Patent Publication Number: US-7725064-B2

Title: Fixing device and image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
   1. The present invention relates to a fixing device capable of favorably removing an electric charge which disturbs a toner image at a time of fixing to cause an image deterioration from a surface of a heating rotary member. 
   2. Description of the Related Art. A fixing device used in an image forming apparatus such as a copying machine and a printer allows a pressing member to come in contact with a heating rotary member such as a fixing drum, a fixing belt or the like to form a fixing nip portion. Then, the fixing device allows a sheet bearing an unfixed toner image to pass through the fixing nip portion in a manner such that the toner image comes in contact with a surface of the heating rotary member, and then applies heat and pressure to the same to fix the toner image onto the sheet surface. 
   Since the unfixed toners are electrically charged at this time, they may be gradually accumulated on the fixing rotating member. In such case, the accumulated toners are likely to disturb a toner image formed on a next sheet to be fixed to cause an image deterioration. 
   Various technologies for preventing such deterioration in an image quality have been proposed. For example, documents 1 through 6 disclose the following contents. 
   Japanese Patent Unexamined Publication No. HEI 3-116176 (document 1) discloses a state where an earthed charge-removing brush is in contact with a heating roller. Japanese Patent Unexamined Publication No. 2003-280420 (document 2) discloses a charge-removing unit capable of moving toward and apart from a heating member, and Japanese Patent Unexamined Publication No. HEI 3-148682 (document 3) discloses a state where a charge-removing roller made of metal comes in contact with a fixing roller having an elastic rubber layer so that it is rotated. Further, Japanese Patent Unexamined Publication 2002-162857 (document 4) discloses a state where an earthed charge-removing brush comes in contact with an exposed part of a metal core of a fixing roller. Japanese Patent Unexamined Publication No. HEI 3-91786 (document 5) discloses an arrangement of a discharge plate so shaped as to have a plurality of sharp-pointed projections to be positioned close to a surface of a heating roller in a non-contact state. Further, Japanese Patent Unexamined Publication 2005-31402 (document 6) discloses a state where a charge-removing sheet is so arranged as to face a surface of a heating roller on upstream of a fixing nip portion along a rotational direction of a heating roller. 
   However, according to the above-described documents 1 to 4, a member adapted to remove an electric charge (the charge-removing brush, the charge-removing unit, the charge-removing roller, and the earthed charge-removing brush) is so arranged as to come in contact with a member whose electric charge is to be removed (the heating roller, the heating member, the fixing roller, and the metal core of the fixing roller). Accordingly, there is a problem that a charge-removing efficiency is lowered drastically in a case where the contact part is contaminated by toners and paper powders. 
   Further in a case where an image forming apparatus is a color image forming machine, and a charge-removing member comes in contact with a sheet-passing area of a surface bearing a toner image (print surface side), a contact line (a line caused by a tight contact) is likely to appear on an image. 
   On the other hand, the discharge plate as a charge-removing member in the documents 5 and the charge-removing sheet in the document 6 are so arranged in non-contact state with respect to the member whose electric charge is to be removed. Accordingly, the lowering in a charge-removing efficiency due to toners and paper powders and occurrence of the contact line in a color image like the ones described above can be prevented. 
   However, there is a problem that the charge-removing members disclosed in the documents 5 and 6 can effectively remove an electric charge gradually accumulated in a wide area of a surface of the heating rotary member, but sufficient removal of electric charge is not possible if the electric charge is locally and remarkably accumulated. In other words, a great difference in electric potential occurs between a toner existing part and a toner non-existing part on a sheet at a time of fixing operation. Further, when a rear end of the sheet is separated from the heating rotary member, a separating discharge occurs. The charge-removing members of the documents 5 and 6 require a long time for removal of locally and remarkably accumulated electric charge. Therefore, an image deterioration is likely to occur, for example, when a second or later sheet is fixed at a time of consecutive sheet transfer. 
   SUMMARY OF THE INVENTION 
   In view of the above, an object of the present invention to provide a fixing device capable of preventing the lowering in a charge-removing efficiency and the occurrence of a contact line in a color image, and also capable of performing in a relatively short time a sufficient removal of electric charge accumulated locally and remarkably due to a separating discharge. 
   (1) The invention in accordance with a first embodiment relates to a fixing device which forms a fixing nip portion by allowing a pressing member to come in contact with a heating rotary member having a surface which is not electrically conductive, and allows a sheet to pass through the fixing nip portion in an orientation such that an unfixed toner image bore on a sheet surface comes in contact with a surface of the heating rotary member, so that the image is heated and pressed to fix on the sheet. The fixing device in accordance with this invention includes: a base member being electrically conductive and positioned with respect to the surface of the heating rotary member; and a separating member which has a separating portion on its leading end side for separating the sheet bearing the fixed toner image from the surface of the heating rotary member, the separating member being retained by the base member at its base end side so that the separating portion is so positioned as to face the surface of the heating rotary member at a first clearance apart; and a charge-removing member retained by the base member for removing electric charge from the surface of the heating rotary member. The charge-removing member has a charge-removing portion which is arranged along a longitudinal direction of the surface of the heating rotary member on further downstream side along a rotational direction of the heating rotary member than the separating portion, and is so positioned as to face the surface of the heating rotary member at a second clearance apart. 
