Patent Publication Number: US-6337967-B1

Title: Image forming apparatus for superposing monochrome images

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-270895, filed Sep. 24, 1999, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     This invention relates to a color image forming apparatus for superposing monochrome images on each other to form a color image. 
     A color image forming apparatus using an electronic photography system is designed to execute color separation of an original document image into three primary colors, thereby forming four monochrome images (i.e. images of four colors in total) using toners of C (cyan), M (magenta) and Y (yellow), as complementary colors, and K (black) for emphasizing the black, superposing the monochrome color images and transferring them onto an output material (i.e. a sheet of paper). The process of superposing monochrome images can use a method for superposing toner images on an intermediate transfer member and then simultaneously transferring them onto a paper sheet, or a method for directly superposing toner images on a paper sheet. 
     Japanese Patent Application KOKAI Publication No. 7-281577, for example, discloses the use of such an intermediate transfer member. Specifically, it discloses an example where an intermediate transfer drum  2  is provided below a photosensitive belt  1 . 
     The method using an intermediate transfer member is employed in many color image forming apparatuses. However, the speed of forming a color image is ¼ the speed of forming a black-and-white image. Further, in color image forming apparatuses, the process of temporarily transferring a toner image onto an intermediate transfer member is included even when forming a black-and-white image. Accordingly, the black-and-white image forming speed (copy speed) of color image forming apparatuses is lower than that of black-and-white image forming apparatuses. The problem in copying speed is conspicuous, particularly in the case of the first copying period that is required for copying only one page. 
     BRIEF SUMMARY OF THE INVENTION 
     It is the object of the invention to provide a color image forming apparatus capable of forming a black-and-white image at high speed. 
     According to a first aspect of the invention, there is provided a color image forming apparatus comprising: 
     a charger unit for charging a photosensitive member; 
     an exposing unit for forming an electrostatic latent image on the charged photosensitive member; 
     a developing unit for visualizing the electrostatic latent image formed on the photosensitive member to form a toner image; 
     a first transfer unit for directly transferring the toner image on the photosensitive member onto an image receiving material; 
     a second transfer unit including an intermediate transfer member for transferring the toner image, formed on the photosensitive member, onto the intermediate transfer member, and for transferring the toner image, transferred to the intermediate transfer member, onto the image receiving material; and 
     a fixing unit for fixing the toner image on the image receiving material transferred to the image receiving material by the first and second transfer units and held thereon, thereby. 
     According to a second aspect of the invention, there is provided a color image forming method in a color image forming apparatus includes a charger unit for charging a photosensitive member; an exposing unit for an electrostatic latent image on the charged photosensitive member charged by the charger unit; a developing unit for visualizing the electrostatic latent image formed on the photosensitive member to formed toner image; an intermediate transfer member for holding the toner image formed on the photosensitive member; a first transfer unit for transferring the toner image on the photosensitive member; a second transfer unit for transferring the toner image on the intermediate transfer member; and a fixing unit for fixing the toner image on an image receiving material; comprising steps of: 
     forming a first image on the photosensitive member; 
     forming a second image on the photosensitive member; 
     directly transferring the first image on the image receiving material by using the first transfer unit; 
     transferring the second image on the intermediate transfer member; and 
     transferring the transferred second image on an image receiving material by using the second transfer unit. 
     According to a third aspect of the invention, there is provided a color image forming method in a color image forming apparatus includes a charger unit for charging a photosensitive member; an exposing unit for an electrostatic latent image on the charged photosensitive member charged by the charger unit; a developing unit for visualizing the electrostatic latent image formed on the photosensitive member to formed toner image; an intermediate transfer member for holding the toner image formed on the photosensitive member; a first transfer unit for transferring the toner image on the photosensitive member; a second transfer unit for transferring the toner image on the intermediate transfer member; a first image receiving material supply mechanism for supplying an image receiving material to the first transfer unit along first route; a second image receiving material supply mechanism for supplying an image receiving material to the second transfer unit along second route different from the first route; and a fixing unit for fixing the toner image on an image receiving material; comprising steps of: 
     forming a first image on the photosensitive member; 
     forming a second image on the photosensitive member; 
     directly transferring the first image on the image receiving material supplied from the first image receiving material supply mechanism by using the first transfer unit; 
     transferring the second image on the intermediate transfer member by the voltage between the photosensitive member and the intermediate transfer member; and 
     transferring the transferred second image on the image receiving material supplied from the second image receiving material supply mechanism by using the second transfer unit. 
     According to a fourth aspect of the invention, there is provided a color image forming method comprising steps of: 
     forming a first image on a photosensitive member; 
     forming a second image on the photosensitive member; 
     directly transferring the first image formed on the photosensitive member on to a first image receiving material; 
     transferring the second image formed on the photosensitive member on to an intermediate transfer member; and 
     transferring the second image transferred on the intermediate transfer member on to a second image receiving material. 
     Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention. 
     FIG. 1 is a schematic view illustrating a color image forming apparatus according to an embodiment of the invention; 
     FIG. 2 is a schematic view illustrating a color image forming apparatus according to another embodiment of the invention; 
     FIG. 3 is a schematic view illustrating a color image forming apparatus according to yet another embodiment of the invention; 
     FIG. 4 is a schematic view illustrating a color image forming apparatus according to a further embodiment of the invention; 
     FIG. 5 is a schematic view illustrating a modification of the color image forming apparatus shown in FIG. 2; 
     FIG. 6 is a schematic view illustrating a modification of the color image forming apparatus shown in FIG. 4; 
     FIGS. 7A to  7 J are views useful in explaining an example of a special operation executed by the image forming apparatus of each of FIGS. 1 to  6 ; and 
     FIGS. 8A to  8 M are views useful in explaining another special operation executed by the image forming apparatus of each of FIGS. 1 to  6 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Image forming apparatuses according to the embodiments of the invention will be described with reference to the accompanying drawings. 
     FIG. 1 is a schematic view illustrating a color image forming apparatus according to the invention, which uses an electronic photography process. 
     As is shown in FIG. 1, a color image forming apparatus  1  includes a photosensitive member  2  in the form of an endless belt, a charger  3  for charging the photosensitive member  2  with a predetermined potential,  4 ; an exposing unit  4  for forming an electrostatic latent image on the charged photosensitive member  2 , first to fourth developing units  5 ,  6 ,  7  and  8  for supplying toner to the electrostatic latent image on the photosensitive member  2  to visualize it, an intermediate transfer member  9  for temporarily carrying a toner image formed on the photosensitive member  2  by the developing units  5  to  8 , and an image forming section  12  that includes an intermediate transfer member cleaner  10  for cleaning the intermediate transfer member  9 , and a cleaning unit  11  for removing toner remaining on the photosensitive member  2 . 
     The photosensitive member  2  is supported, with a predetermined tension, by first and second rollers  2   a  and  2   b  that keep the photosensitive member  2  in tight contact with the outer peripheral surface of the intermediate transfer member  9 , and also by third and fourth rollers  2   c  and  2   d  that keep a predetermined space between the photosensitive member  2  and each developing unit. The photosensitive member  2  is rotated at a predetermined speed in a direction, indicated by the arrow in FIG. 1, in accordance with the rotation of a motor (not shown) connected to one of the rollers. The entire length of the photosensitive member  2  is equal to the circumferential length of the intermediate transfer member  9 , and is longer than twice the width of a paper sheet of A4 size (i.e. than the length of a paper sheet of A3 size). It will be possible the entire length of the photosensitive member  2  is longer than the circumferential length of intermediate transfer member  9 , and possible the entire length of the photosensitive member  2  is shorter than the circumferential length of intermediate transfer member  9 . Voltage is supplied to the circumferential portion of the intermediate transfer member  9  by voltage supply  9   a . The intermediate transfer member  9  is made from hollow aluminium tube covered with rubber member having semi-conductivity and coated with the Teflon (tread name) of the outer surface of the rubber member. The first transfer roller  21  is made from rubber sponge having conductivity. 
     A paper sheet cassette  13  that contains paper sheets (output material sheets) P having a predetermined size is provided below the image forming section  12 . 
     In the vicinity of the paper sheet cassette  13 , there are provided a feed roller  14  for picking up paper sheets in the paper sheet cassette one by one, and a switching mechanism  19  for guiding each picked-up paper sheet toward the photosensitive member  2  or the intermediate transfer member  9 . 
     A first conveyance system  20  for conveying paper sheets P to the intermediate transfer member  9  is provided between the paper sheet cassette  13  and the intermediate transfer member  9 . Similarly, a second conveyance system  30  for conveying paper sheets P to the photosensitive member  2  (the fourth roller  2   d ) is provided between the paper sheet cassette  13  and the fourth roller  2   d  that supports the photosensitive member  2 . 
     A first transfer roller  21  for transferring, onto each conveyed paper sheet P, a color toner image formed on the intermediate transfer member  9  is provided at a location in the first conveyance system  20 , at which each conveyed paper sheet P contacts the intermediate transfer member  9 . A first aligning roller  22  is provided upstream of the first transfer roller  21  (at the paper sheet cassette  13  side) for temporarily stopping each paper sheet P conveyed in the conveyance system  20  to adjust the inclination of each sheet in the transfer direction, and to align the front end of each paper sheet P with the front end of the toner image on the intermediate transfer member  9 . Further, downstream of the first transfer roller  21 , there are provided a separating unit  23  for applying an AC charge to the intermediate transfer member  9  to separate therefrom each paper sheet P with a color toner image formed thereon, and a first fixing unit  24  for fixing on each paper sheet P the color toner image transferred thereon. 
