Abstract:
An electrophotographic printer has neutralizing units and a fixing unit that are disposed in the transport path of a print medium. The neutralizing units neutralize a toner image surface of the print medium entering the fixing unit and a non-printed surface of the print medium exiting the fixing unit. The fixing unit includes two rollers. A fixing roller incorporates a heat source that generates heat for fusing a toner image on the print medium. A pressure roller is in pressure engagement with the fixing roller. At least one of the fixing roller and pressure roller includes a resilient member that covers a metal shaft and an insulation layer that covers the resilient member. The resilient member may contain electrically conductive whiskers that extend radially in three dimensions. The whiskers are dispersed such that a surface of the insulation layer and a surface of the shaft are electrically continuous.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to an electrophotographic printer in which toner is prevented from migrating from a print medium to a heat roller of a fixing unit during fixing operation.  
           [0003]    2. Description of the Related Art  
           [0004]    [0004]FIG. 11 illustrates a relevant portion of a conventional electrophotographic printer.  
           [0005]    [0005]FIG. 12 illustrates a mechanism in which a toner image formed on a photoconductive drum is transferred onto a print medium.  
           [0006]    Referring to FIG. 11, the conventional electrophotographic printer has a charging roller  12 , an LED head  13 , a developing unit  14 , a transfer roller  15 , and a cleaning roller  17 , all of which being disposed around a photoconductive drum  11 . These structural elements are disposed in the order in which a photographic process is carried out.  
           [0007]    A print medium P travels in a direction shown by arrow A. The transfer roller  15  is disposed under the photoconductive drum  11  and rotates in contact with the photoconductive drum  11 , thereby defining a transfer section through which the print medium passes during printing. When the print medium P passes the transfer section, a toner image is transferred from the photoconductive drum  11  onto the print medium P. The charging roller  12  and cleaning roller  17  are disposed downstream of the transfer section with respect to the rotation of photoconductive drum  11 . The LED head  13  is disposed above the photoconductive drum  11 . The developing unit  14  is disposed upstream of the transfer section with respect to the rotation of the photoconductive drum.  
           [0008]    There is provided an endless belt  10  that is entrained about rollers, not shown, and is sandwiched between the photoconductive drums  11  and transfer rollers  15 . The endless belt  10  runs in the direction shown by arrow A. The endless belt  10  attracts the print medium P that is fed from a paper cassette, not shown, and transports the print medium P along a transport path from an upstream end to a downstream end of the transport path.  
           [0009]    The charging roller  12  uniformly charges the surface of the photoconductive drum  11 . An LED array, not shown, of the LED head  13  is selectively energized in accordance with print data to illuminate the surface of the charged photoconductive drum  11  to form an electrostatic latent image on the photoconductive drum  11 .  
           [0010]    In the developing unit  14 , a sponge roller  14   b  supplies toner to a developing roller  14   a . A developing blade  14   c  is in contact with the rotating developing roller  14   a  to form a uniform thin layer of toner on the surface of the developing roller  14   a . The developing roller  14   a  rotates in contact with the photoconductive drum  11 , thereby depositing negatively charged toner on the latent image formed on the photoconductive drum  11  to form a toner image.  
           [0011]    The transfer roller  15  is connected to a high voltage power supply  16  which applies a potential in the range of several hundred volts to several thousand volts across the photoconductive drum  11  and the transfer roller  15 , so that the toner T on the photoconductive drum  11  migrates from the photoconductive drum  11  to the print medium P that is transported by the endless belt  10  between the photoconductive drum  11  and the transfer roller  15 .  
           [0012]    As shown in FIG. 12, dielectric polarization occurs such that toner image-receiving surface of the print medium P is positively charged. When the negatively charged toner T on the photoconductive drum  11  is brought into contact with the positively charged surface of the print medium P, the negatively charged toner T is attracted by the Coulomb force to the positively charged surface of the print medium P. In other words, the toner image is transferred onto the print medium P. A current flows through the print medium P between the print medium P and non-exposed areas, i.e., areas on the photoconductive drum  11  in which no toner exists, so that the residual charges are neutralized. Thus, the toner image-receiving surface of the print medium P is generally negatively charged. The toner transferred onto the print medium P remains attracted to the print medium by weak Coulomb force.  
           [0013]    The cleaning roller  17  shown in FIG. 11 removes the toner that remains on the photoconductive drum  11  after transferring.  
           [0014]    A neutralizing brush  18  is provided in the transport path of print medium downstream of the transfer section. The free end of the neutralizing brush  18  is in contact with the non-printed surface of the print medium P in order to neutralize the charges deposited on the non-printed surface of the print medium P. The base portion of the neutralizing brush  18  is grounded.  
