Patent Publication Number: US-6904262-B2

Title: Double-faced image formation system

Description:
The present application claims the benefit of U.S. Provisional Patent Application No. 60/509,810 filed Oct. 10, 2003. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a double-faced image formation system that forms images on both sides of a sheet, specifically to a type of double-faced image formation system that forms images on one side of a recording medium, and then forms images on the other side of the same recording medium. 
   2. Description of the Related Art 
   Conventionally, an image formation system is known, in which are disposed in parallel plural image formation units provided with image carriers (for example, photosensitive drums) on which toner images are formed and retained, an intermediate transfer belt is provided which moves to circulate along the direction of each image formation units being arrayed, the images of each color components (for example, yellow, magenta, cyan, black) formed by each image formation units are sequentially primarily transferred onto the intermediate transfer belt, and thereafter the images superposed on the intermediate transfer belt are secondarily transferred onto a sheet in a lump. Of this type of image formation system, a double-faced image formation system that forms images on both sides of a sheet is known, in which, when the double face mode is selected, the images superposed on the intermediate transfer belt (front face images) are secondarily transferred onto the front face of the sheet, the front face images are fixed by a fixation device, after the sheet is reversed by means of a sheet reversing conveying mechanism, the sheet is reconveyed to a secondary transfer unit, the images superposed on the intermediate transfer belt (rear face images) are secondarily transferred onto the rear face of the sheet, and the rear face images are fixed by the fixation device. 
   And, the fixation device provides a pair of fixation members that rotate in contact with each other (for example, a heating roll and a pressure roll to press in contact with the heating roll), and passes the sheet through a nipping area between the fixation members to thereby fix the yet-to-be-fixed toner images on the sheet. A conventional fixation device is known which provides an oil coating applicator to apply oil as a lubricant on the surface of the heating roll in order to prevent the so-called offset phenomenon that the toner on the sheet transfers to the heating roll (refer to Patent Reference 1). 
   [Patent Reference 1] 
   Japanese Published Unexamined Patent Application No. 2000-267381 
   In the double-faced image formation system using this sort of fixation device, if only the surface of the heating roll has the oil applied, since the heating roll and the pressure roll rotate in contact with each other before the fixation processing, the surface of the pressure roll will have the oil applied indirectly. Accordingly, when fixing the front face images (yet-to-be-fixed toner images) secondarily transferred on the front face of the sheet, there occurs a situation that the oil is transferred and adhered on the rear face (the face that comes in contact with the pressure roll) of this sheet. At that moment, the oil is electrified to the positive or negative polarity on the energy level of molecule in many cases. Under such a situation, if the sheet is reversed and the rear face images are secondarily transferred on the rear face of this sheet, the oil electrified to the reverse polarity to the toner will be transferred to the surface of the intermediate transfer belt from the rear face of the sheet by the function of the secondary transfer field. However, the oil is difficult to transfer to the area (image area) where the toner images are present on the intermediate transfer belt by the influence of the secondarily transferred toner; accordingly, the oil will transfer only to the area (non-image area) where the toner images are not present on the intermediate transfer belt. 
   Now, supposing a case that one double-faced image is sequentially formed to 1000 sheets, for example, the oil hardly transfers to the area corresponding to the image area, which exists on the same position on the intermediate transfer belt, but the oil sequentially transfers only to the area corresponding to the non-image area. As the quantity of the oil transferred to the intermediate transfer belt increases, the surface energy of the oil-adhered area on the intermediate transfer belt remarkably varies, which influences the transfer performance of the toner images. To explain this in detail, after executing the double-faced image formation sequentially multiple times as to one image, when an image different from the same image, especially a half-tone pattern image is formed on the whole area, the transfer efficiency in the oil adhered area (corresponding to the non-image area) on the intermediate transfer belt is enhanced in comparison to the other area. As the result, the rear face image formed on the formation of the double-faced image directly before appears faintly in a negative image in the half-tone image next formed, thus creating the so-called ‘oil ghost’ phenomenon. 
