Patent Publication Number: US-8126380-B2

Title: Image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus having a plurality of image forming units developing and visualizing a latent image formed on a latent image carrier employed for a photocopier, an image recording apparatus, a printer, a facsimile and the like. 
     2. Description of Related Art 
     In a related art image forming apparatus printing a multi-color image, plural image forming units developing and visualizing a latent image formed on a latent image carrier are disposed in parallel on a transfer belt tightly stretched by tension members such as a drive roller and a tension roller (e.g., Japanese Un-examined Patent Application Publication No. H07-104609). 
     Such a related art image forming apparatus cited in the above document, however, has a longer depth thereof due to a parallel disposition of the plurality of image forming units, resulting in an increase in an overall size thereof. 
     It is an object of the present invention to provide an image forming apparatus capable of reducing a size thereof while reducing occurrences of print quality deterioration due to the reduced size thereof. 
     BRIEF SUMMARY OF THE INVENTION 
     According to one aspect of the invention, an image forming apparatus includes: a plurality of image forming units forming a visible image by attaching charged developer to an electrostatic latent image; a transfer belt member being transferred the visible image formed by each of the plurality of image forming units; a plurality of tension members tightly stretching the transfer belt member; a primary transfer member transferring the visible image formed by the plurality of image forming units to the transfer belt unit; a secondary transfer member transferring the visible image transferred to the transfer belt member on a recording medium; and a fixing unit fixing the visible image transferred to the recording medium. The plurality of image forming units are disposed in at least two transfer regions formed by segmenting the transfer belt member by the plurality of the tension members. 
     Additional features and advantages of the present invention will be more fully apparent from the following detailed description of embodiments, the accompanying drawings and the associated claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the aspects of the invention and many of the attendant advantage thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a cross-sectional view illustrating an image forming apparatus according to a first embodiment of the present invention; 
         FIG. 2  is a schematic diagram illustrating a transfer roller and an image forming unit included in the image forming apparatus of  FIG. 1 ; 
         FIG. 3  is another schematic diagram illustrating a transfer roller and an image forming unit included in the image forming apparatus of  FIG. 1 ; 
         FIG. 4  is a schematic diagram illustrating positions of the image forming units and a transfer belt unit in the image forming apparatus of  FIG. 1 ; 
         FIG. 5  is a cross-sectional view illustrating a related art image forming apparatus; 
         FIG. 6  is a block diagram illustrating a control unit of the image forming apparatus according to the first embodiment; 
         FIG. 7  is a schematic diagram illustrating toner in a curve portion of a transfer belt; 
         FIG. 8  is a schematic diagram illustrating a replacement method for the image forming unit; 
         FIG. 9  is a schematic diagram illustrating a replacement method for the transfer belt; 
         FIG. 10  is a cross-sectional view illustrating an image forming apparatus according to a second embodiment of the present invention; 
         FIG. 11  is a cross-sectional view illustrating an image forming apparatus according to a third embodiment of the present invention; 
         FIG. 12  is a block diagram illustrating a control unit of the image forming apparatus according to the third embodiment of the present invention; 
         FIG. 13  is a cross-sectional view illustrating an image forming apparatus according to a fourth embodiment of the present invention; 
         FIG. 14  is a schematic diagram illustrating an image forming unit included in the image forming apparatus according to the fourth embodiment of the present invention; 
         FIG. 15  is a schematic diagram illustrating an example of a toner conveyance member included in the image forming apparatus according to the fourth embodiment of the present invention; and 
         FIG. 16  is another schematic diagram illustrating an example of a toner conveyance member included in the image forming apparatus according to the fourth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In describing embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner. Reference is now made to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views. 
     First Embodiment 
     Referring to  FIG. 1 , an image forming apparatus  1  according to a first embodiment of the present invention is illustrated in a cross-sectional view. An X-axis, a Z-axis and a Y-axis in XYZ coordinates shown in  FIG. 1  represent a traveling direction of a transfer belt  11  serving as a transfer medium, a vertical direction with respect to the X-axis, and a direction perpendicular to the X and Z-axes, respectively. The XYZ coordinates designate identical or corresponding axes throughout the several views. 
     The image forming apparatus  1  includes a recording medium  5 , a transfer belt  11  serving as a transfer belt member, a transfer roller  12  serving as a primary transfer member, a driver roller  13  serving as a tension member, image forming units  15 Y,  15 M,  15 C and  15 K (color components of yellow, magenta, cyan and black are abbreviated as Y, M, C and K respectively), a sheet cassette  30 , a hopping roller  31 , a registration roller  32 , a pinch roller  33 , an ejection roller  800 , a recording medium stacking unit  801 , a secondary transfer unit  802  including a secondary transfer roller  803 , a secondary transfer roller  803 , a fixing device  900 , a backup roller  902 , a heat roller  903 , a transfer belt unit  910 , a tension roller  950  serving as another tension member, a density sensor  1200 , and a cleaning blade  1700 . 
