Abstract:
An image forming apparatus includes: plural image carriers that are disposed at intervals and in a line, and that are driven to rotate while carrying images; an intermediate transfer belt that have an endless shape, and that is disposed to be laid on plural rolls so as to be able to contact the image carriers; plural transfer rolls that rotate while being brought into contact with the image carriers through the intermediate transfer belt in a first position, respectively, so as to transfer the images on the image carriers to the intermediate transfer belt; and a moving mechanism that moves a part of the plural transfer rolls to a second position keeping apart from the plural image carriers while the part of the plural transfer rolls are kept in contact with the intermediate transfer belt.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present invention relates to printers, copying machines and other image forming apparatuses using an intermediate transfer system. 
         [0003]    2. Related Art 
         [0004]    In recent years, for example, the following apparatus using an intermediate transfer system has been known as an image forming apparatus such as a printer using an electrophotographic system or an electrostatically recording system. 
         [0005]    That is, in such an image forming apparatus, as shown in  FIG. 13  by way of example, four image forming units  10 Y,  100 M,  100 C and  100 K for forming toner images of four colors, yellow (Y), magenta (M), cyan (C) and black (K) dedicatedly on image carriers  101  such as photoconductor drums are disposed in series. Toner images of yellow, magenta, cyan and black formed by the image forming units  100  (Y, M, C and K) respectively are temporarily (primarily) transferred to an endless intermediate transfer belt  201  which is rotating. The intermediate transfer belt  201  is disposed to be able to contact the image carriers  101  of the image forming units individually. Due to the rotation of the intermediate transfer belt  201 , the toner images are conveyed to a secondary transfer position where a recording sheet  300  runs together. Then, the toner images are (secondarily) transferred to the recording sheet  300  in a lump, and fixed thereon. The reference numerals  150  in  FIG. 13  represent primary transfer rolls for rotating the intermediate transfer belt  201  so as to press it against the image carriers  101  respectively, and transfer the images on the image carriers  101  to the intermediate transfer belt  201  respectively. The reference numerals  202 - 204  represent plural support rolls for stretching the intermediate transfer belt  201  so as to rotate and drive it. 
         [0006]    According to this image forming apparatus, a full color (multi-color) image having a configuration where the aforementioned toner images of the four colors have been combined, or a single-color image such as a monochrome image composed of toner images of one or two colors of the aforementioned four colors is formed. 
         [0007]    Of such image forming apparatuses, for example, there is an apparatus designed to operate at least a part of image carriers (for example,  101 Y,  101 M and  101 C) of the plural image carriers  101  (Y, M, C and K) so that the rotation velocities thereof are changed over to velocities different from the rotation velocities of the other image carriers (for example,  101 K) and the rotation velocity of the intermediate transfer belt  201 . Thus, it is possible to obtain an image forming operation satisfying proper image forming conditions corresponding to a difference in kind of image to be formed (for example, a difference between a color image and a single-color image), a difference in kind of recording sheet or the like. 
         [0008]    In a special example of the image forming apparatus, a color image forming mode and a monochrome image forming mode are prepared. When the monochrome image forming mode is selected, the rotation velocity of the image carrier  101 K of the image forming unit  100 K engaged in operation for forming a monochrome image is set to be equal to the rotation velocity of the intermediate transfer belt  201 . On the other hand, the rotation velocities of the image carriers  101 Y,  101 M and  101 C in the other image forming units  100 Y,  100 M and  100 C which are not engaged in the operation for forming a monochrome image are changed over to be set to be lower than the rotation velocity of the image carrier  101 K or the rotation velocity of the intermediate transfer belt  201 . A monochrome image may be formed by operating (activating) the image forming apparatus in such changed conditions. When a monochrome image is formed in such conditions, the image carriers  101 Y,  101 M and  101 C in the image forming units which are not engaged in the operation for forming the monochrome image can be prevented from being driven to rotate needlessly. Thus, the lives of the image carriers  101 Y,  101 M and  101 C can be prevented from being shortened. 