   The above Patent Reference 1 discloses a technique that installs a static eliminator in the sheet reversing conveying mechanism, eliminates static electricity from oil adhered on the front and rear faces of a sheet with images formed on the front face thereof, and thereby restrains the transfer and adhesion of the oil to the intermediate transfer belt. 
   However, even in the case of using the technique disclosed in the Patent Reference 1, there is a possibility that the oil adhered on the sheet will be transferred and adhered to the intermediate transfer belt, which proved that the technique was yet insufficient to prevent the above oil ghost as the measure. To be more concrete, the technique disclosed in the Patent Reference 1 definitely eliminates static electricity from the electrified oil to thereby remove an electrostatic factor. However, since the oil adhered to the sheet comes directly in contact with the intermediate transfer part (image retaining conveyer), it is confirmed that a physical adhesive force makes the oil transfer and adhere from the sheet to the intermediate transfer part (image retaining conveyer). 
   SUMMARY OF THE INVENTION 
   The present invention has been made in view of the above technical problems, and the invention provides a technique that prevents a lubricant adhered to a recording medium from transferring and adhering to the image retaining conveyer or the intermediate transfer part. 
   The invention also provides a technique that prevents a lubricant adhered to a recording medium from transferring and adhering to the transfer member. 
   After earnest examinations by the inventor, by supplying a lubricant with the electrostatic adhesive force that excels in the physical adhesive force, it was found that the lubricant could be retained on a recording medium. This brought the inventor to an idea to solve the above problems, which made up the invention. 
   According to one aspect of the invention, the double-faced image formation system includes an image retaining conveyer that conveys to retain toner images, a transfer part that electrostatically transfers the toner images retained on the image retaining conveyer onto a recording medium, a fixation part that has a pair of fixation members disposed in contact with pressure and a lubricant feed member that supplies a lubricant on surfaces of the fixation members, and fixes the toner images transferred to the recording medium, a reversing conveying part that reverses front and rear faces of the recording medium having the toner images fixed thereon by the fixation part, and conveys it toward the transfer part, and an electrifying part that, when the recording medium is conveyed by the reversing conveying part, electrifies the lubricant adhered on the recording medium to a specific polarity. Here, the electrifying part may electrify the lubricant adhered on the rear face of the recording medium on which the toner images are not fixed to the same polarity as an electrified polarity of the toner images. Further, the electrifying part may electrify the lubricant adhered on the front face of the recording medium on which the toner images are fixed to a reverse polarity to the electrified polarity of the toner images. And, the image retaining conveyer may convey to retain color toner images having toners of plural color components superposed thereon. 
   According to another aspect of the invention, the double-faced image formation system includes one or plural image retaining conveyers, an intermediate transfer part disposed to face to the image retaining conveyer, a primary transfer unit that transfers toner images on the image retaining conveyer to the intermediate transfer part, a secondary transfer unit that transfers toner images on the intermediate transfer part to a recording medium, a fixation unit that fixes toner images transferred on the recording medium to the recording medium, a lubricant supply unit that supplies a lubricant to the fixation unit, a conveyance unit that reverses the front and rear faces of the recording medium having the toner images fixed on one side thereof by the fixation unit, and reconveys it to the secondary transfer unit, and an electrifying unit that electrifies the recording medium conveyed on the conveyance unit. Here, the electrifying unit may include a pair of contact electrifying members that nip the recording medium. And, a magnitude of an electrifying bias applied to the recording medium by the electrifying unit may be determined on the basis of any one of the characteristics of the recording medium, image density of the toner images next recorded on the recording medium, and the environmental conditions. Further the electrifying unit may electrify the front and rear faces of the recording medium to different polarities. And, the lubricant may be amine-denatured silicon oil. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the present invention will be described in detail based on the followings, wherein: 
       FIG. 1  illustrates an image formation system of the embodiment to which the invention is applied; 
       FIG. 2  is a block diagram illustrating a bias setting unit that sets electrifying biases by a front face electrifying unit and a rear face electrifying unit; 
       FIG. 3  is a typical illustration of a sheet with one side image already formed, which passes through the front face electrifying unit and the rear face electrifying unit; and 
       FIGS. 4A and 4B  are typical illustrations of a sheet that is resent into the secondary transfer unit through a sheet reversing conveying mechanism;  FIG. 4A  shows the case of this embodiment, and  FIG. 4B  shows the conventional case. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The preferred embodiment will now be described in detail with reference to the accompanying drawings. 