     In the image forming apparatus  1  as illustrated in  FIG. 1 , the sheet cassette  30  storing the recording medium or media  5  such as a sheet or sheets of paper in an accumulated state therein is attached on an upstream side relative to a conveyance path of the recording medium  5 , and the hopping roller  31  picking up the recording medium  5  is disposed above the sheet cassette  30 . In the image forming apparatus  1 , the pinch roller  33  correcting the recording medium  5  in the diagonal movement with the registration roller  32  is disposed on a downstream side of the hopping roller  31  relative to a sheet conveyance direction of the recording medium  5 . The hopping roller  31  and the registration roller  32  are rotated by the power transmitted from a drive source (not shown) through a transmission mechanism such as a gear. 
     A pair of the drive roller  13  and the tension roller  950  drives the transfer belt  11  tightly stretched thereby. The drive roller  13  is rotated by the power transmitted from a drive source (not shown) through a transmission mechanism such as a gear. The tension roller  950  according to the first embodiment includes a conductive shaft being wound therearound with a conductive resin or rubber, and is applied with voltage of a polarity reverse to the polarity of an electrical potential charged at the toner  4 , used for image formation, being charged with an electrical potential from a power source (described later). Here, the voltage to be applied to the tension roller  950  is substantially the same polarity as a voltage applied to the transfer roller  12  when a toner image formed by the image forming unit  15  is transferred to the transfer belt  11 . 
     The image forming apparatus  1  includes the cleaning blade  1700  collecting a remaining residual transfer toner remained on a surface of the transfer belt  11 , and the density sensor  1200  detecting a toner density and transmitting toner density information to a print control unit  51  (described later). 
     The secondary transfer roller  803  in the secondary transfer unit  802  transfers the toner image transferred to the transfer belt  11  by the primary transfer member to the recording medium  5 . 
     The fixing device  900  includes the back up roller  902  and the heat roller  903 , and fixes the toner transferred to the recording medium  5  by the secondary transfer unit  802  with pressure and heat. Here, the recording medium  5  having the toner image fixed thereon is conveyed to the recording medium stacking unit  801  by the ejection roller  800 . 
     The four image forming units  15  are disposed in sequence of  15 Y,  15 M,  15 C and  15 K from an upper stream side in a print direction in a state of being pressed against the transfer belt  11  as illustrated in  FIG. 1 . 
     Referring to  FIG. 2 , each of the image forming units  15 Y and  15 M disposed above the transfer belt  11  and the transfer roller  12  serving as the primary transfer member are illustrated. Since the image forming units  15 Y and  15 M are substantially similar to each other except for the toner color, an image forming unit  15  illustrated in  FIG. 2  is representative of the image forming units  15 Y and  15 M. The transfer roller  12  is disposed with respect to each of the image forming units  15 . 
     The image forming unit  15  representing each of the image forming units  15 Y and  15 M includes a photosensitive drum  100  forming an electrostatic latent image thereon, a charging roller  2  charging a surface of the photosensitive drum  100 , a light emitting diode (LED) head  3  including LED elements and the like forming the electrostatic latent image on the surface of the photosensitive drum  10  charged by the charging roller  2 , a development roller  6  developing the electrostatic latent image by attaching the toner  4  including toner particles to the electrostatic latent image formed on the surface of the photosensitive drum  100 , a toner supply roller  7  supplying the toner  4  to the development roller  6 , a development blade  8  evenly adjusting a toner layer supplied from the toner supply roller  7 , a cleaning blade  9  removing the toner  4  and the electrostatic latent image remained on the surface of the photosensitive drum  100 , and a housing  10  made of resin housing a development device. 
     The photosensitive drum  100  is rotatable in a direction indicated by an arrow shown in  FIG. 2  at predetermined rotation speed. The charging roller  2  is rotatable in a counterclockwise direction as illustrated in  FIG. 2  and contacts a surface of the photosensitive drum  100  so as to apply predetermined voltage to the surface thereof. The LED head  3  forms the electrostatic latent image on the surface of the photosensitive drum  100 . The development roller  6  supplies the toner  4  to the surface of the photosensitive drum  100  having the electrostatic latent image thereon, thereby developing the electrostatic latent image. The toner supply roller  7  contacts the development roller  6  with certain pressure, and the development blade  8  is disposed to the development roller  6  so as to regulate a thickness of the toner  4  supplied from the toner supply roller  7  at a certain level. The cleaning blade  9  made of an elastic member is disposed to contact the surface of the photosensitive drum  100  with certain pressure using an edge portion thereof. 
     The fixing roller  12  is disposed below the photosensitive drum  100  through the transfer belt  11  in such a manner to contact the photosensitive drum  100 . The fixing roller  12  rotates in a counterclockwise direction as illustrated in  FIG. 2  and applies the voltage of the potential reverse to the electrical potential charged at the toner  4 . Consequently, the toner image developed on the surface of the photosensitive drum  100  is transferred to the transfer belt  11 . The print control unit  51  described later with reference to  FIG. 6  controls such a rotation operation, a voltage application operation and the like of each of such elements. 
     Referring to  FIG. 3 , each of the image forming units  15 C and  15 K disposed below the transfer belt  11  and the transfer roller  12  serving as the primary transfer member are illustrated. Since the image forming units  15 C and  15 K are substantially similar to each other except for the toner color, an image forming unit  15  illustrated in  FIG. 3  is representative of the image forming units  15 C and  15 K. The transfer roller  12  is disposed with respect to each of the image forming units  15 . 