   According to the invention in accordance with the above-described first embodiment, the electrically conductive base member is positioned with respect to the surface of the heating rotary member. Further, the separating member is retained by the base member, so that it is so positioned as to face the surface of the heating rotary member at a first clearance apart. According to this, the separating portion can reserve a size of the first clearance at a high accuracy, and is inserted between the surface of the heating rotary member during rotation and a leading end of the sheet with the toner image formed thereon. Consequently, the sheet can be favorably separated. 
   On the other hand, the charge-removing member is retained by the base member similarly to the separating member, so that the charge-removing portion is so positioned as to face the surface of the heating rotary member along a longitudinal direction of the surface of the heating rotary member at a second clearance apart. Accordingly, the charge-removing portion can reserve a size of the second clearance at a high accuracy. In other words, the charge-removing portion can be arranged close to the heating rotary member in a non-contact state and at a high accuracy. Consequently, the charge-removing portion can favorably remove electric charge from the surface of the heating rotary member during rotation. 
   Further, the separating portion of the charge-removing member is arranged on a further downstream side along the rotational direction of the heating rotary member than the separating portion. In other words, since the separating member and the charge-removing member are retained commonly by the base member, the charge-removing portion of the charge-removing member can be arranged close to the downstream side of the separating portion of the separating member along a rotational direction of the heating rotary member. Meanwhile, an operation of fixing the toner image, the sheet is attached firmly to the surface of the heating rotary member at the fixing nip portion by the pressing member, and separated from the surface of the heating rotary member by the separating portion after the operation of fixing the toner image, the heating rotary member is likely to be charged between the fixing nip portion and the separating portion with electric charge of the toner image at the time of fixing operation or the separating discharge at the rear end of the sheet. In the invention in accordance with the first embodiment, since the charge-removing portion arranged on the immediate downstream side of the separating portion removes an electric charge from the surface of the heating rotary member, in other words, it can remove the electric charge from the surface of the heating rotary member immediately after the charging, an efficient removal of electric charge can be performed. 
   (2) The invention in accordance with the second embodiment relates to a fixing device which forms a nip portion by allowing a pressing member to come in contact with a heating rotary member having a surface which is not electrically conductive, and allows a sheet to pass through the fixing nip portion in an orientation such that an unfixed toner image bore on a sheet surface comes in contact with a surface of the heating rotary member, so that the image is heated and pressed to be fixed on the sheet. The fixing device according to this invention includes: a base member being electrically conductive and positioned with respect to the surface of the heating rotary member surface; and a separating member which has a separating portion on its leading end side for separating the sheet bearing the fixed toner image from the surface of the heating rotary member, the separating member being retained by the base member at its base end side so that the separating portion is so positioned as to face the heating rotary member at a first clearance apart. The base member has a charge-removing portion which is arranged along a generating line of the surface of the heating rotary member on further downstream side along a rotational direction of the heating rotary member than the separating portion, and is so positioned as to face the surface of the heating rotary member at a second clearance apart. 
   According to the invention in accordance with the second embodiment, the base member being electrically conductive is positioned with respect to the surface of the heating rotary member. Further, the separating member is retained by the base member, so that the separating portion is as to face the surface of the heating rotary member at a first clearance apart. Accordingly, the separating portion can reserve a size of the first clearance at a high accuracy, and is inserted between the surface of the heating rotary member during rotation and a leading end of the sheet with the toner image formed thereon. Consequently, the sheet can be favorably separated. 
   On the other hand, the base member is so positioned that the charge-removing portion faces the surface of the heating rotary member at a second clearance apart along a longitudinal direction of the surface of the heating rotary member. Accordingly, the charge-removing portion can reserve a size of the second clearance at a high accuracy. In other words, the charge-removing portion can be arranged close to the heating rotary member in a non-contact state and at a high accuracy. Consequently, the charge-removing portion can favorably remove electric charge from the surface of the heating rotary member surface during rotation. 
   Further, the charge-removing portion is arranged on a further downstream side along the rotational direction of the heating rotary member than the separating portion. In other words, since the charge-removing portion is formed on the base member which retains the separating member, it can be arranged close to the downstream side of the separating portion of the separating member along the rotational direction of the heating rotary member. Meanwhile, during an operation of fixing the toner image, the sheet is attached firmly to the surface of the heating rotary member at the fixing nip portion by the pressing member, and separated from the surface of the heating rotary member by the separating portion after the operation of fixing the toner image, the heating rotary member is likely to be charged between the fixing nip portion and the separating portion with electric charge of the toner image at the time of fixing operation or the separating discharge at the rear end of the sheet. In the invention in accordance with the second embodiment, since the charge-removing portion arranged on the immediate downstream side of the separating portion removes an electric charge from the surface of the heating rotary member, in other words, it can remove the electric charge from the surface of the heating rotary member immediately after the charging, an efficient removal of electric charge can be performed. 