       SUMMARY 
       [0009]    According to a first aspect of the present invention, an image forming apparatus includes: plural image carriers that are disposed at intervals and in a line, and that are driven to rotate while carrying images; an intermediate transfer belt that have an endless shape, and that is disposed to be laid on plural rolls so as to be able to contact the image carriers; plural transfer rolls that rotate while being brought into contact with the image carriers through the intermediate transfer belt in a first position, respectively, so as to transfer the images on the image carriers to the intermediate transfer belt; and a moving mechanism that moves a part of the plural transfer rolls to a second position keeping apart from the plural image carriers while the part of the plural transfer rolls are kept in contact with the intermediate transfer belt. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    Exemplary embodiment of the present invention will be described in detail based on the following figures, wherein: 
           [0011]      FIG. 1  is a schematic explanatory view showing the whole of an image forming apparatus according to a first exemplary embodiment; 
           [0012]      FIG. 2  is a schematic explanatory view showing a main portion (a primary transfer portion, a moving mechanism, etc.) of the image forming apparatus in  FIG. 1 ; 
           [0013]      FIG. 3  is a block diagram showing a main portion (chiefly a control portion connected with a driving system) of a control unit; 
           [0014]      FIG. 4  is a table showing setting conditions of process speeds in each mode; 
           [0015]      FIG. 5  is a schematic explanatory view showing the moving mechanism for moving each intended primary transfer roll in a monochrome mode, and a state of movement thereof; 
           [0016]      FIG. 6A  flow chart showing main control operations in an image forming operation (printing) by the image forming apparatus in  FIG. 1 ; 
           [0017]      FIG. 7  is a main portion schematic explanatory view showing a state of movement of intended primary transfer rolls in the monochrome mode; 
           [0018]      FIG. 8  is a graph showing test results about the condition that depths of scratches appear in photoconductor drum surfaces in Example and Comparative Examples; 
           [0019]      FIG. 9  is a graph showing test results about the relationship between the number of consecutive sheets of prints and the scratch depth in the photoconductor drum surface when there occurs a failure in image; 
           [0020]      FIG. 10  is a main portion schematic explanatory view showing another example of the configuration (how to move and the state of the movement) of the moving mechanism; 
           [0021]      FIG. 11  is a main portion schematic explanatory view showing further another example of the configuration (how to move and the state of the movement) of the moving mechanism; 
           [0022]      FIG. 12  is a main portion schematic explanatory view showing a state of primary transfer rolls to be moved in the monochrome mode when the moving mechanism in  FIG. 11  is used; and 
           [0023]      FIG. 13  is a schematic view showing a main portion of a background-art color image forming apparatus using an intermediate transfer system. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]      FIGS. 1 and 2  show an image forming apparatus according to a first exemplary embodiment of the invention.  FIG. 1  shows the outline of the apparatus as a whole, and  FIG. 2  shows a main portion of the apparatus. 
         [0025]    This image forming apparatus is, for example, designed as a color printer. An image forming device  1  for forming toner images in accordance with image information and then transferring the toner images to a recording sheet is formed inside an apparatus body  2  of the image forming apparatus. The image forming device  1  is constituted by four image forming units  10 Y,  10 M,  10 C and  10 K and an intermediate transfer unit  20 . The image forming units  10 Y,  10 M,  10 C and  10 K form toner images of four colors, that is, yellow (Y), magenta (M), cyan (C) and black (K) dedicatedly. The intermediate transfer unit  20  transfers each color toner image formed by each image forming unit  10  (Y, M, C, K) to an intermediate transfer belt  21  temporarily, and conveys the toner image to a secondary transfer position where the toner image will be transferred to a recording sheet  9  such as given paper. A paper feed unit  3 , a fixing unit  4 , a system control unit  5 , etc. are also disposed inside the apparatus body  2 . The paper feed unit  3  feeds the recording sheet  9  to the secondary transfer position in the intermediate transfer unit  20  of the image forming device  1 . The fixing unit  4  fixes the toner images transferred in the secondary transfer position, onto the recording sheet  9 . The system control unit  5  controls each operation involved in the printer as a whole including these units or other units. 
         [0026]    Each image forming unit  10  (Y, M, C, K) has a cylindrical photoconductor drum  11  which is driven to rotate in the direction of the arrow A as shown in  FIG. 1  or  2 . A charging unit  12  of a charging roll system, an exposure unit  13 , a developing unit  14 , a primary transfer unit  16  of a transfer roll system, a drum cleaning unit  17 , etc. are chiefly disposed around the photoconductor drum  11 . The charging unit  12  charges the surface of the photoconductor drum  11  uniformly. The exposure unit  13  is constituted by a laser scanner or the like for irradiating the charged surface of the photoconductor drum  11  with image light (broken line) based on image information (signal) so as to form an electrostatic latent image having a potential difference and corresponding to each color component. The developing unit  14  transfers and attaches a developing agent (toner component) of one of the aforementioned colors corresponding to the electrostatic latent image so as to form a single-color toner image in the corresponding one of the aforementioned four colors. The primary transfer unit  16  transfers the formed toner image to the intermediate transfer belt  21 . The drum cleaning unit  17  cleans the surface of the photoconductor drum  11  after the primary transfer. 