     The image forming unit  15  representing each of the image forming units  15 C and  15 K illustrated in  FIG. 3  is substantially similar to the image forming unit  15  representing the image forming units  15 Y and  15 M illustrated in  FIG. 2  except for positions of the transfer belt  11 , the transfer roller  12 , and the toner supply roller  7 . The transfer belt  11  and the transfer roller  12  in  FIG. 3  are disposed at an upper portion in the Z-axis direction compared to those shown in  FIG. 2 . Since the toner  4  is accumulated in a lower portion in the Z-axis direction by gravity, the toner supply roller  7  is disposed below the development roller  6  in the Z-axis direction. In this way, even where a remaining amount of the toner  4  decreases, a surface of the toner supply roller  7  can convey the toner  4  as much as possible. 
     Although the development blade  8  regulating the thickness of the toner  4  supplied from the toner supply roller  7  at the certain level is disposed to the development roller  6 , the development blade  8  is preferably disposed in a reverse direction relative to the rotation direction of the development roller  6  as illustrated in  FIG. 3  so as to reduce a variation in regulation of a toner layer depending on a loading weight of the toner  4 . The print control unit  51  described later with reference to  FIG. 6  controls a rotation operation, a voltage application operation and the like of each of such elements. 
     The transfer belt unit  910  includes the image forming units  15 Y,  15 M,  15 C, and  15 K, the transfer belt  11 , each of the transfer rollers  12 , the drive roller  13 , the tension roller  950 , and the cleaning blade  1700 . The transfer belt unit  910  is disposed along a guide unit  1300  in the image forming apparatus  1  as illustrated in  FIG. 4 . 
     It should be noted that the first embodiment of the present invention has been described above with four image forming units  15 Y,  15 M,  15 C and  15 K, but is not limited thereto. For example, the first embodiment of the present invention may be applied to an image forming apparatus including a plurality of image forming units having the toner colors of red, green, and blue or a plurality of image forming units having the toner colors of gold and silver, etc. in addition to the yellow, magenta, cyan, and black according to the first embodiment. In an image forming procedure, the image forming unit having the toner color of black may be disposed at the last, so that a black image is formed on the recording medium at a short time period when a high frequent usage of the black image is printed. Therefore, the image forming unit having the toner color of black is preferably disposed in the most downstream relative to a print direction. 
     Referring to  FIG. 5 , a conventional image forming apparatus  5000  is illustrated. The image forming apparatus  5000  includes image forming units  5015 Y,  5015 M,  5015 C, and  5015 K disposed in parallel at one side of a transfer belt  5011 . The image forming apparatus  1  according to the first embodiment of the present invention illustrated in  FIG. 1  has a shorter length in the X-axis direction in an amount of two image forming units compared to the conventional image forming apparatus  5000 . 
     Since the image forming apparatus  1  according to the first embodiment has the shorter length in the X-axis direction in the amount of two image forming units compared to the conventional image forming apparatus  5000 , a size of the image forming apparatus  1  is reduced. 
     Referring to  FIG. 6 , a control unit of the image forming apparatus  1  according to the first embodiment of the present invention is illustrated in a block diagram. 
     The control unit of the image forming apparatus  1  includes the print control unit  51 , an interface (I/F) control unit  52 , a receiving memory  53 , an image data editing memory  54 , an operation unit  55 , a sensor group  56 , a power source  57  for the charging roller  2 , a power source  58  for the development roller  6 , a power source  59  for the toner supply roller  7 , a power source  60  for the transfer roller  12 , a head drive control unit  61 , a fixing control unit  62 , a conveyance motor drive unit  63 , a drive control unit  64 , a power source  804  for the secondary transfer roller  803 , and a power source  949  for the tension roller  950 . 
     The print control unit  51  includes a microprocessor, a read only memory (ROM), a random access memory (RAM), an input-output port, and a timer  51   a . The print control unit  51  receives print data and a control command form a higher-level device (not shown) through the I/F control unit  52  and controls a sequence of the image forming apparatus  1  as a whole to execute print operation. 
     The receiving memory  53  temporality stores the print data input from the higher-level or host device through the I/F control unit  52 . The image data editing memory  54  both receives the print data stored in the receiving memory  53  and stores image data formed by editing the print data. 
     The operation unit  55  includes LED for displaying a state of the image forming apparatus  1  and a switch for inputting an instruction by an operator. The sensor group  56  includes various sensors, for example, a sheet position sensor, a temperature-humidity sensor, and a density sensor, for monitoring an operation state of the image forming apparatus  1 . 
     The power source  57  for the charging roller  2  applies the voltage to the charging roller  2 . The power source  58  for the development roller  6  applies the voltage to the development roller  6 . The power source  59  for the toner supply roller  7  applies the voltage to the toner supply roller  7 . The power source  60  for the transfer roller and the power source  804  for the secondary transfer roller apply the voltage to the transfer roller  12  serving as the transfer member and the secondary transfer roller  803  respectively. 
     The head drive control unit  61  transmits the image data stored in the image data editing memory  54  to the LED head  3  and drives the LED head  3 . 
