Patent Publication Number: US-9429883-B1

Title: Image forming apparatus and image forming method

Description:
FIELD 
     Embodiments described herein relate generally to an image forming apparatus and an image forming method. 
     BACKGROUND 
     An image forming apparatus is known which fixes a toner on a sheet. In some cases, the image forming apparatus forms images on two sides of a sheet (double-side printing). At this time, the image forming apparatus forms a first toner image to be transferred on the surface of the sheet. After transferring the first toner image onto the surface of the sheet, the image forming apparatus fixes the first toner image on the surface of the sheet using a fixer. The sheet on which the first toner image is fixed is reversed by a reverser. Then, the image forming apparatus forms a second toner image to be transferred on the back of the sheet. The image forming apparatus receives the sheet fed from the reverser. After transferring the second toner image onto the back of the sheet, the image forming apparatus fixes the second toner image on the back of the sheet using the fixer. 
     The fixer is equipped with a heat roller and a press roller which heats and presses the toner images while clamping the sheet therebetween. 
     In double-side printing, the image forming apparatus separately carries out an image formation operation and an image fixation operation twice for a sheet. Thus, double-side printing process is twice as long as single-side printing. Further, the press roller needs to be preheated by the heat roller before the fixer fixes the toner image on a side. Thus, the fixer consumes double power in the double-side printing when compared with that in single-side printing. 
     Thus, double-side printing, when compared with single-side printing, increases both print time and power consumption. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic sectional view exemplifying the whole structure of an image forming apparatus according to a first embodiment; 
         FIG. 2  is a schematic sectional view exemplifying the structure of the transfer fixer of an image forming apparatus according to the first embodiment; 
         FIG. 3  is a block diagram exemplifying the functional structure of an image forming apparatus according to the first embodiment; 
         FIG. 4  is a schematic sectional view illustrating the actions of the transfer fixer of an image forming apparatus according to the first embodiment; and 
         FIG. 5  is a schematic sectional view exemplifying the whole structure of an image forming apparatus according to a second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In accordance with an embodiment, an image forming apparatus comprises: a first image forming unit, a second image forming unit, a first intermediate transfer belt, a second intermediate transfer belt, a conveyance unit and a transfer fixer. The first image forming unit forms a first toner image with a first toner with negative charges. The second image forming unit forms a second toner image with a second toner with positive charges. The first intermediate transfer belt is formed into an endless belt. The first toner image is primarily transferred onto the first intermediate transfer belt from the first image forming unit. The first intermediate transfer belt moves the primarily transferred first toner image to a first secondary transfer position. The second intermediate transfer belt is formed into an endless belt opposite to the first intermediate transfer belt. The second toner image is primarily transferred onto the second intermediate transfer belt from the second image forming unit. The second intermediate transfer belt moves the primarily transferred second toner image to a second secondary transfer position opposite to the first secondary transfer position. The conveyance unit conveys a sheet between the first secondary transfer position and the second secondary transfer position. The transfer fixer secondarily transfers the first toner image on the first intermediate transfer belt onto a first side of the sheet at the first secondary transfer position. The transfer fixer fixes the first toner image transferred to the first side of the sheet. Moreover, the transfer fixer secondarily transfers the second toner image on the second intermediate transfer belt onto a second side of the sheet at the second secondary transfer position. The second side is aside opposite to the first side. The transfer fixer fixes the second toner image transferred to the second side of the sheet. 
     The image forming apparatus and the image forming method disclosed herein are described below with reference to accompanying drawings in which identical reference signs denote identical components unless specified otherwise. 
     First Embodiment 
     The image forming apparatus and the image forming method according to the first embodiment are described below. 
       FIG. 1  is a schematic sectional view exemplifying the whole structure of an image forming apparatus according to the first embodiment.  FIG. 2  is a schematic sectional view exemplifying the structure of the transfer fixer of an image forming apparatus according to the first embodiment.  FIG. 3  is a block diagram exemplifying the functional structure of an image forming apparatus according to the first embodiment. 
     As shown in  FIG. 1 , according to the first embodiment, an image forming apparatus  100  comprises a control panel  1 , a scanner unit  2 , a printer unit  3 , a sheet feed unit  20 , a conveyance unit  5  and a control unit  6 . 
     The control panel  1  is operated by the user to activate the image forming apparatus  100 . 
     The scanner unit  2  reads the image information of a copied object as light intensity and outputs the read image information to the printer unit  3 . 
     The printer unit  3  forms an output image (hereinafter referred to as a toner image) using a toner-containing developing agent according to the image information received from the scanner section  2  or from the outside. 
     The printer unit  3  transfers the toner image onto the surface of a sheet S and heats and presses the toner image on the surface of the sheet S to fix the toner image on the sheet S. 
     The printer unit  3  prints on one or two sides of the sheet S. In single-side printing, an image is formed on either of the surfaces, that is, a first side P 1  of the sheet and a second side P 2  of the sheet S. In double-side printing, images are synchronously formed on both the surfaces, that is, the first side P 1  of the sheet and the second side P 2  of the sheet S. 
     The sheet feed unit  20  feeds sheets S, one by one, to the printer unit  3  matching with the formation of toner images by the printer unit  3 . The sheet feed unit  20  is provided with a paper cassette  20 A which accommodates sheets S. The sheet S is loaded into the paper cassette  20 A with the first side P 1  facing down. 
     The paper cassette  20 A is equipped with a paper feed roller  20   a  which picks up sheets S from the paper cassette  20 A one by one. The sheet S picked up is conveyed to the conveyance unit  5 . In  FIG. 1  which is a schematic diagram, the representation of the paper feed roller  20   a  is simplified. The paper feed roller  20   a  includes, for example, a plurality of rollers such as a pickup roller and a separating roller. 
     In addition to the paper cassette  20 A, the sheet feed unit  20  may be equipped with other paper cassettes to feed sheets S of other types and other sizes. The paper feed roller  20   a  is also arranged in the other paper cassettes like in the paper cassette  20 A. 
     The sheet feed unit  20  may further comprise a manual tray and a manual paper feeder. 
     The conveyance unit  5  comprises a conveyance guider  21 , a conveyance roller  22  and a register roller  23 . The conveyance guider  21  changes the conveyance path of the sheet S fed from the sheet feed unit  20  to be vertically upward. The conveyance roller  22  is configured above the conveyance guider  21 . The conveyance roller  22  conveys the sheet S vertically upwards fed from the conveyance roller  21  along a vertical plane. 
     The register roller  23  is configured over the conveyance roller  22 . The register roller  23  settles the position of the front end of the sheet S. The register roller  23  conveys the position-settled sheet S. The time at which the register roller  23  conveys the sheet is decided by the time at which the printer unit  3  secondarily transfers a toner image onto the sheet S. 