     A second transfer roller  31  for transferring, onto each conveyed paper sheet P, a black toner image formed on the photosensitive member  2  is provided at a location in the second conveyance system  30 , at which each conveyed paper sheet P contacts the photosensitive member  2 . A second aligning roller  32  is provided upstream of the second transfer roller  31  (at the paper sheet cassette  13  side) for temporarily stopping each paper sheet P conveyed in the conveyance system  30  to adjust the inclination of each sheet in the transfer direction, and to align the front end of each paper sheet P with the front end of the toner image on the photosensitive member  2 . Further, downstream of the second transfer roller  31 , there is provided a second fixing unit  33  for fixing on each paper sheet P the black toner image transferred thereon. 
     The operation of the color image forming apparatus shown in FIG. 1 will now be described. 
     When forming a color image, at first, the charger  3  uniformly charges the photosensitive member  2 . Subsequently, the exposing unit  4  exposes the photosensitive member  2  to form thereon an electrostatic latent image corresponding to a BK (black) image as a first color image component. 
     The electrostatic latent image corresponding to the BK image is developed with black toner by the black developing unit (first developing unit)  5 , and transferred as a BK image onto the intermediate transfer member  9  by voltage between the photosensitive member  2  and the intermediate transfer member  9 . Since, each of the photosensitive member  2  and intermediate transfer member  9  are charged, for example,—(minus) 450 volt and 500 volt. 
     After transferring the BK image to the intermediate transfer member  9 , the photosensitive member  2  is deelectrified by a deelectrifying unit (not shown), whereby toner remaining thereon without being transferred onto the intermediate transfer member  9  is removed by the cleaning unit  11 . The removed toner is collected into a used-toner box (not shown). 
     The charger  3  again charges the photosensitive member  2 , and the exposing unit  4  exposes the photosensitive member  2  to thereby form an electrostatic latent image corresponding to a Y (yellow) image as a second color image component. 
     The electrostatic latent image corresponding to the Y image is developed with yellow toner by the yellow developing unit (second developing unit)  6 , and transferred as a Y image onto the intermediate transfer member  9  such that it is superposed upon the previously transferred BK image by voltage between the photosensitive member  2  and the intermediate transfer member  9 . 
     After the photosensitive member  2  is cleaned, it is exposed to form thereon an electrostatic latent image corresponding to an M (magenta) image as a third color image component. 
     The electrostatic latent image corresponding to the M image is developed with magenta toner by the magenta developing unit (third developing unit)  7 , and transferred as an M image onto the intermediate transfer member  9  such that it is superposed upon the previously transferred BK image and Y image by voltage between the photosensitive member  2  and the intermediate transfer member  9 . 
     After the photosensitive member  2  is cleaned, it is exposed to form thereon an electrostatic latent image corresponding to a C (magenta) image as a fourth color image component. 
     The electrostatic latent image corresponding to the C image is developed with cyan toner by the cyan developing unit (fourth developing unit)  8 , and transferred as a C image onto the intermediate transfer member  9  such that it is superposed upon the previously transferred BK image, Y image and M image by voltage between the photosensitive member  2  and the intermediate transfer member  9 . 
     Thus, a color toner image of four colors, including of images sequentially transferred onto the intermediate transfer member  9  and superposed upon each other, is transferred (secondary transfer) by the first transfer roller  21 , charged about 1.2 kV, onto each paper sheet P fed from the paper sheet cassette  13  at a predetermined time point and conveyed in the first conveyance system  20  to the first aligning roller  22  via the switching mechanism  14 . 
     Each paper sheet P with the four-color toner image transferred thereon is separated from the intermediate transfer member  9  by the separating unit  23 , and conveyed to the first fixing unit  24 , where the four-color toner image is fixed thereon. Each paper sheet (output material sheet) P with the color image fixed thereon is discharged to a tray  25 . 
     On the other hand, toner, which could not be transferred to each sheet P and remains on the intermediate transfer member  9 , is removed, after the completion of the secondary transfer, by the cleaner  10  to be brought into contact with the intermediate transfer member  9  by a driving mechanism (not shown). It is a matter of course that the intermediate transfer member cleaner  10  is separated from the intermediate transfer member  9  while the four-color toner image is transferred onto the intermediate transfer member  9 . 
     When forming a black-and-white image, the charger  3  uniformly charges the photosensitive member  2 , and then the exposing unit  4  exposes the photosensitive member  2  to form thereon an electrostatic latent image corresponding to the black-and-white image. 