     The fixing control unit  62  provides an instruction to apply predetermined voltage to the heat roller  903  included in the fixing device  900  from a power source (not shown) so as to fix the toner image transferred to the recording medium  5 . The fixing device  900  includes the heat roller  903  fusing the toner  4  of the toner image transferred to the recording medium  5  and a temperature sensor detecting temperature. The fixing control unit  62  controls the heat roller  903  such that the temperature of the heat roller  903  is at a certain level based on a sensor output from the temperature sensor. 
     The conveyance motor drive unit  63  controls rotation of a sheet conveyance motor  66  for conveying the recording medium  5 . The conveyance motor drive unit  63  conveys the recording medium  5  at a predetermined timing and halts the conveyance of the recording medium  5  based an instruction of the print control unit  51 . The drive control unit  64  controls rotation of a drive roller  67  involving the image forming units  15 . 
     The power source  949  for the tension roller  950  applies predetermined voltage to the tension roller  950  in the course of the print operation. The voltage to be applied to the tension roller  950  has the polarity reverse to the polarity of the electrical potential charged at the toner  4 . 
     Now, the operation of the image forming apparatus  1  is described. The print control unit  51  receives the print data from the higher-level or host device (not shown) through the I/F control unit  52  and temporarily stores the print data in the receiving memory  53 . Subsequently, the print control unit  51  forms the image data by editing the print data stored. The formed image data are stored in the image data editing memory  54 . 
     The print control unit  51  provides an instruction to the conveyance motor drive unit  63  to drive the sheet conveyance motor  66 . The hopping roller  31  begins to rotate by the driving of the sheet conveyance motor  66  and separates a plurality of recording media  5  stored in the sheet cassette  30  in the accumulated state one by one from the top, so that each of the recording media  5  is separately fed into the sheet conveyance path. The diagonal movement of the recording medium  5  is corrected in the course of passing the registration roller  32 , and the recording medium  5  is further conveyed to the secondary transfer unit  802 . The print control unit  51  provides an instruction to the drive control unit  64  to drive the drive motor  66 . The drive motor  66  begins to drive based on the instruction of the drive control unit  64 , and drive power of the drive motor  66  is transmitted to each of the rollers, so that each of the rollers begins to rotate. 
     Simultaneously, the image forming unit  15 Y begins to form a yellow image. The print control unit  51  supplies instructions to the power sources  57 ,  58 ,  59 , and  949  to apply the voltage to the charging roller  2 , development roller  6 , the toner supply roller  7 , and the tension roller  950  respectively. The charging roller  2  with the predetermined voltage applied thereto charges the surface of the photosensitive drum  100  to the certain level. Subsequently, the print control unit  51  provides the instruction to the hard drive control unit  61  based on the image data stored in the image data editing memory  54 . Upon receiving the instruction, the head drive control unit  61  drives the LED head  3 , so that the LED head  3  exposes the surface of the photosensitive drum  100 , thereby forming the electrostatic latent image on the surface thereof. 
     The development roller  6  supplied with the toner  4  from the toner supply roller  7  attaches the toner  4  to the electrostatic latent image formed on the surface of the photosensitive drum  100 , so that the electrostatic latent image is developed and the toner image of yellow is formed. The photosensitive drum  100  having the yellow toner image on the surface thereof continues to rotate, and the print control unit  51  provides the instruction to the power source  60  to apply the predetermined voltage to the transfer roller  12  when the surface of the photosensitive drum  100  having the yellow toner image becomes closer to the surface of the transfer belt  11 . By application of the predetermined voltage to the transfer roller  12 , the toner image on the surface of the photosensitive drum  100  is transferred to the transfer belt  11 . The cleaning blade  9  scrapes the toner  4  being not transferred to the transfer belt  11  from the surface of the photosensitive drum  100 , and the charging roller  2  uniformly charges the surface of the photosensitive drum  100  again. Such image forming processes are sequentially performed in the image forming units  15 Y,  15 M,  15 C, and  15 K. Each of the toner images formed by respective image forming units  15 Y,  15 M,  15 C, and  15 K is sequentially transferred to the transfer belt  11 , thereby overlaying the toner images. 
     When each color of the toner images is formed on the transfer belt  11 , the print control unit  51  provides an instruction to the power source  804  to apply the voltage to the secondary transfer roller  803 . When the recording medium  5  reaches the secondary transfer roller  803  being applied with the voltage, each of the toner images on the transfer belt  11  is transferred on the recording medium  5 . Next, the print control unit  51  provides an instruction to the fixing control unit  62  to heat the heat roller  903  included in the fixing device  900 . The toner image transferred on the recording medium  5  is fixed by heat and pressure applied by the heat roller  903  and the backup roller  902 . The recording medium  5  having the toner imaged fixed thereon is ejected on the recording medium stacking unit  801  disposed outside the image forming apparatus  1 . Therefore, a multi-color image is formed on the recording medium  5  by the process above. 
     In a related art image forming apparatus, on the other hand, a tension roller is not applied with the voltage. In this regard, an upper portion of the toner (i.e., the toner positioned in a relatively longer distance from a transfer belt) is dropped from a transfer belt due to the gravity, causing deterioration of the image. 