     The conveyance roller  22  collides the front end of the conveyance direction of the sheet S with the nip N of the register roller  23 . The conveyance roller  22  settles the position of the front end of the sheet S in the conveyance direction by curving the sheet S. 
     The register roller  23  fits the front end of the sheet S in the nip N. Moreover, the register roller  23  vertically conveys the sheet S upwards. The transfer fixer  24  which is described later is located above the register roller  23 . 
     Next, the detailed structure of the printer unit  3  is described below. 
     The printer unit  3  comprises an image forming section  4 A (a first image forming section), an intermediate transfer belt  16 A (a first intermediate transfer belt), a transfer belt cleaner  18 A, an image forming section  4 B (a second image forming section), an intermediate transfer belt  16 B (a second intermediate transfer belt), a transfer belt cleaner  18 B and a transfer fixer  24 . 
     The image forming section  4 A forms a toner image Tma (a first toner image) with a toner tm (a first toner). The toner tm has negative charges. The toner tm, to which no color limitations are given, is exemplarily black in the first embodiment. 
     The image forming section  4 A includes a photoconductive drum  10 A which has a photoconductive layer on the surface of the metal drum. The potential of the metal drum is the same as the reference potential of the image forming apparatus  100 . The positive/negative polarity of the potential mentioned hereinafter refers to a positive/negative polarity relative to the reference potential of the image forming apparatus  100 , if not specified otherwise. 
     The surface of the photoconductive drum  10 A is charged with negative charges by a charger  11 A which is described later. If the photoconductive layer is irradiated with light, then the charges of the exposed part are removed. 
     In the image forming section  4 A, a charger  11 A, an exposure portion  19 A, a developer  12 A, a primary transfer roller  13 A (a first primary transfer roller), a cleaner  15 A and a destaticizer  14 A are arranged around the photoconductive drum  10 A. The charger  11 A, the developer  12 A, the primary transfer roller  13 A, the cleaner  15 A and the destaticizer  14 A are orderly arranged along the clockwise direction shown in  FIG. 1 . 
     The primary transfer roller  13 A is opposite to the photoconductive drum  10 A, and the intermediate transfer belt  16 A is clamped between the primary transfer roller  13 A and the photoconductive drum  16 A. The external peripheral surface of the intermediate transfer belt  16 A clamped between the primary transfer roller  13 A and the photoconductive drum  16 A is a first primary transfer position. 
     The charger  11 A charges the surface of the photoconductive drum  10 A with negative charges. For example, the charger  11 A is a corona discharged charger, a pin electrode charger, a charging roller or the like. 
     The exposure portion  19 A irradiates the surface of the photoconductive drum  10 A with exposure light LA which is modulated based on an image signal sent from the scanner unit  2  or the outside. The image signal sent from the scanner unit  2  or the outside to the exposure portion  19 A is corresponding to an image formed on the first side P 1  of the sheet S. 
     The exposure portion  19 A forms an electrostatic latent image on the photoconductive drum  10 A according to the image signal. The irradiation position of the exposure light LA is between the configuration position of the charger  11 A and that of the developer  12 A which is described later. No specific limitations are given to the configuration position of the exposure portion  19 A as long as the exposure light LA from the exposure portion  19 A can be transmitted to the irradiation position. 
     For example, the exposure portion  19 A can scan with laser beams. For example, the exposure portion  19 A may be a solid scanner which uses an LED light emitting component. 
     The developer  12 A charges the toner tm with negative charges. If the developer  12 A applies a development bias voltage, then the electrostatic latent image on the photoconductive drum  10 A is developed by the toner tm. The toner tm attached on the photoconductive drum  10 A is formed into a toner image Tma according to the electrostatic latent image. 
     The developer  12 A may be a well-known two-component or one-component developer. 
     The primary transfer roller  13 A primarily transfers the toner image Tma attached on the photoconductive drum  10 A onto the intermediate transfer belt  16 A which is described later. A positive transfer voltage is applied to the primary transfer roller  13 A according to the time of the primary transfer. If the positive transfer voltage is applied to the primary transfer roller  13 A, then the toner image Tma is primarily transferred onto the intermediate transfer belt  16 A from the photoconductive drum  10 A. 
     The cleaner  15 A removes the residual toner left on the photoconductive drum  10 A and recycles the removed residual toner. 
     The destaticizer  14 A irradiates the surface of the photoconductive drum  10 A with light to remove the charges on the surface of the photoconductive drum  10 A. 
     The intermediate transfer belt  16 A consisting of an endless belt is arranged above the photoconductive drum  10 A. The intermediate transfer belt  16 A rotates in synchronization with the photoconductive drum  10 A in an opposite rotation direction. 
     The intermediate transfer belt  16 A made from an insulating material is thermally resistive to the fixation temperature of the transfer fixer  24  which is described later. To fix the toner tm on the sheet S, the sheet S is clamped between the intermediate transfer belt  16 A and an intermediate transfer belt  16 B which is described later. The intermediate transfer belt  16 A is pressed towards the intermediate transfer belt  16 B. Then, a nip with a width enough for the fixation of the toner tm is formed on the intermediate transfer belt  16 A. 
     The primary transfer roller  13 A is propped against the internal peripheral surface of the intermediate transfer belt  16 A. The intermediate transfer belt  16 A is pressed towards the surface of the photoconductive drum  10 A via the primary transfer roller  13 A at least during the primary transfer. 
     Further, a drive roller  16   a  and a transferring and fixing roller  17 A are propped against the internal peripheral surface of the intermediate transfer belt  16 A with the primary transfer roller  13 A clamped therebetween. 
     The drive roller  16   a  is arranged at the upstream side of the rotation direction of the intermediate transfer belt  16 A with respect to the primary transfer roller  13 A. The drive roller  16   a  rotationally drives the intermediate transfer belt  16 A. 
     The transferring and fixing roller  17 A is arranged at the downstream side of the rotation direction of the intermediate transfer belt  16 A with respect to the primary transfer roller  13 A. 
     The drive roller  16   a  and the transferring and fixing roller  17 A are driven by an intermediate transfer belt drive section  202 A (refer to  FIG. 3 ). 
     The intermediate transfer belt drive section  202 A comprises a drive motor (not shown) and a drive transmission portion (not shown). The intermediate transfer belt drive section  202 A matches the rotation direction and the rotational linear velocity with those of the transferring and fixing roller  17 A and the drive roller  16   a.    
     The transferring and fixing roller  17 A is arranged on the side of the first side P 1  of the sheet S conveyed by the register roller  23 . The transferring and fixing roller  17 A is opposite to the first side P 1  of the sheet S across the intermediate transfer belt  16 A. 
     The transferring and fixing roller  17 A constitutes one part of the transfer fixer  24  which is described later. The detailed structure of the transferring and fixing roller  17 A is described later. 