           [0015]    Downstream of the neutralizing brush  18 , there is provided a fixing unit  19  that includes a fixing roller  19   a  and a pressure roller  19   b . The fixing roller  19   a  has a heat source, not shown, that supplies heat to the surface of the print medium onto which the toner image has been transferred. The pressure roller  19   b  is disposed under the fixing roller  19   a  and presses the print medium P against the fixing roller  19   a.    
           [0016]    The heat supplied from the fixing roller  19   a  fuses the toner deposited on the print medium and the pressure applied by the pressure roller  19   b  causes the fused tone to penetrate the print medium P. The fixing roller  19   a  and pressure roller  19   b  have insulating layers formed thereon such that the toner is easily peeled off therefrom.  
           [0017]    With the aforementioned conventional electrophotographic printer, the surfaces of the fixing roller  19   a  and pressure roller  19   b  are covered with insulation that facilitates the peeling off of toner. However, the conventional printer suffers from the “offset” problem, i.e., an unintentional or faulty transfer of toner from a print medium to the fixing roller in contact with the print medium.  
           [0018]    If the offset problem occurs, the toner deposited on the fixing roller  19   a  is then deposited to the pressure roller  19   b  and/or other parts of the print medium. The toner may also be deposited on the following page of print medium such that a ghost image may appear on the following page or the page is simply soiled.  
           [0019]    The offset problem will occur if the negative potential on the toner image surface of the print medium P and the positive potential on the non-toner image surface are excessively high when the print medium P passes the fixing unit. For a tandem type color printer, the yellow, magenta, cyan, and black toner images are formed on the print medium attracted to the transport belt. Thus, the potential of the toner image surface of the print medium is apt to be higher for a tandem type color printer than for a monochrome printer. Especially, in a low temperature and low humidity environment, the impedance of the print medium is high and therefore the both opposed surfaces of the print medium tend to be high potential.  
           [0020]    The surface of the print medium on which a toner image has not been transferred is easily neutralized by the neutralizing brush  18 , the frame or chassis of the printer body. However, the charges on the toner-image surface of the print medium and the non-toner image surface are opposite in polarity and are in equilibrium. Thus, it is very difficult to completely neutralize the charges on the both surfaces of the print medium.  
           [0021]    If the negative potential on the toner image surface of the print medium and the positive potential on the non-toner image surface are excessively high, when the print medium P is brought into contact with the fixing roller  19   a , a potential difference will develop between the fixing roller  19   a  and the print medium P, creating an electric field therebetween. The electric field causes the negatively charged toner on the toner image surface of the print medium to migrate to the fixing roller  19   a , thereby resulting in the offset problem.  
         SUMMARY OF THE INVENTION  
         [0022]    An object of the present invention is to provide an electrophotographic printer in which the offset problem of a fixing unit is effectively prevented.  
           [0023]    Another object of the present invention is to provide an electrophotographic printer in which the charges deposited on the print medium after transfer are promptly neutralized.  
           [0024]    An electrophotographic printer according to the invention incorporates a fixing unit. The fixing unit includes a fixing roller and a pressure roller. The fixing roller has a heat source that generates heat for fusing a toner image transferred by a transfer unit onto a print medium. The pressure roller is in pressure engagement with the fixing roller. At least one of the fixing roller and the pressure roller includes a resilient member that covers a metal shaft and an insulation layer that covers the resilient member. The resilient member contains electrically conductive whiskers that extend radially in three dimensions. The whiskers are dispersed such that a surface of the insulation layer and a surface of the shaft are electrically continuous.  
           [0025]    The electrophotographic printer may further include a neutralizing brush that is disposed downstream of the fixing unit in proximity to the fixing unit. The neutralizing brush neutralizes a first surface of the print medium on which the toner image is transferred.  
           [0026]    The electrophotographic printer may include a neutralizing roller instead of the neutralizing brush. The neutralizing roller is disposed downstream of the fixing unit and neutralizes a first surface of the print medium on which the toner image is transferred.  
           [0027]    The electrophotographic printer may include a metal rod that has a sharp point and is disposed downstream of the transfer unit. When the print medium advances from the transfer unit to the fixing unit, the sharp point is in proximity to a first surface of the print medium to neutralize an excessive charge deposited thereon, the first surface being a surface on which the toner image is transferred.  
           [0028]    The electrophotographic printer may further include:  
           [0029]    moisture detector that detects a moisture in the air;  
           [0030]    moisture calculator that determines an amount of moisture that should be given to the print medium;  
           [0031]    a humidifier disposed downstream of the transfer unit and upstream of the fixing unit, the humidifier applying an amount of moisture calculated by the moisture calculator to a second surface of the print medium opposite to a first image on which a toner image is transferred.  