     The toner images of yellow and magenta formed on the transfer belt  11  tend to drop from the transfer belt  11  due to the gravity exerted at a curve portion of the transfer belt  11  curved by the tension roller  950  tightly stretching the transfer belt  11 . In other words, the toner particles of the toner  4  forming the image on the transfer belt  11  increase a distance therebetween at the curve portion of the transfer belt  11  as illustrated in  FIG. 7 , causing a decrease in an interaction of the toner particles adjacent to each other. In other words, a surface layer of the transfer belt  11  decreases power to hold the toner  4  thereon by losing the Van der Waals force generated by attraction of the toner particles, causing deterioration of the toner image by the gravity. 
     According to the present invention, the voltage having the polarity reverse to the polarity of the electrical potential charged at the toner  4  is applied to the tension roller  950  so as to attract the toner  4  to a side of the transfer belt  11 , thereby holding the toner  4  on the transfer belt  11  by reducing (if not eliminating) an occurrence of dropping the toner  4  from the transfer belt  11 . 
     After the toner images of yellow and magenta are formed by the image forming units  15 Y and  15 M respectively, the toner images of cyan and black are sequentially transferred on the transfer belt  11  in the course of passing through the image forming units  15 C and  15 K having the toner colors of cyan and black respectively. The toner images overlaid on the transfer belt  11  are transferred on the recording medium  6  in the secondary transfer unit  802  by the voltage applied to the secondary transfer roller  803 . 
     When at least one of the image forming units  15 Y,  15 M,  15 C, and  15 K is replaced, a cover  1600  is opened for pulling out the image forming unit  15  along a portion extended toward inside of the image forming apparatus  1  in a rail shape with the guide unit  1300  rubbing the outside of the image forming unit  15  as illustrated in  FIG. 8 . Here, the LED head  3  is separated from the image forming unit  15  by a separation mechanism (not shown). 
     When transfer belt unit  910  is replaced, the cover  1600  is opened for pulling out the transfer belt unit  910  using a handle  1401  along a portion extended toward inside of the image forming apparatus  1  in a rail shape with the guide unit  1300  rubbing the outside of the image forming unit  15  as illustrated in  FIG. 9 . 
     When the image forming unit  15  is pulled out, a handle  1400  of the image forming unit  15  is held and pulled out in the Y-axis direction. For example, when the image forming units  15 C and  15 K are withdrawn and inserted in the Z-axis direction, the image forming units  15 Y and  15 M and the transfer unit  910  need to be removed. When the image forming units  15 M and  15 C are withdrawn and inserted in the X-axis direction, the image forming units  15 Y and  15 K need to be removed. Therefore, the image forming units  15 Y,  15 M,  15 C, and  15 K are preferably withdrawn and inserted in the Y-axis direction with ease. 
     According to the first embodiment described above, the image forming apparatus  1  has the shorter length in the X-axis direction in the amount of two image forming units compared to the related art image forming apparatus, thereby reducing the size thereof. Moreover, the tension roller  950  of the first embodiment is employed, so that the toner  4  remains held on the surface layer of the transfer belt  11  even when the interaction of the toner particles adjacent to each other is decreased in the curve portion of the transfer roller  11 , thereby reducing (if not eliminating) the deterioration of the toner image. 
     Second Embodiment 
     Referring to  FIG. 10 , an image forming apparatus  2650  according to a second embodiment of the present invention is illustrated. An element and a configuration of the image forming apparatus  2650  similar to those of the image forming apparatus  1  of the first embodiment described above will be given the same reference numerals as the image forming apparatus  1  of the first embodiment and description thereof will be omitted. 
     In the image forming apparatus  2650  according to the second embodiment, a transfer belt  11  is tightly stretched by three tension members, a tension roller  951 , the tension roller  950  applied with the voltage as described in the first embodiment, and the drive roller  13 , and the image forming units  15 M,  15 Y,  15 C, and  15 K are disposed in two transfer regions being not arranged most adjacent to the fixing device  900  among three transfer regions formed by the three tension members as illustrated in  FIG. 10 . 
     The image forming apparatus  2650  includes a cleaning blade  1000 , scraping the toner  4  remained on the transfer belt  11 , disposed in the transfer region most adjacent to the fixing device  900 . The other configurations of the image forming apparatus  2650  are substantially similar to those of the image forming apparatus  1  of the first embodiment. Since the print operation of the second embodiment is also substantially similar to that of the first embodiment, description thereof will be omitted while an element, a configuration and an operation of the image forming apparatus  2650  that differ from those of the above first embodiment will be described. 
     The print data are received, and the cleaning blade  1000  scrapes the toner  4  remained on the transfer belt  11  beginning to rotate in a direction aa indicated by an arrow shown in  FIG. 11  by an instruction of the print control unit  51 . Here, the toner image is not transferred on the transfer belt  11  although the surface of the transfer belt  11  cleaned by the cleaning blade  1000  passes a portion curved by the tension roller  951 . 
     The toner images of yellow and magenta are formed when the surface of the transfer belt  11  cleaned by the cleaning blade  1000  passes through the image forming units  15 Y and  15 M respectively. According to the second embodiment, the voltage having the polarity reverse to the polarity of the electrical potential charged at the toner  4  is applied in a position of the tension roller  950 , that is, a position in which the surface of the transfer belt  11  having the toner images transferred thereon by the image forming units  15 Y and  15 M passes the curve portion. 