     The drive roller  16   a  and the transferring and fixing roller  17 A stretch the intermediate transfer belt  16 A. A tension roller  16   b  is propped against the internal peripheral surface of the intermediate transfer belt  16 A between the drive roller  16   a  and the transferring and fixing roller  17 A. The internal peripheral surface propped against the tension roller  16   b  is a side (the upper side shown in  FIG. 1 ) opposite to the internal peripheral surface propped against the primary transfer roller  13 A. 
     The tension roller  16   b  presses the internal peripheral surface of the intermediate transfer belt  16 A outwards. The tension roller  16   b  keeps the tension of the intermediate transfer belt  16 A fixed. 
     The transfer belt cleaner  18 A is arranged on the external periphery of the intermediate transfer belt  16 A. The transfer belt cleaner  18 A is opposite to the drive roller  16   a , and the intermediate transfer belt  16 A is clamped between the transfer belt cleaner  18 A and the drive roller  16   a.    
     The transfer belt cleaner  18 A removes the residual toner left on the external periphery of the intermediate transfer belt  16 A and recycles the residual toner. 
     The image forming section  4 B forms a toner image Tpa (a second toner image) with a toner tp (a second toner) with positive charges. The toner tp, to which no color limitations are given, has the same color with the toner tm in the first embodiment. 
     The image forming section  4 B includes a photoconductive drum  10 B which has a photoconductive layer on the surface of the metal drum thereof. The potential of the metal drum is the same as the reference potential of the image forming apparatus  100 . 
     The surface of the photoconductive drum  10 B is charged with positive charges by a charger  11 B which is described later. If the photoconductive layer is irradiated with light, then the charges of the exposed part are removed. 
     In the image forming section  4 B, a charger  11 B, an exposure portion  19 B, a developer  12 B, a primary transfer roller  13 B (a second primary transfer roller), a cleaner  15 B and a destaticizer  14 B are arranged around the photoconductive drum  10 B. The charger  11 B, the developer  12 B, the primary transfer roller  13 B, the cleaning portion  15 B and the destaticizer  14 B are orderly arranged along the counterclockwise direction shown in  FIG. 1 . 
     The primary transfer roller  13 B is opposite to the photoconductive drum  10 B, and the intermediate transfer belt  16 B is clamped between the primary transfer roller  13 B and the photoconductive drum  10 B. The position where the external peripheral surface of the intermediate transfer belt  16 B is clamped between the primary transfer roller  13 B and the photoconductive drum  10 B is a second primary transfer position. 
     The charger  11 B charges the surface of the photoconductive drum  10 B with positive charges. For example, the charger  11 B is a corona discharged charger, a pin electrode charger, a charging roller or the like. 
     The exposure portion  19 B irradiates the surface of the photoconductive drum  10 B with exposure light LB which is modulated based on an image signal sent from the scanner unit  2  or the outside. The image signal sent from the scanner unit  2  or the outside to the exposure portion  19 B is corresponding to an image formed on the second side P 2  of the sheet S. 
     The exposure portion  19 B forms an electrostatic latent image on the photoconductive drum  10 B according to the image signal. The irradiation position of the exposure light LB is between the configuration position of the charger  11 B and that of the developer  12 B which is described later. No specific limitations are given to the configuration position of the exposure portion  19 B as long as the exposure light LB from the exposure portion  19 B can be transmitted to the irradiation position. 
     The exposure portion  19 B is structurally identical to the exposure portion  19 A except for part placement. 
     The developer  12 B charges the toner tp with positive charges. If the developer  12 B applies a development bias voltage, then the electrostatic latent image on the photoconductive drum  10 B is developed by the toner tp. The toner tp attached on the photoconductive drum  10 B is formed into a toner image Tpa according to the electrostatic latent image. 
     The developer  12 B may be a well-known two-component or one-component developer. 
     The primary transfer roller  13 B primarily transfers the toner image Tpa attached on the photoconductive drum  10 B onto the intermediate transfer belt  16 B which is described later. A negative transfer voltage is applied to the primary transfer roller  13 B according to the time of the primary transfer. If the negative transfer voltage is applied to the primary transfer roller  13 B, then the toner image Tpa is primarily transferred onto the intermediate transfer belt  16 B from the photoconductive drum  10 B. 
     The cleaner  15 B removes the residual toner left on the photoconductive drum  10 B and recycles the removed residual toner. 
     The destaticizer  14 B irradiates the surface of the photoconductive drum  10 B with light to remove the charges on the surface of the photoconductive drum  10 B. 
     The intermediate transfer belt  16 B consisting of an endless belt is arranged above the photoconductive drum  10 B. The intermediate transfer belt  16 B rotates in synchronization with the photoconductive drum  10 B in an opposite rotation direction. 
     The intermediate transfer belt  16 B made from an insulating material is thermally resistive to the fixation temperature of the transfer fixer  24  which is described later. The intermediate transfer belt  16 B has elasticity. To fix the toner tp on the sheet S, the sheet S is clamped between the intermediate transfer belt  16 B and an intermediate transfer belt  16 A. The intermediate transfer belt  16 B is pressed towards the intermediate transfer belt  16 A. Then, a nip with a width enough for the fixation of the toner tp is formed on the intermediate transfer belt  16 B. 
     The primary transfer roller  13 B is propped against the internal peripheral surface of the intermediate transfer belt  16 B. The intermediate transfer belt  16 B is pressed towards the surface of the photoconductive drum  10 B via the primary transfer roller  13 B at least during the primary transfer. 
     Further, a drive roller  16   c  and a transferring and fixing roller  17 B are propped against the internal peripheral surface of the intermediate transfer belt  16 B with the primary transfer roller  13 B clamped therebetween. 
     The drive roller  16   c  is arranged at the upstream side of the rotation direction of the intermediate transfer belt  16 B with respect to the primary transfer roller  13 B. The drive roller  16   c  rotationally drives the intermediate transfer belt  16 B. 
     The transferring and fixing roller  17 B is arranged at the downstream side of the rotation direction of the intermediate transfer belt  16 B with respect to the primary transfer roller  13 B. 
     The drive roller  16   c  and the transferring and fixing roller  17 B are driven by an intermediate transfer belt drive section  202 B (refer to  FIG. 3 ). 
     The intermediate transfer belt drive section  202 B comprises a drive motor (not shown) and a drive transmission portion (not shown). The intermediate transfer belt drive section  202 B matches the rotation direction and the rotational linear velocity with those of the transferring and fixing roller  17 B and the drive roller  16   c.    
     The transferring and fixing roller  17 B is arranged on the side of the second side P 2  of the sheet S conveyed by the register roller  23 . The transferring and fixing roller  17 B is opposite to the second side P 2  of the sheet S across the intermediate transfer belt  16 B. 