           [0032]    Another electrophotographic printer according to the invention incorporates a fixing unit and a neutralizing unit disposed in a transport path of a print medium. The fixing unit includes a fixing roller and a pressure roller. The fixing roller has a heat source that generates heat for fusing a toner image transferred onto a print medium. The pressure roller is in pressure engagement with the fixing roller to form a nip between the fixing roller and the pressure roller. The neutralizing unit is disposed downstream of the fixing unit and neutralizes a charged surface of the print medium discharged from the fixing unit.  
           [0033]    The neutralizing unit is disposed to oppose a first surface of the print medium on which the toner image is transferred.  
           [0034]    The neutralizing unit is a neutralizing brush.  
           [0035]    The neutralizing unit is a neutralizing roller.  
           [0036]    Another neutralizing unit may be disposed upstream of the fixing unit and downstream of a transfer section. The another neutralizing unit is disposed to oppose a second surface of the print medium opposite to the first surface.  
           [0037]    Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0038]    The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limiting the present invention, and wherein:  
         [0039]    [0039]FIG. 1 illustrates a configuration of a first embodiment of a fixing roller according to the present invention;  
         [0040]    [0040]FIG. 2 illustrates a configuration of a pressure roller of the first embodiment;  
         [0041]    [0041]FIG. 3 illustrates a pertinent portion of an electrophotographic printer of the first embodiment;  
         [0042]    [0042]FIG. 4 illustrates an image force that is developed between the surface of an electrically conductive roller and the toner on the print medium P;  
         [0043]    [0043]FIG. 5 illustrates a pertinent portion of an electrophotographic printer according to a second embodiment;  
         [0044]    FIGS.  6 A- 6 C illustrate the neutralizing brush that is connected to the ground through a varistor, a Zener diode, and a resistor;  
         [0045]    [0045]FIG. 7 illustrates a pertinent portion of an electrophotographic printer according to a third embodiment;  
         [0046]    [0046]FIG. 8 illustrates a pertinent portion of an electrophotographic printer according to a fourth embodiment;  
         [0047]    [0047]FIG. 9 illustrates a pertinent portion of an electrophotographic printer according to a fifth embodiment;  
         [0048]    [0048]FIG. 10 is a table that list moisture in the air for various temperature and humidity;  
         [0049]    [0049]FIG. 11 illustrates a relevant portion of a conventional electrophotographic printer; and  
         [0050]    [0050]FIG. 12 illustrates a mechanism in which a toner image formed on a photoconductive drum is transferred onto a print medium. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0051]    Embodiments of the invention will be described in detail with reference to the accompanying drawings.  
         [0052]    First Embodiment  
         [0053]    In the specification, the term toner image surface is used to cover a surface of a print medium on which a toner image is carried when the print medium enters a fixing unit. The term non-toner image surface is used to cover a surface of the print medium on which a toner image is not carried when the print medium enters the fixing unit. The toner image surface is a surface opposite to the non-toner image surface.  
         [0054]    [0054]FIG. 1 illustrates a configuration of a first embodiment of a fixing roller according to the present invention.  
         [0055]    [0055]FIG. 2 illustrates a configuration of a pressure roller of the first embodiment.  
         [0056]    [0056]FIG. 3 illustrates a pertinent portion of an electrophotographic printer of the first embodiment.  
         [0057]    Referring to FIG. 3, the electrophotographic printer according to the first embodiment is of a tandem type color printer. The color printer includes image-forming sections  21 ,  22 ,  23 , and  24  for yellow, magenta, cyan, and black images. The image-forming sections  21 ,  22 ,  23 , and  24  are disposed along a path of a print medium P indicated by arrow A and operate to print the images on the print medium P supplied from a paper cassette, not shown.  
         [0058]    The image-forming sections  21 ,  22 ,  23 , and  24  each include an LED (light emitting diode) type exposing unit. The image-forming section is an integral unit that takes the form of an image drum cartridge  25 . The image drum cartridge  25  includes a charging unit, a developing unit, and a cleaning unit, and can be replaced on a cartridge basis. The image-forming section carries out an electrophotographic process: charging, exposing, developing and transferring.  
         [0059]    A transport belt  10  is entrained about an attraction roller  32 , drive roller  33 , tension roller  34 , and driven roller  35 , and runs in the direction shown by arrow A between the photoconductive drums  11  and the transfer rollers  15 . Another attraction roller  31  is disposed over the attraction roller  32  such that the transport belt  10  is sandwiched between the attraction rollers  31  and  32 .  