     The latent image corresponding to the black-and-white image and formed on the photosensitive member  2  is developed by the black developing unit (first developing unit)  5 , thereby forming a black toner image. 
     The black toner image on the photosensitive member  2  is transferred by the second transfer roller  31  onto each paper sheet P conveyed in the second conveyance system  30 , as a result of switching by the switching mechanism  19 , from the paper sheet cassette  13  to the second aligning roller  32 . The second transfer roller  31  is usually separated from the photosensitive member  2 . Only when forming a black-and-white image, the roller  31  is urged against the fourth roller  2   d  and hence brought into contact with the photosensitive member  2  by a driving mechanism (not shown). 
     Each paper sheet P with the black-and-white image transferred thereon is conveyed to the second fixing unit  33 , where the black-and-white image is fixed thereon. Each sheet (output material sheet) P with the black-and-white image fixed thereon is discharged to a tray  34  via a surface reversing mechanism, which will not be described later in detail. 
     On the other hand, toner, which could not be transferred to each sheet P and remains on the intermediate transfer member  9 , is removed by the cleaning unit  11 . 
     Thus, in the color image forming apparatus  1  having the intermediate transfer member  9 , the second conveyance system  30  and the second transfer roller  31 , which do not require the secondary transfer using the intermediate transfer member  9 , are provided for forming a black (BK) image. Accordingly, a black (BK) image can be output in a short time since a corresponding toner image formed on the photosensitive member  2  is directly transferred onto a paper sheet P. 
     FIG. 2 is a schematic view showing a color image forming apparatus according to another embodiment of the invention. In the color image forming apparatus of FIG. 2, similar elements to those in FIG. 1 are denoted by corresponding reference numerals, and no detailed description will be given thereof. 
     As shown in FIG. 2, a color image forming apparatus  101  has a photosensitive member  2 , a charger  3 , an exposing unit  4 , first to fourth developing units  5 ,  6 ,  7  and  8 , an intermediate transfer member  9 , an image forming section  12  that includes an intermediate transfer member cleaner  10  and a cleaning unit  11 , a first paper sheet cassette  13 , a first conveyance system  120 , a second conveyance system  130  and a second paper sheet cassette  150 . 
     The first conveyance system  120  includes a first feed roller  121  provided at the intermediate transfer member  9  side of the paper sheet cassette  13  for feeding each paper sheet P toward the intermediate transfer member  9 , and can supply the intermediate transfer member  9  with each paper sheet P through a short conveyance path. 
     The second conveyance system  130  includes a second transfer roller  31  to be opposed to the fourth roller  2   d  of the photosensitive member  2  by a driving mechanism (not shown) only when forming a black-and-white image, an aligning roller  32 , a second fixing unit  33  and a second feed roller  131 . The system  130  supplies each paper sheet P from the second paper sheet cassette  150  to the transfer roller  31  when forming a black-and-white image. 
     In the color image forming apparatus  101  shown in FIG. 2, a color image is formed in the same process as in the process shown in FIG.  1 . As aforementioned, the second feed roller  131  provided at the paper sheet cassette  13  shortens the time period required for the supply of each paper sheet P to the intermediate transfer member  9 , as compared with the structure of FIG.  1 . 
     On the other hand, a black-and-white image formed by developing a black (BK) latent image by the black developing unit  5  is transferred at the transfer roller  31  onto each paper sheet P fed from the second paper sheet cassette  150 . 
     As is evident from FIG. 2, the distance between the second paper sheet cassette  150  and the aligning roller  32  is very short. In the case of the color image forming apparatus of FIG. 1, to form a black-and-white image, it is necessary to feed a paper sheet from the paper sheet cassette  13  to the aligning roller  32  before exposure and development processes. On the other hand, in the structure of FIG. 2, it is not necessary to feed a paper sheet P in advance. Accordingly, this structure is free from a delay in starting exposure due to the time period required for the conveyance of the sheet P to the aligning roller  32 . 
     Thus, the color image forming apparatus of FIG. 2 requires a shorter first copy period when forming a black-and-white image. 
     FIG. 3 is a schematic view illustrating a color image forming apparatus according to yet another embodiment. In the color image forming apparatus of FIG. 3, similar elements to those in FIG. 1 are denoted by corresponding reference numerals, and no detailed description will be given thereof. 
     As shown in FIG. 3, a color image forming apparatus  201  includes a photosensitive member  2 , a charger  3 , an exposing unit  4 , first to fourth developing units  5 ,  6 ,  7  and  8 , an intermediate transfer member  9 , an image forming section  12  that includes an intermediate transfer member cleaner  10  and a cleaning unit  11 , a first conveyance system  220 , a second conveyance system  230  and a paper sheet cassette  250 . 