     Further, the transferring and fixing roller  17 B is opposite to the transferring and fixing roller  17 A, and the intermediate transfer belts  16 B and  16 A are clamped between the transferring and fixing rollers  17 A and  17 B. The transferring and fixing rollers  17 A and  17 B are opposite to each other in the horizontal direction vertical to the conveyance direction of the sheet. 
     The transferring and fixing rollers  17 B and  17 A together constitute one part of the transfer fixer  24  which is described later. The detailed structure of the transferring and fixing roller  17 B is described later. 
     The drive roller  16   c  and the transferring and fixing roller  17 B stretch the intermediate transfer belt  16 B. A tension roller  16   d  is propped against the internal peripheral surface of the intermediate transfer belt  16 B between the drive roller  16   c  and the transferring and fixing roller  17 B. The internal peripheral surface propped against the tension roller  16   d  is a side (the upper side shown in  FIG. 1 ) opposite to the internal peripheral surface propped against the primary transfer roller  13 B. 
     The tension roller  16   d  presses the internal peripheral surface of the intermediate transfer belt  16 B outwards. The tension roller  16   d  keeps the tension of the intermediate transfer belt  16 B fixed. 
     The transfer belt cleaner  18 B is arranged on the external periphery of the intermediate transfer belt  16 B. The transfer belt cleaner  18 B is opposite to the drive roller  16   c , and the intermediate transfer belt  16 B is clamped between the transfer belt cleaner  18 B and the drive roller  16   c.    
     The transfer belt cleaner  18 B removes the residual toner left on the external periphery of the intermediate transfer belt  16 B and recycles the residual toner. 
     The transfer fixer  24  secondarily transfers the toner image Tmb on the intermediate transfer belt  16 A onto the first side P 1  of the sheet S and fixes the secondarily transferred toner image Tmb on the first side P 1  of the sheet S. Moreover, the transfer fixer  24  secondarily transfers the toner image Tpb on the intermediate transfer belt  16 B onto the second side P 2  of the sheet S. The transfer fixer  24  fixes the secondarily transferred toner image Tpb on the second side P 2  of the sheet S. 
     The toner image Tmb and the toner image Tpb are secondarily transferred and fixed at the same time in the transfer fixer  24 . 
     As shown in  FIG. 2 , the transfer fixer  24  comprises: transferring and fixing rollers  17 A and  17 B and intermediate transfer belts  16 A and  16 B passing between the transferring and fixing rollers  17 A and  17 B. The intermediate transfer belts  16 A and  16 B passing between the transferring and fixing rollers  17 A and  17 B are pressed by the transferring and fixing rollers  17 A and  17 B in opposite directions. Thus, the intermediate transfer belts  16 A and  16 B are deformed. A nip Nf is formed between the intermediate transfer belts  16 A and  16 B. 
     The nip Nf extends along the length direction of the transferring and fixing rollers  17 A and  17 B. 
     On the nip Nf, the surface of the intermediate transfer belt  16 A is the first secondary transfer position where the toner image Tmb is secondarily transferred onto the first side P 1  of the sheet S. Moreover, on the nip Nf, the surface of the intermediate transfer belt  16 B is the second secondary transfer position where the toner image Tpb is secondarily transferred onto the second side P 2  of the sheet S. 
     The path between the first primary transfer position and the first secondary transfer position which may not be as long as the path between the second primary transfer position and the second secondary transfer position is, however, as long as the path between the second primary transfer position and the second secondary transfer position in the image forming apparatus  100 . 
     Besides, the nip Nf is also an area in which the toner image Tpb is fixed on the second side P 2  of the sheet S while the toner image Tmb is fixed on the first side P 1  of the sheet S. 
     The width of the nip Nf in the conveyance direction F of sheet S is width within which the toner images Tmb and Tpb can be heated and pressurized to be synchronously fixed. 
     A conveyance guider  24   a  for guiding the conveyance of the sheet S is arranged below the nip Nf to guide the sheet S on a vertical plane passing the nip Nf. 
     Thermally insulating covers (not shown) may also be arranged around the transferring and fixing rollers  17 A ad  17 B and the intermediate transfer belts  16 A and  16 B. The thermally insulating covers are arranged at proper positions so as not to obstruct the conveyance of the sheet S. 
     The transferring and fixing roller  17 A comprises a metal roller  17 Aa and a heater  17 Ab. 
     The metal roller  17 Aa is formed into a hollow cylinder. The metal roller  17 Aa is rotated by the intermediate transfer belt drive section  202 A (not shown) in the counterclockwise direction shown in  FIG. 2 . The metal roller  17 Aa is rotated in synchronization with the drive roller  16   a.    
     Pressure springs (not shown) are arranged at two ends of the metal roller  17 Aa in the length direction of the metal roller  17 Aa to press the metal roller  17 Aa towards a metal roller  17 Ba. 
     A negative transfer voltage is applied to the metal roller  17 Aa. If the negative transfer voltage is applied to the metal roller  17 Aa, then the toner image Tmb is secondarily transferred onto the sheet S. The application of a voltage to the metal roller  17 Aa is controlled by the control unit  6  which is described later. 
     The control unit  6  secondarily transfers the toner image Tmb by applying a negative transfer voltage to the metal roller  17 Aa. 
     The metal roller  17 Aa is made of a metal having excellent conductivity. The higher the thermal conductivity of the metal roller  17 Aa is, the better the metal roller  17 Aa is. For example, the metal roller  17 Aa is made from aluminum alloy, ferrous alloy or copper alloy. 
     The metal roller  17 Aa is propped against the intermediate transfer belt  16 A serving as an insulator. It is not needed to coat the surface of the metal roller  17 Aa with an insulating coat. 
     However, the surface of the metal roller  17 Aa may be coated with various coatings. For example, a coating may be coated on the surface of the metal roller  17 Aa to change the frictional property between the surface of the metal roller  17 Aa and the intermediate transfer belt  16 A. For example, a protective coating may be coated on the surface of the metal roller  17 Aa to reduce the abrasion caused by the intermediate transfer belt  16 A. 
     The heater  17 Ab heats the metal roller  17 Aa. The quantity of the heat emitted from the heater  17 Ab is controlled by the control unit  6  which is described later. During a fixation process, the control unit  6  at least controls the temperature of the nip Nf to be a temperature needed for the fixation of the toner image Tmb. 
     The heater  17 Ab may be, for example, a halogen heater lamp, an induction heater (IH) or the like. 
     The transferring and fixing roller  17 B comprises a metal roller  17 Ba and a heater  17 Bb. 
     The metal roller  17 Ba and the heater  17 Bb are structurally identical to the metal roller  17 Aa and the heater  17 Ab of the transferring and fixing roller  17 A. 
     However, the metal roller  17 Ba is rotated by the intermediate transfer belt drive section  202 B (not shown) in the clockwise direction shown in  FIG. 2 . The metal roller  17 Ba is rotated in synchronization with the drive roller  16   c.    
