Patent Publication Number: US-8995895-B2

Title: Fixing apparatus for an image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a fixing apparatus and an image forming apparatus that incorporates the fixing apparatus. 
     2. Description of the Related Art 
     Conventional image forming apparatus include printers, copying machines, facsimile machines, and multifunction peripherals. For example, an electrophotographic printer includes a charging roller, a photoconductive drum, a light emitting diode (LED) head, a developing roller, and a transfer roller. The charging roller uniformly charges the surface of the photoconductive drum. The LED head illuminates the charged surface of the photoconductive drum to form an electrostatic latent image in accordance with print data. The developing roller supplies a thin layer of toner to the electrostatic latent image as the photoconductive drum rotates, thereby developing the electrostatic latent image with the toner into a toner image. The transfer roller transfers the toner image onto print paper. After transfer, the print paper passes through a fixing unit so that the toner image on the print paper is fixed into a permanent image by heat and pressure. The print paper is then discharged onto a stacker. 
     One such printer is disclosed in Japanese Patent Application No. 2005-275371 and employs a belt-nip fixing unit in which a pressure pad is pressed against a fixing roller with an endless belt sandwiched between the pressure pad and the fixing roller. 
       FIG. 12  is a cross-sectional view of a conventional belt-nip fixing unit  101 . 
     The fixing unit  101  includes a pressure belt assembly  102  and a fixing roller  103 . The pressure belt assembly  102  includes a pressure roller  104 , a pressure belt  105 , a pressure pad  106 , a spring  107 , and a holder  108 . The spring  107  urges the pressure pad  106  against the fixing roller  103  with the pressure belt  105  sandwiched between the pressure pad  106  and the fixing roller  103 . The holder  108  holds the pressure pad  106  in position. The pressure pad  106  is supported such that the pressure pad  106  is slidable on the inner surface of the pressure belt  105  and can extend and retract within the holder  108  relative to the fixing roller  103 . A heater  109  is disposed inside the fixing roller  103 . 
     A nip N 3  is formed between the fixing roller  103  and the pressure belt assembly  102 . When the print paper carrying a toner image thereon passes through the nip N 3 , the toner image is fused by heat and is pressed by the pressure belt assembly  102 , thereby being fixed into the print paper. 
     The conventional fixing unit  101  suffers from a problem in that when the pressure belt  105  runs, a friction resistance is developed between the inner surface of the pressure belt  105  and the upper surface of the pressure pad  106 , and in that the friction resistance may cause the pressure pad  106  to decline, vibrate, or rattle. This may result in seizure of the pressure pad  106  within the holder  108 , the pressure pad  106  becoming unable to reliably urge the pressure belt  105  against the fixing roller  103 . The unstable urging force of the pressure pad  106  causes poor print quality such as disturbance, shifting, uneven quality of image or increases the load on the fixing motor (not shown) that drives the fixing roller  103  in rotation, or may cause complete stoppage of the fixing unit  101 . 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to solve the aforementioned drawbacks of the conventional fixing unit and to provide a fixing unit capable of pressing a pressure member against a fixing member while also preventing image quality from degrading or the fixing unit from stopping. 
     A fixing apparatus is capable of pressing a pressure member against a fixing member while also preventing image quality from degrading or the fixing unit from stopping. 
     A fixing member is rotatably supported. A first pressing member is disposed to face the fixing member. A running member is sandwiched between the fixing member and the first pressing member, and runs when the fixing member rotates. A first pressing mechanism urges the first pressing member against the fixing member so that a first nip is defined between the fixing member and the running member. A second pressing member is disposed to face the fixing member with the running member sandwiched between the fixing member and the second pressing member. A second pressing mechanism urges the second pressing member against the fixing member with the running member sandwiched between the fixing member and the second pressing member so that a second nip is defined between the fixing member and the running member. The second nip is adjacent to the first nip. The second pressing mechanism includes a holding member that holds the second pressing member and an urging member therein. The urging member urges the second pressing member against the fixing member. The second pressing member and the holding member abut each other through a plurality of projections. 
     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 
       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: 
         FIG. 1  illustrates a printer according to a first embodiment; 
         FIG. 2  is a cross-sectional view of a fixing unit according to the first embodiment; 
         FIG. 3  is a rear perspective view, illustrating the rear of a pressure pad as seen substantially in a direction shown by arrow E in  FIG. 2 ; 
         FIG. 4  is a perspective view illustrating the front of the pressure pad as seen substantially in the opposite direction to arrow E; 
         FIG. 5A  illustrates the operation of the fixing unit according to the first embodiment; 
         FIG. 5B  illustrates a pertinent portion of the fixing unit; 
         FIG. 6  is a rear view of the pressure pad according to the second embodiment; 
         FIG. 7A  is a cross-sectional view of a fixing unit according to a second embodiment; 
         FIG. 7B  is an expanded view of a pertinent portion of the fixing unit according to a second embodiment; 
         FIG. 8A  is an exploded perspective view illustrating a pressure pad, springs, and a holder according to the second embodiment; 
         FIG. 8B  is a rear view of the pressure pad according to the second embodiment; 
         FIG. 9A  is a cross-sectional view illustrating the pertinent portion of the fixing unit according to a third embodiment; 
         FIG. 9B  is an expanded view of a pertinent portion of the fixing unit; 
         FIG. 10  is a rear view of a pressure pad according to the third embodiment; 
         FIG. 11  is a cross-sectional view of the pressure pad; and 
         FIG. 12  is a cross-sectional view of a conventional belt-nip fixing unit. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will be described in detail by way of preferred embodiments with reference to the accompanying drawings. The embodiments will be described in terms of a printer as an image forming apparatus. 