         [0027]    Of these parts, a charger used as the charging unit  12  is of a contact charging system by which a charging roll applied with a charging bias is brought into contact with the photoconductor drum  11  so as to be charged. A not-shown image processing unit installed in the apparatus body  2  performs predetermined image processing (for each color component) upon image information transmitted from a not-shown externally connected device such as an image reader or a personal computer. An image signal obtained thus is supplied to the exposure unit  13 . Toner of a predetermined color is supplied to each developing unit  14  (Y, M, C, K) from a toner cartridge  19  (Y, M, C, K). A roll-system transferor in which a primary transfer roll applied with a primary transfer bias is pressed onto an intermediate transfer belt  21  from its inner circumferential surface side toward the photoconductor drum  11  so as to perform transferring is used as the primary transfer unit  16 . 
         [0028]    The intermediate transfer unit  20  rotates the intermediate transfer belt  21  in the direction of the arrow B while stretching the intermediate transfer belt  21  over plural rolls  22 - 24  so as to pass through a primary transfer position between the photoconductor drum  11  and the primary transfer unit  16  of each image forming unit  10  (Y, M, C, K). The intermediate transfer belt  21  is constituted by an endless belt made of a resin film prepared to have a predetermined volume resistivity due to a conductive agent contained therein. The roll  22  is a driving roll. The roll  23  is a tension roll for giving a constant tension to the intermediate transfer belt  21 . The roll  24  is a secondary transfer backup roll. In addition, a secondary transfer roll  26  is disposed on the intermediate transfer belt  21  supported by the secondary transfer backup roll. The reference numeral  27  in  FIG. 1  represents a belt cleaning unit for cleaning the intermediate transfer belt  21  after the secondary transfer. 
         [0029]    The paper feed unit  3  has a paper feed cassette  31 , a feeder  32 , and a sheet conveyance path  35 . The paper feed cassette  31  receives recording sheets  9 . The feeder  32  sends out the recording sheets  9  from the paper feed cassette  31  one by one. The sheet conveyance path  35  is constituted by plural-conveyance rolls  33 , guide members, etc. The conveyance rolls  33  convey the recording sheets  9  sent out from the feeder  32 . A manual paper feed tray  38  is also provided in the apparatus body  2 . The recording sheets  9  on which images will be formed can be also supplied from the manual paper feed tray  38 . 
         [0030]    The fixing unit  4  is chiefly constituted by a heating roll  41  to be driven to rotate, and a pressure roll  42 . The heating roll  41  has a heating source. The pressure roll  42  rotates while having pressure contact to the heating roll  41  with a predetermined pressure. A recording sheet  9  to be applied to fixation is introduced into a pressure contact portion between the heating roll  41  and the pressure roll  42 , and passed through the pressure contact portion. Thus, fixation is performed on the recording sheet  9 . In addition, a discharge roll  36  is provided so that, as a conveyance path of the recording sheet, the recording sheet  9  applied to fixation and passing through the fixing unit  4  is discharged by the discharge roll  36  to a discharge reception portion  37  formed in the exterior of the apparatus body  2 . 
         [0031]    The control unit  5  has a control portion connected with a driving system as shown in  FIG. 3 . This control unit  5  has a control circuit  50  constituted by a central processing unit, a memory, a storage, etc. The control circuit  50  controls operations of constituent parts of the driving system according to a control program stored in the storage. 
         [0032]    The control circuit  50  in this example controls a main motor  51  for driving the fixing unit  4 , a sheet conveyance system including the paper feed unit  3  and the sheet conveyance path, the black developing unit  14 K, etc., an intermediate-transferor driving motor  52  for driving the intermediate transfer belt  21  of the intermediate transfer unit  20 , etc., a color photoconductor driving motor  53  for driving the photoconductor drums  11 Y,  11 M and  11 C in the color image forming units  10 Y,  10 M and  10 C excluding the black image forming unit  10 K, a black photoconductor driving motor  54  for driving the photoconductor drum  11 K in the black image forming unit  11 K, a color developing unit driving motor  55  for driving the color developing units  14 Y,  14 M and  14 C, etc. individually. The control circuit  50  also controls a clutch  56  which is connected to the main motor  51  so as to change over the driving of the sheet conveyance system, and a clutch  57  which is connected to the main motor  51  so as to change over the driving of the black developing unit  14 K. Further, the control circuit  50  controls a primary transfer roll moving motor  58  for driving a primary transfer roll moving unit ( 6 ) which will be described later. That is, the operations of the aforementioned kinds of motors and clutches are controlled by the control circuit  50  so as to adjust the operation timing of each part of the printer. Thus, an image forming operation is implemented. 
         [0033]    In this printer, a mode (full color mode) to form a full color image composed of toner images of the aforementioned four colors (Y, M, C and K) and a mode (monochrome mode) to form a monochrome image composed of only a toner image of black (K) can be selected. An image forming (printing) operation can be carried out in accordance with a selected one of the modes. 
