Patent Publication Number: US-6701100-B2

Title: Image forming apparatus including an image carrier and a polarization uniforming structure

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a copier, printer, facsimile apparatus or similar image forming apparatus of the type including an intermediate image transfer body intervening between an image carrier and a sheet or recording medium as to image transfer. 
     2. Description of the Background Art 
     An image forming apparatus of the type described is implemented as, e.g., a color copier or a color laser printer in which toner images of different colors are transferred from an image carrier to an intermediate image transfer body one above the other (primary transfer) and then collectively transferred to a sheet (secondary transfer). The intermediate image transfer body is usually formed of a high-molecular material having a preselected mechanical characteristic and a preselected electrostatic characteristic. The problem with this type of image forming apparatus is that toner scatters around a toner image transferred from the image carrier to the intermediate image transfer body. 
     To obviate the scattering of the toner at the time of primary transfer, the intermediate image transfer body may include a high-resistance layer having a volume resistivity of 10 10  Ω·cm or above. Such an intermediate image transfer body allows the potential of a latent image to be transferred from the image carrier thereto and held thereon together with the toner image. The transferred potential prevents the toner from scattering around the toner image transferred to the intermediate image transfer body. 
     However, the intermediate image transfer body with the high-resistance layer brings about the following problem. When a plurality of toner images are transferred to the same area of the intermediate image transfer body one above the other, the history of potential contrast images remain in the high-resistance layer in accordance with the presence/absence of toner on the image carrier and sheet. A potential contrast image left in the high-resistance layer is difficult to discharge and is apt to remain up to the next image forming cycle. As a result, when a highlight image or similar image with low image density (ID) is formed later, a residual image corresponding to the potential contrast image is likely appear in the low ID image. 
     We found by a series of researches and experiments that even when the intermediate image transfer body was discharged from the outside, a potential distribution remained in the body and caused a residual image to appear in an image later. Further, the potential distribution was apt to remain in the high-potential layer, which formed part of a laminate structure. 
     The residual charge in the intermediate image transfer body is difficult to remove with charging means that applies a DC voltage opposite to the conventional image transfer bias to the intermediate image transfer body. If the size of the DC voltage is increased, then the residual charge in the intermediate image transfer body may be discharged to a certain degree. However, such a DC voltage is likely to damage the surface of the intermediate image transfer body to a critical degree. While an AC voltage with a great amplitude may effectively discharge the residual charge, it increases the current to 1 mA or so, which is greater than several microamperes to several ten microamperes of the DC voltage. This is also likely to damage the surface of the intermediate image transfer body, and moreover increases the cost. 
     Particularly, as for the intermediate image transfer body with the high-resistance layer, the conventional discharging means described above simply causes the surface potential of the body to vary and cannot directly apply a bias to the high-resistance layer. It is therefore difficult to discharge the high-resistance layer with the conventional discharging means. Moreover, the conventional discharging means is apt to critically damage the surface layer. 
     Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 6-194967, 9-204107 and 11-231687. 
     SUMMARY OF THE INVENTION 
     It is a first object of the present invention is to provide an image forming apparatus capable of surely obviating, even when an intermediate image transfer body with a high-resistance layer is used, a residual image ascribable to polarization, which is left in the high-resistance layer, before primary image transfer. 
     It is a second object of the present invention to provide an image forming apparatus capable of surely obviating, even when an intermediate image transfer body of the kind is used, a residual image ascribable to polarization, which is left in the high-resistance layer, after an image forming operation. 
     An image forming apparatus of the present invention includes an image carrier, a latent image forming device for forming a latent image on the image carrier, and a developing device for developing the latent image with toner to thereby form a corresponding toner image. An intermediate image transfer body has a movable surface and includes an high-resistance layer whose volume resistivity is 10 10  Ω·cm or above. A primary image transferring device transfers the toner image from the image carrier to the intermediate image transfer body. A secondary image transferring device transfers the toner image from the intermediate image transfer body to a recording medium. A polarization uniforming device uniforms, at the beginning of an image forming operation, polarization left in the high-resistance layer while preserving its polarity after the surface of the intermediate image transfer body has started moving, but before the toner image is transferred from the image carrier to the secondary image transfer body. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description taken with the accompanying drawings in which: 
     FIG. 1 is a front view showing a color copier embodying the present invention; 
     FIG. 2 is a fragmentary front view of the color copier; 
     FIG. 3 is a fragmentary section of an intermediate image transfer belt included in the illustrative embodiment; 
     FIG. 4 is a view showing a specific arrangement for measuring a potential left an intermediate image transfer belt formed of PVDF (polyvinylidene fluoride); 
     FIG. 5 is a graph showing a relation between the duration of a discharging bias (log T) and the surface potential measured with the arrangement of FIG. 4 on the elapse of a preselected period of time since the application of the above bias; 
     FIG. 6 is a table listing various biases for initial saturation polarization and discharging biases; 
     FIG. 7 is a schematic block diagram showing essential part of a control system included in the illustrative embodiment; 
     FIG. 8 is a timing chart demonstrating a specific operation of the illustrative embodiment; 
     FIG. 9 is a table listing residual image ranks determined at two points on an image by varying pre-bias unique to the illustrative embodiment; 
     FIGS. 10A through 10C are views for describing how the pre-bias obviates a residual image; 
     FIGS. 11A through 11C are views demonstrating a mechanism in which a comparative example causes a residual image to appear; 
     FIG. 12 is a front view showing a modification of the illustrative embodiment; 
     FIG. 13 is a front view showing another modification of the illustrative embodiment; 
     FIG. 14 is a timing chart demonstrating a specific operation of an alternative embodiment of the present invention; and 
     FIG. 15 is a table comparing the alternative embodiment and a comparative example with respect to residual image. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1 of the drawings, an image forming apparatus embodying the present invention is shown and implemented as an electrophotographic color copier by way of example. This embodiment is mainly directed toward the first object stated earlier. As shown, the copier is generally made up of a color scanner or color image reading device  1 , a printer or color image recording device  2 , and a sheet bank  3 . 