     FIG. 1  illustrates an image formation system relating to this embodiment. The image formation system illustrated in  FIG. 1  is the so-called tandem type, so-called intermediate decal type image formation system, which includes plural image formation units  10  ( 10 Y,  10 M,  10 C,  10 K) that form toner images of each color components by means of the electro-photographic system, an intermediate transfer belt (image retaining conveyer, intermediate transfer part  15  that allows the toner images of each colors formed by each of the image formation units  10  to be sequentially transferred (primary transfer) and retained thereon, a secondary transfer unit  20  that transfers in a lump the superimposed toner images transferred on the intermediate transfer belt  15  onto a sheet P being a recording medium, and a fixation unit  60  that fixes the secondary transferred images on the sheet P. Further, this image formation system includes a control unit  40  that controls the operations of each device (each unit) and a user interface UI  41  through which the instruction of operations by a user is implemented. 
   Each of the image formation units  10  ( 10 Y,  10 M,  10 C,  10 K) is provided with electro-photographic devices on the periphery of a photosensitive drum  11  as the image carrier that rotates in the direction of the arrow A, such as an electrifier  12  that electrifies the photosensitive drum  11 , a laser exposure device  13  that writes electrostatic latent images into the photosensitive drum  11  (the exposure beams are shown by the symbol Bm in the drawing), a development device  14  that contains the toner of each color and makes visible the electrostatic latent images on the photosensitive drum  11  by the toner, a primary transfer roll  16  as the primary transfer unit that transfers the toner image of each color formed on the photosensitive drum  11  onto the intermediate transfer belt  15 , and a drum cleaner  17  that cleans off a residual toner on the photosensitive drum  11 , etc. These image formation units  10  are disposed substantially linearly in the order of yellow (Y) magenta (M), cyan (C), black (K). This embodiment uses the color toners that are electrified to the negative polarity. 
   The intermediate transfer belt  15  being the intermediate transfer part (image retaining carrier of the toner image) uses a polyimide or polyamide resin having an appropriate amount of the antistatic agent such as a carbon black contained therein. The intermediate transfer belt  15  is made such that the volume resistivity thereof is within 10 6  to 10 14  Ω·cm, and is formed in a film-like endless belt with a thickness of about 0.1 mm. The intermediate transfer belt  15  is driven to circulate at a specific speed by means of various rolls in the direction B illustrated in the drawing. The various rolls include a drive roll  31  that drives to circulate the intermediate transfer belt  15 , which is driven by a motor exceeding in the constant speed performance (not illustrated), a support roll  32  that supports the intermediate transfer belt  15  extending substantially linearly along the direction of the photosensitive drums  11  being aligned, a tension roll  33  functioning as a compensation roll, which gives a constant tension to the intermediate transfer belt  15 , and prevents meandering of the intermediate transfer belt  15 , a backup roll  25  installed in the secondary transfer unit  20 , and a cleaning backup roll  34  that scratches off a residual toner on the intermediate transfer belt  15 . 
   Each of the primary transfer rolls  16  is arranged to face to each of the photosensitive drums  11  inside the intermediate transfer belt  15  extending substantially linearly. A voltage of reverse polarity to the electrified polarity of the toner (positive polarity in this embodiment) is applied to each of the primary transfer rolls  16 . Thereby, the toner images on each of the photosensitive drums  11  are sequentially electro-statically attracted into the intermediate transfer belt  15 , thus forming a superposed toner image on the intermediate transfer belt  15 . 
   The secondary transfer unit  20  is made up with a secondary transfer conveyance belt  21  positioned on the toner image retaining side of the intermediate transfer belt  15 , and the backup roll  25 , etc. The backup roll  25  is made of a blended rubber tube of EDPM and NBR, the surface thereof has a carbon dispersed, and the inside thereof has an EDPM rubber contained. The backup roll  25  is formed such that the surface resistivity thereof is 7 to 10 log Ω/□, and the diameter thereof is 28 mm, and the hardness thereof is 70°, for example, (ASCOR type C) The backup roll  25  is positioned on the rear side of the intermediate transfer belt  15 , and forms an electrode facing to the secondary transfer conveyance belt  21 . A metal feed roll  26  to which a secondary transfer bias is stably applied is attached in contact with the backup roll  25 . 