         [0034]    This mode selection is performed by inputting selection information into printer command information or image information inputted or transmitted from an external device such as a personal computer connected to the printer, or by performing a selection operation on an operation panel provided in the apparatus body  2  of the printer. This selection information of the mode is supplied to the control circuit  50  of the control unit  5  through a mode determination portion  59  in the image processing unit or the like as shown in  FIG. 3 . 
         [0035]    When the full color mode or the monochrome mode is selected, settings are done so that the intermediate transfer belt  21  and the fourth photoconductor drums  11  (Y, M, C and K) are driven at rotation velocities (process speeds) respectively in accordance with the selected mode as shown in  FIG. 4 . That is, in the full color mode, settings are done so that the intermediate transfer belt  21  and the four photoconductor drums  11  are driven to rotate at the same velocity (e.g. 104 mm/sec) as each other. In the monochrome mode, settings are done so that the intermediate transfer belt  21  and the black photoconductor drum  11 K are driven to rotate at the same high velocity (e.g. 194 mm/sec) as each other, while the color photoconductor drums  11  (Y, M and C) are driven to rotate at a low velocity (e.g. 52 mm/sec). 
         [0036]    In this printer, when the monochrome mode is selected, the rotation velocities of the intermediate transfer belt  21  and the four photoconductor drums  11  are set as described above. In addition thereto, a moving mechanism  6  is provided. The primary transfer rolls  16  (Y, M and C) opposed to the photoconductor drums  11  (Y, M and C) in the color image forming units (Y, M and C) which are not engaged in the formation of a black toner image are moved to predetermined positions by the moving mechanism  6 . 
         [0037]    As shown in  FIG. 5 , the moving mechanism  6  moves the primary transfer roll  16  (Y, M, C) from a transfer position (P 0 ) to a first position (P 1 ). In the transfer position (P 0 ), the primary transfer roll  16  (Y, M, C) is in contact with the photoconductor drum  11  (Y, M, C) through the intermediate transfer belt  21 . In the first position (P 1 ), the primary transfer roll  16  (Y, M, C) is kept away from the photoconductor drum  11  (Y, M, C) while the intermediate transfer belt  21  is kept in contact with both the primary transfer roll  16  (Y, M, C) leaving the photoconductor drum  11  (Y, M, C) and the photoconductor drum  11  (Y, M, C) and swells toward the outer circumferential surface of the belt so as to make a detour. The straight line expressed by the two-dot chain line in  FIG. 5  designates the (virtual) state of the intermediate transfer belt  21  when the primary transfer roll  16  (Y, M, C) is in the transfer position (P 0 ). This line substantially corresponds to a tangent in the transfer position (P 0 ) of the photoconductor drum  11  on the primary transfer roll  16  (Y, M, C). 
         [0038]    In this case, the moving mechanism  6  is attached so that a rotation shaft  16   a  of the primary transfer roll  16  (Y, M, C) or a support frame thereof can be slid on a guide member (guide rail or the like) for guiding the rotation shaft  16   a  or the support frame thereof from the transfer position P 0  to the first position P 1 . The rotation shaft  16   a  or the support frame thereof is used directly or through suitable conversion (including power conversion using a cam) as driving force for moving the rotating power of the primary transfer roll moving motor  58  or the linear power of a solenoid, so as to move the rotation shaft  16   a  or the support frame thereof to both the positions (P 0  and P 1 ). This example uses a configuration in which the moving mechanism  6  moves between the transfer position P 0  and the first position P 1  through two moving steps (first moving step E 1  and a second moving step E 2 ) as shown by the broken-like two-way arrow in  FIG. 5 . The first moving step E 1  corresponds to a step of moving in a process direction (direction such as a belt moving direction B). The second moving step E 2  corresponds to a step of moving in a downward direction substantially perpendicular to the direction of the first moving step. The first position P 1  can be located in a position on the opposite side to the process direction in view from the transfer position P 0 . However, in the case where the first position P 1  is located in such a position, there is a fear that the intermediate transfer belt  21  may crease when the primary transfer roll  16  moves to the opposite side to the process direction, or deflection (very small deformation) may occur in the intermediate transfer belt  21  due to creases generated unevenly in a direction crossing the process direction. It is therefore preferable that the first position P 1  is located on the same side as the process direction. 
         [0039]    Next, description will be made on a fundamental image forming (printing) operation using the printer configured thus. 
         [0040]    When the control unit  5  receives a print command inputted or transmitted from a not-shown operation panel or an external device such as a personal computer connected to the printer, the control unit  5  first determines whether the print command designates the full color mode or the monochrome mode as shown in  FIG. 6  (Steps S 10  and S 11 ). Here, first, description will be made on the assumption that the received print command designates the full color mode. 