     First Embodiment 
     {Configuration of Image Forming Apparatus} 
       FIG. 1  illustrates a printer  10  as an image forming apparatus according to a first embodiment. 
     The printer  10  includes a body  40 , image forming units, a transport path  25  in which print paper (not shown) as a print medium is transported, and transport rollers  26 - 29  in the transport path  25 . The image forming units (ID units) Bk, Y, M, and C that form black, yellow, magenta, and cyan images, respectively, are aligned along the transport path  25 . 
     A belt nip fixing unit  35  is disposed downstream of the image forming units BK, Y, M, and C, and fixes a toner image on the print paper by heat and pressure. 
     Each of the image forming units BK, Y, M, and C may be substantially identical; for simplicity, only the image forming apparatus BK will be described, it being understood that other image forming units Y, M, and C may work in a similar fashion. 
     The image forming unit BK includes a photoconductive drum  11 , an LED head  23 , and a developing unit  36 . A charging roller  12  rotates in pressure contact with the photoconductive drum  11  in a direction opposite to the photoconductive drum  11 , and uniformly charges the surface of the photoconductive drum  11 . An LED head  23  is disposed above the photoconductive drum  11  and parallels the photoconductive drum  11 . The LED head  23  illuminates the charged surface of the photoconductive drum  11  to form an electrostatic latent image (not shown) . A transfer unit  34  is disposed under the image forming units BK, Y, M, and C, and transfers the toner images of the respective colors onto the print paper. The transfer unit  34  is driven to run by a belt driving mechanism. 
     A developing unit  36  includes a developing roller  16 , a developing blade (nth sown), and a toner supplying roller  18 . The developing roller  16  supplies toner to the electrostatic latent image formed on the photoconductive drum  11 . The developing blade forms a thin layer of the toner on the developing roller  15 . The toner supplying roller  18  supplies the toner to the developing roller  16 . The developing roller  16  is in pressure contact with the photoconductive drum  11 , and rotates in an opposite direction to the photoconductive drum  11 . The toner supplying roller  18  is in pressure contact with the developing roller  16 , and rotates in the same direction as the developing roller  16 . The photoconductive drum  11 , developing roller  16 , toner supplying roller  18 , LED head  23  and charging roller  12  and so on form a print engine. 
     The photoconductive drum  11 , charging roller  12 , and developing unit  36  are housed in a body of the image forming unit  20 . A toner cartridge  15  is detachably attached to the top of the image forming unit  20 . 
     The transfer unit  34  includes a transfer belt  21  and transfer rollers  22 . The transfer belt  21  is disposed about the transfer rollers  22  so that the transfer belt  21  is sandwiched between the photoconductive drum  11  and the transfer rollers  22 . The transfer belt  21  carries the print paper thereon, and runs through the image forming units  20 . The transfer belt  21  and transfer rollers  22  receive voltage from a power supply (not shown) and transfer the toner image of the respective colors from the photoconductive drums onto the print paper in sequence. 
     The printer  40  includes a body  38   a  and a top cover  38   b  configured to pivot about a shaft Sh 1  so that the top cover  38   b  can be opened and closed relative to the body  38   a . When the top cover  38   b  is closed, the top cover  38   b  closes the upper opening of the body  38   a  depicted at a plane A-A. The top cover  38   b  includes a stacker  31  formed on an outer surface thereof, the stacker  31  receiving the discharged sheets of print paper thereon. A paper cassette  30  is located under the transfer unit  34 , and holds a stack of sheet of paper. The paper feeding mechanism  32  is disposed in the vicinity of the paper cassette  30 , and feeds the print paper into the transport path on a sheet-by-sheet basis. 
     {Operation of Image Forming Apparatus} 
     The operation of the printer  10  of the aforementioned configuration will be described. 
     The charging roller  12  charges the surface of the photoconductive drum  11 . The LED head  23  illuminates the charged surface of the photoconductive drum  11  in accordance with the print data to form an electrostatic latent image. The developing unit  36  supplies the toner to the electrostatic latent image, thereby developing the electrostatic latent image with the toner to form a toner image. 