   On the other hand, the secondary transfer conveyance belt  21  is a semi-conductive endless belt having the volume resistivity of 10 6  to 10 10  Ω·cm, for example, which is stretched by a drive roll  22  and an idle roll  23 . The secondary transfer conveyance belt  21  is driven by the drive roll  22 , and is given a specific tension by the idle roll  23 . The drive roll  22  is positioned to press the backup roll  25 , in a state that the secondary transfer conveyance belt  21  and the intermediate transfer belt  15  are put between the two rolls, thus functioning as a secondary transfer roll that performs the secondary transfer to the sheet P carried on the secondary transfer conveyance belt  21 . The feed roll  26  is connected to a power supply  27  for the secondary transfer bias, which applies a specific negative secondary transfer bias to the feed roll  26 , and the drive roll  22  is grounded. 
   A belt cleaner  35  is located on the downstream viewing from the secondary transfer unit  20  of the intermediate transfer belt  15  so as to face to the cleaning backup roll  34  with the intermediate transfer belt  15  put in-between, which is in contact with and freely detachable from the intermediate transfer belt  15 . The belt cleaner  35  removes residual toners or paper powders on the intermediate transfer belt  15  after the secondary transfer, and cleans the surface of the intermediate transfer belt  15 . On the other hand, a reference sensor (home position sensor)  42  is located on the upstream of the yellow image formation unit  10 Y, which generates a reference signal for making exact timings with the image formations in the image formation units  10  ( 10 Y,  10 M,  10 C,  10 K). An image density sensor  43  for regulating the image quality is located on the downstream of the black image formation unit  10 K. The reference sensor  42  generates the reference signal on recognition of a specific mark attached on the rear side of the intermediate transfer belt  15 . The image formation units  10  ( 10 Y,  10 M,  10 C,  10 K) are designed to start the image formations according to the instructions from the control unit  40  based on the recognition of the reference signal. 
   Further, this embodiment includes, as the sheet conveyance system, a sheet tray  50  that contains the sheet P, a pickup roll  51  that takes out the sheet P accumulated in the sheet tray  50  at a specific timing, and sends it out to a conveyance path  55 , conveyance rolls  52  that convey the sheet P sent out by the pickup roll  51 , a conveyance shoot  53  that sends the sheet P conveyed by the conveyance rolls  52  into the secondary transfer unit  20 , and a conveyance belt  54  that conveys the sheet P toward the fixation unit  60  after the secondary transfer is performed by the secondary transfer conveyance belt  21 . Further, the image formation system contains a temperature/humidity sensor  57  that measures the temperature and humidity inside thereof. 
   The fixation unit  60  includes a heating roll  61  installed rotatably, which incorporates a heating source not illustrated, a pressure roll  62  placed rotatably in contact with pressure to the heating roll  61 , and an oil feeder (lubricant feeder, lubricant feed member)  63  that comes in contact with the heating roll  61  to supply oil (silicon oil) as a lubricant to the surface of the heating roll  61 . The heating roll  61  and the pressure roll  62  constitute a pair of fixation member. Here, this embodiment uses amine-denatured silicon oil for the silicon oil, which has a significant affinity with a fluoro-rubber and exhibits a high release property. 
   Especially, when the double face mode is selected, this embodiment provides a sheet reversing conveying mechanism  70  that reverses the sheet P having already completed one side of fixation by the fixation unit  60 , and returns it again to the secondary transfer unit  20 . This sheet reversing conveying mechanism  70  includes a branch path  71  branching downward against a discharge path  56  from the fixation unit  60 , a reversing path  72  extending to the right, provided with the branch path  71 , and a return path  73  formed to curve from this reversing path  72 , which returns to the conveyance path  55  starting with the sheet tray  50 ; and these paths are connected to communicate with each other. The mechanism  70  provides these paths with an appropriate number of conveyance rolls  74 , as the occasion arises. Further, the mechanism  70  provides a gate  75  on the exit of the fixation unit  60 , which switches the conveyance direction of the sheet P after fixation into the discharge path  56  or the branch path  71 , and provides a gate  76  on the branch point of the branch path  71  and the return path  73 , which switches the conveyance direction of the sheet P to the right before reversing, or to the left after reversing. The reversing path  72  is also provided with a reciprocally rotatably switchback roll  77  attached thereon. 