         [0060]    A potential difference exists between the attraction rollers  31  and  32 . The potential difference creates dielectric polarization in the print medium P that is transported on the transport belt  10 . The dielectric polarization produces an electrostatic force so that the transport belt  10  attracts the print medium P. The transport belt  10  carries the print medium P thereon, passing through the transfer sections defined between the photoconductive drums  11  and the transfer rollers  15  of the respective image-forming sections. As the print medium P passes through the transfer sections, the toner images of the respective colors are transferred one over the other onto the print medium P in sequence.  
         [0061]    The transport belt  10  is made of, for example, a high resistance semiconductive plastic film. The resistance of the transport belt  10  is selected such that the transport belt  10  is charged to sufficiently attract the print medium P and neutralized by itself after the print medium P has left the transport belt  10 .  
         [0062]    When the print medium P reaches the drive roller  33 , the transport belt  10  releases the print medium P. There is provided a neutralizing device, not shown, above the drive roller  33 . The neutralizing device neutralizes the print medium P so that the print medium P is no longer electrostatically attracted to the transport belt  10  but is separated from the transport belt  10 .  
         [0063]    A neutralizing brush  18  is disposed downstream of the drive roller  33  in the transport path of the print medium P. The neutralizing brush  18  has a base portion that is grounded and a free end that contacts the non-toner image surface of the print medium P to dissipate charges on the non-toner image surface.  
         [0064]    A fixing unit  36  is located downstream of the neutralizing brush  18  and includes a fixing roller  40  and a pressure roller  50 . The fixing roller  40  supplies heat to the toner image on the print medium P. The pressure roller  50  is under the fixing roller  40  to oppose the fixing roller  40  such that the toner-image surface of the print medium P fed between pressure roller  50  and the fixing roller  40  is pressed against the fixing roller  40 .  
         [0065]    The toner images of the respective colors on the print medium P are fused by the heat supplied from the fixing roller  40 , and the pressure roller  50  causes the fused toner to penetrate the print medium P, so that the toner images are fixed. Thereafter, the print medium P is discharged into a stacker, not shown.  
         [0066]    The construction of the fixing roller  40  will be described with reference to FIG. 1.  
         [0067]    Referring to FIG. 1, the fixing roller  40  incorporates a heater  41  in the middle thereof. The heater  41  is accommodated in a metal pipe  42  that has an inner circumferential surface in proximity to the heater  41  and one longitudinal end connected to the ground, not shown. The heater  41  may be provided on an outside surface of the metal pipe  42 . The metal pipe  42  is covered with a resilient member  43  whose surface is covered with an insulation layer  44 . The resilient material  43  is made of a rubber material such as silicone rubber and fluoro rubber. The insulation layer  44  is formed of an insulation material having a very low electrical conductivity, for example, fluororesin such as PFA (tetrafluoroethylene/perfluoroalxylvinylether).  
         [0068]    Whiskers  45  made of zinc oxide are in the shape of a tetrapod that has legs extending in three dimensions. The whiskers  45  are electrically conductive and dispersed in the resilient member  43  such that the surface of the fixing roller  40  is electrically continuous to the metal pipe  42 . The whiskers  45  are dispersed randomly on the surface of the insulation layer  44 , thereby maintaining the metal pipe  42  and the entire surface of the fixing roller  40  at the same potential. In the first embodiment, the whisker  45  is not limited to the shape of a tetrapod but can be of any shape that extends in three dimensions.  
         [0069]    Regular tetrahedrons, defined by four vertexes of the whisker  45 , should preferably fill the resilient member  43  uniformly, so that the electrical conductivity between the surface of the fixing roller  40  and the metal pipe  42  is uniform across the length of the fixing roller  40 .  
         [0070]    A preferable number of whiskers  45  is in the range of 1×10 10 ×V to 1×10 20 ×V, and more preferably in the range of 1.6V×10 13  to 2.4×10 17 μV, provided that the resilient member  43  has a volume of V. The number of whiskers  45  required for filling the entire resilient member  43  and insulation layer  44  can be calculated as follows:  
         [0071]    A length a of legs of the whisker is equal to the distance between the center of gravity of the tetrahedron and the vertex of the tetrahedron. The height and edge of the tetrahedron is given by  
           4   3        a                 and                   2   3          6        a     ,                         
 
         [0072]    respectively. The height of a triangle that defines the base of the tetrahedron is given by {square root}{square root over (2α)}. Thus, the area of the base can be calculated by  
           2   3          6        a   ×     2        a   ×     1   2       =       2   3          3          a   2                   and                           
 
         [0073]    therefore the volume of the tetrahedron is obtained by  
           2   3          3          a   2     ×     4   3        a   ×     1   3       =       8   27          3            a   3     .                             