     The paper feeding mechanism feeds the sheets of print paper from the paper cassette  30  on a sheet-by-sheet basis, the sheet being transported by the transport rollers  26  and  29  to the transfer belt  21 . The print paper is attracted to the transfer belt  21  by Coulomb force. As the transfer belt  21  runs through the respective image forming units BK, Y, M, and C in sequence, the toner images of the respective colors are transferred onto the print paper in registration, thereby transferring a full-color toner image onto the print paper. The print paper then advances to the fixing unit  35  where the full-color toner image is fixed into the print paper by heat and pressure. After fixing, the print paper is further transported by the transport rollers  28  and  29  and is discharged onto the stacker  31 . 
     The printer  10  includes an interface and a controller. The interface communicates print data with an external apparatus (not shown). The controller performs the overall control of the printer  10 . 
     {Fixing Unit} 
     The fixing unit  35  will now be described. 
       FIG. 2  is a cross-sectional view of the fixing unit  35  according to the first embodiment. Referring to  FIG. 2 , the fixing unit  35  includes a fixing roller  37  as a heating member and a pressure belt assembly  41 . The fixing roller  37  is rotatably supported by bearings (not shown) and is driven in rotation by a fixing motor (not shown). The fixing motor is disposed on the body  40 , and serves as a drive source for the fixing roller  37 . The fixing roller  37  rotates at a predetermined circumferential speed in a direction shown by arrow A. A drive gear (not shown) is attached to an output shaft of the fixing motor. A driven gear (not shown) is attached to one longitudinal end portion of the fixing roller  37 . The drive gear is in mesh with the driven gear, thereby transmitting the drive force to the fixing roller  37 . 
     The fixing roller  37  is a hollow roller having an outer diameter of about 28 mm, and includes a core metal formed of iron, covered with silicone rubber. A heat resistant, elastic layer having a thickness of 1.2 mm is formed on the silicone rubber. A toner releasing layer is formed of fluoroplastic with a thickness of 41 μm, and covers the heat resistant, elastic layer. The core metal may also be formed of other metal, e.g., aluminum. 
     The fixing roller  37  includes built-in heaters  42 . The heaters  42  are energized by a power source (not shown). A thermistor (not shown) is disposed in the vicinity of the surface of the fixing roller  37 , and serves as a temperature sensor. The thermistor detects the surface temperature of the fixing roller  37 , and feeds the sensor output signal to a temperature control circuit (not shown). The heater  42  may be implemented with a halogen lamp or an induction heater. 
     A pressure belt assembly  41  includes a pressure roller  44  that rotates in a direction shown by arrow B, a pressure belt  45 , a pressure pad  46 , an urging mechanism  81 , and an urging mechanism  82 . The pressure belt  45  is an endless belt, and runs in contact with the fixing roller  37  in a direction shown by arrow F. The pressure pad  46  is disposed upstream of the pressure roller  44  with respect to the direction in which the pressure belt  45  runs, and serves as a second pressure member. The urging mechanism  81  serves as a first urging mechanism that urges the pressure roller  44  against the fixing roller  37  with the pressure belt  45  sandwiched between the pressure roller  44  and the fixing roller  37 . The urging mechanism  82  is mounted on the body  40  of the printer  10  and serves as a second pressure mechanism, urging the pressure pad  46  against the fixing roller  37  with the pressure belt  45  sandwiched between the fixing roller  37  and the pressure pad  46 . 
     The pressure belt  45  is disposed about the pressure roller  44 , pressure pad  46 , and urging mechanism  82  . The pressure roller  44  and pressure pad  46  are urged against the fixing roller  37  with the pressure belt  45  sandwiched between the pressure roller  44  and the pressure pad  46 . A nip N is formed between the pressure belt  45  and the fixing roller  37  so that when the print paper P passes through the nip N, the color toner image is fused by heat and pressure. As the fixing roller  37  rotates, the pressure belt  45  is rotated in the C direction due to the friction between the pressure belt  45  and the fixing roller  37 , thereby transporting the print paper P. 
     The pressure roller  44  is a hollow roller having an outer diameter of about 23 mm, and includes a core metal formed of iron covered with silicone rubber. A covering layer formed on the core metal is formed of fluoroplastic has a thickness of 20 μm. The core metal may also be formed of other metal, e.g., aluminum. 
     The pressure belt  45  has a two-layer structure, and includes a diameter of about 41 mm and a thickness of about 90 μm. The two-layer structure includes a base layer formed of polyimide as a heat resistant resin and a toner releasing layer formed of perfluoroalkoxy alkane (PFA) and having a thickness of about 20 μm. 
     The urging mechanism  81  is disposed at each longitudinal end portion of the pressure roller  44 , and includes a spring  43  and a pressure lever  83 . The pressure lever  83  is rotatably supported on the shaft Sh 11  so that the pressure lever  83  is rotatable relative to the chassis Cs 1  of the fixing unit  35 . The spring  43  urges the pressure lever  83  counter clockwise. The spring  43  is mounted across an engagement portion  84  formed at a distance from the shaft Sh 11  and a pressing portion P 1  on the chassis Cs 1 . The spring  43  according to the first embodiment takes the form of a coil spring. 