   Further, the return path  73  is provided with, as the electrifying unit, a front face electrifying unit  80  that electrifies the front face (the face of the toner image having already been fixed) of the reversed conveyed sheet P, and a rear face electrifying unit  90  that electrifies the rear face (the face of the toner image being next formed) of the reversed conveyed sheet P on the downstream in the sheet conveyance direction of the front face electrifying unit  80 . The front face electrifying unit  80  includes a front face electrifying roll  81  that is placed to come in contact with the front face of the sheet P, and a rear face roll  82  that is placed to face to the front face electrifying roll  81  and to come in contact with the rear face of the sheet P. Further, a front face electrifying power supply  83  applies a positive bias to the front face electrifying roll  81 , and the rear face roll  82  is grounded. On the other hand, the rear face electrifying unit  90  includes a rear face electrifying roll  91  that is placed to come in contact with the rear face of the sheet P, and a front face roll  92  that is placed to face to the rear face electrifying roll  91  and to come in contact with the front face of the sheet P. Further, a rear face electrifying power supply  93  applies a negative bias to the rear face electrifying roll  91 , and the front face roll  92  is grounded. Here, the front face electrifying roll  81 , rear face roll  82 , rear face electrifying roll  91 , and front face roll  92  as the contact electrifying member can be selected appropriately among metal rolls, rubber rolls, and so forth. 
     FIG. 2  illustrates a block diagram of a bias setting unit  100  that sets the electrifying biases by the front face electrifying unit  80  and the rear face electrifying unit  90 . The bias setting unit  100  forms a function of the control unit  40 . A CPU  101  of the bias setting unit  100  follows the program stored in a ROM  102 , and executes the processing while exchanging data appropriately between the ROM  10 . 2  and a RAM  103 . The bias setting unit  100  is supplied with information such as the sheet type, basic weight, and size of the sheet P inputted from the UI  41 , the image density measured by the image density sensor  43 , and the temperature/humidity measured by the temperature/humidity sensor  57 , by way of an input interface  104 . On the other hand, the bias setting unit  100  controls the magnitudes of the electrifying biases of the front face electrifying power supply  83  and the rear face electrifying power supply  93 , by way of an output interface  105 . 
   Next, the basic process of the image formation in the image formation system relating to this embodiment will be described. The image data outputted from an image reading device not illustrated, a personal computer not illustrated, and the like are inputted to the image formation system as illustrated in FIG.  1 . The image formation system executes operations to form images by the image formation units  10 , after completion of a specific image processing by an image processing system (IPS) not illustrated. The image processing system (IPS) executes to inputted reflectance data the given image processing of various image editing such as shading correction, position error correction, brightness/color space conversion, gamma correction, frame erasing and color editing, and motion editing, etc. The image data having the image processing applied thereto are converted into the color material gradation data of the four colors: yellow (Y), magenta (M), cyan (C), black (B), and the converted data are outputted to the laser exposure device  13 . 
   The laser exposure device  13  irradiates, according to the color material gradation, for example, each of the photosensitive drums  11  of the image formation units  10 Y,  10 M,  10 C, and  10 K data with the exposure beams Bm emitted from a semiconductor laser. After the electrifier  12  electrifies the surface of each of the photosensitive drums  11  of the image formation units  10 Y,  10 M,  10 C, and  10 K, the laser exposure device  13  applies a scanning exposure to the surface thereof to form the electrostatic latent images. The electrostatic latent images formed are developed into the toner images of each of the colors: yellow (Y), magenta (M), cyan (C), and black (B), by the image formation units  10 Y,  10 M,  10 C, and  10 K. 