     The first conveyance system  220  is designed to feed each paper sheet P, contained in the paper sheet cassette  250 , to the intermediate transfer member  9 . The system  220  can feed each sheet P, using a first feed roller  221 , to the intermediate transfer member  9  and the first transfer roller  21  through a short conveyance path. 
     On the other hand, the second conveyance system  230  is designed to feed each paper sheet P, contained in the paper sheet cassette  250 , to the photosensitive member  2 , when forming a black-and-white image. The system  230  can feed each sheet P to the second transfer roller  31  (to the aligning roller  32 ), using a second feed roller  231 . 
     The paper sheet cassette  250  includes first to third springs  251 ,  252  and  253  provided therein for raising paper sheets P. These springs enable the conveyance in respective directions of paper sheets P contained in the paper sheet cassette  250 , together with the first and second feed rollers  221  and  231  of the first and second conveyance systems  220  and  230 . The feed rollers  221  and  231  may be formed integral with the paper sheet cassette  250  as one body. In this case, each of conveyance rollers located (usually in the conveyance systems) downstream of the feed rollers may be formed integral with the paper sheet cassette  250  as one body. 
     In this structure, the conveyance path of paper sheets P to the intermediate transfer member  9  when forming a color image can be shortened, and paper sheets P can be fed from the same paper sheet cassette when forming a color image and a black-and-white image. 
     FIG. 4 is a schematic view illustrating a color image forming apparatus according to a further embodiment. In the color image forming apparatus of FIG. 4, similar elements to those in FIG. 1 are denoted by corresponding reference numerals, and no detailed description will be given thereof. 
     As shown in FIG. 4, a color image forming apparatus  301  has a photosensitive member  2 , a charger  3 , an exposing unit  4 , first to fourth developing units  5 ,  6 ,  7  and  8 , an intermediate transfer member  9 , an image forming section  12  that includes an intermediate transfer member cleaner  10  and a cleaning unit  11 , a first conveyance system  320 , a second conveyance system  330  and a paper sheet cassette  350  that can contain two types of paper sheets. 
     The first conveyance system  320  includes a first feed roller  321  provided at the intermediate transfer member  9  side of the paper sheet cassette  350  for feeding each paper sheet P in group of paper sheets Pa, contained in one chamber  351  of the paper sheet cassette  350 , toward the intermediate transfer member  9 , and can supply the intermediate transfer member  9  with each paper sheet P through a short conveyance path. 
     The second conveyance system  330  is designed to convey each paper sheet P toward the second transfer roller  31  when forming a black-and-white image, and can feed each paper sheet P in the other group of paper sheets Pb, contained in the other chamber  352  of the paper sheet cassette  350 , toward the transfer roller  31  (the aligning roller  32 ), using a second feed roller  331 . 
     As described above, in the color image forming apparatus of FIG. 4, the paper sheet cassette  350  can simultaneously contain two types of group of paper sheets Pa and Pb. The group of paper sheets Pa and Pb contained therein can be supplied to the intermediate transfer member  9  and the photosensitive member  2  independently of each other by the first and second conveyance systems  320  and  330 , respectively. 
     FIG. 5 is a schematic view illustrating a color image forming apparatus according to a modification of the embodiment of FIG.  2 . In the color image forming apparatus of FIG. 5, similar elements to those in FIG. 2 are denoted by corresponding reference numerals, and no detailed description will be given thereof. 
     As shown in FIG. 5, a color image forming apparatus  401  includes a photosensitive member  2 , a charger  3 , an exposing unit  4 , first to fourth developing units  5 ,  6 ,  7  and  8 , an intermediate transfer member  9 , an image forming section  12  that includes an intermediate transfer member cleaner  10  and a cleaning unit  11 , a first paper sheet cassette  13 , a first conveyance system  120 , a second conveyance system  130  and a second paper sheet cassette  150 . The color image forming apparatus  401  of FIG. 5 employs only one fixing unit  460 . 
     More specifically, as is evident from FIG. 5, the surface of a paper sheet P, onto which a color image formed on the intermediate transfer member  9  is transferred, and the surface of a paper sheet Pb, onto which a color image formed on the photosensitive member  2  is transferred, are brought into contact with the rollers  461  and  462  of the fixing unit  460 , respectively. Accordingly, if one of the rollers  461  and  462  is formed of an elastic roller, it is possible that one of a color image and a black-and-white image will be insufficiently fixed, or the color reproductivity of a color image will fall out of an appropriate range. To avoid this, each of the rollers  461  and  462  has, for example, a hollow aluminum roller that has its outer peripheral surface coated with silicon rubber having a thickness of 4 mm, and a PFA tube with an outer diameter φ of 41 mm, which covers on the silicon rubber on the aluminum roller. 