 
         [0074]    The number n of whiskers that fill the entire resilient member  43  and insulation layer  44  is given by  
       n   =       9   8          3     ×       (     V   /     a   3       )     .                             
 
         [0075]    The length a of fibers of the whisker  45  varies from 2 to 50 μm. A length of fiber of 2 μm provides an optimum value of 2.4×10 17 μV. A length of fiber of 50 μm provides an optimum value of 1.6×10 13 ×V. The range of 1×10 10 μV to 1×10 20 μV is an acceptable range taking into consideration variation in the amount of whisker  45  dispersed in the roller, the thickness and shape of the resilient member  43 , and the insulation layer  44 .  
         [0076]    The construction of the pressure roller  50  will be described with reference to FIG. 2.  
         [0077]    Referring to FIG. 2, the pressure roller  50  has a cylindrical metal shaft  51  with one end thereof grounded. The metal shaft  51  is covered with a resilient member  52  whose surface is covered with an insulation layer  53 . Just like the resilient member  43  of the fixing roller  40 , the resilient member  52  is formed of a rubber material such as silicone rubber and fluoro rubber. Just like the insulation layer  44  of the fixing roller  40 , the insulation layer  53  is formed of an insulation material having a very low electrical conductivity, for example, fluororesin such as PFA (tetrafluoroethylene/perfluoroalxylvinylether).  
         [0078]    Just as in the fixing roller  40 , whiskers  54  of zinc oxide are in the shape of a tetrapod that extends in three dimensions. The whiskers  54  are electrically conductive and dispersed in the resilient member  52  and the insulation layer  53  such that the surface of the pressure roller  50  is electrically continuous to the metal shaft  51 . The whiskers  54  are also dispersed at random on the surface of the insulation layer  44 . Thus, the surface of the fixing roller  40  is maintained at the same potential as the metal pipe  42 . In the first embodiment, the whisker  54  is not limited to the shape of a tetrapod but can be of any shape that extends in three dimensions.  
         [0079]    Regular tetrahedrons, defined by four vertexes of the whisker  54 , should preferably fill the resilient member  53  fully uniformly, so that the pressure roller  40  has a uniform electrical conductivity across the length of the pressure roller  50  between the surface of the pressure roller  50  and metal pipe  51 .  
         [0080]    A preferable amount of whisker  54  is in the range of V×10 6  to V×10 16  and more preferably in the range of 6V×10 9  to 9V×10 13 , provided that the resilient member  53  has a volume of V (including the insulation layer  53 ). A most preferable amount of whisker  54  is in the range of V×10 9  to V×10 13 .  
         [0081]    As previously described, the length of fiber of the whisker  45  varies from 2 to 50 μm. A length of fiber of 2 μm provides an optimum value of 6V×10 9 . A length of fiber of 50 μm of an optimum value of 9V×10 13 . The range of V×10 6  to V×10 16  is an acceptable range taking into consideration variation in the amount of whisker  45  dispersed in the roller and thickness and shape of the resilient member  43  and the insulation layer  44 .  
         [0082]    Generally, the surfaces of a fixing roller and a pressure roller are covered with an insulation layer having a low electric conductivity in order to facilitate smooth peeling off of toner, i.e., to prevent “offset” problem. Use of an insulation layer having a high conductivity minimizes an image force. Image force is a force that acts on a charge when the charge approaches a metal surface. The image force attracts the charge to the metal surface.  
         [0083]    In contrast, in order to remove excessive charges on the print medium, the surface of the roller should be electrically conductive or semiconductive such that the charges thereon are sufficiently dissipated. However, the existence of electrically conductive materials such as metal, carbon black, and graphite on the surface of the roller causes an image force that acts between the roller and the print medium P, thereby inducing offset of toner.  
         [0084]    [0084]FIG. 4 illustrates an image force that is developed between the surface of an electrically conductive roller and the toner on the print medium P. It is assumed that the surface of the roller is made of a metal material. An amount ω of charge will be developed which has a density given by the following equation.  
         ω= q·a /(2π( a   2   +R   2 ) {fraction (3/2)})    
         [0085]    where -q (q&gt;0) is an amount of charge on a spherical toner particle, a is a distance between the surface of the roller and the center of the toner particle, S and Q are positions away from the surface of the roller by the distance a, and R is a distance on the roller surface from the point O. Positions S and Q are located in a three dimensional space defined by X-, Y-, and Z-axis such that one is a mirror image of the other. The Z-axis is perpendicular to the page of FIG. 4.  