     The toner images are sequentially formed by the image forming units  15 C and  15 K on the surface of the transfer belt  11  having passed the curve portion. When the transfer belt  11  having the toner images formed and transferred by all of the image forming units reaches the secondary transfer unit  802 , the toner images on the transfer belt  11  are transferred on the recording medium  5 . Substantially, the recording medium  5  having the toner images transferred thereon is conveyed to the fixing device  900 . The toner images on the recording medium  5  are fixed by the fixing device  900 , and the recording medium  5  is stacked on the stacking tray  851 . 
     In a case where such a print operation as described above is repeated, inside temperature of the image forming apparatus  2650  increases due to the heat generated from the fixing device  900 . Such an increase of the inside temperature is particularly notable in a portion near the fixing device  900 . Moreover, in a case where a size of an image forming apparatus is smaller, such an increase of the inside temperature of the apparatus is more notable compared to an image forming apparatus of a general size. 
     In a case where such an increase of the temperature is occurred in an image forming apparatus, toner inside an image forming unit softens. Consequently, toner particles of the toner slightly fuse one another, resulting in deterioration of print quality. 
     In the image forming apparatus  2650  according to the second embodiment of the present invention, the image forming unit is not disposed to the surface of the transfer belt in a position most adjacent to the fixing device  900 . Therefore, each of the image forming units can reduce an occurrence of being influenced by the heat generated by the fixing device  900  in the course of a successive print operation, thereby reducing (if not eliminating) the deterioration of the print image. 
     According to the second embodiment of the present invention described above, the image forming apparatus  2650  reducing the influence to the image forming units  15  by the heat generated by the fixing device  900  can be provided while the size of the image forming apparatus  2650  can be reduced. 
     Third Embodiment 
     Referring to  FIG. 11 , an image forming apparatus  2675  according to a third embodiment of the present invention is illustrated. The image forming apparatus  2675  according to the third embodiment is substantially similar to the image forming apparatus  2650  according to the second embodiment above, and an element and a configuration of the image forming apparatus  2675  similar to those of the image forming apparatus  2650  of the second embodiment will be given the same reference numerals and description thereof will be omitted. However, the image forming unit  15 K having the toner color of black is disposed at an upstream side relative to the sheet conveyance path of the recording medium  5  as illustrated in  FIG. 11 . A drive roller  2100  forms a curve portion at a downstream side of the image forming unit  15 C relative to the print direction, and is applied with the voltage having the same polarity as the electrical potential charged at the toner  4 , included in the image forming unit  15 K, being charged with the electrical potential. Moreover, the image forming apparatus  2675  includes a cleaning blade  1001  scraping the toner  4 . The cleaning blade  1001  is disposed at the downstream side of the image forming unit  15 C relative to the print direction. 
     The other configurations of the image forming apparatus  2675  are substantially similar to those of the image forming apparatus  2650  of the second embodiment. Since the print operation of the third embodiment is also the substantially similar to that of the second embodiment, description thereof will be omitted while an element, a configuration and an operation of the image forming apparatus  2675  that differ from those of the above second embodiment will be described. 
     The drive roller  2100  includes a conductive shaft wound semiconductive resin or rubber and the like therearound and is applied with the electrical potential of same polarity as the electrical potential charged at the toner  4 , included in the image forming unit  15 K, being charged with the electrical potential from a power source  1100  (described later) for the driver roller  2100 . 
     The cleaning blade  100  is disposed in such a manner to be movable to contact and separate from the transfer belt  11  by an actuator (not shown). 
     The toner  4  used for the third embodiment is charged to a negative potential by triboelectric charge. 
     Referring to  FIG. 12 , a control unit of the image forming apparatus  2675  according to the third embodiment is illustrated in a block diagram. The control unit of the image forming apparatus  2675  includes a second image data editing memory  65  and the power source  1100  for the driver roller in addition to the control unit of the image forming apparatus  1  according to the first embodiment described above with reference to  FIG. 6 . An element and a configuration illustrated in  FIG. 12  similar to those of  FIG. 6  will be given the same reference numerals and description thereof will be omitted. 
     Although the second image data editing memory  65  is substantially the same as the first image data editing memory  54 , the second image data editing memory  65  serving as a memory is used when the toner image of black is formed on the recording medium  5 . 
     The power source  1100  applies a predetermined voltage to the drive roller  2100  based on an instruction of the print control unit  51 . 
     Now, a description is given of formation of a multi-color image on the recording medium  5  by the image forming apparatus  2675 . After receiving the print data, the print control unit  51  forms the image data by editing the print data, involving the black toner, stored in the receiving memory  53 . The formed image data are stored in the image data editing memory  65 . Subsequently, the print control unit  51  provides an instruction to the head drive unit  61  based on the image data stored in the image editing memory  65 . Upon receiving the instruction, the head drive unit  61  drives the LED head  3  disposed inside the image forming unit  15 K, and the LED head  3  exposes the surface of the photosensitive drum  100 , thereby forming the electrostatic latent image on the surface thereof. 