     FIG. 6 is a schematic view illustrating a color image forming apparatus according to a modification of the embodiment of FIG.  4 . In the color image forming apparatus of FIG. 6, similar elements to those in FIG. 4 are denoted by corresponding reference numerals, and no detailed description will be given thereof. 
     As shown in FIG. 6, a color image forming apparatus  501  has a photosensitive member  2 , a charger  3 , an exposing unit  4 , first to fourth developing units  5 ,  6 ,  7 , and  8 , an intermediate transfer member  9 , an image forming section  12  that includes an intermediate transfer member cleaner  10  and a cleaning unit  11 , a first conveyance system  320 , a second conveyance system  330  and a paper sheet cassette  350  capable of containing two types of paper sheets. The color image forming apparatus  501  of FIG. 6 employs only one fixing unit  570 . 
     Like the case of FIG. 4, the paper sheet cassette  350  in this modification can contain two types of group of paper sheets Pa and Pb (of the same size in FIG.  6 ). The sheets Pa and Pb are supplied to the intermediate transfer member  9  and the photosensitive member  2  independently of each other by the first and second conveyance systems  320  and  330 , respectively. 
     The fixing unit  570  has substantially the same structure as the fixing unit  460  shown in FIG.  5 . 
     A description will now be given of a special image forming operation (the operation of interrupting color image forming to execute black-and-white image forming) executed by the color image forming apparatuses shown in FIGS. 1 to  6 . 
     For example, to interrupt the process of forming A4-size color images so as to form a black-and-white image, a series of characteristic operations as below are executed. 
     As aforementioned, the circumferential length of the intermediate transfer member  9  is twice or more the width of an A4 portrait size paper sheet. Accordingly, usually, two color images of A4 portrait size can be formed while the intermediate transfer member  9  makes rotates through 360 degrees (one revolution). 
     When forming an image on the entire paper sheet of A3 size, images of four colors having of a BK image, a Y image, an M image and a C image are sequentially transferred onto the intermediate transfer member  9  each time the body  9  rotates through 360 degrees. On the other hand, when forming an image of A4 portrait size, for example, a BK image included in one surface image of a double-sided document and a BK image included in the reverse surface of the document can be simultaneously output (transferred) onto the circumferential surface of the intermediate transfer member  9 . After that, a Y image, an M image and a C image are sequentially transferred onto each BK image. This type of control enables reduction of the total time period required for forming a plurality of A4-size color images. 
     Using this function, a BK image included in a color image is formed on the photosensitive member  2  (FIG.  7 A), and then an interrupting image (black-and-white image) B 1  is formed thereon (FIG.  7 B). In these two processes, the black developing unit  5  forms two toner images. Concerning a color image, suppose that a BK image, a Y image, an M image and a C image are formed in this order. 
     After that, a Y image included in the color image is formed on the photosensitive member  2  while the BK image formed on the photosensitive member  2  is transferred onto the intermediate transfer member  9  by voltage between the photosensitive member  2  and the intermediate transfer member  9  (FIG.  7 C). 
     Subsequently, a second interrupting image (black-and-white image) B 2  is formed on the photosensitive member  2  while the first black-and-white image B 1  formed on the photosensitive member  2  is transferred, using the transfer roller  31 , onto a paper sheet conveyed on the path  30  ( 130 ,  230 , . . .  530 ) (FIG.  7 D). 
     Similarly, an M image included in the color image is formed on the photosensitive member  2  while the Y image formed on the photosensitive member  2  is transferred onto the intermediate transfer member  9  such that it is superposed on the previously transferred BK image by voltage between the photosensitive member  2  and the intermediate transfer member  9  (FIG.  7 E). 
     Subsequently, a third interrupting image (black-and-white image) B 3  is formed on the photosensitive member  2  while the second black-and-white image B 2  formed on the photosensitive member  2  is transferred, using the transfer roller  31 , onto a paper sheet conveyed on the path  30  ( 130 ,  230 , . . .  530 ) (FIG.  7 F). 
     Similarly, a C image included in the color image is formed on the photosensitive member  2  while the M image formed on the photosensitive member  2  is transferred onto the intermediate transfer member  9  such that it is superposed upon the previously transferred BK and Y images by voltage between the photosensitive member  2  and the intermediate transfer member  9  (FIG.  7 G). 
     Thereafter, a fourth interrupting image (black-and-white image) B 4  is formed on the photosensitive member  2  while the third black-and-white image B 3  formed on the photosensitive member  2  is transferred, using the transfer roller  31 , onto a paper sheet conveyed on the path  30  ( 130 ,  230 , . . .  530 ) (FIG.  7 H). 