   These and other objects, features and advantages of the present invention will become apparent upon reading of the following detailed description along with the accompanied drawings. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  shows a schematic configuration of an image forming apparatus including a fixing device in accordance with the present invention, which is viewed from a front side. 
       FIG. 2  is an enlarged view showing the fixing device, which is viewed from the front side. 
       FIG. 3  is an enlarged sectional view illustrating a layer configuration of a heating belt. 
       FIG. 4  is an enlarged view showing a separating and charge-removing member constituting the fixing device, which is viewed from the front side. 
       FIG. 5  is a perspective view showing the separating and charge-removing member, which is viewed from a back side (a side of the charge-removing member). 
       FIG. 6  is an enlarged view showing another separating and charge-removing member constituting the fixing device, which is viewed from the front side. 
       FIG. 7  is an enlarged view of a charge-removing portion showing a base member shown in  FIG. 6 , which is viewed from a direction of an arrow B. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. It should be understood that members identified with the same reference numerals in the drawings have the same configuration, and duplicate descriptions of those will be properly omitted. Further, members and the like which are not necessary to be described are properly omitted from each of the drawings. 
   Embodiment 1 
   An image forming apparatus including a fixing device  1  in accordance with the present invention will be described with reference to  FIG. 1 .  FIG. 1  shows a schematic configuration of an image forming apparatus  2  including the fixing device  1  which is viewed from a front side (front side: a side on which an operator is positioned at a time of operating the image forming apparatus  2 ). 
   As shown in  FIG. 1 , the image forming apparatus  2  includes a sheet-conveying section  10 , an image forming section  11 , and a fixing section  12 . In the sheet-conveying section  10 , a sheet-feeding roller  14  supplies sheets P stacked and accommodated in a sheet-feeding cassette  13 , and a conveying roller  15  and a retard roller  16  separate the sheets P one another. Further, a pair of conveying rollers  17  allows the sheet P to strike against a pair of registration rollers  18  which is suspended, so that an oblique motion of the sheet P is corrected. 
   In the image forming section  11 , a charging roller  21  charges a photoconductive drum  20  rotated in a direction of an arrow R 20  to a predetermined polarity and electric potential. An exposure device  22  performs exposure in accordance with image information to a charged surface of the photoconductive drum  20  to form an electrostatic latent image thereon. A developing device  23  allows toners to attach to the electrostatic latent image to develop the electrostatic latent image as a toner image. 
   The toner image is conveyed by rotation of the photoconductive drum  20  to a transfer nip T formed between the photoconductive drum  20  and a transferring roller  24 . In synchronization with this, the sheet P is supplied from the pair of registration rollers  18  to the transfer nip T, and then the transferring roller  24  transfers the toner image onto the sheet P. 
   After the toner image is transferred, a cleaning device  25  removes toners remained on the surface of the photoconductive drum  20 , and then the photoconductive drum  20  is used for the next image forming operation. On the other hand, the toner image transferred onto the sheet P is fixed by the fixing device  1  arranged in the fixing section  12 . The fixing device  1  will be described in detail hereinafter. The sheet P bearing the fixed toner image is discharged by a pair of sheet-discharging rollers  26  to a sheet-discharging tray  27 . Accordingly, the image forming with respect to one side (front side) of one sheet P is terminated. 
   The fixing device  1  will be described in detail with reference to  FIGS. 2  to  5 .  FIG. 2  is an enlarged view showing the fixing device  1 , which is viewed from the front side.  FIG. 3  is an enlarged sectional view illustrating a layer configuration of a heating belt  32 .  FIG. 4  is an enlarged view showing a separating and charge-removing member  34  constituting the fixing device  1 , which is viewed from the front side.  FIG. 5  is a perspective view showing the separating and charge-removing member  34 , which is viewed from a back side (a side of the charge-removing member). 
   As shown in  FIG. 2 , the fixing device  1  includes a fixing roller  30 , a heating roller  31 , the heating belt  32  as a heating rotary member extending between the fixing roller  30  and the heating roller  31 , a pressing roller (pressing member)  33  as a pressing member, and the separating and charge-removing member  34 . 
   Among these, the fixing roller  30  has a metal core  35  as a rotational center and a cylindrical roller portion  36  (for example, a sponge roller) surrounding an outer peripheral surface of the metal core  35  and having an elasticity. The metal core  35  of the fixing roller  30  extends in forward and backward directions and is freely rotatably supported by a main body frame  37  which is a strength member of a main body of the image forming apparatus. The fixing roller  30  is rotated in a direction of an arrow R 30  by a driving unit (unillustrated) provided in the main body of the image forming apparatus. 
   The heating roller  31  is arranged on the right hand side of the above-described fixing roller  30  and in parallel with the fixing roller  30 . The heating roller  31  includes a cylindrical drum  38  made of metal, and provided therein with rod-like heaters  40  e.g. halogen heaters so oriented that respective longitudinal axes extends in forward and backward directions. In the present embodiment, two heaters  40  are used. The heating roller  31  is rotatably supported by the main body frame  37  and is rotated in a direction of an arrow R 31  by rotation of the heating belt in a direction of an arrow R 32 . The heating roller  31  is heated by the heaters  40 , so that it heats the heating belt  32 . 