     The development roller  6  supplied with the toner  4  from the toner supply roller  7  attaches the toner  4  to the electrostatic latent image formed on the surface of the photosensitive drum  100 , so that the electrostatic latent image is developed and the toner image of black is formed. The photosensitive drum  100  having the black toner image on the surface thereof continues to rotate, and the print control unit  51  provides the instruction to the power source  60  to apply the predetermined voltage to the transfer roller  12  when the surface of the photosensitive drum  100  having the black toner image becomes closer to the surface of the recording medium  5 . By application of the predetermined voltage to the transfer roller  12 , the toner image on the surface of the photosensitive drum  100  is transferred to the recording medium  5 . 
     On the other hand, each of the image forming units  15 Y,  15 M, and  15 C begins to form the toner image, and each of the toner images formed by the image forming units  15 Y,  15 M, and  15 C is transferred on the transfer belt  11 . 
     When the surface of the transfer belt  11  having thereon the transferred toner images formed by the image forming units  15 Y,  15 M, and  15 C and the recording medium  5  having the black toner image transferred thereon reach the secondary transfer roller  803 , the toner images on the surface of the transfer belt  11  are transferred on the recording medium  5 , thereby forming a multi-color toner image on the recording medium  5 . When the surface of the transfer belt  11  having thereon the transferred toner images formed by the image forming units  15 Y,  15 M, and  15 C reaches the secondary transfer roller  803 , the cleaning blade  1001  is moved by the actuator to a position in which the cleaning blade  1001  does not contact the transfer belt  11 . 
     The toner image transferred on the recording medium  5  is fixed by the fixing operation that is substantially the same as the operation described above in the first and second embodiments. 
     Now, a description is given of formation of the image having only the black toner image on the recording medium  5 . In such a case, the operation of the image forming unit  15 K is substantially the same as that of the multi-color image formation described above. Here, the toner images involving the image forming units  15 Y,  15 M, and  15 C are not formed on the transfer belt  11 . The surface of the transfer belt  11  is cleaned by the cleaning belt  1001  being moved by the actuator to a potion in which the cleaning belt  1001  contacts the transfer belt  11 , the cleaned surface of the transfer belt  11  contacts the recording medium  5  by the secondary transfer roller  803 . 
     Here, the print control unit  51  provides an instruction to the power source  1100  to apply the polarity reverse to the polarity of the electrical potential charged at the black toner to the drive roller  2100 . 
     The toner image of black transferred on the recording medium  5  is fixed by the fixing operation that is substantially the same as the operation described above in the first and second embodiments. 
     In a case where the print operation is repeated, reverse charge toner is generated in each color of the toner  4  included in the image forming units  15 Y,  15 M,  15 C, and  15 K due to deterioration or charge shortage. Such a repeat of the print operation causes the toner  4  inside the image forming units  15 Y,  15 M,  15 C, and  15 K to continue to be damaged mechanically resulting in deterioration of a charge characteristic and in being charged to a reverse potential (reverse potential toner) in a contact area between the development blade  8  and the development roller  6  or in a contact area between the development roller  6  and the toner supply roller  7  in the process of image formation. Such reverse potential toner is moved on the photosensitive drum  100  regardless of the electrostatic latent image formed thereon and is also moved on the transfer belt  11  by the Van der Waals force. Therefore, in a case where the cleaning blade  1001  is not disposed, the reverse charge toner exists on the transfer belt  11  regardless of the toner image formed thereon and is moved on the recording medium  5  in the secondary transfer unit  802 , resulting in deterioration of the print quality. 
     According to the third embodiment, where the image having only the black toner image is formed, the cleaning blade  1001  scrapes the reverse charge toner existed on the transfer belt by contacting the transfer belt  11  so as to reduce (if not prevent) an occurrence of moving the reverse charge toner on the transfer belt  11  to the recording medium  5 . However, in a case where the cleaning blade  1001  constantly contacts the transfer belt  11 , the toner forming the multi-color image excluding the black color is scraped. Therefore, a position of the cleaning blade  1001  is adjusted by the actuator in such a manner not to contact the transfer belt  11  when the multi-color image is formed. 
     Moreover, the reverse charge toner has a positive polarity according to the third embodiment. Such the reverse charge toner might be moved to the recording medium  5  in the secondary transfer unit  802  as described above. Therefore, the image forming apparatus  2675  according to the third embodiment includes the drive roller  2100  being applied with the negative voltage attracting the reverse charge toner to a side of the transfer belt  11  through the transfer belt  11  in the secondary transfer unit  802 . Consequently, the reverse charge toner, being charged to the positive, developed on the recording medium  5  in the course of the image formation with the black toner is moved to the side of the transfer belt  11  by the negative voltage applied to the drive roller  2100  while the toner serving as normal charge toner forming the image remains held on the recording medium  5 . In other words, the reverse charge toner attached to the recording medium  5  in the image forming unit  15  can be collected by the secondary transfer unit  802 . 
     Therefore, according to the third embodiment described above, where the image having only the black toner image is formed, the image forming apparatus  2675  can reduce (if not prevent) a mixture of the toner used in another image forming unit and can reduce (if not prevent) deterioration of the print quality caused by the reverse charge toner in addition to the advantage of the second embodiment. 