     The color scanner  1  includes a lamp  122 , mirrors  123   a ,  123   b  and  123   c , and a lens  124 . While the lamp  122  illuminates a document  4  laid on a glass platen  121 , the resulting reflection representative is focused on a color sensor  125  via the mirrors  123   a  through  123   c  and lens  124 . The color sensor  125  reads color image information color by color, e.g., on a R (red), G (green) and B (blue) basis while outputting a corresponding electric image signal. Specifically, the color sensor  125  is made up of R, G and B color separating means and a CCD (Charge Coupled Device) array or similar photoelectric transducer and reads R, G and B image data separated by the separating means at the same time. An image processing section, not shown, converts the R, G and B image signals to Bk (black), C (cyan), M (magenta) and Y (yellow) color image data. 
     More specifically, in response to a scanner start signal synchronous to the operation of the color printer  2 , the color scanner  1  causes the lamp  122  and mirrors  123   a  through  123   c  to move in a direction indicated by an arrow in FIG. 1 while scanning the document  4 . By one time of scanning, the color scanner  1  outputs four different color image data. The color printer  2  sequentially forms toner images of four different colors and superposes them on each other to thereby produce a four-color or full-color toner image. 
     The color printer  2  includes a photoconductive drum or image carrier  200 , an optical writing unit  220 , developing means implemented as a revolver  230 , an intermediate image transferring unit  500 , a secondary image transferring device  600 , and a fixing device  270 . The drum  200  is rotatable counterclockwise, as indicated by an arrow in FIG.  1 . Arranged around the drum  200  are a drum cleaner  201 , a discharge lamp  202 , a charger  203 , a potential sensor  204  and a density pattern sensor  205  as well as the intermediate image transfer unit  500  and secondary image transferring device  600 . In addition, the revolver  230  is located such that one of its developing sections, which will be described specifically later, adjoins the drum  200 . The charger  203  and optical writing unit  220  constitute latent image forming means. 
     The writing unit  200  converts the color image data output from the color scanner  1  to a corresponding optical signal and scans the drum  200  with the optical signal to thereby form a latent image, which is representative of the document image. The writing unit  220  includes a semiconductor laser or light source  221 , a laser driver, not shown, a polygonal mirror  222 , a motor  223  for rotating the mirror  222 , an f/θ lens  224 , and a mirror  225 . 
     The revolver  230  includes a Bk developing section  231 K, a C developing section  231 C, an M developing section  231 M, and a Y developing section  231 Y. A driveline, not shown, causes the revolver  230  to revolve counterclockwise, as indicated by an arrow in FIG. 1. A bias power supply, not shown, applies a DC voltage biased by an AC voltage Vac to a sleeve, which is included in each developing section, as a bias for development. The bias biases the sleeve to a preselected potential relative to a base included in the drum  200 . 
     In the illustrative embodiment, when the copier body is in a stand-by state, the Bk developing unit  231  of the revolver  230  is positioned upstream of a developing position in the direction of rotation of the revolver  230  by an angle of 30°. At the developing position, the revolver  230  faces the drum  200 . On the start of a copying cycle, the color scanner  1  starts outputting Bk color data at a preselected timing. The color printer  2  starts forming a latent image in accordance with the Bk color data. Let the latent image derived from the Bk color data be referred to as a Bk latent image hereinafter. This is also true with the other colors M and Y. 
     Before the leading edge of the Bk latent image arrives at the developing position, the revolver  230  rotates to locate the Bk developing section  231  at the developing position while causing a Bk sleeve included in the Bk developing section  231  to start rotating. In this condition, the Bk developing section  231  develops the Bk latent image with Bk toner. As soon as the trailing edge of the Bk latent image moves away from the developing position, the revolver  230  again revolves to bring the next developing section thereof to the developing section. This rotation completes at least before the leading edge of the next latent image arrives at the developing section. 
     FIG. 2 shows the intermediate image transferring unit  500  specifically. As shown, an intermediate image transfer belt (simply belt hereinafter)  501  is passed over a plurality of rollers. Arranged around the belt  501  are a bias roller  605 , a belt cleaner  504 , and a brush  505 . The bias roller  605  constitutes a secondary image transfer member (secondary image transfer charge applying means) included in a secondary image transferring device  600 . The belt cleaner or intermediate image transfer body cleaning means  504  cleans the belt  501 . The brush or lubricant coating means  505  coats a lubricant on the belt  501 . 
     A mark, not shown, is positioned on the inner surface of the outer surface of the belt  501  for allowing the position of the belt  501  to be sensed. The mark should preferably be positioned on the inner surface of the belt  501  because the mark positioned on the outer surface of the belt  501  must avoid a belt cleaning blade  504  and therefore makes layout difficult. An optical sensor or mark sensor  514  is positioned between a bias roller  507  and a drive roller  508  over which the belt  501  is passed. 