     Then, a BK image included in the next color image is formed while the C image formed on the photosensitive member  2  is transferred onto the intermediate transfer member  9  such that it is superposed upon the previously transferred BK, Y and M images by voltage between the photosensitive member  2  and the intermediate transfer member  9  (FIG.  7 I). 
     Thereafter, a fifth interrupting image (black-and-white image) B 5  is formed on the photosensitive member  2  while the fourth black-and-white image B 4  formed on the photosensitive member  2  is transferred, using the transfer roller  31 , onto a paper sheet conveyed on the path  30  ( 130 ,  230 , . . .  530 ) (FIG.  7 J). If the next color image is not formed, the second BK image is not formed at the step shown in FIG.  7 I. Further, where the number of interrupting images is less than  5 , the operation of forming an interrupting image is finished before the step shown in FIG. 7J,  7 H,  7 F or  7 D. 
     In other words, in the image forming operations shown in FIGS. 7A to  7 J, at the first revolution of the intermediate transfer member  9 , the BK image of a first color image and a non-output image corresponding to a first interrupting image (black-and-white image) are transferred onto the body  9 . Since the interrupting image is directly transferred and output to a paper sheet from the photosensitive member  2  independently of a color image, the image corresponding to the interrupting image and transferred to the intermediate transfer member  9  is an offset image, which is not transferred to a paper sheet. The “non-output image” indicates this offset image. At the second revolution of the intermediate transfer member  9 , the Y image of the first color image and a non-output image corresponding to a second interrupting image (black-and-white image) is transferred onto the body  9 . At the third revolution of the intermediate transfer member  9 , the M image of the first color image and a non-output image corresponding to a third interrupting image (black-and-white image) is transferred onto the body  9 . At the fourth revolution of the intermediate transfer member  9 , the C image of the first color image and a non-output image corresponding to a fourth interrupting image (black-and-white image) is transferred onto the body  9 . At this point in time, the first color image is re-transferred from the intermediate transfer member  9  to a paper sheet, thereby forming a color image thereon. 
     Thus, interrupting images can be output independently by alternately forming, on the photosensitive member  2 , each color component image included in a color image and each interrupting image, transferring each color component image onto the half of the circumferential surface of the intermediate transfer member  9 , which is in the direction of rotation, and transferring each interrupting image onto the other half of the circumferential surface. Accordingly, the speed of forming a color image reduces to ½, but interrupting images can be continuously formed (copied) without stopping the color image forming operation. 
     Referring then to FIGS. 8A to  8 M, a description will be given of another embodiment in which an A4-size monochrome document is copied interruptedly while an A4-size color image is copied. 
     For example, when a request for copying a black-and-white image has been issued while color images are continuously printed, a half of that image area of the intermediate transfer member  9 , which is equal to the total area of two paper sheets of A4 portrait size, is assigned to black-and-white image copying. 
     First, the charger  3  uniformly charges the photosensitive member  2 , and then the exposing unit  4  exposes the photosensitive member  2  to form thereon an electrostatic latent image corresponding to a BK image included in a color image. The developing unit (BK)  5  develops the latent image. (FIG. 8A) 
     Subsequently, the exposing unit  4  exposes the photosensitive member  2  to form thereon an electrostatic latent image corresponding to a B 1  image (a first interrupting image), and the BK developing unit  5  develops the latent image on the photosensitive member  2 . The previously developed BK image is conveyed to an intermediate transfer position in which the intermediate transfer member  9  is in contact with the photosensitive member  2  by voltage between the photosensitive member  2  and the intermediate transfer member  9 , without being transferred by the transfer roller  31 . In the intermediate transfer position, the B 1  image is transferred onto the intermediate transfer member  9 . (FIG. 8B) 
     After that, the photosensitive member  2  is exposed to form thereon an electrostatic latent image corresponding to a Y image included in the color image, and the Y developing unit  6  develops the latent image, thereby obtaining a Y toner image. At this time, the B 1  image is transferred onto the intermediate transfer member  9  downstream of the previously transferred BK image by voltage between the photosensitive member  2  and the intermediate transfer member  9  (FIG.  8 C). In this state, the B 1  toner image included in the color image is held on a color image area (situated at the upstream Bide of the intermediate transfer member  9 ), and the black toner (B 1 ) image is held on a black-and-white image area situated downstream of the color image area. 
     The black toner image (B 1 ), which is a result of interrupting copying, is transferred onto a paper sheet P by the transfer roller  21  (FIG.  8 D), and then fixed by the fixing unit  24 . The paper sheet with the black toner image is guided, through a path selected by a tray switching mechanism (not shown), to, for example, the tray  34 , to which black-and-white images are usually output. Accordingly, the B 1  toner image (included in the color image) is left on the color image area of the intermediate transfer member  9 . While the B 1  toner image is left thereon, the photosensitive member  2  is exposed to form thereon an electrostatic latent image corresponding to a second interrupting black-and-white image B 2 , and the developing unit  5  develops the latent image, thereby forming a second black toner image B 2  (FIG.  8 E). 