   The heating belt  32  extends between the above-described fixing roller  30  and heating roller  31  endlessly.  FIG. 3  shows a layer configuration of the heating belt  32 .  FIG. 3  schematically shows an enlarged sectional view of a part “A” of the heating belt  32  shown in  FIG. 2 . In  FIG. 3 , a lower side corresponds to an inner side of the heating belt  32 , and an upper side corresponds to an outer side of the heating belt  32 . As shown in  FIG. 3 , the heating belt  32  has a metal layer  42  as a base layer, a rubber layer  41  covering an outer peripheral surface of the metal layer  42 , and a surface layer  43  covering an outer peripheral surface of the rubber layer  41 . As material, for example, the rubber layer  41  is formed of a silicone rubber having a thickness of 270 μm, and a metal layer  42  is formed of nickel (Ni) coated by electroforming and having a thickness of 30 μm, and the surface layer  43  is formed of a PFA tube having a thickness of 30 μm. The metal layer  42  is grounded. The heating belt  32  is formed to have a thickness of 0.3 mm as a whole. A width of the heating belt  32  in forward and backward directions (hereinafter, referred to as a “belt width”) is slightly shorter than lengths of the above-described fixing roller  30  and heating roller  31  in forward and backward directions (length in an axial direction) and is set to be longer than a sheet width of a sheet P having the longest sheet width of sheets P bearing the fixed toner image. The heating belt  32  is rotated in a direction of the arrow R 32  with rotation of the fixing roller  30  in a direction of the arrow R 30 . 
   The pressing roller  33  is formed of a cylindrical drum  44  made of metal and is arranged on the left hand side of the above-described fixing roller  30  in parallel with the fixing roller  30 . The pressing roller  33  is in contact with a part of the above-described heating belt  32  around the fixing roller  30  to form a fixing nip portion N with the heating belt  32 . Since an outer peripheral side of the above-described fixing roller  30  is a sponge roller, and the base layer  41  of the heating roller  32  is made of rubber, a surface of the pressing roller  33  digs into the fixing roller  30  and forms a fixing nip portion N having a predetermined width (nip width) along a rotational direction of the heating belt  32 . The unfixed toner image bore on the sheet P in the above-described image forming section  11  (refer to  FIG. 1 ) comes in contact with the heating belt  32  at a time of passing through the fixing nip portion N, so that the unfixed toner image is heated and pressed to fix on the surface of the sheet P. 
   As shown in  FIGS. 2 and 4 , the separating and charge-removing member  34  is arranged above the fixing roller  30  in the present embodiment. As shown in  FIGS. 4 and 5 , the separating and charge-removing member  34  has a base member  50 , a separating member  51  and a charge-removing member  52  retained by the base member  50 , a spacer  53 , and a tension spring (biasing member)  54 . Among those, the first three members, in other words, the base member  50 , the separating member  51 , and the charge-removing member  52  are formed to be longer than a belt width of the heating belt  32 , and the remaining spacer  53  and tension spring  54  are provided respectively at a front end portion and a rear end portion in forward and backward directions (hereinafter, desirably referred to as “longitudinal direction”) of the base member  50 . Hereinafter, the members are described in order starting from the base member  50 . 
   As shown in  FIGS. 4 and 5 , the base member  50  has an attachment portion  55  and arm portions  56 . The attachment portion  55  has an L-shaped cross section of a direction perpendicular to the longitudinal direction and is long in forward and backward directions. The arm portions  56  are provided respectively at the front and rear end portions in a longitudinal direction of the attachment portion  55 . However,  FIGS. 4 and 5  shows only the arm portion  56  provided at the rear end. As shown in  FIG. 4 , the attachment portion  55  has a retaining portion  57  and a reinforcing portion  58  so formed as to be a rectangular plate and is so formed as to have a cross section of an L-shape in a direction perpendicular to the longitudinal direction. The reinforcing portion  58  is formed by bending a base end of the retaining portion  57  toward the downstream of a rotational direction of the heating belt  32  so as to be at almost right angle. The retaining portion  57  is so arranged that its leading end portion tilts toward the heating belt  32 , in other words, tilts in a counter direction with respect to a rotational direction of the heating belt  32 . An edge  60  being a part of the leading end portion of the retaining portion  57  and close to the heating belt  32  comes close to the surface of the heating belt  32  at a clearance G 3  apart and is arranged along a generating line of the heating belt  32 . A surface of the retaining portion  57 , in other words, a surface on an upstream along a rotational direction of the heating belt  32  is a separating member attachment surface  57   a  for attaching the separating member  51  described hereinafter. On the other hand, a back surface of the retaining portion  57 , in other words, a surface on downstream along a rotational direction of the heating belt  32  is a charge-removing member attachment surface  57   b  for attaching the charge-removing member  52  described hereinafter. 