         [0041]    In this case, it is concluded in Step S 11  that the monochrome mode is not selected (Step S 11 ). After that, the printer is operated with the intermediate transfer belt  21  and the four photoconductor drums  11  (Y, M, C and K) being driven to rotate in the velocity conditions ( FIG. 4 ) in the full color mode (Step S 20 ). In this example, all the intermediate transfer belt  21  and the four photoconductor drums  11  (Y, M, C and K) are driven to rotate at a rotation velocity of “104 mm/sec”. 
         [0042]    In the full color mode, toner images of the aforementioned four colors are first formed respectively in the image forming units  10  (Y, M, C and K) of the image forming device  1 , and then transferred to the intermediate transfer unit  20 . 
         [0043]    That is, in each image forming unit  10 , the photoconductor drum  11  staring rotating is charged to a predetermined potential by the charging unit  12 . The charged photoconductor drum  11  is scanned and exposed to light in accordance with an image signal by the exposure unit  13 . Thus, an electrostatic latent image corresponding to a corresponding color component is formed. After that, the electrostatic latent image is developed with corresponding color toner in the developing unit  14 . Thus, toner images of the respective colors (Y, M, C and K) are formed. The toner images formed in the image forming units  10  respectively are electrostatically transferred to the intermediate transfer belt  21  in the intermediate transfer unit  20  of the image forming device  1  in the primary transfer positions where the photoconductor drums  11  are opposed to the primary transfer units  16 . Thus, the toner images are transferred sequentially so as to be superimposed on one another. After that, the intermediate transfer belt  21  is conveyed to the secondary transfer position where the intermediate transfer belt  21  is opposed to the secondary transfer roll  26 . The primary transfer is performed by a transfer electric field formed between the intermediate transfer belt  21  and the photoconductor drum  11  by transfer bias having polarity opposite to the toner charge polarity and applied to the primary transfer roll of the primary transfer unit  16 . 
         [0044]    On the other hand, recording sheets  9  required by the aforementioned print command are sent out one by one from the paper feed cassette  31 , where the paper feed unit  3  is received, by the feeder  32 . Each sent-out recording sheet  9  is conveyed in the sheet conveyance path  35  by the plural conveyance rolls  33  and so on. The recording sheet  9  is then sent and supplied into the secondary transfer position at predetermined timing by a registration roll  34 . In the secondary transfer position, the toner images transferred onto the intermediate transfer belt  21  are electrostatically transferred onto the supplied recording sheet  9  in a lump. The secondary transfer is, for example, performed in a transfer electric field formed between the recording sheet  9  and the secondary transfer roll  26  through the intermediate transfer belt  21  by transfer bias having the same polarity as the toner charge polarity and applied to the secondary transfer backup roll  24 . 
         [0045]    Successively the recording sheet  9  having the toner images transferred thereto is conveyed toward the fixing unit  4  and introduced into a pressure contact portion between the heating roll  41  and the pressure roll  42 . Due to heat and pressure applied to the recording sheet  9  passing through the pressure contact portion, the toner images are fixed onto the recording sheet  9 . After the completion of the fixation, the recording sheet  9  is discharged to the discharge reception portion  37  by the discharge roll  36 . By the process described above, a full color image composed of toner images of the aforementioned four colors is formed on one recording sheet  9 . 
         [0046]    Printing in the full color mode as described above is proceeded with till all the print commands (jobs: number of prints) are terminated (Step S 21 ). When the printing is proceeded, it is confirmed whether the mode is changed or not (Step S 21 ). In this event, when the mode is not changed, the next print is executed in the full color mode. When the mode is changed, the routine of processing moves to the step of determining the mode (Step S 10 ). 
         [0047]    On the other hand, when it is concluded in Step S 11  that the print command designates the monochrome mode, the moving mechanism  6  is operated to move the primary transfer rolls  16  (Y, M and C) of the color image forming units  10  (Y, M and C) excluding the black image forming unit  10 K from their transfer positions P 0  to their first positions P 1  respectively (Step S 12 ). As a result, the primary transfer rolls  16  (Y, M and C) are moved to the first positions P 1  with respect to the photoconductor drums  11  (Y, M and C) respectively as shown in  FIG. 7  ( FIG. 5 ). The two-dot chain line in  FIG. 7  ( FIG. 5 ) designates the state of the intermediate transfer belt  21  when the primary transfer rolls  16  (Y, M and C) are located in the transfer positions P 0 . 
         [0048]    This movement of the primary transfer rolls  16  (Y, M and C) is followed by driving the intermediate transfer belt  21  and the four photoconductor drums  11  (Y, M, C and K) so as to rotate the intermediate-transfer belt  21  and the four-photoconductor drums  11  (Y, M, C and K) in the velocity conditions ( FIG. 4 ) in the monochrome mode and thereby operate the printer (Step S 13 ). In this example, as described previously, the intermediate transfer belt  21  and the black photoconductor drum  11 K are driven to rotate at the same high rotation velocity of “194 mm/sec”. On the other hand, the color photoconductor drums  11  (Y, M and C) are driven to rotate at the low rotation velocity of “52 mm/sec”. 