     The belt  501  is passed over a bias roller or primary image transferring means  507 , a tension roller  509  and rollers  510 ,  511  and  512  as well as over the bias roller  507  and drive roller  508 . The rollers  510  and  511  join in secondary image transfer and belt cleaning, respectively. The roller  512  is used to sense a feedback current, as will be describe specifically later. The rollers other than the roller  507  are connected to ground. 
     A power supply  801  assigned to primary image transfer applies to the bias roller  507  a bias for primary image transfer, which is a current or a voltage controlled to a preselected size matching with the number of toner image to be superposed. In the illustrative embodiment, constant current control is effected to apply a constant bias to the bias roller  507  without regard to the electric resistance of the belt  501 . Also, control is effected such that a current flowing from the bias roller  507  to the roller  512  via the belt  501  remains constant (e.g. 22 μA). 
     A motor, not shown, causes the belt  507  to move in a direction indicated by an arrow in FIG. 2 via the drive roller  508 . The belt  501  is formed of a conductor or an insulator and has a laminate structure, which will be described later. The belt  501  has a size greater than the maximum sheet size applicable to the copier in order to superpose toner images of different colors. 
     A moving mechanism or moving means, not shown, selectively moves the bias roller  605  for secondary image transfer into or out of contact with part of the belt  501  passed over the roller  510 . A sequence controller, which will be described later, controls the moving mechanism via a clutch, which will also be described later. The moving means may be implemented by a solenoid, if desired. A constant-current power supply  802  for secondary image transfer applies a bias, which is a preselected current, to the bias roller  605 . The sequence controller monitors the current of the secondary image transfer bias. 
     The bias roller  605  may be provided with a conductive high-molecular film having an electric resistance of 10 4 Ω to 10 8 Ω on its surface and may have a diameter of 30 mm. Likewise, the roller  510  may be provided with a conductive high-molecular film on its surface and may have a diameter of 40 mm. 
     A registration roller pair  610  (see FIG. 1) feeds a sheet or recording medium P to a nip between the bias roller  605  and the roller  510  at a preselected timing. A cleaning blade or cleaning means  608  is held in contact with the bias roller  605  in order to remove toner and impurities deposited on the bias roller  605 . 
     In operation, when an image forming cycle begins, the motor mentioned earlier rotates the drum  200  counterclockwise. In this condition, a Bk, C, an M and a Y toner image are sequentially formed on the drum  200 . The drive roller  508  causes the belt  501  to move clockwise. The bias applied to the bias roller  507  causes the Bk, C, M and Y toner images to be sequentially transferred from the drum  200  to the belt  501  one above the other (primary image transfer). As a result, a full-color image is completed on the belt  501 . 
     How the Bk toner image, for example, is formed will be described with reference to FIG.  2 . The charger  203  uniformly charges the surface of the drum  200  to a preselected potential with a negative charge. The optical writing unit  220 , FIG. 1, scans the charged surface of the drum  200  with a laser beam by raster scanning in accordance with the Bk image signal on the basis of a signal representative of the mark of the belt sensed. The scanned or exposed portion of the drum  200  loses the charge by an amount corresponding to the quantity of incident light, so that the Bk latent image is formed as a potential distribution. The Bk developing unit  231  develops the Bk latent image with Bk toner deposited on the Bk sleeve. More specifically, the Bk toner deposits on the exposed portion of the drum  200 , but does not deposit on the unexposed portion where the charge is left, forming a Bk toner image corresponding to the Bk latent image. 
     The Bk toner image is transferred from the drum  200  to the belt  500 , which is moving at a constant speed in contact with the drum  200 . The drum cleaner  201  removes some toner left on the drum  200  after the primary image transfer to thereby prepare the drum  200  for the next image forming cycle. After the formation of the Bk toner image, the color scanner  220  starts reading Y image data out of the document  4 , FIG.  1 . The writing unit  220  forms a Y latent image on the surface of the drum  200  in accordance with the resulting Y image data. 
     The revolver  230  revolves to locate the Y developing section  231 Y at the developing position after the trailing edge of the Bk latent image has moved away from the developing position, but before the leading edge of the Y latent image arrives thereat. The Y developing section  231 Y then develops the Y latent image with Y toner. The revolver  230  again revolves to locate the C developing section  231 C at the developing position after the trailing edge of the Y latent image has moved away from the developing position, but before the leading edge of the next or C latent image arrives at the same. This revolution also completes before the leading edge of the C latent image arrives at the developing position. C and M image forming steps are identical with the Bk and Y image forming steps except for the color and will not be described specifically. 
     The Bk, Y, C and M toner images sequentially formed on the drum  200  are sequentially transferred to the belt  501  one above the other, completing a full-color image on the belt  501 . 
     At the time when the image forming cycle beings, the sheet P is fed from any one of sheets cassettes  207 , sheet cassettes  300   a  through  300   c , and a manual feed tray  240 . The registration roller pair  610  once stops the sheet P fed thereto. 
     When the leading edge of the full-color toner image on the belt  501  is about to reach the nip between the belt  501  and the bias roller  605  (secondary image transfer position), the registration roller pair  610  drives the sheet P. The leading edge of the sheet P therefore accurately meets the leading edge of the toner image. 