   The arm portions  56  are plate-like members formed by bending opposite end portions in forward and backward directions (longitudinal direction) of the retaining portion  57  toward the back surface side and is arranged on outer side than the frond and rear ends of the above-described fixing roller  30 . The arm portion  56  has a through hole  61  at its substantially center portion, and the through hole  61  is engaged with a boss (supporting portion)  62  which projects from a main body frame  37  inwardly. The base member  50  as a whole is supported swingably about the boss  62  as a swing center. The through hole  61  of the arm portion  56  is so positioned that a frictional force applied to after-mentioned spacers  53  by rotation of the heating belt  32  in a direction of the arrow R 32  is applied toward the through hole  61 . This prevents a change in a contact pressure applied against the spacers  53  which comes in contact with the heating belt  32  or fixing roller  30 . On the rear end side (the right side in  FIG. 4 ) of the through hole  61 , there is formed a bent portion  63  which is bent inwardly. At a lower end of the bent portion  63 , there is formed a recess  64  to which a leading end (lower end) of a tension spring  54  is engaged. Between the recess  64  and an engaging portion  65  of the main body frame  37 , there is provided a tension spring  54  as a biasing member in a stretched state. This applies a bias to the base member  50  in a direction of an arrow R 50  shown in  FIG. 4 . 
   The separating member  51  is attached to the separating member attachment surface  57   a  on the front surface side of the retaining portion  57  of the above-described base member  50 . The separating member  51  is so formed as to have a rectangular shape which is long in forward and backward directions, and its length in forward and backward directions is set to be slightly shorter than that of the retaining portion  57 . On the other hand, a length of a short side (a length in a vertical direction in  FIG. 4 ) is so formed as to be longer than that of the retaining portion  57 , and its leading end portion  66  is so attached as to project from a leading end of the retaining portion  57 . The separating member  51  as a whole is oriented toward the direction same as that of the retaining portion  57 , in other words, toward a counter direction with respect to the rotational direction of the heating belt  32 , and a separating portion  67  adapted to separate the sheet P bearing the fixed toner image is provided at a line edge of the separating member  51 . The separating portion  67  is so arranged that it comes close to a surface of the heating belt  32  at a clearance G 1  apart, and follows a generating line of the surface of the heating belt  32 . The clearance G 1  is set to be smaller than the clearance G 3  between the edge  60  of the above-described retaining portion  57  and the surface of the heating belt  32 . For example, it is set to be a distance between 0.3 mm and 0.5 mm. The separating member  51  is formed of an electrically conductive thin plate such as a thin plate made of metal. The clearance G 1  is maintained by after-mentioned spacers  53  at a high accuracy. The separating portion  67  of the separating member  51  is coated with a member such as a PTFE (polytetrafluoroethylene) having an excellent separability to prevent attachment of toners and the like. 
   The charge-removing member  52  is attached to the charge-removing member attachment surface  57   b  on the back surface side of the retaining portion  57  of the above-described base member  50 . The charge-removing member  52  is so formed as to have a rectangular-like shape which is long in forward and backward directions, and its length in the forward and backward directions is set to be slightly shorter than that of the retaining portion  57 . On the other hand, a length of a short side (a length in substantially vertical direction in  FIG. 4 ) is formed to be slightly longer than that of the retaining portion  57 , and a leading end portion of the short side slightly projects from the leading end of the retaining portion  57 . The separating member  52  as a whole is so arranged as to be oriented toward a direction same as that of the retaining portion  57 , in other words, arranged in a counter direction with respect to the rotational direction of the heating belt  32 , and a leading edge as a charge-removing portion  68  is so arranged as to come close to the surface of the heating belt  32  at a clearance G 2  apart and follow the generating line direction of the surface of the heating belt  32 . The clearance G 2  is set smaller than the clearance G 3  between the edge  60  of the above-described retaining portion  57  and the surface of the heating belt  32 , but same as or larger than the clearance G 1  between the separating portion  67  of the separating member  51  and the surface of the heating belt  32 . For example, it is set to have a distance between 0.3 mm and 1.0 mm. In other words, a size relation between the clearances G 1 , G 2 , G 3  is G 1 ≦G 2 &lt;G 3 . Accordingly, toners which pass through the clearance G 1  do not attach to the edge  60  or charge-removing portion  68 . Material to be used for the charge-removing member  52  may include a non-woven fabric including an electrically conductive filler, a sheet woven with electrically conductive fiber, and a brush-like member formed of electrically conductive fiber. In other words, it is preferable that charge-removing portion  68  arranged on the surface of the heating belt  32  at a clearance apart is not linear like metal, and the electrically conductive filler or electrically conductive fiber properly project from the charge-removing portion  68 . This enables even and fine removal of electric charge from the surface of the heating belt  32 . Especially, it is effective in removal of electric charge with respect to local electric charges such as the separating discharge and the like. The removed electric charge is grounded through the base member  50  and main body frame  37 . 