         [0049]    In the monochrome mode, first, a black toner image is formed by the black image forming unit  10 K of the image forming device  1 , and then transferred to the intermediate transfer unit  20 . That is, a black toner image is formed on the photoconductor drum  11 K of the black image forming unit  10 K as described previously. The black toner image is primarily transferred onto the intermediate transfer belt  21  by the primary transfer roll  16 K. 
         [0050]    In this event, in the color image forming units  10  (Y, M and C), the operation of forming toner images of their corresponding colors (Y, M and C) is not performed, but their corresponding photoconductor drums  11  (Y, M and C) are driven to rotate with a difference in velocity (at a low velocity) with respect to the intermediate transfer belt  21 . This is intended to prevent the lowering of the lives of the photoconductor drums  11  (Y, M and C). 
         [0051]    In the color image forming units  10  (Y, M and C) at this time, their primary transfer rolls  16  (Y, M and C) move to their first positions P 1  respectively ( FIGS. 5 and 7 ) so as to leave the photoconductor drums  11  (Y, M and C). In addition, the intermediate transfer belt  21  is kept in contact with both the primary transfer rolls  16  (Y, M and C) leaving the photoconductor drums  11  (Y, M and C) and the photoconductor drums  11  (Y, M and C) and swelling toward the outer circumferential surface side of the belt so as to be bent. 
         [0052]    Thus, the intermediate transfer belt  21  is prevented from moving and running vibrating. As a result, the portion of the intermediate transfer belt  21  where there are particulates or the like adhering thereto can be prevented from vibrating and contacting the photoconductor drums  11  (Y, M and C) with high pressure. In addition, due to the movement of the transfer rolls  16  (Y, M and C), the tension of the intermediate transfer belt  21  increases in accordance with the swelling detour to the outer circumferential surface side of the belt, while the intermediate transfer belt  21  contacts each photoconductor drum  11  (Y, M, C) opposed to its corresponding transfer roll as if it were wrapped around the surface of the photoconductor drum  11  (Y, M, C) (the wrapped belt portion is illustrated by the region designated by the reference sign W in  FIG. 5 ). Thus, the intermediate transfer belt  21  is more surely prevented from moving and running vibrating. As a result, even if a portion to which comparatively hard particulates or the like adhere is present in the outer circumferential surface of the intermediate transfer belt  21 , the portion can be prevented from contacting the photoconductor drum  11  (Y, M, C) with pressure increased by the vibrating running. Thus, the drum surface is more hardly scratched. 
         [0053]    The black toner image primarily transferred to the intermediate transfer belt  21  is secondarily transferred to a required recording sheet  9  in the secondary transfer position in the same manner as in printing in the full color mode. The black toner image carried on the recording sheet  9  is introduced into the fixing unit  4  as it is, so as to be fixed onto the recording sheet  9 . Finally, the recording sheet  9  after the fixation is discharged to the discharge reception portion  37 . As a result of the process described above, a monochrome image composed of the black toner image is formed on one recording sheet  9 . 
         [0054]    Printing in the monochrome mode as described above is proceeded with till all the print commands (jobs: number of prints) are terminated (Step S 14 ). When the printing is proceeded, it is confirmed whether the mode is changed or not (Step S 15 ). In this event, when the mode is not changed, the next print is executed in the monochrome mode. When the mode is changed, the routine of processing moves to the step of determining the mode (Step S 10 ). 
       (Evaluation Test) 
       [0055]    Description will be made below on an evaluation test using the printer according to the first exemplary embodiment. 
         [0056]    Particulates (seven kinds of loamy layers of the Kanto Plain: JIS Z8901) are made to adhere to a rotary brush roll. The rotary brush roll is kept to be driven with the rotation of the intermediate transfer belt  21  of the printer, and bring into contact with the outer circumferential surface of the belt  21 . A test monochrome image (half tone image) is printed on 400 sheets continuously in the monochrome mode. 
         [0057]    In the monochrome mode in this test, the intermediate transfer belt  21  and the black photoconductor drum  11 K are driven to rotate at the same high rotation velocity of “208 mm/sec”. The color photoconductor drums  11  (Y, M and C) are driven to rotate at the low rotation velocity of “52 mm/sec”. Each color primary transfer roll  16  (Y, M, C) is moved to its first position P 1  where the length (wrap length W: see  FIG. 5 ) of the portion where the intermediate transfer belt  21  is wrapped around the outer circumferential surface of the photoconductor drum  11  (Example). In this event, a roll whose outer diameter is 30 mm is used as each photoconductor drum  11 . For comparison, Comparative Example 1 and Comparative Example 2 were prepared. In Comparative Example 1, each color primary transfer roll  16  (Y, M, C) is placed in its transfer position P 0  (in a contact state) where the color primary transfer roll  16  (Y, M, C) is pressed against the corresponding photoconductor drum  11  through the intermediate transfer belt  21 . In Comparative Example 2, each color primary transfer roll  16  (Y, M, C) is placed in a position (in a retract state) where the color primary transfer roll  16  (Y, M, C) is at a distance (about 5 mm) from the intermediate transfer belt  21 . Printing is performed in Comparative Examples 1 and 2 in the same manner as in Example. 