     The power supply  802  applies the bias for secondary image transfer to the bias roller  605  when the sheet P passes the nip between the belt  501  and the bias roller  605 . As a result, the full-color image is transferred from the belt  501  to the sheet P (secondary image transfer). Separating means, not shown, positioned downstream of the secondary image transfer position in the direction of sheet conveyance separates the sheet P off the belt  501  by discharge. Belt conveyors  210  and  211  shown in FIG. 1 sequentially convey the sheet with the full-color image to the fixing device  270 . The fixing device  271  and  272  includes a heat roller  271  and a press roller  272 . The heat roller  271  and press roller  272  fix the toner image on the sheet P with heat and pressure. An outlet roller pair  212  drives the sheet P with the fixed toner image, or print, out of the copier body to a copy tray, not shown, face up. 
     The separating means mentioned above is implemented by discharge needles  611  and a bias power supply  803 . The discharge needles  611 , which constitute a separating member, are positioned such that their tips face the sheet P coming out of the secondary image transfer position. The bias power supply  803  applies a bias to the discharge needles  611  for causing it to separate the sheet P from the belt  501 . 
     The drum cleaner  201  cleans the surface of the drum  200  after the primary image transfer. A quenching lamp, not shown, discharges the cleaned surface of the drum  200 . The moving means presses the belt cleaning blade  504  against the belt  501  in order to remove the toner left on the belt  501  after the secondary image transfer. 
     In a repeat copy mode, after the formation of the first M or fourth-color image, the  1  and printer  2  start forming the second Bk or first-color toner image at a preselected timing. After the secondary transfer of the first full-color image from the belt  501  to the sheet P, the second Bk toner image is transferred from the drum  200  to part of the belt  501  cleaned by the belt cleaning blade  504 . This is followed by the sequence of steps described in relation to the first full-color image. 
     The procedure described above has concentrated on a full-color copy mode. In a tricolor or a bicolor copy mode, the above procedure is repeated a number of times corresponding to the number of colors and the desired number of copies. Further, in a monochromatic copy mode, only the developing section of the revolver  230  corresponding to the desired color is operated while the belt cleaning blade  504  is continuously held in contact with the belt  501 . 
     Hereinafter will be described a specific configuration of the belt  501  and an arrangement for uniforming, before the primary image transfer, the charge (polarization left on the belt  501 . As shown in FIG. 3, the belt  501  has a laminate structure having a thickness of 150 m, a width of 368 mm, and an inner circumferential length of 565 mm. The belt  501  moves at a linear velocity of 245 mm/sec by way of example. The laminate structure is made up of an outer layer  501   a  for carrying the toner, an intermediate layer  501   b , and an inner layer or base layer  501   c . The outer layer  501   a  and intermediate layer  501   b  have high resistance each. The three layers  501   a  through  501   c  are mainly formed of PVDF. Suitable additives including a conductive material are dispersed in the layers  501   a  through  501   c.    
     The outer layer  501   a  has a thickness of 1 μm and a volume resistivity of 10 10  Ω·cm to 10 16  Ω·cm. The intermediate layer  501   b  has a thickness of about 75 μm and a volume resistivity ρv of 10 10  Ω·cm to 10 10  Ω·cm. Further, the inner layer  501   c  has a thickness of 75 μm and a volume resistivity ρv of 10 8  Ω·cm to 10 11  Ω·cm. The resistance of the entire belt  501  is adjusted on the bases of the amount of the conductive material and thickness of each layer. 
     The materials and configuration of the belt  501  described above are only illustrative. The crux is that the volume resistivity of the entire belt  501  be as high as 10 10  Ω·cm or above. 
     As for the belt  501  with three layers including high-resistance layers, when a toner image is transferred from the drum  200  to the belt  501 , part of a latent image (potential distribution) is also transferred from the drum  200  to the belt  501 . When an electric field of about 50 MV/m (field resistance value) is applied to PVDF or similar ferroelectric material, the material automatically polarizes in the opposite direction to the electric field, saturates, and then stabilizes. A voltage of 100 V or above acts on the surface layer  501   a , which is about 1 μm thick, and raises the electric field inside the layer  501   a  above the field resistance layer. As a result, the outer layer  501   a  immediately polarizes and then stabilizes. The belt  501  with such a unique surface layer  501   a  can erase, at the time of transfer of a toner image from the drum  200 , the potential contrast of the previous latent image and hold the potential contrast of a new latent image on its surface. The potential contrast is essential for reducing the previously discussed toner scattering and obviating a residual image. 
     However, assume that the same toner image is repeatedly transferred to the same area of the belt  501  as in the full-color copy mode or the repeat copy mode using a single document. Then, the intermediate layer  501   b  below the outer layer  501   a  polarizes and remains in the polarized state for the following reason. At the time of primary image transfer, the strength of the electric field acting on the intermediate layer  501   b  is short of the field resistance value. As a result, the inside of the intermediate layer  501   b  polarizes little by little due to the electric field for primary transfer. The electric field formed in the intermediate layer  501   b  by the above electric field is weaker than the electric field formed in the outer layer  501   a . It follows that a longer period of time is necessary to cancel or invert the polarization of the intermediate layer  501   b  than to cancel or invert the polarization of the surface layer  501   a . For details, reference may be made to, e.g., Tajitsu and Furukawa “Basics of Ferroelectrics”, Journal of Institute of Electrostatics Japan, Vol. 13, No. 2 (1989), pp. 74-81 and Odajima “Piezoelectricity and Ferroelectricity of Polyvinylidene Fluoride”, Journal of The Japan Society of Applied Physics, Vol. 50, No. 12 (1981), pp. 79-83. 