   The spacers  53  are provided on the retaining portion  57  of the base member  50  respectively at positions inner than the arm portions  56  provided at the front end side and rear end side in the longitudinal direction. Each spacer  53  is so arranged that it extends from the back side of the retaining portion  57  (the side of the charge-removing member attachment surface  57   b ) to the surface side (separating member attachment surface  57   a ) through the leading end side, and is fixed to the retaining portion  57  with a screw  70 . A contact surface  71  at the leading end of the spacer  53  which comes in contact with the surface of the heating belt  32  is so formed as to be an arc-shape heating belt  32  following the surface. Further, the end portions in the contact surface  71  on the upstream side and downstream side along the rotational direction of the heating belt  32  include mildly curved surfaces  72 ,  73 . The base member  50  is biased in a direction of an arrow R 50  about the boss  62  as a base by a biasing force of the tension spring  54 , so that each spacer  53  allows the contact surface  71  to come in contact with the surface of the heating belt  32 . The contact pressure at this time can be set to a favorable pressure by properly setting a length and spring constant of the tension spring  54 . Further, a length of the contact surface  71  along the rotational direction of the surface of the heating belt  32  is set to be relatively long, in other words, set to be long relative to a contact length of a case where the charge-removing member  52  is not provided, so that a contact pressure per unit area is reduced, and thereby preventing irrelevant digging of the spacer  53  into the heating belt  32  and fixing roller  30 . In such a manner, in a state where the spacers  53  are in contact with the surface of the heating belt  32 , clearances (distance) of the surface of the heating belt  32  with respect to the separating portion  67  of the separating member  51 , the edge  60  of the retaining portion  57 , and the charge-removing portion  68  of the charge-removing member  52  are G 1 , G 3 , G 2  in this order, and the distances have the relation of G 1 ≦G 2 &lt;G 3  as described above. Accordingly, the spacer  53  comes in contact with the surface of the heating belt  32 , so that the clearance G 1 , G 2 , G 3  can be maintained at a high accuracy while following an irregularity of the surface of the heating belt  32 . Further, in the present embodiment, in a state of viewing from the direction shown in  FIG. 4 , either the separating portion  67  of the separating member  51  and the charge-removing portion  68  of the charge-removing member  52  are within a predetermined area between the spacers  53  (not projecting from an outer contour of the spacer  53 ), it can be effectively prevented that the surface of the heating belt  32  irrelevantly come in contact with the separating portion  67  of the separating member  51  or the charge-removing portion  68  of the charge-removing member  52 . 
   As described above, according to the fixing device in accordance with the present embodiment, allowing the spacer  53  to come in contact with the heating belt  32  positions the base member  50  at a high accuracy. Further, since the separating member  51  and charge-removing member  52  are retained by the base member  50 , the separating portion  67  and the charge-removing portion  68  can respectively retain the clearances G 1 , G 2  with respect to the surface of the heating belt  32  at a high positional accuracy. Thus, the separating portion  67  can finely separate the sheet P bearing the fixed toner image from the surface of the heating belt  32 . Further, since the charge-removing portion  68  is arranged at immediate downstream of the separating portion  67 , electric charge can be removed efficiently immediately after charging even in a case where the heating belt  32  is locally and drastically charged by the separating discharge and the like at a portion between the fixing nip portion N and separating portion  67 . 
   Embodiment 2 
   With reference to  FIGS. 6 and 7 , another example of the charge-removing member will be described.  FIG. 6  is a drawing which corresponds to  FIG. 4  of the above-described embodiment 1.  FIG. 7  shows a view of  FIG. 6  from a direction of an arrow B. In the present embodiment, the charge-removing member  52  of the embodiment 1 is omitted, and a function as a charge-removing member is added to a retaining portion  57 A of a base member  50 A. The base member  50 A is formed of a metal plate as described above. A surface of the retaining portion  57 A includes a separating member attachment surface  57   a  like the above-described embodiment 1. In the present embodiment, a leading end of the retaining portion  57 A is so formed as to have a shape of saw teeth as shown in  FIG. 7 . A charge-removing portion  75  being a leading edge of the saw-teeth  74  is arranged along a generating line of the surface of the heating belt  32 , and faces the surface of the heating belt  32  at a clearance G 2  apart. The clearance G 2  and a clearance G 1  between the surface of the heating belt  32  and the separating portion  67  of the separating member  51  have a relation of G 1 ≦G 2 , like above-described embodiment 1. Therefore, toners passed through the separating portion  67  of the separating member  51  do not attach to the charge-removing portion  75  of the saw-teeth  74 . In the present embodiment, the clearance G 2  is set to be within a range between 0.3 mm and 1.0 mm. It should be understood that the spacers  53  are the same as those of the above-described embodiment 1. 
   According to the present embodiment, a charge-removing member for exclusive use is omitted, and the base member  50 A is provided with a configuration and function as a charge-removing member. Accordingly, the number of parts and assembling steps can be reduced. 
   In the above-described embodiments 1 and 2, the spacers  53  come in contact with end portions in a width direction of the surface of the heating belt  32 . However, the present invention is not limited to this. The spacers  53  may be so configured as to be apart from the heating belt  32  and come in contact with the surface of fixing roller  30 . 
   In the above-described embodiments 1 and 2, the heating rotary member is a belt-like heating belt  32 . However, the present invention is not limited to this, and it can be applied also in a case where the heating rotary member is a drum-like heating drum. Similar effect can be obtained also in this case. 