   The toner images formed on the photosensitive drums  11  of the image formation units  10 Y,  10 M,  10 C, and  10 K are transferred on the intermediate transfer belt  15  at respective positions where the photosensitive drums  11  come in contact with the intermediate transfer belt  15 . To be more concrete, in the primary transfer unit, a voltage of the reverse polarity to the electrified polarity of the toners is applied to the base material of the intermediate transfer belt  15  at each of the primary transfer rolls  16 , and the toner images not yet fixed are sequentially superposed on the surface of the intermediate transfer belt  15 ; thus, the primary transfer is carried out. 
   The yet-to-be-fixed toner images thus primarily transferred are conveyed to the secondary transfer unit  20  accompanied with the rotation of the intermediate transfer belt  15 . 
   On the other hand, in the sheet conveyance system, the pickup roll  51  rotates in exact timings with the image formations to supply the sheet P of a specific size from the sheet tray  50 . The sheet P supplied by the pickup roll  51  is conveyed on the conveyance path  55  by the conveyance rolls  52  to pass through the conveyance shoot  53  and reach the secondary transfer unit  20 . Before reaching the secondary transfer unit  20 , the sheet P is temporarily halted. In an exact timing with the move of the intermediate transfer belt  15  having the toner images retained thereon as mentioned above, a resist roll (not illustrated) rotates, whereby the sheet P is aligned with the toner images. 
   In the secondary transfer unit  20 , the drive roll  22  presses the backup roll  25  through the semi-conductive secondary transfer conveyance belt  21  and the intermediate transfer belt  15 . At that moment, the sheet P conveyed in an exact timing slides in between the intermediate transfer belt  15  and the secondary transfer conveyance belt  21 . Here, as a voltage of the same polarity (negative polarity in this embodiment) with the electrified polarity of the toner is applied to the feed roll  26 , a transfer field is generated with the secondary transfer conveyance belt  21  as the facing electrode, and the yet-to-be-fixed toner images retained on the intermediate transfer belt  15  are electrostatically transferred to the sheet P at the secondary transfer position where the yet-to-be-fixed toner images are pressed by the drive roll  22  and the backup roll  25 . 
   Thereafter, the secondary transfer conveyance belt  21  conveys the sheet P having the toner images electrostatically transferred thereon, while it is peeled off the intermediate transfer belt  15 , to the conveyance belt  54  provided on the downstream in the sheet conveyance direction of the secondary transfer conveyance belt  21 . The conveyance belt  54  conveys the sheet P to the fixation unit  60  to keep pace with the optimum conveyance speed in the fixation unit  60 . The yet-to-be-fixed toner images on the sheet P having been conveyed to the fixation unit  60  undergo fixation processing with heat and pressure by the fixation unit  60 . Thereby, the yet-to-be-fixed toner images are fixed on the sheet P, and the sheet P with the fixed images formed is forwarded to the discharge path  56  according to the gate  75  to be discharged outside the system by a discharge roll (not illustrated). On the other hand, after completing the transfer to the sheet P, the residual toner remaining on the intermediate transfer belt  15  is conveyed to the cleaning unit with the rotation of the intermediate transfer belt  15 , where the residual toner is removed from the intermediate transfer belt  15  by the cleaning backup roll  34  and the belt cleaner  35 . 
   When the images are to be formed on both the faces of the sheet P, the front tip of the sheet P passed through the fixation unit  60  enters into the branch path  71  according to the gate  75 . After conveyed on the branch path  71 , the sheet P enters into the reversing path  72  according to the gate  76 . After the sheet P is conveyed toward the inner part of the reversing path  72  by the switchback roll  77 , the sheet P temporarily stops at the timing when the back tip of the sheet P has just passed the gate  76 . Thereafter, the switchback roll  77  is reverse rotated at a specific timing, whereby the sheet P is now conveyed toward the reverse direction. The sheet P enters into the return path  73  according to the gate  76 . After passing through the front face electrifying unit  80  and the rear face electrifying unit  90 , the sheet P conveyed on the return path  73  is made to return to the conveyance path  55 . Here, the front and rear faces of the sheet P is reversed to the state that the sheet P is initially placed on the conveyance path  55 . And, according to the abovementioned process, the yet-to-be-fixed toner images are electrostatically transferred to the rear face of the sheet P this time, and are fixed by the fixation unit  60 . The sheet P with the toner images fixed is discharged through the discharge path  56  to the outside of the system. 