     The pressure roller  44  is supported by bearings (not shown) and is rotatable relative to the pressure lever  83 . When the spring  43  urges the pressure lever  83  to rotate counterclockwise about the shaft Sh 11 , the pressure roller  44  is firmly pressed against the fixing roller  37 . As a result, a first nip N 1  is formed on a portion of the pressure belt  45  in contact with the fixing roller  37 . 
     In the first embodiment, the fixing roller  37  includes a heat resistant, elastic layer while the pressure roller  44  does not include a heat resistant, elastic layer but has a higher hardness than the fixing roller  37 . Therefore, the fixing roller  37  elastically deforms at the first nip N 1 , thereby preventing the print paper P from wrapping around the fixing roller  37  to release the print paper P reliably. 
     The shaft Sh 11  is located downstream of the nip N 1  with respect to the direction of travel (arrow D) of the print paper P, and is adjacent to the pressure roller  44 . The line passing through the rotational axes of the urging mechanism  83  and the pressure roller  44  is substantially perpendicular to the line passing through the rotational axes of the fixing roller  37  and the pressure roller  44 . 
     The pressure pad  46  includes a base member  85 , an elastic layer  86  and a low friction layer (not shown) formed on the elastic layer  86 . The elastic layer  86  serves as a heat resistant, elastic layer formed at the tip of the base member  85 . The pressure pad  46  holds the low friction layer in contact with the loop-like pressure belt  45  from inside so that the low friction layer is in position in the direction of travel of the print paper P. The pressure pad  46  is extendable and retractable relative to the fixing roller  37 . 
     The base member  85  extends in such a direction as to go away from the reader and is formed in a two-stage process. A metal material, e.g., aluminum is subjected to drawing process or extrusion process, and then to press working. The elastic layer  86  is formed of silicone rubber, and has a hardness of 15 to 41 degrees when measured according to JIS-A, and a thickness of about 1 mm. The low friction layer covers the elastic layer  86  in order to reduce the friction between the inner surface of the pressure belt  45  and the elastic layer  86 . The low friction layer is formed of a silicone material containing graphite, and has a thickness of 20 μm. 
     A longitudinally extending base member  85  includes an upper half portion  85   a  and a plurality of lower half portions  85   d  aligned along the upper half portion  85   a . The lower half portions  85   d  extend downwardly from the upper half portion  85   a . A pressing portion  85   b  extends from the upper end portion of the upper half portion  85   a  toward the first nip N 1 , and has a substantially triangular cross section. An abutting portion  85   c  extends from the middle portion of the base member  85  in such a direction as to go away from the base member  85 , upstream of the base member  85  with respect to the direction of travel of the print paper P. The elastic layer  86  is disposed on the pressing portion  85   b , substantially all over the upper surface of the pressing portion  85   b.    
     The urging mechanism  82  includes a holder  48  and springs  47 . The holder  48  holds the pressure pad  46  so that the pressure pad  46  parallels the fixing roller  37  and is extendable and retractable relative to the fixing roller  37 . The springs  47  are aligned along the holder  48  and urge the pressure pad  46  against the fixing roller  37 , thereby creating a second nip N 2  in an area of the pressure pad  46  in contact with the fixing roller  37 . The second nip N 2  is adjacent to the first nip N 1 . 
     The springs  47  may also take the form of a resilient body or a driver member that causes the pressure pad  46  to extend and retract relative to the fixing roller  37 . 
       FIG. 3  is a rear perspective view illustrating the rear of a pressure pad  46  as seen substantially in a direction shown by arrow E in  FIG. 2 .  FIG. 4  is a perspective view illustrating the front of the pressure pad as seen substantially in the opposite direction to arrow E. 
     Referring to  FIG. 3 , a plurality of cylindrical projections  46   a  are formed in one piece construction with the base member  85  on a surface Sa of the base member  85  that faces the inner wall  48   a  of the holder  48 . A plurality of projections  46   b  (second projections) are formed on a surface Sb of the lower half portion  85   d  that faces the wall w 1 . The projections  46   a  are slidable on the inner wall  48   a , and the projections  46   b  are slidable on the wall w 1 . The projections  46   a  and  46   b  are formed by pressing work. 
       FIG. 5A  illustrates the operation of the fixing unit  35  according to the first embodiment.  FIG. 5B  illustrates a pertinent portion of the fixing unit. 
     Referring to  FIGS. 5A and 5B , the holder  48  is formed of a metal material and has a gutter-like hollow body which has a generally U-shaped cross section and is open at its upper end. The holder  48  is mounted on the chassis of the printer  10  and is disposed close to the pressure roller  44 . The holder  48  receives the abutting portion  85   c  and the lower half portions  85   d  therein so that the abutting portion  85   c  and the lower half portions  85   d  are slidable on the inner walls  48   a  and  48   b , and  48   c . The inner walls  48   a ,  48   b , and  48   c  define a room rm that accommodates the abutting portion  85   c , lower half portions  85   d , and the springs  47  therein. The springs  47  are disposed between a bottom wall  48   c  and the abutting portion  85   c.    