     Fourth Embodiment 
     Referring to  FIG. 13 , an image forming apparatus  2700  according to a fourth embodiment of the present invention is illustrated in a block diagram. An element and a configuration of the image forming apparatus  2700  similar to those of the image forming apparatus  1  of the first embodiment described above will be given the same reference numerals as the image forming apparatus  1  of the first embodiment and description thereof will be omitted. As illustrated in  FIG. 13 , the image forming apparatus  2700  according to the fourth embodiment includes an image forming unit  151 K having the black color of the toner  4 , and the image forming unit  151 K is disposed in facing to other image forming units. The other configurations of the image forming apparatus  2700  are substantially similar to those of the image forming apparatus  1  of the first embodiment. Since the print operation of the fourth embodiment is also substantially similar to that of the first embodiment, description thereof will be omitted while an element, a configuration and an operation of the image forming apparatus  2700  that differ from those of the above first embodiment will be described. 
     The image forming unit  151 K is relatively long in the X-axis direction as illustrated in  FIG. 14  compared to the image forming unit  15 K of the first embodiment, thereby capable of having a relatively large volume of the black toner  4  accumulated therein. 
     A toner conveyance member  2500  conveys the toner  4  of black accumulated in the X-axis direction to the toner supply roller  7 , and includes a shaft  2501  and resin film  2502  that is integrally formed with the shaft  2501  as illustrated in  FIG. 15 . A material for each of the shaft  2501  and the resin film  2502  is not particularly defined. However, the shaft  2501  may be prepared using an ABS (acrylonitrile-butadiene-styrene) resin or metal, and the resin film  2501  may be prepared using, for example, PET (polyethylene terephthalate). 
     The toner conveyance member  2500  may include, for example, a resin film  2503  having a chevron shape protruding toward a tip of a middle portion thereof in a Z-axis minus direction as illustrated in  FIG. 16  in such a manner to reduce a difference of a charge characteristic between the toner  4  of black accumulated both ends of the image forming unit in the Y-axis direction and the toner  4  of black accumulated at a middle portion in the image forming unit. 
     The print data are received, and the shaft  2501  provided with drive force through a driving source, a gear, and the like (not shown) based on an instruction of the print control unit  51  begins to rotate by the drive force. Subsequently, the resin film  2502  integrally formed with the shaft  2501  begins to rotate, and the toner  4  of the black accumulated in the X-axis direction is conveyed to the supply roller  7  by the rotation force of the resin film  2502 . The other print operations are substantially the same as the operations described in the first embodiment. 
     Generally, regarding toner inside an image forming unit, the toner accumulated in a middle portion of the image forming unit is more frequently used. Therefore, the toner accumulated in the middle portion of the image forming unit is often used before deterioration thereof while the toner accumulated in both ends of the image forming unit increases a likelihood of deterioration thereof before being used. Consequently, the toner accumulated in both ends inside the image forming unit is preferably brought to the middle portion and not accumulated in both end portions. 
     Therefore, the toner conveyance member  2600  is shaped as illustrated in  FIG. 16 , so that the toner is supplied from both end sides of the resin film  2503  in the Y-axis direction and then is supplied from a middle portion of the resin film  2503  regarding a toner conveyance sequence with respect to the toner supply roller  7 . That is, the toner is supplied to the toner supply roller  7  from the both end sides of the resin film  2503  before being supplied from the middle portion of the resin film  2503 . Therefore, use of the conveyance member  2600  simultaneously allows the supply of the toner to the toner supply roller  7  and movement of the toner accumulated in both ends of the image forming unit to the middle portion thereof. 
     Therefore, according to the fourth embodiment described above, the image forming apparatus  2700  can reduce a number of replacement times of the image forming unit including the toner of black being frequently used. Since the image forming apparatus  2700  includes the toner conveyance member  2500  disposed inside the image forming unit  151 K, the toner can be evenly conveyed in the X-axis direction inside the image forming unit  151 K. Therefore, the image forming apparatus  2700  can reduce (if not eliminate) an occurrence of the toner deterioration and can reduce (if not prevent) the deterioration of the print quality when an image forming unit being long in the X-axis direction such as the image forming unit  151 K is employed. 
     Moreover, the image forming apparatus  2700  employs the toner conveyance member  2600  shaped as illustrated in  FIG. 16 , so that the toner accumulated in both ends of the image forming unit is conveyed to the middle portion of the image forming unit, thereby reducing (if not eliminating) an occurrence of deteriorating the toner inside the image forming unit. 
     In addition to the configurations described in the first through fourth embodiments, an electrical potential to be applied to a transfer roller disposed such that the toner is transferred to the transfer belt from a lower portion or in a horizontal direction may be higher than an electrical potential to be applied to a transfer roller disposed such that the toner is transferred to the transfer belt from an upper portion. Such a configuration allows the toner transferred to the transfer belt from the lower portion to be stably attached on the transfer belt. 
     As can be appreciated by those skilled in the art, numerous additional modifications and variation of the present invention are possible in light of the above-described teachings. It is therefore to be understood that, within the scope of the appended claims, the disclosure of this patent specification may be practiced otherwise than as specifically described herein.