         [0086]    Thus, the image force causes the surface of the roller  40  and  50  having electrical conductivity to attract a negatively charged toner T.  
         [0087]    However, the resilient member  43  and the insulation layer  44  contain whiskers  45  dispersed therein and exposed on the surface of the insulation layer  44 . Thus, the PFA resin fills most of the surface of the insulation layer  44  and the whiskers are sparse, facilitating effective peeling-off of toner T.  
         [0088]    The whiskers  45  are dispersed such that the whiskers  45  are electrically continuous with one another. Thus, the surface of the fixing roller  40  and metal pipe  42  are of the same potential. Thus, when the toner image surface of the print medium P is brought into contact with the fixing roller  40 , the charges on the toner image surface enter the resilient member  43  and reaches the metal pipe  42 . Thus, the charges are dissipated. This prevents the offset of toner to the fixing unit  40 .  
         [0089]    With a conventional fixing roller whose surface is covered with an insulating material, it takes a long time for heat generated by a heat source to be transferred to the fixing roller because the insulating material has poor thermal conductivity. In contrast, the resilient member  43  of the fixing roller  40  of the invention contains whiskers  45  dispersed therein and therefore the heat generated by the heat source is transferred quickly through the whiskers  45  to the surface of the fixing roller  40 . This allows temperature control of the heat source  41  in such a way that, for example, heat lost to the print medium P in contact with the fixing roller  40  can be quickly supplied. This leads to high-speed printing.  
         [0090]    With the fixing roller  50 , the insulation layer  53  and resilient member  52  contain the whiskers  54  therein. The PFA resin fills most of the surface of the fixing roller  40 , and the whiskers are sparse, thereby facilitating effective peeling-off of toner T.  
         [0091]    The whiskers  54  are dispersed in the resilient member  52  such that the whiskers  54  are electrically continuous with one another. Thus, the surfaces of the fixing roller  40  and metal pipe  42  are of the same potential. Thus, when the toner image surface of the print medium P is brought into contact with the fixing roller  40 , the charges on the toner image surface enter the resilient member  43  and then reaches the metal pipe  42 . Thus the charges are neutralized.  
         [0092]    Thus, just like the neutralizing brush  18  located downstream of the transfer section and upstream of the fixing unit  36 , the whiskers  54  neutralizes the charges deposited on the non-toner image surface of the print medium P. This prevents the offset of toner to the fixing unit  40 .  
         [0093]    If the leakage current that flows from the transfer roller  15  to the fixing roller  40  and pressure roller  50  becomes large enough to cause an offset problem, current limiting elements such as varistor  71  may be inserted between the ground and the metal pipe  42  and the metal shaft  51  of the pressure roller  50 .  
         [0094]    Second Embodiment  
         [0095]    [0095]FIG. 5 illustrates a pertinent portion of an electrophotographic printer according to a second embodiment.  
         [0096]    Referring to FIG. 5, the electrophotographic printer according to the second embodiment is of a conventional tandem type color printer. The color printer includes image-forming sections  21 ,  22 ,  23 , and  24  for yellow, magenta, cyan, and black images, which are disposed along a path of a print medium P indicated by arrow A and operate to print images of corresponding colors on the image-carrying surface of the print medium.  
         [0097]    A transport belt  10  is entrained about an attraction roller  32 , drive roller  33 , tension roller  34 , and driven roller  35 , and runs in the direction shown by arrow A between the photoconductive drums  11  and the transfer rollers  15  of the image forming sections  21 - 24 . Another attraction roller  31  is disposed over the attraction roller  32  such that the transport belt  10  is sandwiched between the attraction rollers  31  and  32 .  
         [0098]    A potential difference exists between the attraction rollers  31  and  32 . The potential difference creates dielectric polarization in the transport belt  10  and the print medium P that is transported on the transport belt  10 . The dielectric polarization produces an electrostatic force so that the transport belt  10  attracts the print medium P. The transport belt  10  carries the print medium P thereon, passing through the transfer sections defined between the photoconductive drums  11  and the transfer rollers  15  of the respective image-forming sections  21 - 24 . Thus, the toner images of the respective colors are transferred one over the other onto the print medium P as the print medium P passes through the respective transfer sections in sequence.  
         [0099]    The transport belt  10  is made of, for example, a high resistance semiconductive plastic film. The resistance of the transport belt  10  is selected such that the transport belt  10  is charged to sufficiently attract the print medium P and neutralized by itself after the print medium P has left the transport belt  10 .  