     Besides, the metal roller  17 Ba rotates at the same speed with the metal roller  17 Aa. 
     Pressure springs (not shown) are arranged at two ends of the metal roller  17 Ba in the length direction of the metal roller  17 Ba. The pressure springs press the metal roller  17 Ba towards the metal roller  17 Aa. 
     A positive transfer voltage is applied to the metal roller  17 Ba. If the positive transfer voltage is applied to the metal roller  17 Ba, then the toner image Tpb is secondarily transferred onto the sheet S. The application of a voltage to the metal roller  17 Ba is controlled by the control unit  6 . 
     The control unit  6  secondarily transfers the toner image Tpb by applying a positive transfer voltage to the metal roller  17 Ba. 
     The heater  17 Bb heats the metal roller  17 Ba. The quantity of the heat emitted from the heater  17 Bb is controlled by the control unit  6  which is described later. During a fixation process, the control unit  6  at least controls the temperature of the nip Nf to be a temperature needed for the fixation of the toner image Tpb. 
     In the image forming apparatus  100 , the exposure position of the exposure line LA, the first primary transfer position and the first secondary transfer position are set on an image formation path at predetermined intervals. Moreover, the exposure position of the exposure line LB, the second primary transfer position and the second secondary transfer position are set on the image formation path according to the same relative position relation. 
     The printer unit  3  further comprises a paper discharging guider  25 , a paper discharging roller  26  and a paper discharging tray  27 . 
     The paper discharging guider  25  changes the conveyance direction of the fixed sheet S. 
     The fixed sheet S is vertically moved upwards towards the paper discharging guider  25  via the transfer fixer  24 . In the image forming apparatus  100 , the paper discharging guider  25  changes the conveyance direction of the fixed sheet S into a horizontal direction. 
     The paper discharging roller  26  discharges the sheet S passing the paper discharging guiders  25  onto the paper discharging tray  27 . 
     The paper discharging tray  27  holds the sheet S discharged from the paper discharging roller  26 . In an embodiment, the paper discharging tray  27  positioned above the image forming section  4 A horizontally holds the sheets S. 
     As shown in  FIG. 3 , the control unit  6  is connected with each unit of the image forming apparatus  100  in a communicable manner so as to control each unit of the image forming apparatus  100 . 
     The control carried out by the control unit  6  includes: the control on the scanner unit  2 , the control on the printer unit  3  and the control on the feed, the conveyance and the discharging of the sheet S. 
     The control unit  6  is connected with an input unit  200 , the printer unit  3 , the conveyance unit  5  and the sheet feed unit  20  in a communicable manner. The control unit  6  controls an image formation action based on an instruction input from the input unit  200 . 
     The input unit  200  comprises: a printer interface  201 , the foregoing control panel  1  and the foregoing scanner unit  2 . 
     The printer interface  201  is an interface serving when the image forming apparatus  100  functions as a printer. The printer interface  201  is connected with a communication line to send, via the communication line, an action instruction given from the image forming apparatus  100  and an image signal to be printed to the control unit  6 . 
     The control unit  6  controls actions of the exposure portion  19 A and the image forming section  4 A based on the one of the image signals input from the input unit  200  which indicates that a printing operation is to be implemented on the first side P 1  of the sheet S. 
     Meanwhile, the control unit  6  controls actions of the exposure portion  19 B and the image forming section  4 B based on the one of the image signals input from the input unit  200  which indicates that a printing operation is to be implemented on the second side P 2  of the sheet S. 
     The control unit  6  drives the drive rollers  16   a  and  16   c  and the metal rollers  17 Aa and  17 Ba through the intermediate transfer belt drive sections  202 A and  202 B. 
     The control unit  6  controls the transfer voltages applied to the primary transfer rollers  13 A and  13 B and the metal rollers  17 Aa and  17 Ba and the application time of the transfer voltages. 
     The control unit  6  changes the amount of the heat emitted from the heaters  17 Ab and  17 Bb based on an output from a temperature sensor (not shown) to control the temperature of the nip Nf. 
     The detailed content of the control carried out by the control unit  6  and the actions of the image forming apparatus  100  are described below together. 
     The control unit  6  structurally consists of proper hardware and a computer equipped with a CPU, a memory, an input/output interface and an external memory. The control unit  6  enables the computer to execute a control program to realize the foregoing control function. Alternatively, the control unit  6  activates the proper hardware to realize the foregoing control function. 
     The image forming apparatus  100  with the foregoing structure is an apparatus using an image forming method of the first embodiment. The actions of the image forming apparatus  100  are described below centering on those related to the image forming method of the first embodiment. 
       FIG. 4  is a schematic sectional view illustrating the actions of the transfer fixer of an image forming apparatus according to the first embodiment. 
     In the image forming apparatus  100 , an image formation instruction is input to the control unit  6  from the control panel  1  or the outside. 
     The control unit  6  determines to implement a single-side printing or a double-side printing based on an instruction from the input unit  200 . 
     A double-side printing action is described below first. A single-side printing action is described later. 
     In double-side printing, the image forming apparatus  100  forms an image on the surfaces, that is, a first side  21  of the sheet S and a second side P 2  of the sheet S. In the image forming apparatus  100 , the front ends of the images formed by the image forming sections  4 A and  4 B, after being primarily transferred, reach the nip Nf synchronously. 
     If the control unit  6  receives an image formation starting instruction from the input unit  200 , then a sheet is fed and conveyed by the sheet feed unit  20  and the conveyance unit  5 . The front end of the sheet reaching the register roller  23  is settled at a nip N. 
     After the front end of the sheet is settled at a nip N, the control unit  6  starts the image formation in the image forming sections  4 A and  4 B. The image formation process in the image forming section  4 A is merely different from the image formation process in the image forming section  4 B in charge characteristics of the toners tm and tp. 
     The following actions are carried out under the control of the control unit  6 . 
     The photoconductive drum  10 A ( 10 B) is rotated in the clockwise (counterclockwise) direction. The charger  11 A ( 11 B) charges the surface of the photoconductive drum  10 A ( 10 B) with the negative (positive) charges. The exposure unit  19 A ( 19 B) illuminates the surface of the photoconductive drum  10 A ( 10 B) with exposure light LA (LB) modulated based on an image signal. The charges of the part exposed by the exposure light LA (LB) are removed according to the quantity of illumination. The potential of the part exposed by the exposure light LA (LB) is changed from a charged potential to an exposed potential. In this way, an electrostatic latent image is formed on the surface of the photoconductive drum  10 A ( 10 B) based on the image signal. 
     The time at which the front end of the image is allowed to be exposed is marked as time t 0 . The position of the photoconductive drum  10 A ( 10 B) at which the exposure light LA (LB) arrives at t 0  is the front end of an image formation range. 