     Subsequently, the photosensitive member  2  is exposed to form thereon an electrostatic latent image corresponding to an M image included in the color image, and the M developing unit  7  develops the latent image, thereby obtaining an M toner image. At this time, each of the Y toner image and the second black toner image B 2  are transferred onto the intermediate transfer member  9  by voltage between the photosensitive member  2  and the intermediate transfer member  9 , in series (FIG.  8 F). 
     After that, the photosensitive member  2  is exposed to form thereon an electrostatic latent image corresponding to a third interrupting black-and-white image, and the developing unit  5  develops the latent image, thereby forming a third black toner image B 3 . While the third black toner image B 3  is formed, the second black-and-white image B 2  transferred to the intermediate transfer member  9  is further transferred onto a paper sheet by the transfer roller  21  and discharged (FIG.  8 G). At the same time, the Y image is transferred onto the intermediate transfer member  9  such that it is superposed upon the BK image held thereon by voltage between the photosensitive member  2  and the intermediate transfer member  9  (FIG.  8 H). 
     A latent image corresponding to a C image included in the color image is formed and developed in the same manner as above, and then a latent image corresponding to a fourth black-and-white image B 4  is formed on the photosensitive member  2  (FIG.  8 I). 
     Thereafter, the M image is transferred onto the intermediate transfer member  9  such that it is superposed upon the BK image and the Y image previously transferred thereon, while the third black-and-white image B 3  is transferred onto the remaining area of the intermediate transfer member  9  by voltage between the photosensitive member  2  and the intermediate transfer member  9  (FIG.  8 J). 
     Then, the third black toner image B 3  is transferred onto a paper sheet by the transfer roller  21  and sent to the fixing unit. At this time, a toner image consisting of the BK image, the Y image and the M image are superposed upon each other is left on the intermediate transfer member  9 . In this state, a latent image corresponding to the BK image included in the next color image, if any, is formed on the photosensitive member  2  (FIG.  8 K). 
     Then, the C image formed on the photosensitive member  2  is transferred onto the intermediate transfer member  9  such that it is superposed upon the BK image, the Y image and the M image previously transferred thereon, while the fourth black-and-white image B 4  is transferred onto the remaining area of the intermediate transfer member  9  by voltage between the photosensitive member  2  and the intermediate transfer member  9 . While the fourth black-and-white image B 4  is transferred onto the intermediate transfer member  9 , a color image formed by superposing the BK, Y, M and C toner images is transferred onto a paper sheet by the transfer roller  21  and sent to the fixing unit (FIG.  8 L). 
     After that, the fourth black-and-white image B 4  is transferred onto a paper sheet by the transfer roller  21  and sent to the fixing unit. At this time, if there is a fifth black-and-white image to be copied, a latent image corresponding to the fifth black-and-white image is formed on the photosensitive member  2  (FIG.  8 M). 
     If there is no more color image to be copied, the next BK image is not formed in the state shown in FIG.  8 K. Further, if the number of interrupting images is less than  5 , the operation of forming an interrupting image is finished before the step shown in FIG. 8L,  8 J, . . . , or  8 D. 
     Each paper sheet, onto which a four-color toner image has been transferred, is sent to the fixing unit  24 , where the toner image is fixed thereon. Each paper sheet with a color toner image fixed thereon is guided to the tray  25  through a path selected by a tray switching mechanism (not shown). 
     On the other hand, each paper sheet, onto which a black toner image has been transferred, is sent to the fixing unit  24 , where the toner image is fixed thereon. Each paper sheet with a black toner image fixed thereon is guided to the tray  34  through a path selected by the tray switching mechanism. 
     Thus, one black-and-white image can be output each time the intermediate transfer member  9  makes one revolution, and one color image can be output each time the intermediate transfer member  9  makes four revolutions. 
     Accordingly, when, for example, forming two copies of a black-and-white image while forming two copies of a color image, the above control enables formation of a black-and-white image while forming a color image each time the intermediate transfer member  9  makes two revolutions. This control method also enables formation of a color image and a black-and-white image in a single paper sheet. 
     As described above, in the image forming apparatus of the invention, the process of transferring a color image onto a paper sheet is separated from the process of transferring a black-and-white image onto a paper sheet, thereby enabling transfer of a black-and-white image onto a paper sheet and fixing of the transferred black-and-white image immediately after development. As a result, the first copying period of a black-and-white image can be shortened. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.