     Holes h 1  are formed in the bottom wall  48   c  near the inner wall  48   a , allowing the lower half portions  85   d  to extend through the holes h 1 . A wall w 1  rises from the bottom wall  48   c  to cooperate with the inner wall  48   a  to define a groove therebetwen. 
     Since the base member  85  and holder  48  are formed of metal materials, the holder  48  may be connected to the ground so that the charges on the base member  85  are dissipated and the base member  85  is prevented from acting as an antenna that radiates electrical noise. 
     As the pressure belt  45  runs in the C direction, a frictional resistance is developed between the inner surface of the pressure belt  45  and the elastic layer  86 , which causes the pressure pad  46  to incline, vibrate, or rattle. When the pressure pad  46  inclines, vibrates, or rattles, the lower half portions  85   d  of the pressure pad  46  and the inner wall of the holder  48  move into contact with each other. 
     Excessive temperatures of the pressure pad  46  and the holder  48  will lead to increased friction at their interface, which may result in seizure of the pressure pad  46  in a high temperature environment and fail to reliably urge the pressure pad  46  against the fixing roller  37 . The seizure of the pressure pad  46  causes damaged images, positional shifts of images, and uneven image quality, leading to deteriorated image quality, increasing the load on the fixing motor, or even causing complete stoppage of the fixing unit. 
     Referring to  FIG. 5B , when the pressure pad  46  abuts the projections  46   a  and the pressure mechanism  82  abuts the projections  46   b , and the pressure pad  46  extends and retracts relative to the pressure belt  45 , the projections  46   a  slide on the inner wall  48   a  and the projections  46   b  slide on the wall w 1 . 
     In the first embodiment, the lower half portions  85   d  and projections  46   a  and  46   b  are formed along the upper half portion  85   a  in a direction perpendicular to the direction of travel of the print paper P. The number of the lower half portions  85   d  and projections  46   a  and  46   b  may be selected so that the printer  10  is capable of printing on the print paper P of a variety of sizes including A5, A4 and A3 sizes. 
       FIG. 6  is a rear view of the pressure pad. The printer  10  according to the present invention supports A5, A4, and A3 size paper. The A5 size print paper P has a width Wh 1  (148 mm) and the A3 size print paper P has a width Wh 2  (297 mm). Two laterally centered first lower half portions  85   d  are disposed within the width Wh 2 . Two laterally centered second lower half portions  85   d  are disposed between the two laterally centered first lower half portions  85   d . Two laterally centered third lower half portions  85   d  are disposed between the two laterally centered second lower half portions  85   d . One projection  46   a , shown in solid line, is formed substantially at the longitudinal middle of the upper half portion  85   a . Each of four projections  46   a , shown in solid lines, is formed on the surface Sa immediately above a corresponding one of the first and third lower half portions  85   d . Each of four projections  46   b , shown in dotted lines, is formed on the surface Sb of a corresponding one of the first and third lower half portions  85   d . When printing is performed on A5 size paper, the A5 size paper is advanced in the area Wh 1 . When printing is performed on A3 size paper, the A3 size paper is advanced in the area Wh 2 . 
     The projections  46   a  and  46   b  have been described in terms of a cylindrical shape but may take the form of a rib. 
     {Operation of Fixing Unit} 
     The operation of the fixing unit  35  will be described with reference to  FIGS. 5A and 5B . 
     When the printer  10  starts printing, the fixing motor is energized to drive the fixing roller  37  into rotation, the fixing roller  37  causing the pressure belt  45  to run by means of the frictional force developed at the nip N between the pressure belt  45  and the fixing roller  37 . 
     The heaters  42  are energized by the power supply to generate heat, which in turn heats the fixing roller  37  from inside. The power supply adjusts the amount of current supplied to the heater in accordance with the output of the thermistor, thereby controlling the temperature of the fixing roller  37  to a predetermined temperature. 
     When the temperature of the fixing roller  37  has reached the predetermined temperature, the print paper P is fed into the nip N. As the pint paper P passes through the nip, i.e., nips N 1  and N 2 , the toner image T on the print paper P is fused by heat and pressure into a permanent image. After fixing, the print paper P is discharged outside the chassis Cs 1  through a discharge port (not shown). 
     When the fixing roller  37  rotates in the A direction and the pressure belt  45  is driven to run in the C direction, a force acts on the pressure pad  46  in the E direction so that the pressure pad  46  inclines, causing the lower end portion  46   a  of the upper half portion  85   a  to press the inner wall  48   a , and the lower end portion  46   b  of the lower half portion  85   d  to press the wall w 1 . Since the projections  46   a  and  46   b  have a small surface area, the contact area between the inner wall  48   a  and the upper half portion  85   a  and the contact area between the wall w 1  and the lower half portion  85   d  are small. 