     The developer  12 A ( 12 B) develops the electrostatic latent image formed with the toner tm (tp). The toner tm (tp) is attached on the part of the photoconductive drum  10 A ( 10 B) where the potential is changed after an exposure. A toner image Tma (Tpa) is formed on the surface of the photoconductive drum  10 A ( 10 B) with the toner tm (tp). 
     The front end of the image formation range on the photoconductive drum  10 A ( 10 B) arrives at a first (second) primary transfer position after a given time elapses from t 0 . The time of the arrival of the front end of the image formation range on the photoconductive drum  10 A ( 10 B) at the first (second) primary transfer position is marked as t 1  (t 1 &gt;t 0 ). 
     On the other hand, the drive roller  16   a  ( 16   c ) and the transferring and fixing roller  17 A ( 17 B) are driven by the intermediate transfer belt drive section  202 A ( 202 B). Thus, the intermediate transfer belt  16 A ( 16 B) rotates at the same linear speed in an opposite direction with respect to the photoconductive drum  10 A ( 10 B). 
     The quantity of the heat emitted from the heater  17 Ab ( 17 Bb) is controlled by the control unit  6 . The heater  17 Ab ( 17 Bb) heats the metal roller  17 Aa ( 17 Ba). The heater  17 Ab ( 17 Bb) heats the intermediate transfer belt  16 A ( 16 B) propped against the metal roller  17 Aa ( 17 Ba) via the metal roller  17 Aa ( 17 Ba). The temperature of the nip Nf is a preset fixation temperature. 
     At this time, the metal roller  17 Aa and the intermediate transfer belt  16 A in the area of the nip Nf are mainly heated by the heater  17 Ab. The metal roller  17 Ba and the intermediate transfer belt  16 B in the area of the nip Nf are mainly heated by the heater  17 Bb. 
     For example, in the case where the fixer consists of a heat roller and a press roller, the heat of the heater in the heat roller is transferred to the press roller via the nip. The heat roller heats the press roller provided with no heat source. To stabilize the fixation temperature of the nip, it is needed to increase the temperature of the press roller to be equal to that of the heat roller. Apart from being consumed to melt a toner, the heat energy of the heat roller is also consumed to increase the temperature of the press roller and maintain the temperature of the press roller. As the press roller has a relatively large heat capacity, the heat energy consumed to increase the temperature of the press roller and maintain the temperature of the press roller is relatively large. 
     However, in the image forming apparatus  100  not equipped with a press roller having no heat source, the heaters  17 Ab and  17 Bb need not to heat a press roller. Thus, the power totally consumed by the heaters  17 Ab and  17 Bb is less than that consumed by the heater of a fixer consisting of a heat roller and a press roller. 
     A positive (negative) transfer voltage is applied to the primary transfer roller  13 A ( 13 B) when the front end of the image formation range reaches the first (second) primary transfer position (time t 1 ). The toner image Tma (Tpa) of the photoconductive drum  10 A ( 10 B) is charged with negative (positive) charges. The primary transfer roller  13 A ( 13 B) primarily transfers the toner image Tma (Tpa) onto the intermediate transfer belt  16 A ( 16 B). The toner image Tma (Tpa) is moved orderly on the intermediate transfer belt  16 A ( 16 B) to successively form the toner image Tmb (Tpb). 
     The residual toner left on the surface of the photoconductive drum  10 A ( 10 B) on which the toner image Tma (Tpa) are transferred is removed by the cleaner  15 A ( 15 B). The removed residual toner is recycled into the cleaner  15 A ( 15 B). 
     The photoconductive drum  10 A ( 10 B) is irradiated by light emitted from the destaticizer  14 A ( 14 B) so that the residual charges on the photoconductive drum  10 A ( 10 B) are removed. 
     The photoconductive drum  10 A ( 10 B) carries out the foregoing image formation action repeatedly. 
     On the other hand, the toner image Tmb (Tpb) transferred to the intermediate transfer belt  16 A ( 16 B) is moved together with the intermediate transfer belt  16 A ( 16 B). 
     As shown in  FIG. 2 , the front ends Iaf and Ibf of the image formation ranges in the intermediate transfer belts  16 A and  16 B are close to and the same path length from the nip Nf. The time of the arrival of the front ends Iaf and Ibf of the image formation ranges at the nip Nf is marked as time t 3  (t 3 &gt;t 2 ). 
     The control unit  6  drives the register roller  23  at ts in a range from the time t 0  to the time t 3 . The sheet S settled at the nip N of the register roller  23  is conveyed towards the transfer fixer  24 . 
     The time ts is the moment at which the front end Sf of the sheet S starts to be conveyed so that the front end Sf reaches the nip Nf at the moment t 3 ′, wherein t 3 ′=t 3 −Δt (Δt&gt;0), in which Δt is the conveyance error of the sheet S as well as the time needed for the formation of a margin on the front end of the sheet S. 
     Thus, the front end Sf of the sheet S is slightly clamped on the nip Nf at the time t 3 . 
     The control unit  6  starts to apply a negative (positive) transfer voltage to the metal roller  17 Aa ( 17 Ba) before the moment t 3 ′. If the metal roller  17 Aa ( 17 Ba) is applied with the transfer voltage, then an electric field is generated between the metal rollers  17 Aa and  17 Ba. The direction of the electric field is from the metal roller  17 Ba to the metal roller  17 Aa. 
     After the time t 3 , the toner image Tmb, the sheet S and the toner image Tpb are orderly clamped between the intermediate transfer belts  16 A and  16 B, as shown in  FIG. 4 . 
     The toner image Tmb with negative charges accepts an external force fA applied from the electric field between the metal rollers  17 Aa and the  17 Ba towards the first side P 1  so that the toner image Tmb is transferred onto the first side P 1 . 
     The toner image Tpb with positive charges accepts an external force fB applied from the electric field between the metal rollers  17 Aa and the  17 Ba towards the second side P 2  so that the toner image Tpb is transferred onto the second side P 2 . 
     Such a secondary transfer is started from an area nearby the nip Nf according to the magnitude of a transfer voltage. 
     On the other hand, the heat applied to the toner images Tmb and Tpb increases as the toner images Tmb and Tpb approaches the nip Nf. The toner images Tmb and Tpb are orderly softened. 
     In the nip Nf, the toner images Tmb and Tpb clamped by the intermediate transfer belts  16 A and  16 B are heated to a fixation temperature while being pressurized. Thus, the toner images Tmb and Tpb are melted in the nip Nf. The melted toner images Tmb and Tpb are fixed on the sheet S under the pressure in the nip Nf. 
     The sheet S is moved upwards from the nip Nf as the transferring and fixing rollers  17 A and  17 B are rotated. In this case, a fixed toner image TAc resulting from the fixation of the toner image Tmb is formed on the first side P 1  of the sheet S, and a fixed toner image TBc resulting from the fixation of the toner image Tpb is formed on the second side P 2  of the sheet S. 