     For the reason described above, even when a bias implemented by a DC voltage is applied to the belt  501  for discharging it, the intermediate layer  501   b  cannot be easily discharged. On the other hand, assume that a discharging bias implemented by a DC voltage of opposite polarity is applied to the belt  501 . Then, the net bias cannot act on the intermediate layer  501   b  because the surface layer  501   a  polarizes soon due to its short time constant of the variation of polarization. Although a high bias may apply a preselected voltage even on the intermediate layer  501   b , it is undesirable for the surface layer  501   a.    
     FIG. 4 shows a specific arrangement for measuring a potential left on the belt formed of PVDF. As shown, the arrangement includes a conductive base  900  connected to ground. The belt, labeled  901 , is laid on the conductive base  900 . A probe electrode  902 , which is a substitute for the bias roller, is held in contact with the belt  901 . An electrometer and a pen recorder, not shown, are connected to the probe electrode  902 . In this condition, a switch  903  is operated to apply a bias voltage, which a DC voltage, from a high-tension power supply  904  to the belt  901  via the probe electrode  902 . First, a bias of V 0  (+250 V or +500 V) for initial saturation polarization is applied to the belt  901 , thereby causing polarization to saturate. 
     Subsequently, the switch  903  is operated to bring the belt  901  into a floating state. The electrometer and pen recorder record the resulting attenuation of the surface potential of the belt  901 . Usually, an extremely long period of time is necessary for the surface potential to attenuate. Thereafter, a discharging bias of V 1  (V) (0V or −250 V) is applied to the belt  901  for a period of time of Δt, which is 0.1 second to 10 seconds. The switch  903  is again operated to bring the belt  901  into a floating state in order to record the attenuation of the surface potential. FIG. 5 plots the surface potential of the belt  901  on the elapse of a preselected period of time, i.e., 6 seconds necessary for the belt to complete one turn. 
     Specifically, FIG. 5 shows a relation between the duration (log T) of the discharging bias V 1  and the surface potential of the belt  901  measured in the preselected period of time (6 seconds) since the application of the discharging bias V 1 . FIG. 6 lists the various values of the initial bias V 0  for saturation and those of the discharging bias V 1  derived the data shown in FIG.  5 . As shown in FIG. 5, when the bias V 0  of +250 V and the bias V 1  of −250 V were sequentially applied to the belt  901  in this order, 10 seconds was necessary for the belt  901  to be actually discharged to −250 V. Further, the greater the absolute value of the bias V 0  (the higher the initial surface potential of the belt  901  itself) or the higher the potential contrast (the higher the latent image contrast), the longer the discharging time. This probes that it is difficult to discharge the belt  901  to 0 V with the DC voltage after causing the polarization of the belt  901 , which is ferroelectric, to saturate. 
     As stated above, even after the surface potential of the belt  901  has been discharged to 0 V, polarization corresponding to the potential contrast of the previous toner image remains in the intermediate layer  501   b . This is also true when the surface layer  901   a  is formed of a material other than PVDF because of the tunnel effect particular to a thin layer. Consequently, it is difficult to discharge the belt  501  including the high-resistance intermediate layer  501   b  with a DC voltage. While a high AC bias with a great amplitude may surely discharge the entire laminate of the belt  501 , it not only increases the power supply cost, but is apt to bring about damage to the belt  501  and cause banding to appear in an image. Moreover, discharge using an AC bias must be accompanied by postprocessing to deal with ozone. 
     In light of the above, the illustrative embodiment uniforms polarization left in the high-resistance intermediate layer  501   b  while maintaining its polarity. This is done after the belt  501  has started moving at the beginning of an image forming operation, but before the primary transfer of a toner image from the drum  200  to the belt  501 . Uniforming the polarization of the intermediate layer  501   b  is successful to reduce the potential contrast left in the belt  501  for thereby obviating a residual image. Particularly, by applying a pre-bias to the belt  501  in a direction in which the polarization of the intermediate layer  501   b  saturates, it is possible to more rapidly, easily reduce the potential contrast. 
     Specifically, in the illustrative embodiment, the secondary image transferring device  600  plays the role of polarization uniforming means at the same time. FIG. 7 shows major part of a control system for controlling the secondary image transferring device  600  to apply the pre-bias. As shown, the sequence controller mentioned earlier, labeled  850 , includes a CPU (Central Processing Unit)  851 , a RAM (Random Access Memory)  852 , a ROM (Read Only Memory)  853 , and an I/O (Input/Output) interface  854 . A secondary image transfer clutch  855  and the power supply  802  for secondary image transfer are connected to the sequence controller  850  via the I/O interface  854 . 
     FIG. 8 demonstrates a specific image forming operation including the control over the application of the pre-bias. The specific operation sequentially forms two monochromatic toner images of different colors on the belt  501  one after the other within the circumferential length of the belt  501 . The toner images are transferred to sheets P of size A 3  one after the other. A halftone image with low image density (ID) is transferred to the second sheet P. 
     As shown in FIG. 8, after a copy button, for example, has been pressed to cause the drum  200  and belt  501  to start moving, a belt cleaning clutch is coupled to cause the belt cleaning blade  504  to start cleaning the belt  501 . As soon as the optical sensor  514  senses the mark provided on the belt  501 , the sequence controller  850  couples the secondary image transfer clutch  855  and thereby brings the bias roller  605  into contact with the belt  501 . At the same time, the sequence controller  850  causes the power supply  802  to apply the pre-bias (e.g. +70 μA), which is current controlled, to the bias roller  605 . The pre-bias starts uniforming the polarization of the intermediate layer  501   b . On the elapse of a preselected period of time since the detection of the mark, FGATE signals corresponding to the consecutive images are sequentially output. 
