  36  to move the position of the abutting switch roller  34  to a second roller position Q 2  (refer to  FIG. 4 ). The second roller position Q 2  is a position that sets the belt abutting state to the second state of abutting. The second roller position Q 2  is stored in advance on the storage unit of the control unit  6 . 
     The second tensioned area  27   a , as in the first roller position Q 1 , is parallel to the plane A in the second roller position Q 2 . The second tensioned area  27   a , as in the first roller position Q 1 , is interposed between the primary transfer rollers  42   y ,  42   m ,  42   c , and  42   k  and the photoconductive drums  41   y ,  41   m ,  41   c , and  41   k.    
     Furthermore, in the second roller position Q 2 , the third tensioned area  27   b  abuts the photoconductive drum  41   s  (refer to the position P 5 ). At this time, the primary transfer roller  42   s  is pressed from the transfer roller pressing unit  43   s  and abuts the inner peripheral face of the third tensioned area  27   b . The third tensioned area  27   b  is interposed between the primary transfer roller  42   s  and the photoconductive drum  41   s.    
     The abutting switch roller  34  is assumed to be moved from the first roller position Q 1  to the second roller position Q 2 . In this case, the abutting switch roller moving unit  36  may move the abutting switch roller  34  in a parallel manner in a direction along the plane A by a certain distance toward the photoconductive drum  41   s.    
     The second roller position Q 2  is a position to which the first roller position Q 1  is moved in a parallel manner in a direction along the plane A by a certain distance toward the photoconductive drum  41   s.    
     ACT 5 ends, and ACT 1 is performed after ACT 5. 
     In ACT 6, the control unit  6  determines whether the operational mode specifying signal for the first color mode occurs. 
     ACT 7 is performed when the control unit  6  determines that the operational mode specifying signal for the first color mode occurs (YES in ACT 6). 
     ACT 8 is performed when the control unit  6  determines that the operational mode specifying signal for the first color mode does not occur (NO in ACT 6). 
     In ACT 7, the control unit  6  causes the abutting switch roller moving unit  36  to move the position of the abutting switch roller  34  to a third roller position Q 3  (refer to  FIG. 5 ). The third roller position Q 3  is a position that sets the belt abutting state to the third state of abutting. The third roller position Q 3  is stored in advance on the storage unit of the control unit  6 . 
     In the third roller position Q 3 , the second tensioned area  27   a  abuts only the photoconductive drum  41   k  (refer to the position P 1 ). Furthermore, in the third roller position Q 3 , the third tensioned area  27   b  is separated from the photoconductive drum  41   s.    
     The abutting switch roller  34  is assumed to be moved from the first roller position Q 1  to the third roller position Q 3 . In this case, the abutting switch roller moving unit  36  increases the distance between the abutting switch roller  34  and the plane A. The abutting switch roller moving unit  36  causes the intermediate transfer belt  27  tensioned between the passive roller  37  and the abutting switch roller  34  to be inclined upward with the passive roller  37  as the center. 
     The third roller position Q 3  is a position to which the first roller position Q 1  is moved in a parallel manner in a direction that is orthogonal with respect to the plane A by a certain distance. 
     In the third roller position Q 3 , the intermediate transfer belt  27  abuts only the photoconductive drum  41   k  of the photoconductive drums. The second tensioned area  27   a  is interposed between the primary transfer roller  42   k  and the photoconductive drum  41   k.    
     ACT 7 ends, and ACT 1 is performed after ACT 7. 
     In ACT 8, the control unit  6  determines whether the operational mode specifying signal for the fifth color mode occurs. 
     ACT 9 is performed when the control unit  6  determines that the operational mode specifying signal for the fifth color mode occurs (YES in ACT 8). 
     ACT 1 is performed when the control unit  6  determines that the operational mode specifying signal for the fifth color mode does not occur (NO in ACT 8). 
     In ACT 9, the control unit  6  causes the abutting switch roller moving unit  36  to move the abutting switch roller  34  to a fourth roller position Q 4  (refer to  FIG. 6 ). The fourth roller position Q 4  is a position that sets the belt abutting state to the fourth state of abutting. The fourth roller position Q 4  is stored in advance on the storage unit of the control unit  6 . 
     In the fourth roller position Q 4 , the second tensioned area  27   a  is separated from the photoconductive drums  41   y ,  41   m ,  41   c , and  41   k . Furthermore, in the fourth roller position Q 4 , the third tensioned area  27   b  abuts the photoconductive drum  41   s  (refer to the position P 5 ). 
     The abutting switch roller  34  is assumed to be moved from the first roller position Q 1  to the fourth roller position Q 4 . In this case, the abutting switch roller moving unit  36  moves the abutting switch roller  34  in a direction along the plane A slightly closer to the photoconductive drum  41   s  than the second position Q 2 . The abutting switch roller moving unit  36  further increases the distance between the abutting switch roller  34  and the plane A in a direction that is orthogonal with respect to the plane A than in the third roller position Q 3 . 