     As shown in  FIG. 1 , the sheet S pulled off the transfer fixer  24  is changed in conveyance direction by the paper discharging guider  25 . Once reaching the front end of the paper discharging guider  25 , the sheet S is discharged to the paper discharging tray  27  by the paper discharging roller  26 . In the image forming apparatus  100 , the sheet S is discharged with the first side P 1  thereof facing the bottom side of the paper discharging tray  27 . 
     At this time, fixed toner images TAc and TBc are formed on the first side P 1  and the second side P 2  of the sheet S corresponding to image signals. 
     The above is description on double-side printing. 
     Then, a single-side printing action is described below. 
     In the image forming apparatus  100 , single-side printing is implemented by stopping the image formation action of the one of the image forming sections  4 A and  4 B which is not required to form an image. Which one of the image forming sections  4 A and  4 B is used to form an image can be selected through the input unit  200 . Alternatively, an image is formed by a fixed one of the image forming sections  4 A and  4 B. 
     As an example, in the image forming apparatus  100 , only the image forming section  4 A is used for single-side printing. 
     The single-side printing of the mage forming apparatus  100  is described below centering on the difference from double-side printing. 
     In single-side printing, the control unit  6  stops all the actions of the image formation section  4 B, including rotating the photoconductive drum  10 B. In single-side printing, the intermediate transfer belt drive section  202 B implements the same action as that in double-side printing. However, the primary transfer roller  13 B is freed from pressing the photoconductive drum  10 B. Thus, the intermediate transfer belt  16 B is not contacted with the photoconductive drum  10 B. 
     The control unit  6  applies no transfer voltage to the primary transfer roller  13 B. 
     In single-side printing, the transfer voltage is applied to the metal roller  17 Ba in the same way as that in double-side printing. 
     The control unit  6  reduces the quantity of the heat emitted from the heater  17 Bb in single-side printing. 
     In single-side printing, it is not needed to melt the toner image on the second side P 2 . Moreover, the heater  17 Bb only needs to heat the intermediate transfer belt  16 B which is in close contact with the metal roller  17 Ba having excellent thermal conductivity. Thus, the temperature of the intermediate transfer belt  16 B rises faster when compared with a case where a heater heats a press roller having a large thermal capacity from a nip. Thus, the temperature of the nip Nf can be kept at a proper fixation temperature even if the quantity of the heat emitted from the heater  17 Bb is reduced. 
     As stated above, the image forming apparatus  100  can print on both sides of a sheet S by causing the sheet S to pass the transfer fixer  24  once. Thus, the image forming apparatus  100  spends as much time in completing double-side printing as in completing single-side printing. In the image forming apparatus  100 , the time taken to reverse a sheet S one side of which is printed is saved. Thus, the time spent by the image forming apparatus  100  in completing double-side printing is half of that spent by the image forming apparatus  100  in the case where a sheet S passes a fixer twice. 
     Further, the transferring and fixing rollers  17 A and  17 B of the image forming apparatus  100  are equipped with heaters  17 Ab and  17 Bb, respectively. Thus, in both double-side printing and single-side printing, the power consumption of the image forming apparatus  100  is reduced when compared with the case where a heat roller and a press roller are combined in a fixer. Especially, the power consumption of the image forming apparatus  100  is nearly reduced by half in double-side printing. 
     Further, in the image forming apparatus  100 , secondary transfer and fixation are synchronously carried out in the transfer fixer  24 . Thus, the apparatus is smaller than an apparatus in which secondary transfer and fixation are carried out in different units. Further, as the transferring and fixing rollers  17 A and  17 B additionally function as secondary transfer rollers and fixing rollers, the number of components is reduced. 
     In the image forming apparatus  100 , transfer voltages which are opposite in polarity are applied to the metal rollers  17 Aa and  17 Ba, respectively. The toner image Tmb is transferred by a repulsive force from the metal roller  17 Aa in combination with an attractive force from the metal roller  17 Ba. The toner image Tpb is transferred by a repulsive force from the metal roller  17 Ba in combination with an attractive force from the metal roller  17 Aa. 
     Thus, the toner images Tmb and Tpb can be secondarily transferred effectively, thus preventing an uneven transfer. 
     In the image forming apparatus  100 , as the image forming section  4 B is stopped in single-side printing, the power consumed in single-side printing is reduced when compared to that consumed in double-side printing. 
     Second Embodiment 
     The image forming apparatus of a second embodiment is described. 
       FIG. 5  is a schematic sectional view exemplifying the whole structure of an image forming apparatus according to the second embodiment. 
     As shown in  FIG. 5 , the transfer fixer  24  of the image forming apparatus  100  of the first embodiment is replaced by a transfer fixer  44  in the image forming apparatus  101  of the foregoing the second embodiment. 
     The transferring and fixing rollers  17 A and  17 B in the transfer fixer  24  are replaced by secondary transfer rollers  37 A and  37 B in the transfer fixer  44 . Moreover, the transfer fixer  44  comprises an inlet guider plate  38  and fixing rollers  39 A and  39 B. 
     The second embodiment is described below centering on the difference from the first embodiment. 
     The secondary transfer roller  37 A ( 37 B) secondarily transfers the toner image Tmb (Tpb) onto the first side P 1  (the second side P 2 ) of the sheet S. If the toner image Tmb (Tpb) is secondarily transferred, then a toner image Tmc (Tpc) is formed on the first side P 1  (second side P 2 ). 
     The secondary transfer roller  37 A ( 37 B) is applied with a negative (positive) transfer voltage corresponding to the time of the secondary transfer. 
     The secondary transfer rollers  37 A and  37 B may be structured by deleting the heaters  17 Ab and  17 Bb from the transferring and fixing roller  17 A and  17 B of first embodiment. However, the secondary transfer roller  37 A ( 37 B) is not limited to be structured like this. For example, the secondary transfer roller  37 A ( 37 B) may be a well-known secondary transfer roller which applies a secondary transfer voltage from the inner side of an intermediate transfer belt. 
     The secondary transfer rollers  37 A and  37 B are arranged opposite to each other, like the transferring and fixing rollers  17 A and  17 B. The secondary transfer rollers  37 A and  37 B press each other in opposite directions, like the transferring and fixing rollers  17 A and  17 B. Thus, a nip Nt is formed between the intermediate transfer belts  16 A and  16 B clamped by the transferring and fixing rollers  37 A and  37 B. 
     The width of the nip Nt can be optionally set as long as the toner images Tmb and Tpb are secondarily transferred onto the sheet S while the sheet S is vertically driven upward. Thus, the mutual pressure between the secondary transfer rollers  37 A and  37 B is lower than that between the transferring and fixing rollers  17 A and  17 B. 