         [0100]    When the print medium P reaches the drive roller  33 , the transport belt  10  releases the print medium P. There is provided a neutralizing device, not shown, above the drive roller  33 . The neutralizing device neutralizes the print medium P so that the print medium P is no longer electrostatically attracted to the transport belt  10  but is separated from the transport belt  10 .  
         [0101]    A neutralizing brush  18  is provided in the transport path of print medium downstream of a final one of the transfer sections defined between the photoconductive drum  11  for black and transfer roller  15 . The free end of the neutralizing brush  18  is in contact with a non-toner image surface of the print medium P in order to neutralize the charges deposited on the non-toner image surface. The base portion of the neutralizing brush  18  is grounded.  
         [0102]    Downstream of the neutralizing brush  18 , there is provided a fixing unit  19  that includes a fixing roller  19   a  and a pressure roller  19   b . The fixing roller  19   a  has a heat source, not shown, that supplies heat to the surface of the print medium P onto which a toner image has been transferred. The pressure roller  19   b  is disposed under the fixing roller  19   a  and presses the print medium P against the fixing roller  19   a.    
         [0103]    The fixing unit  19  is housed in an upper case  119  and a lower case  120 . The fixing unit  19  may be of a conventional type, but the fixing unit  36  of the first embodiment will be very effective. The neutralizing brush  60  is mounted to a part of the upper case  119  and is located downstream of the fixing unit  19  in the transport path of the print medium P. The neutralizing brush  60  has a free end that contacts the toner image surface of the print medium P to remove the charges deposited on the toner image surface of the print medium P, and a base portion that is grounded.  
         [0104]    The neutralizing brush  60  is preferably disposed as close to a nip defined between the fixing roller  19   a  and the pressure roller  19   b  as possible. The shorter the distance between the neutralizing brush  60  and the nip, the more effectively the neutralizing brush  60  begins to neutralize the print medium P from the forward end of the print medium P to effectively prevent the offset problem. The fixing roller  19   a  and pressure roller  19   b  have finite dimensions and therefore the neutralizing brush  60  is disposed such that the free end of the neutralizing brush  60  contacts the print medium P about 30 mm downstream of the nip.  
         [0105]    As described previously, the non-toner image surface of the print medium P that enters the fixing unit  19  has been subjected to positive dielectric polarization and the toner image surface of the print medium P has been subjected to negative dielectric polarization. The charges deposited on the non-toner image surface of the print medium P are easily dissipated by the brush  18 , pressure roller  19   b , and the frame and chassis of the printer. However, the charges on the image carrying surface and charges on the non-toner image surface are opposite in polarity and the same in amount, and therefore the charges on the respective surfaces of the print medium P cannot easily be removed.  
         [0106]    Providing the neutralizing brush  60  downstream of the fixing unit  36  in addition to the brush  18  allows eliminating of the charges on the both surfaces of the print medium P in equilibrium condition. Eliminating the charges with the neutralizing brush  60  reduces the potential difference between the fixing roller  40  and the print medium P, thereby effectively preventing the offset of the toner carried on the toner image surface of the print medium P.  
         [0107]    FIGS.  6 A- 6 C illustrate cases when the neutralizing brush  60  is grounded through a varistor, a Zener diode, and a resistor, respectively. Arrows A indicate the direction of travel of the print medium P.  
         [0108]    If a large leakage current flows from, for example, the transfer roller  15  to the neutralizing brush  60  to cause the offset problem of toner image, the neutralizing brush  60  may be grounded through a varistor (FIG. 6A) to limit the current that flows through the neutralizing brush  60 . Alternatively, the neutralizing brush  60  may be grounded through a Zener diode (FIG. 6B) or a resistor (FIG. 6C) instead of the varistor in order to limit current. Limiting the current prevents noise from being generated and offers a reliable printer.  
         [0109]    A pair of transport rollers  121   a  and  121   b  is disposed downstream of the neutralizing brush  60 . After the print medium P has passed the neutralizing brush  60 , the transport rollers  121   a  and  121   b  advance the print medium P through the guides  122 - 125  to discharge rollers  126  and  127 . The print medium P is then further advanced by the discharge rollers  126  and  127  to an upper stacker  128   a  defined on a part of an upper cover  128 . A photo sensor  109  detects a rearward end of the print medium.  
         [0110]    Third Embodiment  
         [0111]    [0111]FIG. 7 illustrates a pertinent portion of an electrophotographic printer according to a third embodiment.  