         [0058]    After monochrome printing is performed upon consecutive 400 sheets, depths of scratches present in the surfaces of the color photoconductor drums  11  (Y, M and C) in Example and Comparative Examples are measured and analyzed using a laser microscope individually. The results (average values of maximum depths of scratches present in the three photoconductor drums) are shown in  FIG. 8 . 
         [0059]    As is apparent from the results of  FIG. 8 , when the color primary transfer rolls  16  (Y, M, C) are moved to their first positions P 1  as in this Example, the depths of scratches formed in the surfaces of the color photoconductor drums  11  (Y, M and C) are extremely shallow not only in comparison with those in Comparative Example 1 where the primary transfer rolls  16  (Y, M and C) are brought into a contact state, but also in comparison with those in Comparative Example 2 where the primary transfer rolls  16  (Y, M and C) are brought into a retract state. Assume that the primary transfer rolls  16  (Y, M and C) are brought into a contact state as in Comparative Example 1. In this case, when particulates adhering to the intermediate transfer belt  21  pass through the first transfer portions for color, the surfaces of the color photoconductor drums  11  (Y, M and C) suffer high pressure due to the primary transfer rolls  16  (Y, M and C) pressed thereon respectively. Thus, it is guessed that the surfaces are apt to be scratched, and the depths of the scratches increase. Now assume that the primary transfer rolls  16  (Y, M and C) are brought into a retract state as in Comparative Example 2. In this case, when particulates adhering to the intermediate transfer belt  21  pass through the first transfer portions for color, the surfaces of the color photoconductor drums  11  (Y, M and C) are prevented from suffering high pressure due to the primary transfer rolls  16  (Y, M and C) pressed thereon respectively. However, the surfaces of the color photoconductor drums  11  (Y, M and C) suffer pressure due to contact with the intermediate transfer belt  21  running vibrating. Thus, it is guessed that there occur scratches grown with depth corresponding to the suffered pressure. 
         [0060]    Next, single-color halftone images of the colors (Y, M and C) were printed one by one whenever a monochrome halftone image had been printed on consecutive 1,000 sheets in the aforementioned conditions (Example and Comparative Examples 1 and 2). Each print was examined as to whether a failure in image (streaky image formed in the rotation-direction A of each photoconductor drum  11  which was also a process direction) on a level high enough to cause a practical use problem occurred in an image of each color. When it was confirmed that the failure in image occurred, the cumulated number of prints obtained till then was counted, and depths of scratches caused by the aforementioned particulates present in each color photoconductor drum  11  where the failure in image occurred were measured. The results are shown in  FIG. 9 . 
         [0061]    From the results of  FIG. 9 , it is understood that when the primary transfer rolls are set in a contact state as in Comparative Example 1, a failure in image occurs in a stage between the time to start printing and the time to reach about 5,000 sheets of prints, and the depths of scratches present in the photoconductor drums  11  at that time reach a relatively deep level. It was also confirmed that when the primary transfer rolls are set in a retract state as in Comparative Example 2, a failure in image can be prevented from occurring till printing on about consecutive 15,000 sheets, and the depths of scratches present in the photoconductor drums  11  at that time become shallower than those in Comparative Example 1. It is understood that when the primary transfer rolls have been moved as in Example of the invention, the state where a failure in image hardly occurs can be kept from the time to start printing to the time to reach 46,000 sheets. It could be also confirmed that the depths of scratches present in the photoconductor drums  11  at that time are extremely shallow if any. In this test, it was confirmed that when there is a deeper scratch in a photoconductor drum, damage caused by toner or the like finally reaches the charging roll surface of the charging unit  12 , with the result that a failure in image-is-apt to occur. 
       Other Embodiments 
       [0062]    The first exemplary embodiment shows by way of example the moving mechanism  6  has a configuration where an intended primary transfer roll  16  (Y, M, C) is moved from the transfer position P 0  to the first position P 1  through two moving steps (a first moving step E 1  and a second moving step E 2 : see  FIG. 5 ). As shown by the broken-like two-way arrow M in  FIG. 10 , the primary transfer roll  16  (Y, M, C) may be moved to the first position P 1  by one moving step. In this case, for example, with reference to a concentric circle whose diameter is larger than the diameter of each photoconductor drum  11 , the trajectory (M) of the moving step can be set as a trajectory composed of a curve which swells gradually to the outside of the concentric circle and out of the concentric circle as it is closer to the first position P 1 . 