     When at least a period of time necessary for the belt  501  to complete one turn expires, the pre-bias is replaced with a usual bias for secondary image transfer (e.g. +40 μA) to sequentially transfer the two toner images from the belt  501  to two sheets P. 
     With the procedure described above, the illustrative embodiment uniforms polarization left in the intermediate layer  501   b  for thereby canceling a potential contrast left in the layer  501   b . This successfully obviates a residual image ascribable to polarization left in the intermediate layer  501   b  by the previous image forming cycle. Particularly, uniforming polarization while maintaining the polarity of polarization uniforms the polarization more rapidly and more easily than uniforming it by canceling polarization left in the intermediate layer  501   b  or inverting the polarity thereof. 
     FIG. 9 compares the illustrative embodiment and a comparative example with respect to a residual image rank determined by varying the pre-bias. The comparative example did not apply the pre-bias. A residual image was estimated at two positions in five ranks; the greater the numerical value, the lower the degree of a residual image, i.e., the higher the image quality. As FIG. 9 indicates, when the pre-bias is 40 μA or above, high image quality belonging to residual image rank 3.5 or above is achievable. 
     Further, before the primary transfer of the first toner image to the first sheet P, the illustrative embodiment uniforms polarization left in the intermediate layer  501   b . As a result, a residual image ascribable to polarization left in the intermediate layer  501   b  at the time of the secondary transfer of the first toner image appears little in the second toner image transferred to another area of the belt  501 . This will be described specifically with reference to FIGS. 10A through 10C. 
     FIGS. 10A,  10 B and  10 C respectively demonstrate the primary transfer of the first toner image from the drum  200  to the belt  501 , the secondary transfer of the same image from the belt  501  to the sheet P, and the primary transfer of the second toner image. The three layers  501   a  through  510   c  of the belt  510  are shown as being separate from each other for the sake of illustration. Arrows P 1   a , P 2   a  and P 3   a  indicate the directions and sizes of polarization of the outer layer  501   a . Likewise, arrows P 1   b , P 2   b  and P 3   b  indicate the directions and sizes of polarization of the intermediate layer  501   b.    
     As shown in FIG. 10 a , a relatively great amount of negative true charge is present on the background portion of the outer side (top in the figure) of the outer layer  501   a  due to the background potential VD of the drum  200 . Also, a relatively small amount of negative charge is present at a toner portion T on the same side of the outer layer  501   a . At this instant, the pre-bias effected beforehand injects positive true charge into the belt  501  via the outer layer  501   b  beforehand, causing downward polarization, as viewed in the figure, to occur in the intermediate layer  501   b . Therefore, even the bias for primary transfer (positive) applied to the bias roller  507  causes only a small potential difference to act on the intermediate layer  501   b . Consequently, the polarization P 1   b  directed upward, as viewed in the figure, is smaller than conventional one. 
     Subsequently, as shown in FIG. 10B, the polarization of the outer layer  501   a  immediately inverts due to the secondary transfer of the first toner image. This, coupled with the fact that the upward polarization P 1   b  is small, causes the polarization of the intermediate layer  501   b  to invert, too. As a result, the polarization P 2   b , which is relatively small and directed downward, occurs. 
     As shown in FIG. 10C, on the primary transfer of the second toner image, the polarization of the intermediate layer  501   b  resulting from the bias (positive) applied to the bias roller  507  is reduced because the positive true charge deposited by the secondary transfer still remains on the upper side of the intermediate layer  501   b . In this manner, although the polarization derived from the first toner image is left in the intermediate layer  501   b , the size of the polarization is smaller than the conventional size and therefore effects the primary transfer little. This successfully prevents the transfer ratio from varying, i.e., prevents the first toner image from appearing in the second toner image as a residual image. 
     FIGS. 11A through 11B pertain to the comparative example not using the pre-bias and respectively show the primary transfer of the first toner image, the secondary transfer of the same image, and the primary transfer of the second toner image. As shown in FIG. 11A, downward polarization is absent in the intermediate layer  501   b . Therefore, polarization P 1   b ′ more intense than in the illustrative embodiment occurs in the intermediate layer  501   b  due to the primary transfer of the first toner image. As a result, as shown in FIG. 11B, although the secondary transfer of the first toner image inverts the polarization P 2   a ′ in the outer layer  501   a , it does not invert the polarization P 2   b ′ in the intermediate layer  501   b . The polarization P 2   b ′ therefore remains in the intermediate layer  501   b  although slightly decreasing. Subsequently, as shown in FIG. 11C, the primary bias (positive) applied to the bias roller  507  for the primary transfer of the second toner image further intensifies the polarization P 3   b ′ in the intermediate layer  501   b . Such intense polarization P 3   b ′ remaining in the intermediate layer  501   b  causes the first toner image appear in the second toner image as a residual image. 
     In the illustrative embodiment, the pre-bias is subjected to constant-current control. Therefore, even when the resistance of the belt  501  varies, the intermediate layer  501   b  can evenly polarize to preselected intensity. Because the secondary image transferring device  600  plays the role of polarization uniforming means at the same time, the copier is low cost and small size. The current value of the pre-bias should preferably be equal to or greater than the current value of the bias for secondary image transfer. This successfully enhances the effect of charge injection in the belt  501  for thereby more surely uniforming the polarization of the intermediate layer  501   b.    