   Further, the case where image forming apparatus is adapted to form a monochromatic image is described. However, the present invention may be similarly applied also in a case where the image forming apparatus is adapted to form a multiple color image or a full-color image of four colors. 
   INDUSTRIAL APPLICABILITY 
   According to the present invention, a positional accuracy of a charge-removing member is improved by retaining at a high accuracy the charge-removing member with a base member which retains a separating member with respect to a heating rotary member for fixing a toner image onto a sheet. However, in a case where there exists another member which is arranged at a high positional accuracy with respect to the heating rotary member, the charge-removing member can be retained by the member. 
   In summary, the present invention in accordance with the above-described embodiments and the drawings is summarized as follows. 
   (1) The invention in accordance with a first embodiment relates to a fixing device which forms a fixing nip portion by allowing a pressing member to come in contact with a heating rotary member having a surface which is not electrically conductive, and allows a sheet to pass through the fixing nip portion in an orientation such that an unfixed toner image bore on a sheet surface comes in contact with a surface of the heating rotary member, so that the image is heated and pressed to fix on the sheet. The fixing device in accordance with this invention includes: a base member being electrically conductive and positioned with respect to the surface of the heating rotary member; and a separating member which has a separating portion on its leading end side for separating the sheet bearing the fixed toner image from the surface of the heating rotary member, the separating member being retained by the base member at its base end side so that the separating portion is so positioned as to face the surface of the heating rotary member at a first clearance apart; and a charge-removing member retained by the base member for removing electric charge from the surface of the heating rotary member. The charge-removing member has a charge-removing portion which is arranged along a direction of a generating line of the surface of the heating rotary member on further downstream side along a rotational direction of the heating rotary member than the separating portion, and is so positioned as to face the surface of the heating rotary member at a second clearance apart. 
   According to the invention in accordance with the above-described first embodiment, the electrically conductive base member is positioned with respect to the surface of the heating rotary member. Further, the separating member is retained by the base member, so that it is so positioned as to face the surface of the heating rotary member at a first clearance apart. According to this, the separating portion can reserve a size of the first clearance at a high accuracy, and is inserted between the surface of the heating rotary member during rotation and a leading end of the sheet with the toner image formed thereon. Consequently, the sheet can be favorably separated. 
   On the other hand, the charge-removing member is retained by the base member similarly to the separating member, so that the charge-removing portion is so positioned as to face the surface of the heating rotary member along a longitudinal direction of the heating rotary member at a second clearance apart. Accordingly, the charge-removing portion can reserve a size of the second clearance at a high accuracy. In other words, the charge-removing portion can be arranged close to the heating rotary member in a non-contact state and at a high accuracy. Consequently, the charge-removing portion can favorably remove electric charge from the surface of the heating rotary member during rotation. 
   Further, the separating portion of the charge-removing member is arranged on a further downstream side along the rotational direction of the heating rotary member than the separating portion. In other words, since the separating member and the charge-removing member are retained commonly by the base member, the charge-removing portion of the charge-removing member can be arranged close to the downstream side of the separating portion of the separating member along a rotational direction of the heating rotary member. Meanwhile, an operation of fixing the toner image, the sheet is attached firmly to the surface of the heating rotary member at the fixing nip portion by the pressing member, and separated from the surface of the heating rotary member by the separating portion after the operation of fixing the toner image, the heating rotary member is likely to be charged between the fixing nip portion and the separating portion with electric charge of the toner image at the time of fixing operation or the separating discharge at the rear end of the sheet. In the invention in accordance with the first embodiment, since the charge-removing portion arranged on the immediate downstream side of the separating portion removes an electric charge from the surface of the heating rotary member, in other words, it can remove the electric charge from the surface of the heating rotary member immediately after the charging, an efficient removal of electric charge can be performed. 
   (2) The invention in accordance with the second embodiment relates to a fixing device which forms a nip portion by allowing a pressing member to come in contact with a heating rotary member having a surface which is not electrically conductive, and allows a sheet to pass through the fixing nip portion in an orientation such that an unfixed toner image bore on a sheet surface comes in contact with a surface of the heating rotary member, so that the image is heated and pressed to be fixed on the sheet. The fixing device according to this invention includes: a base member being electrically conductive and positioned with respect to the surface of the heating rotary member surface; and a separating member which has a separating portion on its leading end side for separating the sheet bearing the fixed toner image from the surface of the heating rotary member, the separating member being retained by the base member at its base end side so that the separating portion is so positioned as to face the heating rotary member at a first clearance apart. The base member has a charge-removing portion which is arranged along a generating line of the surface of the heating rotary member on further downstream side along a rotational direction of the heating rotary member than the separating portion, and is so positioned as to face the surface of the heating rotary member at a second clearance apart. 
   According to the invention in accordance with the second embodiment, the base member being electrically conductive is positioned with respect to the surface of the heating rotary member. Further, the separating member is retained by the base member, so that the separating portion is as to face the surface of the heating rotary member at a first clearance apart. Accordingly, the separating portion can reserve a size of the first clearance at a high accuracy, and is inserted between the surface of the heating rotary member during rotation and a leading end of the sheet with the toner image formed thereon. Consequently, the sheet can be favorably separated. 