         [0112]    Referring to FIG. 7, the electrophotographic printer according to the third embodiment differs from the second embodiment in that the neutralizing roller  70  is used in place of the neutralizing brush  60  and brought into contact with the toner image surface of the print medium P. The neutralizing roller  70  is grounded. Just as in the neutralizing brush  60 , the neutralizing roller  70  should be as close to the nip defined between the fixing roller  40  and the pressure roller  50  as possible.  
         [0113]    The neutralizing brush  60  according to the second embodiment deteriorates over time due to the friction between the print medium P and the neutralizing brush  60 . The neutralizing roller  70  is not driven in rotation by any drive means and is freely rotatable such that when the print medium P is advanced, the neutralizing roller  70  rotates in rolling contact with the print medium P. Thus, the neutralizing roller  70  does not impose a drag on the print medium P and therefore the neutralizing roller  70  need not be replaced for a new, unused one during the lifetime of the printer. In other words, the neutralizing roller  70  is maintenance free.  
         [0114]    Fourth Embodiment  
         [0115]    [0115]FIG. 8 illustrates a pertinent portion of an electrophotographic printer according to a fourth embodiment.  
         [0116]    Referring to FIG. 8, the fourth embodiment differs from the first embodiment in that a neutralizing member  80  is added to the electrophotographic printer of FIG. 3.  
         [0117]    The neutralizing member  80  is in the shape of a metal rod having a sharp point. The neutralizing member  80  is disposed downstream of the transfer section for black image and upstream of the fixing unit  36 , and connected to the ground. The neutralizing member  80  is mounted such that the sharp point is very close to the toner image surface of the print medium P when the print medium P passes under the neutralizing member  80 .  
         [0118]    When the print medium P with a toner image transferred thereon passes the neutralizing member  80 , electrostatic induction creates charges on the sharp point of the neutralizing member  80 , the charges being opposite in polarity to those deposited on the toner image surface of the print medium P. Attraction is developed between the negative charges on the toner image surface of the print medium P and the positive charges on the sharp point of the neutralizing member  80 .  
         [0119]    Therefore, when the toner image surface of the print medium P is excessively charged, a discharge occurs between the sharp point of the neutralizing member  80  and the toner image surface, reducing the negative charges on the toner image surface. A decrease in charge reduces the potential difference between the fixing roller  40  and the printed medium P, thereby effectively preventing the offset of the toner T deposited on the toner image surface of the print medium P to the fixing roller  40 .  
         [0120]    Fifth Embodiment  
         [0121]    [0121]FIG. 9 illustrates a pertinent portion of an electrophotographic printer according to a fifth embodiment.  
         [0122]    The fifth embodiment differs from the first embodiment in that a humidifier  92  is provided in place of the neutralizing member  18  and a moisture detector  90  and a moisture-supply calculator  91  are added.  
         [0123]    The print medium P contains less moisture therein when the print medium P is placed in a low humidity environment, and therefore the print medium P has good insulation such that both the toner image surface and the non-toner image surface of the print medium P are apt to be at high potential. As is clear from FIG. 10, the moisture in the air is determined by the temperature and humidity.  
         [0124]    An experiment conducted for various values of humidity at a temperature of 20° C. revealed that the offset problem was not observed when the absolute humidity was higher than 70%. It is considered that if the moisture in the air is more than 10 g/m 3 , the electrostatic polarization of the water molecules contained in the print medium P dissipates excessive charges on the toner image surface and non-toner image surface of the print medium P to prevent the offset of toner at the fixing unit  36 . For example, the dielectric constant of water is about 80 times that of vacuum.  
         [0125]    The moisture detector  90  detects the temperature and humidity of the air surrounding the print medium P and determines an amount of moisture in the air using the table of FIG. 10 that list moisture values for humidity values. In accordance with the moisture in the air detected by the moisture detector  90 , the moisture-supply calculator  91  determines an amount of moisture required for increasing the moisture content in the print medium P to a value higher than 10 g/m 3 .  
         [0126]    The humidifier  92  is disposed downstream of the transfer section for black image and upstream of the fixing unit  36 . The humidifier  92  humidifies the toner image surface of the print-medium P to supply an amount of moisture determined by the moisture-supply calculator  91 . If the moisture in the air detected by the moisture detector  90  is more than 10 g/m 3 , then the amount of moisture calculated by the moisture-supply calculator  91  is 0 g/m 3 . Thus, the humidifier  92  does not humidify.  
         [0127]    Thus, in the fifth embodiment, the print medium P is humidified such that the moisture content is more than 10 g/m 3 . Therefore, the excessive charge will be dissipated so that the offset problem nof toner at the fixing unit  36  is effectively prevented.  
         [0128]    The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art intended to be included within the scope of the following claims.