     When the print paper P is fed into the nip N, the pressure pad  46  retracts into the holder  48  in accordance with the thickness of the print paper P. At this time, the small contact areas between the pressure pad  46  and the holder  48  can be small enough. 
     As described above, the. projections  46   a  and  46   b  serve to reduce the contact area between the inner wall  48   a  and the projection  46   a , and the contact area between the wall w 1  and the projection  46   b , reducing the frictional resistance developed between the pressure pad  46  and the holder  48 . 
     As a result, seizure of the pressure pad  46  does not occur in the holder  48  even if the pressure pad  46  inclines, vibrates, or rattles. This helps the pressure pad  46  urge the fixing roller  37 , preventing the printed image from suffering from damaged images, shifted images, and uneven image quality, leading to deteriorated image quality and increasing the load on the fixing motor or even complete stoppage of the fixing unit. 
     The projections  46   a  are formed on the lower end portion of the upper half portion  85   a  so that the area of the base member  85  in pressure contact with the wall  48   a  may be small and the projections  46   b  are formed on the lower end portion of the lower half portion  85   d  so that the area of the base member  85  in pressure contact with the inner wall w 1  may be small. This minimizes the total contact area of the base member  85  with the holder  48  so that the frictional resistance between the pressure pad  46  and the holder  48  is small when the base member  85  extends and retracts within the holder  48 . 
     The projections  46   a  may also be formed at arbitrary locations where the lower half portion  85   d  faces the inner wall  48   a . Still alternatively, the projections  46   b  may be formed at arbitrary locations on the surface Sb ( FIGS. 2 and 4 ) of the lower half portion  85   d.    
     Also, instead of the projections  46   a  and  46   b , projections may be formed either on the inner wall  48   a  and the wall w 1  or on the base member  85 , inner wall  48   a , and wall w 1 . 
     Second Embodiment 
     Elements similar to those of the first embodiment have been given similar reference numerals and their description is omitted. 
       FIG. 7A  is a cross-sectional view of a fixing unit according to a second embodiment.  FIG. 7B  is an expanded view of a pertinent portion of the fixing unit.  FIG. 8A  is an exploded perspective view illustrating a pressure pad, springs, and holder according to the second embodiment. 
     A longitudinally extending base member  85  includes an upper half portion  85   a , a pressing portion  85   b , an abutting portion  85   c , and a plurality of lower half portions  85   d . The lower half portions  85   d  extend downwardly from the upper half portion  85   a . The pressing portion  85   b  extends from the upper end portion of the upper half portion  85   a  toward a first nip N 1  and has a substantially triangular cross section. The abutting portion  85   c  horizontally extends from a middle portion of the base member  85  in such a direction as to go away from the base member  85 , upstream of the base member  85  with respect to the direction of travel of the print paper P. A heat resistant, elastic layer  86  is disposed on the pressing portion  85   b  substantially all over the upper surface of the pressing portion  85   b.    
     The urging mechanism  82  includes a holder  48  and springs  47 . The holder  48  holds the pressure pad  46  therein so that the pressure pad  46  parallels the fixing roller  37  and is extendable and retractable relative to the fixing roller  37 . The springs  47  are disposed in the holder  48  along the holder  48  and urge the pressure pad  46  against the fixing roller  37 . 
     The holder  48  is formed of a metal material and has a gutter-like hollow body which has a generally U-shaped cross section and is open at its upper end. The holder  48  is disposed close to the pressure roller  44 . The holder  48  receives the abutting portion  85   c  and the lower half portions  85   d  therein so that the abutting portion  85   c  and the lower half portions  85   d  are slidable on opposing inner walls  48   b  and  48   a . The inner walls  48   a  and  48   b  and a bottom wall  48   c  define a room rm that accommodates the abutting portion  85   c , the lower half portion  85   d , and the springs  47  therein. The springs  47  are disposed between a bottom wall  48   c  and the abutting portion  85   c.    
     Holes h 1  are formed in the bottom wall  48   c  near the inner wall  48   a , allowing the lower half portions  85   d  to extend therethrough. A wall w 1  rises from the bottom wall  48   c , cooperating with the inner wall  48   a  to define a narrow groove between the wall w 1  and the inner wall  48   a  in which the lower half portions  85   d  are sandwiched and are slidable. 
     As the pressure belt  45  runs in the F direction, a frictional resistance is developed between the inner surface of the pressure belt  45  and the elastic layer  86 , which causes the pressure pad  46  to incline, vibrate, or rattle. When the pressure pad  46  inclines, vibrates, or rattles, the lower half portions  85   d  of the pressure pad  46  and the inner wall of the holder  48  move into contact with each other. 
     Excessive temperatures of the pressure pad  46  and the holder  48  will lead to increased friction at their interface, which may result in seizure of the pressure pad  46  in a high temperature environment and fail to reliably urge the pressure pad  46  against the fixing roller  37 . 