   Next, the operations of the front face electrifying unit  80  and the rear face electrifying unit  90  that are provided on the return path  73  will be described in detail.  FIG. 3  typically illustrates the sheet P with an image already formed on one side, which passes through the front face electrifying unit  80  and the rear face electrifying unit  90  that are provided on the return path  73 . 
   The sheet P with the images fixed on one side has oil Q adhered, when the sheet P passed the fixation unit  60  (see FIG.  1 ). Here in this description, the front side (the side with the toner image G already fixed) of the sheet P is called ‘front face P 1 ’, and the backside (the side with the toner image G not yet formed) is called ‘rear face P 2 ’. As to the oil Q adhered to the sheet P, the oil adhered to the front face P 1  is called ‘front-face-adhered oil Q 1 ’, and the oil adhered to the rear face P 2  is called ‘rear-face-adhered oil Q 2 ’. 
   The amine-denatured silicon oil used for the oil Q in this embodiment has the property that the oil is electrified to the positive polarity when it adheres to the sheet P. Accordingly, on the upstream side of the front face electrifying unit  80 , both the front-face-adhered oil Q 1  and the rear-face-adhered oil Q 2  are electrified to the positive (the symbol ‘+’ is attached in the drawing). When the sheet P passes through the front face electrifying unit  80 , positive charges are injected from the front face electrifying roll  81  into the front-face-adhered oil Q 1  adhered on the front face P 1  of the sheet P, which further increases the positively electrified charges of the front-face-adhered oil Q 1 . When the sheet P passes through the rear face electrifying unit  90 , negative charges are injected from the rear face electrifying roll  91  into the rear-face-adhered oil Q 2  adhered on the rear face P 2  of the sheet P, and the electrified polarity of the rear-face-adhered oil Q 2  changes from the positive polarity into the negative (the symbol ‘−’ is attached in the drawing) Therefore, after the sheet P passes through the front face electrifying unit  80  and the rear face electrifying unit  90 , the front-face-adhered oil Q 1  adhered on the front face P 1  is electrified to the positive polarity, and the rear-face-adhered oil Q 2  adhered on the rear face P 2  is electrified to the negative polarity. That is, the front face P 1  and the rear face P 2  of the sheet P are to be electrified to different polarities. 
   Here, the bias setting unit  100  illustrated in  FIG. 2  sets the magnitude of the front face electrifying bias applied by the front face electrifying power supply  83  of the front face electrifying unit  80 , and the magnitude of the rear face electrifying bias applied by the rear face electrifying power supply  93  of the rear face electrifying unit  90 . To explain this concretely, when receiving the information of sheet that the sheet P is thick paper from the UI  41 , when receiving the information of image density that the image density is high from the image density sensor  43 , and when receiving the information of environment that the ambient temperature and humidity are high from the temperature/humidity sensor  57 , the bias setting unit  100  sets the absolute values of the front face electrifying bias and the rear face electrifying bias. 
     FIG. 4A  typically illustrates the sheet P that is resent into the secondary transfer unit  20  through the sheet reversing conveying mechanism  70 . In this embodiment, the toner T is electrified to the negative polarity, as mentioned above, the secondary transfer bias of the negative polarity is applied to the backup roll  25  (see  FIG. 1 ) provided on the rear side of the intermediate transfer belt  15 , and the drive roll  22  (see  FIG. 1 ) provided on the rear side of the secondary transfer conveyance belt  21  is grounded; and thereby, a secondary transfer field E is generated, and this functions to transfer the toner T to the sheet P. Since the rear-face-adhered oil Q 2  adhered on the rear face P 2  of the sheet P facing to the intermediate transfer belt  15  is electrified to the negative polarity, the oil Q 2  receives an electrostatic force directed toward the sheet P by the secondary transfer field E, thereby maintaining the state that the oil Q 2  remains adhered on the sheet P. Therefore, the transfer and adhesion of the rear-face-adhered oil Q 2  to the intermediate transfer belt  15  can be prevented. On the other hand, since the front-face-adhered oil Q 1  adhered on the front face P 1  of the sheet P facing to the secondary transfer conveyance belt  21  is electrified to the positive polarity, the oil Q 1  receives an electrostatic force directed toward the sheet P by the secondary transfer field E, which also maintains the state that the oil Q 1  remains adhered on the sheet P. Therefore, the transfer and adhesion of the front-face-adhered oil Q 1  to the secondary transfer conveyance belt (transfer belt)  21  can be prevented as well. 