   On the other hand, the base member is so positioned that the charge-removing portion faces the surface of the heating rotary member at a second clearance apart along a longitudinal direction of the surface of the heating rotary member. Accordingly, the charge-removing portion can reserve a size of the second clearance at a high accuracy. In other words, the charge-removing portion can be arranged close to the heating rotary member in a non-contact state and at a high accuracy. Consequently, the charge-removing portion can favorably remove electric charge from the surface of the heating rotary member surface during rotation. 
   Further, the charge-removing portion is arranged on a further downstream side along the rotational direction of the heating rotary member than the separating portion. In other words, since the charge-removing portion is formed on the base member which retains the separating member, it can be arranged close to the downstream side of the separating portion of the separating member along the rotational direction of the heating rotary member. Meanwhile, during an operation of fixing the toner image, the sheet is attached firmly to the surface of the heating rotary member at the fixing nip portion by the pressing member, and separated from the surface of the heating rotary member by the separating portion after the operation of fixing the toner image, the heating rotary member is likely to be charged between the fixing nip portion and the separating portion with electric charge of the toner image at the time of fixing operation or the separating discharge at the rear end of the sheet. In the invention in accordance with the second embodiment, since the charge-removing portion arranged on the immediate downstream side of the separating portion removes an electric charge from the surface of the heating rotary member, in other words, it can remove the electric charge from the surface of the heating rotary member immediately after the charging, an efficient removal of electric charge can be performed. 
   (3) In the fixing device in accordance with the above-described first and second embodiments, the base member has a spacer fixed on its leading end, and the spacer comes in contact with the surface of the heating rotary member to reserve the first clearance and the second clearance. 
   Providing the above-described feature allows the base member to have a spacer fixed on its leading end. Accordingly, allowing the spacer to come in contact with the surface of the heating rotary member positions the separating portion and the charge-removing portion, so that a high size accuracy of the first clearance and the second clearance can be reserved. 
   (4) In the fixing device provided with the feature (3) above, there may be provided a supporting portion for pivotally supporting the base member, and a biasing member for swinging the base member about the supporting portion as a base to allow the spacer to come in contact with the surface of the heating rotary member surface in a biasing state. 
   If the above-described feature is adopted, the biasing member swings the base member. Accordingly, the spacer fixed on the leading end of the base member comes in contact with the surface of the heating rotary member. This enables the separating portion and charge-removing portion positioned by the base member to follow the surface of the heating rotary member surface. Consequently, for example, even when an irregularity of the heating rotary member surface changes, the respective sizes of the first clearance and second clearance can be retained at a high accuracy. 
   (5) In the fixing device provided with the above-described feature of (4), the supporting portion is so positioned that a frictional force applied to the spacer by rotation of the surface of the heating rotary member at a contact portion between the heating rotary member surface and the spacer is oriented toward the supporting portion. 
   If the above-described feature is adopted, the spacer comes in contact with the heating rotary member surface in a biasing state, and rotation of the heating rotary member applies a frictional force to the spacer. The frictional force is then applied to the supporting portion which pivotally supports the base member. Therefore, even when the frictional force changes, a contact pressure of the spacer with respect to the heating rotary member does not change. Consequently, the spacer comes in contact with the heating rotary member surface at a stable contact pressure. 
   (6) In the fixing device provided with any one of the above-described features (1) through (5), it is preferable that the second clearance is so set as to be as large as or larger than the first clearance. 
   If the above-described feature is adopted, the second clearance is set to be as large as or larger than the first clearance. Therefore, even when toners pass through the first clearance formed between the heating rotary member surface and separating portion, the toners easily pass through the second clearance, so that they are unlikely to attach to the charge-removing portion. 
   (7) In the fixing device provided with any one of the above-described features (3) through (6), it is preferable that in a state of viewed from a longitudinal end of the circumferential surface of the heating rotary member, a leading end of the separating portion and a leading end of the charge-removing portion are arranged within an outer contour of the spacer. 
   When the above-described feature is adapted, the leading end of the separating portion and the leading end of the charge-removing portion which are so arranged as to face the heating rotary member at the first clearance and the second clearance apart, respectively, become more unlikely to come in contact with the surface of the heating rotary member. 
   (8) In the fixing device provided with any one of the above-described features (1) through (7), the heating rotary member is a belt-like member. 
   (9) In the fixing device provided with any one of the above-described features (1) through (8), the heating rotary member is a drum-like member. 
   Adopting the feature (8) or (9) defines that the heating rotary member is a belt-like member or a drum-like member, so that the present invention bring about the effect described in (1) through (7) to the rotating member of either shape. 
   This application is based on Japanese Patent Application Serial No. 2006-353904 filed in Japan Patent Office on Dec. 28, 2006, the contents of which are hereby incorporated by reference. 
   Although the present invention has been fully described by way of example with reference to the accompanying drawings, it is to be understood that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the present invention hereinafter defined, they should be construed as being included therein.