     In the second embodiment, a plurality of projections  51  are disposed on a surface Sa of the lower portion of upper half portion  85   a  facing the inner wall  48   a . The projections  51  are a cylinder of a resin with a large diameter portion and a small diameter portion. The projections  51  are press-fitted into the surface Sa so that a surface  51   a  of the large diameter portion projects from the surface Sa. 
     Referring to  FIG. 8A , the holder  48  accommodates abutting members  52  as a second sliding member, which are slidable in contact with the base member  85  and holds the base member  85  between the wall  48   a  and the abutting member  52 . The abutting members  52  are formed of a resin material, and are disposed at a plurality of locations (two locations in the second embodiment) on the bottom wall  48   c  and beside the lower half portions  85   d , and aligned in the longitudinal direction of the holder  48 . Each of the abutting members  52  has four upwardly extending spring supporting projections  52   a , and a spring  47  fits over a corresponding spring supporting projection  52   a . Each spring  47  is disposed between the supporting projections  52   a  and the abutting portion  85   c.    
     The abutting member  52  includes a surface Sc that faces the lower half portion  85   d  and is located between adjacent supporting projections  52   a . A plurality of projections  53  are a cube or a rectangular parallelepiped of a resin material. The projections  53  are press-fitted into the surface Sc so that the projection  53  partially projects from the surface Sc. Projected surfaces  53   a  of the projections  53  project from the surface Sc and are in the shape of a rectangular parallelopiped. When the pressure pad  46  extends out of and retracts into the holder  48 , the projections  51   a  slide on the inner wall  48   a  and the projected surfaces  53   a  slide on the lower portion of the lower half portion  85   d.    
     In the second embodiment, the number of the lower half portions  85   d  and projections  51   a  and projected surfaces  53   a  is selected to support a variety of paper sizes including A5, A4, and A3 sizes. 
       FIG. 8B  is a rear view of the pressure pad according to the second embodiment. The printer  10  according to the present invention supports A5, A4, and A3 size paper. Two laterally centered first lower half portions  85   d  are disposed within the width Wh 2 . Two laterally centered second lower half portions  85   d  are disposed between the two laterally centered first lower half portions  85   d . Two laterally centered third lower half portions  85   d  are disposed between the two laterally centered second lower half portions  85   d . Each of four projected surface  51   a  is formed on the surface Sa of the upper half portion  85   a  immediately above a corresponding one of the laterally centered first and third lower half portions  85   d . One projected surface  51   a  is formed at the longitudinal middle of the upper half portion  85   a . Each of four projected surfaces  53   a  is formed on the surface Sc of a corresponding one of two abutting members  52 . When printing is performed on A5 size paper, the A5 size paper is advanced in the area Wh 1 . When printing is performed on A3 size paper, the A3 size paper is advanced in the area Wh 2 . 
     The projections  51  and  53  are formed of a heat resistant resin that can withstand the temperature at which the fixing unit  35  operates. The heat resistant resin is preferably electrically conductive so that the charges developed on the base member  85  can be dissipated. If the charges on the base member  85  can be conducted to surrounding members in some way, the projections  51  and  53  need not be formed of an electrically conductive resin. 
     In the second embodiment, the projected surface  51   a  is formed on the lower portion of the upper half portion  85   a . Instead, the projected surface  51   a  may be formed at an arbitrary position on the surface of the lower half portion  85   d  that faces the inner wall  48   a . For example, the projected surface  51   a  may be formed on the lower end portion of the lower half portions  85   d.    
     In the second embodiment, the projected surface  51   a  is formed on the base member  85  and the projected surface  53   a  is formed on the abutting member  52 . Instead, projections may be formed on the inner wall  48   a  of the holder  48  and the surface Sb of the lower half portion  85   d.    
     The projected surface  51   a  reduces the area of the upper half portion  85   a  in contact with the inner wall  48   a  and the projected surface  53   a  reduces the area of the lower half portion  85   d  in contact with the abutting member  52 . This configuration reduces the friction resistance developed between the pressure pad  46  and the holder  48  when the pressure pad  46  extends and retracts within the holder  48 . 
     When the projected surfaces  51   a  and  53   a  formed of a resin material rub on the inner wall  48   a  and lower half portion  85   d , respectively, the friction resistance between the pressure pad  46  and the holder  48  is relatively small. Even when the pressure pad  46  inclines, vibrates, or rattles, seizure of pressure pad will not occur in a high temperature environment, preventing damaged images, positional shifts of images, and uneven image quality that would otherwise lead to deteriorated image quality. Further, the load on the fixing motor that drives the fixing roller  37  is minimized, preventing complete stoppage of the fixing unit. 
     When the pressure pad  46  inclines, the lower end portion, i.e., projected surface  51   a  of the upper half portion  85   a  is pressed against the inner wall  48   a , and the projected surface  53   a  of the abutting member  52  is pressed against the lower end portion of the lower half portion  85   d . Therefore, the areas of the base member  85  in contact with the holder  48  and abutting member  52  may be sufficiently small. This further minimizes the friction resistance between the abutting member  52  and the pressure pad  46  and the friction resistance between the pressure pad  46  and the holder  48 . 