     FIG. 4B  illustrates a case that does not contain the front face electrifying unit  80  and the rear face electrifying unit  90 , namely, a conventional example in which the sheet P is resent into the secondary transfer unit  20  in a state that the front-face-adhered oil Q 1  and the rear-face-adhered oil Q 2  are electrified to the positive polarity. In this example, the rear-face-adhered oil Q 2  is electrified to the positive polarity, and the oil Q 2  receives an electrostatic force directed toward the intermediate transfer belt  15  by the secondary transfer field E. It is therefore understood that the oil Q 2  is transferred and adhered to the intermediate transfer belt  15  from the rear face P 2  of the sheet P. 
   In this embodiment, the rear-face-adhered oil Q 2  adhered on the rear face P 2  of the sheet P is electrified to the same polarity as the toner T (the reverse polarity to the electrified polarity of the oil Q), and thereafter the secondary transfer is executed to the rear face P 2 . Thereby, the transfer and adhesion of the rear-face-adhered oil Q 2  to the intermediate transfer belt  15  can be restricted, and the occurrence of the oil ghost can be prevented. 
   In this embodiment, the front-face-adhered oil Q 1  adhered on the front face P 1  of the sheet P is electrified at a high level to the reverse polarity to the toner T (the same polarity as the electrified polarity of the oil Q), and thereafter the secondary transfer is executed to the rear face P 2 . Thereby, the transfer and adhesion of the front-face-adhered oil Q 1  to the secondary transfer conveyance belt  21  can be restricted, and the occurrence of discrepancies accompanied with the adhesion of the oil Q to the secondary transfer conveyance belt  21  (for example, degradation of the secondary transfer conveyance belt  21 , and difficulty in removing the toner adhered on the secondary transfer conveyance belt  21 ) can be prevented. 
   Further in this embodiment, since the front face electrifying unit  80  and rear face electrifying unit  90  use roll materials for the electrifying member, the system generates very little ozone and so forth, which proves the system safe. 
   And, in this embodiment, according to the information of the sheet P, image density formed, and temperature/humidity, the front face electrifying bias applied to the sheet P by the front face electrifying unit  80  and the rear face electrifying bias applied to the sheet P by the rear face electrifying unit  90  are designed to be adjustable appropriately. Therefore, the system is able to implement a satisfactory double-faced image formation, while selecting the conditions of obstructing the transfer and adhesion of the oil Q adhered on the sheet P to the intermediate transfer belt  15  and the secondary transfer conveyance belt  21 . 
   In this embodiment, the tandem type image formation system has been described, however the invention is not limited to this. The invention is also applicable to the so-called four-cycle type image formation system, in which the intermediate transfer belts are placed to face to one photosensitive drum, and the toner images of each colors are sequentially formed on the photosensitive drum to be transferred and superposed sequentially on the intermediate transfer belts. 
   Also in this embodiment, the intermediate decal type image formation system has been described, however the invention is not limited to this. The invention is also applicable to a type of image formation system, in which four photosensitive drums are arranged in parallel, sheet conveyance belts are provided to face to each of these photosensitive drums, and the toner images formed on each of the photosensitive drums are sequentially transferred to the sheet conveyed on the sheet conveyance belts. 
   According to the invention, the transfer and adhesion of a lubricant adhered on a recording medium to the image retaining conveyer and the intermediate transfer part can be prevented. 
   The entire disclosure of Japanese Patent Application No. 2003-041066 filed on Feb. 19, 2003 including specification, claims, drawings and abstract is incorporated herein by reference in its entirety.