     The fourth roller position Q 4  is a position to which the first roller position Q 1  is moved further toward the photoconductive drum  41   s  than the second roller position Q 2  and further upward than the third roller position Q 3 . 
     In the fourth roller position Q 4 , the intermediate transfer belt  27  abuts only the photoconductive drum  41   s  of the photoconductive drums. The third tensioned area  27   b  is interposed between the primary transfer roller  42   s  and the photoconductive drum  41   s.    
     ACT 9 ends, and ACT 1 is performed after ACT 9. 
     In ACT 10, the image forming apparatus  100  initiates image formation. 
     The image forming operation of the image forming apparatus  100  differs depending on the operational mode. 
     In the four-color mode, the image forming unit  40 S does not perform the image forming operation. The light exposure unit  26  stops emitting the exposure light ray L 0 . 
     In the five-color mode, all of the image forming units  40 S,  40 Y,  40 M,  40 C, and  40 K perform the image forming operation. The light exposure unit  26  modulates all of the exposure light rays L 0 , L 1 , L 2 , L 3 , and L 4  according to the image signal. 
     In the first color mode, only the image forming unit  40 K performs the image forming operation. The light exposure unit  26  stops emitting exposure light rays other than the exposure light ray L 1 . 
     In the fifth color mode, only the image forming unit  40 S performs the image forming operation. The light exposure unit  26  stops emitting exposure light rays other than the exposure light ray L 0 . 
     Hereinafter, the image forming operation will be described, excluding the above differences unless otherwise specified. The parts or the members of the apparatus performing the image forming operation will be illustrated by changing the last character of the reference sign thereof to X or x. For example, an image forming unit  40 X (photoconductive drum  41   x ) indicates the image forming units  40 Y,  40 M,  40 C, and  40 K (photoconductive drums  41   y ,  41   m ,  41   c , and  41   k ) in the four-color mode. For example, the image forming unit  40 X (photoconductive drum  41   x ) indicates the image forming unit  40 K (photoconductive drum  41   k ) in the first color mode. 
     In ACT 10, the control unit  6  causes the printer unit  3  to initiate image formation. The printer unit  3  feeds the appropriate size sheet S from the sheet feeding unit  4  to the resist roller  24 . 
     The control unit  6  charges the photoconductive drum  41   x  of the image forming unit  40 X. The control unit  6  forms an electrostatic latent image with the light exposure unit  26  on the photoconductive drum  41   x.    
     The control unit  6  causes the developing device of the image forming unit  40 X to develop the electrostatic latent image formed on the photoconductive drum  41   x . A toner image corresponding to the electrostatic latent image is formed on the surface of the photoconductive drum  41   x.    
     The control unit  6  causes the intermediate transfer belt drive unit  39  to rotate the intermediate transfer belt  27  in a certain direction. The intermediate transfer belt  27  rotates in synchronization with the rotation of the photoconductive drum  41   x.    
     The control unit  6  applies a transfer bias to a primary transfer roller  42   x  and primarily transfers the toner image on the photoconductive drum  41   x  onto the intermediate transfer belt  27 . When the image forming unit  40 X operating at this time includes multiple image forming units, the transfer timing is appropriately shifted depending on the position of arrangement of each image forming unit of the image forming unit  40 X. Thus, each toner image is sequentially overlaid without causing variations in color along with the movement of the intermediate transfer belt  27 . 
     The toner image on the intermediate transfer belt  27  is carried to the transfer unit  28 . The sheet S moves to the secondary transfer position at the timing when the tip end of the toner image moves to the secondary transfer position. 
     The control unit  6  applies a transfer bias to the secondary transfer roller  28   b . The secondary transfer roller  28   b  secondarily transfers the toner image at the secondary transfer position onto the sheet S. The secondarily transferred toner image is fixed to the sheet S by the fixer  29 . The sheet S on which the toner image is fixed is discharged outside the image forming apparatus  100 . 
     Remaining toner that is not transferred by the transfer unit  28  onto the sheet S is scraped by the transfer belt cleaning unit  31 . The intermediate transfer belt  27  is cleaned so as to be usable again. 
     Image formation on one sheet S is ended. 
     The image forming apparatus  100  includes the image forming units  40 S,  40 Y,  40 M,  40 C, and  40 K that can form toner images in five colors. In the image forming apparatus  100 , the image forming units  40 S,  40 Y,  40 M,  40 C, and  40 K can print images in each color and in a combined color of each on the sheet S. 