     The position of the intermediate transfer belt  16 A in the nip Nt is a first secondary transfer position. The position of the intermediate transfer belt  16 B in the nip Nt is a second secondary transfer position. 
     The inlet guider plate  38  guides the front end of the sheet S which passes the nip Nt and is vertically conveyed upward to the nip NF of the fixing rollers  39 A and  39 B which are described later. 
     The fixing roller  39 A ( 39 B) fixes the toner image Tmc (Tpc) on the first side P 1  (second side P 2 ) of the sheet S. For the fixation, the fixing roller  39 A ( 39 B) heats and pressurizes the toner image Tmc (Tpc). 
     The fixing rollers  39 A and  39 B are both provided with a metal roller the surface of which is coated with a rubber layer and a heater which emits heat inside the metal roller. The fixing rollers  39 A and  39 B press each other to form the nip NF. The fixing rollers  39 A and  39 B are rotated towards opposite directions by drivers (not shown). The fixing rollers  39 A and  39 B are capable of vertically conveying the sheet S clamped on the nip NF upward. 
     The rubbers on the surfaces of the fixing rollers  39 A and  39 B may be equal in hardness. In this case, as the first side P 1  and the second P 2  of the sheet S are equally pressed, the nip NF is flat. Thus, the sheet S is prevented from curving during a double-side printing process. 
     The sheets S conveyed by the fixing rollers  39 A and  39 B are conveyed into the paper discharging guider  25 . 
     Like the heaters of the transferring and fixing rollers  17 A and  17 B, the heaters in the fixing rollers  39 A and  39 B heat the nip NF so as to maintain a fixation temperature. 
     In the foregoing transfer fixer  44 , the secondary transfer rollers  37 A and  37 B and the fixing rollers  39 A and  39 B are configured adjacent to on the conveyance path of the sheet S. 
     The secondary transfer rollers  37 A and  37 B clamp the first side P 1  and the second P 2  of the sheet S across the intermediate transfer belts  16 A and  16 B. The secondary transfer rollers  37 A and  37 B secondarily transfer the toner image Tmb and Tpb onto the sheet S at the same time during double-side printing. 
     The fixing rollers  39 A and  39 B synchronously fix the secondarily transferred toner image Tmc and Tpc on the sheet S during double-side printing. 
     In this way, in the transfer fixer  44 , the first and the second secondary transfer positions are separated from the fixation position of the secondarily transferred toner images. 
     Thus, the image forming apparatus  101  is merely different from the image forming apparatus  100  in a secondary transfer action and a fixation action. 
     In the transfer fixer  44  of the image forming apparatus  101 , a fixation action is orderly carried out from where the secondary transfer action is ended. There is a time difference between the secondary transfer action and the fixation action. 
     However, the secondary transfer action is synchronously carried out for the first side P 1  and the second side P 2  at positions opposite to the secondary transfer rollers  37 A and  37 B. The fixation action is synchronously carried out for the first side P 1  and the second side P 2  at positions where the fixing rollers  39 A and  39 B are opposite to each other. 
     Through the secondary transfer action and the fixation action, the image forming apparatus  101  prints on two sides of the sheet S by causing the sheet S to pass the transfer fixer  44  once. Thus, the image forming apparatus  101  spends as much time in completing double-side printing as in completing single-side printing. In the image forming apparatus  101 , the time taken to reverse a sheet S one side of which is printed is saved. Thus, the time spent by the image forming apparatus  101  in completing double-side printing is half of that spent by the image forming apparatus  100  in the case where a sheet S passes a fixer twice. 
     Further, in the image forming apparatus  101 , opposite transfer voltages are applied to the second transfer rollers  37 A and  37 B in the secondary transfer action, which achieves the same effect as in the first embodiment. That is, the toner image Tmb is transferred by a repulsive force from the secondary transfer roller  37 A in combination with an attractive force from the secondary transfer roller  37 B. The toner image Tpb is transferred by a repulsive force from the secondary transfer roller  37 B in combination with an attractive force from the secondary transfer roller  37 A. 
     Thus, the toner images Tmb and Tpb can be secondarily transferred effectively, thus preventing an uneven transfer. 
     Further, the secondary transfer rollers  37 A and  37 B are symmetrical with respect to a plane across the sheet S. Thus, in the secondary transfer action, the pressures and the electric fields generated during the period the sheet S passes the secondary transfer rollers  37 A and  37 B are symmetrical by taking the sheet S as a symmetry plane. As a result, the sheet S is hardly conveyed in a disordered direction after the secondary transfer, thus preventing the sheet S from shaking with respect to the nip NF of the fixing rollers  39 A and  39 B and guaranteeing the entrance of the sheet S into the substantially center part of the inlet guider plate  38 . Consequentially, the friction of the non-transferred toner is prevented. 
     Further, the image forming apparatus  101  carries out the same actions with the image forming apparatus  100  in the single-side printing. Like the image forming apparatus  100 , the image forming apparatus  101  stops the image forming section  4 B during single-side printing. Thus, less power is consumed in single-side printing than in double-side printing. 
     Variations of the foregoing embodiments are described below. 
     In each foregoing embodiment, the image forming sections  4 A and  4 B separately form a monochrome image. However, a plurality of image forming sections  4 A and a plurality of primary transfer rollers  13 A may be separately arranged along the intermediate transfer belt  16 A. In this case, multicolor toner images can be formed by changing the color of each toner. For example, a full-color toner image can be formed by forming toner images of four colors of yellow, magenta, cyan and black. 
     If a plurality of image forming sections  4 B and a plurality of primary transfer rollers  13 B are separately arranged on the intermediate transfer belt  16 B, then a multicolor or a full-color image can also be formed in double-side printing. 
     It is described in each foregoing embodiment that the relative position relations among the exposure position, the primary transfer position and the secondary transfer position are the same. However, if the front ends of the image forming ranges on both sides of a sheet can reach a first and a second secondary transfer position at the same time, then the exposure position and each first primary transfer position may be different on different paths. 
     In accordance with at least one of the foregoing embodiments, an image forming apparatus comprises: a first image forming unit, a second image forming unit, a first intermediate transfer belt and a second intermediate transfer belt. The first image forming unit forms a first toner image with negative charges. The second image forming unit forms a second toner image with positive charges. The first intermediate transfer belt and the second intermediate transfer belt are opposite to each other at a first secondary transfer position and a second secondary transfer position. Further, the image forming apparatus comprises: a transfer fixer configured to transfer and fix the toner images on the first side and the second side of a sheet at a first transfer position and a second transfer position. Thus, the image forming apparatus can print on two sides of the sheet by causing the sheet to pass the transfer fixer once. The image forming apparatus shortens printing time and reduces the power consumed during a fixation process. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the invention. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the invention. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the invention.