     Assume that the pre-bias is applied for a period of time corresponding to one and half turns of the belt  501  by way of example. Then, a step occurs in the polarization of the intermediate layer  501   b  and causes a strip-like defect appear in the resulting image. To solve this problem, the duration of the pre-bias should preferably be an integral multiple of a period of time corresponding to one turn of the belt  501 . 
     FIG. 12 shows a modification of the illustrative embodiment. As shown, an exclusive bias roller  960  for the pre-bias is positioned downstream of the secondary image transferring device  600  in the direction of rotation of the belt  501 . The bias roller  960  is held in contact with the roller  510  with the intermediary of the belt  510 . A power supply  961  applies the pre-bias controlled to a preselected current to the bias roller  960 . The bias roller  960  is simpler in configuration and lower in cost than the relatively expensive bias roller for secondary image transfer used in the illustrative embodiment. 
     In the modification shown in FIG. 12, the bias roller  960  should preferably have medium electric resistance, so that the current does not concentrate when the film of the belt  501  is defective. The kind of conductivity for providing the bias roller  960  with medium resistance may be implemented by either one of electronic conduction and ion conduction. A moving mechanism, not shown, selectively moves the bias roller  960  into or out of contact with the belt  501 . The moving means may bring the bias roller  960  into contact with the belt  501  at the same time when the belt cleaning blade  504  contacts the belt  501 . 
     In the illustrative embodiment, the material of the belt  501  varies in electric resistance by the order of one figure because it is susceptible to humidity. Therefore, in a low temperature, low humidity environment, the current of the pre-bias adequate in a normal temperature, normal humidity environment may be excessively high in a normal temperature, normal humidity atmosphere. FIG. 13 shows another modification of the illustrative embodiment additionally including a humidity sensor or humidity sensing means  970 . The humidity sensor  970  is responsive to absolute humidity inside of the copier. The current of the pre-bias is switched in accordance with absolute humidity sensed by the humidity sensor  970 . For example, when absolute humidity decreases below a preselected reference value (low temperature, how humidity environment), the current of the pre-bias switched to a smaller value. More specifically, the current of the pre-bias is set at 70 μA in a normal temperature, normal humidity environment and set at 50 A in a low temperature, low humidity environment in which absolute humidity is lower than 4.7 g/m 3 . 
     The illustrative embodiment additionally includes a duplex-copy unit  207  for forming images on both sides of the sheet P. Specifically, the sheet P carrying an image on one side or first side thereof and come out of the fixing device  270  is steered to the duplex-copy unit  207 . A pickup roller  208  again pays out the sheet P toward the image forming section, so that another image is formed on the other side or second side of the sheet P. In this case, the electric resistance of the sheet P differs from the time when an image formed on one side, but is not fixed, to the time when an image formed on the other side after the fixation of the image on one side. 
     On the other hand, at the secondary image transferring station, the bias for secondary image transfer is divided with the result that a potential difference acts on the sheet P. This potential difference, i.e., the strength of electric field acting on the sheet P is dependent on the electric characteristic of the sheet P. Consequently, for a given bias for secondary transfer, the strength of electric field to act on the sheet P differs from the time when an image is formed on one side of the sheet P, but is not fixed, to the time when an image formed on the other side after the fixation of the image on the first side. 
     In light of the above, the pre-bias may apply a particular bias to each of the transfer of an image to the first side of the sheet P and the transfer of an image to the second side of the same sheet P. More particularly, a current of 70 μA and a current of 30 μA or below may be respectively assigned to the transfer of an image to the first side of the sheet P and the transfer of an image to the second side of the sheet P. 
     While the illustrative embodiment has concentrated on a color copier, the present invention is similarly applicable to any other image forming apparatus, e.g., a monochromatic copier, a printer or a facsimile apparatus. This is also true with an alternative embodiment to be described later. 
     As stated above, the illustrative embodiment achieves various unprecedented advantages, as enumerated below. 
     (1) The illustrative embodiment can uniform polarization left in a high-resistance layer more rapidly and more easily that an apparatus of the type canceling or inverting the polarity of such polarization. Therefore, even when use is made of an intermediate image transfer body including a high-resistance layer, which desirably obviates toner scattering, a residual image ascribable to polarization left before primary image transfer can be surely obviated. This is true even when the electric resistance of the intermediate image transfer body is irregular. 
     (2) Polarization can be uniformed more efficiently because the polarization of the high-resistance layer polarizes in a single direction. 
     (3) The distribution of polarization of the high-resistance layer does not include a step. This more surely obviates a residual image ascribable to the polarization left in the high-resistance layer. 
     (4) Even when humidity varies, the polarization of the high-resistance layer is increased to a preselected size, insuring desirable secondary image transfer. 
     (5) In a duplex print mode, the size of the polarization is adjusted with respect to the first and second sides of a sheet, so that desirable image transfer can be effected with both sides of the sheet. 
     (6) The illustrative embodiment reduces the cost and size of an image forming apparatus. 
     An alternative embodiment of the present invention, which is mainly directed toward the second object mentioned earlier, will be described hereinafter. The illustrative embodiment is also constructed and operated as described with reference to FIGS. 1 and 13. Description made with reference to FIGS. 2 through 7,  10 A through  10 C,  11 A through  11 C and  12  also applies to the illustrative embodiment and will not be described specifically in order to avoid redundancy. 