     The image forming apparatus  100  separates an image forming unit that does not perform image formation from the intermediate transfer belt  27  in the four-color mode, the first color mode, and the fifth color mode. Thus, wear due to contact between the intermediate transfer belt  27  and the photoconductive drum of the image forming unit that does not perform image formation can be reduced. Furthermore, since the photoconductive drum of the image forming unit that does not perform image formation does not abut the intermediate transfer belt  27 , an aberration of the primarily transferred toner image due to abutting of the photoconductive drum can be prevented. 
     The image forming apparatus  100  can switch the belt abutting state among the first, the second, the third, and the fourth states of abutting. At this time, the image forming apparatus  100  changes only the position of the abutting switch roller  34  with the abutting switch roller moving unit  36 . Thus, the apparatus configuration of the image forming apparatus  100  can be simplified. 
     For example, a configuration of causing five photoconductive drums to abut the tensioned intermediate transfer belt  27  by using two rollers is considered when image formation is performed in five colors. In this case, the operational mode can be switched between the five-color mode and the first color (fifth color) mode when the belt abutting state is changed by moving one of the two rollers. In this configuration, unlike in the four-color mode of the image forming apparatus  100 , a full color image cannot be formed by separating the photoconductive drum for the fifth color from the intermediate transfer belt. The frequency of use of the fifth color is low in comparison with the four-color full color mode. Thus, degradation of the fifth color image forming unit is comparatively fast despite of the low frequency of use. 
     Changing the belt abutting state by moving both of the two rollers is also considered. In this case, the belt abutting state can be switched among the five-color mode, the first color mode, and the fifth color mode. However, the four-color mode of the image forming apparatus  100  cannot be realized in this configuration. In addition, since this configuration requires two moving mechanisms, the apparatus configuration is complicated in comparison with the image forming apparatus  100 . The number of components is also increased in comparison with the image forming apparatus  100 . 
     In the image forming apparatus  100 , the photoconductive drums  41   y ,  41   m ,  41   c , and  41   k  are arranged on the plane A, and the photoconductive drum  41   s  is arranged to be shifted from the plane A. The abutting switch roller moving unit  36  moves the abutting switch roller  34  between the photoconductive drum  41   y  and the photoconductive drum  41   s . The abutting switch roller  34  changes the tensioned shape of the intermediate transfer belt  27  between the transfer belt roller  32  and the passive roller  37 . 
     According to the configuration, the image forming apparatus  100  can selectively switch the belt abutting state among the first, the second, the third, and the fourth states of abutting by moving only the abutting switch roller  34 . Thus, the image forming apparatus  100  can switch among the four-color mode, the five-color mode, the first color mode, and the fifth color mode with a simple and cheap configuration. Since the fifth color having a low frequency of use is used in image formation when necessary in the image forming apparatus  100 , the life of the image forming unit  40 S can be prolonged. 
     Furthermore, since the image forming apparatus  100  includes the first color mode, the life of the second color to the fifth color photoconductive drums having relatively low frequencies of use can be prolonged when the frequency of use of monochrome printing is high. 
     Hereinafter, modification examples of the above embodiment will be described. The image forming apparatus  100  includes the five-color mode in the description of the above embodiment. However, the image forming apparatus  100  may not include the five-color mode. For example, the five-color mode is not necessary when the fifth color toner is an erasable toner that is erased by heating at a lower temperature than the temperatures for fixing the first color to the fourth color. 
     The first color is described as black and the fifth color as a gold toner and the like in the description of the above embodiment. However, these are merely examples. Colors to assign to the first color to the fifth color can be freely selected. 
     The image forming apparatus  100  is described as including four photoconductive drums  41   y ,  41   m ,  41   c , and  41   k  as the first type photoconductive drums in the description of the above embodiment. However, the number of first type photoconductive drums may be two or three or be five or more as long as two or more of first type photoconductive drums are provided. 
     The photoconductive drums  41   s ,  41   y ,  41   m ,  41   c , and  41   k  are described as having the same drum radius in the description of the above embodiment. However, the drum radius of each photoconductive drum may not be the same. The photoconductive drums  41   y ,  41   m ,  41   c , and  41   k  are arranged to have the common tangent plane T to one drum surface when the drum radius of each photoconductive drum is different. In this case, the height of the rotational axis line of the photoconductive drum from the common tangent plane changes depending on the drum radius. The photoconductive drum  41   s  is arranged at a position that is not tangent to the common tangent plane from the same direction as the photoconductive drums  41   y ,  41   m ,  41   c , and  41   k.    
     According to at least one embodiment described above, the image forming apparatus includes the intermediate transfer belt, the multiple first type photoconductive drums, the first type transfer roller, the second type photoconductive drum, the second type transfer roller, the roller, and the roller moving unit. Thus, the image forming apparatus can separate the photoconductive drum of the image forming unit that does not perform image formation from the intermediate transfer belt with a simple configuration. 
     While certain embodiments have been described these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms: furthermore various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and there equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the invention.