     Third Embodiment 
     Elements similar to those of the first and second embodiments have been given the same reference numerals and their description is omitted. 
       FIG. 9A  is a cross-sectional view illustrating the pertinent portion of a fixing unit  35  according to a third embodiment.  FIG. 9B  is an expanded view of a pertinent portion of the fixing unit  35 .  FIG. 10  is a front view of a pressure pad according to the third embodiment.  FIG. 11  is a cross-sectional view of anther pressure pad. 
     Referring to  FIG. 9A , bearings  98  each include an outer race  98   a  and an inner race  98   b . The shaft  44   a  of a pressure roller  44  is fitted into the inner race  98   b.    
     Referring to  FIG. 10 , a plurality of projections  61   a - 61   c  in the shape of a cylinder are formed on a surface Sa of the base member  85 , the surface Sa facing an inner wall  48   a  of a holder  48 . 
     In order for the printer  10  to support a variety of paper sizes including A5, A4, and A3 sizes, the number of lower half portions  85   d  and projections  61 - 61   c  disposed along the longitudinal direction of the pressure pad  46  may be selected in accordance with the paper size to support. 
     In the third embodiment, the number of the lower half portions  85   d  and projections  61   a - 61   c  is selected to support a variety of paper sizes including A5, A4, and A3 sizes. Six lower half portions  85   d  are disposed within the width Wh 2  of the paper. Two laterally centered first lower half portions  85   d  are disposed within the Wh 2 . Two laterally centered second lower half portions  85   d  are formed between the two laterally centered first lower half portions  85   d . Two laterally centered third lower half portions  85   d  are formed between the two laterally centered second lower half portions  85   d . One projection  61   a  is formed on the surface Sa and substantially at the longitudinal middle of the upper half portion  85   a . Each of two projections  61   c  is formed on the surface Sa immediately above a corresponding one of the two laterally centered first lower half portions  85   d . Each of another two projections  61   b  is formed on the surface Sa immediately above a corresponding one of the two laterally centered second lower half portions  85   d . When printing is performed on A5 size paper, the A5 size paper is advanced in the area Wh 1 . When printing is performed on A3 size paper, the A3 size paper is advanced in the area Wh 2 . 
     The outer race  98   a  of the bearing  98  that rotatably supports the pressure roller  44  abuts projections  85   m  formed at both longitudinal end portions of the upper half portion  85   a , creating a clearance between the pressure roller  44  and a pressing portion  85   b  of the base member  85  so that the base member  85  does not interfere with the pressure roller  44 . Thus, the pressure pad  46  is supported at its longitudinal end portion by the projections  85   m.    
     When the fixing roller  37  rotates in a direction shown by arrow A and the pressure belt  45  runs in a direction shown by arrow F, the friction resistance between the inner surface of the pressure belt  45  and the surface of the elastic layer  86  exerts a force on the pressure pad  46  that acts in a direction shown by arrow E. Since the pressure pad  46  is supported at its longitudinal end portions by the projections  85   m , the lower end portion of the upper half portion  85   a  is firmly pressed against the inner wall  48   a  and the lower end portions of the lower half portions  85   d  are firmly pressed against the abutting member  52 . The pressing force decreases with increasing distance from the longitudinal end portions of the pressure pad  46 . 
     It is to be noted that if the pressure pad  46  fails to press the fixing roller  37  uniformly across the length of the fixing roller  37 , non-uniform pressing force causes distorted images, shifted images, and uneven density of images, leading to degraded image quality. 
     In the third embodiment, the projections  61   a - 61   c  are formed with different heights from the surface Sa in accordance with the positions of the projections  61   a - 61   c  in the longitudinal direction. The projections  61   a - 61   c  are higher nearer the longitudinally middle portion of the pressure pad  46 , and therefore lower further away from the middle portion. The heights of the projections  61   a - 61   c  are related as follows:
 
Ha&gt;Hb&gt;Hc
 
where Ha is the height of the projection  61   a , Hb is the height of the projection  61   b , Hc is the height of the projection  61   c.  
 
     The selection of height of the projections  61   a - 61   c  allows the pressure pad  46  to press the fixing roller  37  uniformly across the length of the fixing roller  37 , thus preventing distorted images, shifted images, and uneven density of images to provide good image quality. 
     The aforementioned configuration, in which the projections  61   a - 61   c  are higher nearer the middle portion of the pressure pad  46 , may also be applied to the second embodiment. 
     The first to third embodiments have been described in terms of a tandem printer  10  that incorporates black, yellow, magenta and cyan image forming units aligned in tandem, the present invention may also be applied to a variety of image forming apparatus including a mono color printer, a copying machine, a facsimile machine, and a multifunction printer. 
     The invention is not limited to the described the embodiments, and may be modified in a variety of ways without departing from the scope of the invention.