         [0063]    In the first exemplary embodiment, as shown in  FIG. 11 , the moving mechanism  6  may move each primary transfer roll  16  (Y, M, C) to a second position (P 2 ) where the primary transfer roll  16  (Y, M, C) is kept away from the photoconductor drum  11  (Y, M, C) in the monochrome mode, while the intermediate transfer belt  21  is kept in contact with both the primary transfer roll  16  (Y, M, C) kept away from the photoconductor drum  11  (Y, M, C) and the photoconductor drum  11  (Y, M, C). In this illustrated example, as shown by the broken-like two-way arrow, the moving mechanism  6  is designed to move an intended primary transfer roll  16  in the process direction (moving direction B of the belt) by a predetermined distance. The straight line designated by the two-dot chain line in  FIG. 11  designates the (virtual) state of the intermediate transfer belt  21  when the primary transfer roll  16  (Y, M, C) is located in the transfer position P 0 . 
         [0064]    In this case, in the monochrome mode, the moving mechanism  6  is operated to move each color primary transfer roll  16  (Y, M, C) excluding the black primary transfer roll  16  from the transfer position P 0  to the second position P 2 . As a result, as shown in  FIG. 11  or  12 , the primary transfer roll  16  (Y, M, C) leaves the photoconductor drum  11  (Y, M, C), and the intermediate transfer belt  21  is kept in contact with both each primary transfer roll  16  (Y, M, C) leaving the photoconductor drum  11  (Y, M, C) and the photoconductor drum  11  (Y, M, C). 
         [0065]    When each color primary transfer roll  16  (Y, M, C) is moved to the second position P 2  in this manner, particularly the intermediate transfer belt  21  is kept in contact with both the moved primary transfer roll  16  (Y, M, C) and the photoconductor drum  11  (Y, M, C). Thus, the intermediate transfer belt  21  is prevented from moving and running vibrating. Accordingly, in this case, even if a portion to which comparatively hard particulates or the like adhere is present in the outer circumferential surface of the intermediate transfer belt  21 , the portion can be prevented from contacting the photoconductor drum  11  (Y, M, C) with pressure increased by the vibrating running. Thus, the drum surface is more hardly scratched. 
         [0066]    In this case, the intermediate transfer belt  21  is not retained in a detour swelling on the outer circumferential surface side of the belt by the moved primary transfer rolls  16  (Y, M and C) as in the first exemplary embodiment. Accordingly, the intermediate transfer belt  21  does not contact the photoconductor drums  11  (Y, M and C) opposed to the transfer rolls due to increased tension and in a state (wrapped state) where the intermediate transfer belt  21  is wrapped around the surfaces of the photoconductor drums  11  (there is no belt portion or no wrapped portion as designated by the reference sign W in  FIG. 5 ). Correspondingly the intermediate transfer belt  21  is prevented from strongly contacting the surfaces of the photoconductor drums  11  (Y, M and C). 
         [0067]    The first exemplary embodiment shows the configuration in which when a black toner image is formed (in the monochrome mode), the color primary transfer rolls  16  (Y, M and C) are moved from their transfer positions P 0  to their first positions P 1  (or second positions P 2 ). In a single-color print mode where a single-color toner image composed of another color (Y, M, C) is formed or in a single-color print mode where a single-color toner image composed of two other colors (two of Y, M and C) is formed, the primary transfer rolls other than the primary transfer rolls  16  to be used for the single-color mode may be moved to their first positions P 1  (or second positions P 2 ) by the moving mechanism  6 . Not to say, in this case, the photoconductor drums  11  in the image forming units  10  other than the image forming units  11  to be used for the single-color mode are designed so that their rotation velocities can be changed into velocities different from that of the intermediate transfer belt  21 . 
         [0068]    The first exemplary embodiment shows by way of example an image forming apparatus in which the image forming units  10  are disposed in a lower-side running portion of the intermediate transfer belt  21 . However, the present invention may be applied to an image forming apparatus in which the image forming units  10  are disposed in an upper-side running portion of the intermediate transfer belt  21 . The first exemplary embodiment shows an example of a configuration in which the intermediate transfer belt  21  is disposed to be stretched horizontally as the intermediate transfer unit  20 . However, the present invention can be applied to a configuration where the intermediate transfer belt  21  is disposed to be stretched vertically or in an inclined direction. Further, the number of image forming units  10  is not limited to four. The present invention may be applied to a configuration where two or three image forming units  10  are disposed, or five or more image forming units  10  are disposed by increasing the number of image forming units  10  that hold different chromatic colors other than black color.