     In the illustrative embodiment, at the end of an image forming operation, polarization left in the intermediate or high-resistance layer  501   b  is uniformed while preserving its polarity after the secondary image transfer, but before the stop of movement of the belt  501 . This successfully reduces potential contrast left in the belt  501  to thereby obviate a residual image at the time of the next image forming operation. Particularly, a post-bias is applied in a direction in which the polarization of the intermediate layer  501   b  saturates, so that the potential contrast rapidly, easily decreases. 
     FIG. 14 shows a specific image forming procedure including the application of the post-bias. The procedure assumes that toner images of different colors are sequentially formed on the belt  501  within the circumferential length of the belt  501  and sequentially transferred to consecutive sheets P of size A 3 . 
     As shown in FIG. 14, after a copy button, for example, has been pressed to cause the drum  200  and belt  501  to start moving, a belt cleaning clutch is coupled to cause the belt cleaning blade  504  to start cleaning the belt  501 . After the optical sensor  514  has sensed the mark provided on the belt  501 , FGATE signals corresponding to the consecutive images are sequentially output. Subsequently, the usual bias for secondary transfer is replaced with the post-bias (e.g. +30 μA). The post-bias uniforms polarization left in the intermediate layer  501   b  while preserving its polarity, thereby canceling potential contrast ascribable to the polarization. It follows that the next image formation to be effected later is free from a residual image otherwise brought about by polarization left in the intermediate layer  501   b.    
     Particularly, in a full-color copy mode, toner images are sequentially formed on the drum  200  and then transferred to the belt  501  with the mark on the belt  501  being sensed toner image by toner image. Therefore, the toner images of the same size, but different in color, are transferred to the same area of the belt  501 , so that potential contrast ascribable to polarization is apt to increase. The post-bias unique to the illustrative embodiment uniforms the above polarization left in the intermediate layer  501   b  to thereby obviate a residual image at the next image formation to be effected layer. 
     Further, the illustrative embodiment uniforms the polarization of the intermediate layer  501   b  while preserving the polarity provided by the bias for secondary image transfer applied immediately before. This rapidly, easily uniforms the polarization left in the intermediate layer  501   b , compared to the case wherein the polarization is canceled or inverted in polarity. 
     FIG. 15 compares the illustrative embodiment and a comparative example with respect to a residual image rank determined by varying the post-bias. The comparative example did not apply the post-bias. A residual image was estimated in five ranks; the greater the numerical value, the lower the degree of a residual image, i.e., the higher the image quality. As FIG. 15 indicates, when the post-bias is 30 μA or above, which is 60% of the current of the usual bias for secondary transfer or above, high image quality belonging to residual image rank 3.5 or above is achievable. 
     Assume that the post-bias is applied for a period of time corresponding to one and half turns of the belt  501  by way of example. Then, a step occurs in the polarization of the intermediate layer  501   b  and causes a strip-like defect appear in the resulting image. To solve this problem, the duration of the post-bias should preferably be an integral multiple of a period of time corresponding to one turn of the belt  501 . 
     Generally, potential contrast ascribable to polarization to remain in the intermediate layer  501   b  at the end of an image forming operation increases with an increase in the number of toner images sequentially transferred to the same area of the belt  501  one above the other. In light of this, the sequence controller  850 , FIG. 7, may selectively turn on or turn off the post-bias in accordance with the number of toner images transferred to the same area of the belt  501  one above the other. 
     For example, in a black-and-white or similar monochromatic mode, potential contrast ascribable to polarization left in the intermediate layer  501   b  is low. In this mode operation, the sequence controller  850  turns off the post-bias after the secondary image transfer. On the other hand, in a bicolor or a full-color mode in which the above potential contrast is high, the sequence controller  850  turns on the post-bias because the potential contrast tends to increase. 
     With the selective application of the post-transfer, the sequence controller  850  not only obviates a residual image at the next image formation, but also avoids wasteful application of the post-bias to thereby prevent productivity from decreasing. 
     Potential contrast ascribable to polarization left in the intermediate layer  510  at the end of an image forming operation tends to increase with an increase in the number of sheets to which the same image is transferred as well. In light of this, the sequence controller  85  may count the sheets P to which the same image is transferred by a sequence of image forming cycles and selectively turn on or turn off the post-bias in accordance with the count. For example, when the same color image is transferred to four sheets P or less, the sequence controller  850  turns off the post-bias because potential contract is relatively low. On the other hand, the number of sheets P to which the same color image transferred is five or more, the sequence controller  850  turns on the post-bias because potential contrast tends to increase. With this scheme, too, the sequence controller  850  not only obviates a residual image at the next image formation, but also avoids wasteful application of the post-bias to thereby prevent productivity from decreasing. 
     As stated above, the illustrative embodiment achieves various unprecedented advantages in addition to the advantages of the previous embodiment. The illustrative embodiment can uniform polarization left in a high-resistance layer after an image forming operation more rapidly and more easily that an apparatus of the type canceling or inverting the polarity of such polarization. Therefore, even when use is made of an intermediate image transfer body including a high-resistance layer, which desirably obviates toner scattering, a residual image ascribable to polarization left after an image forming operation can be surely obviated. This is true even when the electric resistance of the intermediate image transfer body is irregular. In addition, the illustrative embodiment not only obviates the residual image, but also avoids wasteful application of a post-transfer and thereby prevents productivity from decreasing. 
     Various modifications will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope thereof.