Patent Publication Number: US-10324402-B2

Title: Belt type fixing apparatus and image forming apparatus having the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority benefit of Korean Patent Application No. 10-2016-0090896 filed on Jul. 18, 2016, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety. 
     BACKGROUND 
     1. Field 
     The disclosure relates to a fixing apparatus for an image forming apparatus. More particularly, the disclosure relates to a belt type fixing apparatus and an image forming apparatus having the same. 
     2. Description of the Related Art 
     Generally, an electrophotographic image forming apparatus such as a laser printer forms a developer image corresponding to print data on a printing medium, and uses a fixing apparatus to permanently fix the developer image on the printing medium by applying predetermined heat and pressure to the developer image. 
     The fixing apparatus may include a pair of rollers, that is, a heating roller that generates a predetermined heat to be applied to the printing medium and a fixing roller that applies a predetermined pressure to the printing medium. 
     In recent years, in image forming apparatuses capable of high-speed printing, a belt type fixing apparatus using a fixing belt, which is an endless belt, is widely used instead of the heating roller. 
     As illustrated in  FIG. 1 , a conventional belt type fixing apparatus guides the rotation of a fixing belt  210  by using guide bushes  200  provided at the opposite ends of the fixing belt  210 . A first surface  201  of the guide bush  200  restricts an axial movement of the fixing belt  210 , and a second surface  203  of the guide bush  200  supports the rotation of the fixing belt  210  inside the fixing belt  210 . The first surface  201  and the second surface  203  of the guide bush  200  are formed to be perpendicular to each other. 
     At this time, at a connecting portion  205  between the first surface  201  and the second surface  203  of the guide bush  200  forming the right angle, there exists a tool shape (or tool trace) which is generated when the guide bush  200  is machined. For example, as illustrated in  FIG. 2 , a round having a predetermined curvature is formed at the connecting portion  205  between the first surface  201  and the second surface  203  of the guide bush  200 . 
     While the fixing belt  210  rotates, the fixing belt  210  receives an axial force B. Then, as illustrated in  FIG. 3 , the fixing belt  210  is moved in the axial direction along the second surface  203  by the axial force B, so that one end  210   a  of the fixing belt  210  climbs up along the round shape  205  formed between the first surface  201  and the second surface  203 . Then, the one end  210   a  of the fixing belt  210  receives a force that acts from the inside of the fixing belt  210  to the outside to cause the one end  210   a  of the fixing belt  210  to be spread out. Accordingly, when the fixing belt  210  repeatedly rotates along the guide bush  200 , the one end  210   a  of the fixing belt  210  is cracked and broken. 
     Accordingly, the development of a belt type fixing apparatus capable of suppressing fatigue cracks at opposite ends of the fixing belt  210  has been demanded. 
     SUMMARY 
     Additional aspects and/or advantages will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of the disclosure. 
     The disclosure has been developed in order to overcome the above drawbacks and other problems associated with the conventional arrangement. An aspect of the disclosure relates to a belt type fixing apparatus that can minimize occurrence of cracks at opposite ends of a fixing belt and an image forming apparatus having the same. 
     According to an aspect of the disclosure, a belt type fixing apparatus may include a fixing belt, a fixing roller disposed to face the fixing belt, a nip forming member that is disposed inside the fixing belt and presses the fixing belt to the fixing roller to form a fixing nip, first and second regulating surfaces that are provided at opposite ends of the nip forming member and restrict an axial movement of the fixing belt, and first and second guide surfaces that are provided inside the fixing belt and guide rotation of the fixing belt. The first regulating surface and the second regulating surface may include at least one regulating step portion which is not in contact with one end of the fixing belt, respectively. The first guide surface and the second guide surface may include at least one guide step portion which is not in contact with an inner surface of the fixing belt. The at least one regulating step portion and the guide step portion may be formed to be staggered from each other. 
     The first regulating surface and the first guide surface may be formed integrally with a first guide bush which is provided at the one end of the fixing belt, and the second regulating surface and the second guide surface may be formed integrally with a second guide bush which is provided at another end of the fixing belt. 
     An entire portion of each of the first and second guide surfaces may be substantially perpendicular to each of the first and second regulating surfaces in an axial direction of the fixing belt, and at least portion of each of opposite ends of a bottom surface of the fixing belt that is in contact with the opposite ends of the fixing belt contacting the first and second regulating surfaces may be in contact with each of the first and second guide surfaces. 
     The first regulating surface and the second regulating surface may include at least one sub-regulating surface that is in contact with the one end of the fixing belt, respectively, the first guide surface and the second guide surface may include at least one sub-guide surface that is in contact with the inner surface of the fixing belt, and the at least one sub-regulating surface and the at least one sub-guide surface may be staggered so as not to intersect each other. 
     The first regulating surface and the second regulating surface may include an entrance end where the fixing belt enters, respectively, and an angle between the entrance end and a tangent line of the fixing belt at a point where the fixing belt enters the entrance end may be in a range of about 85 degrees to about 95 degrees. 
     According to another aspect of the disclosure, a belt type fixing apparatus may include a fixing belt, a fixing roller provided to face the fixing belt, a nip forming member that is provided inside the fixing belt and presses the fixing belt to the fixing roller to form a fixing nip, first and second regulating surfaces that are provided at opposite ends of the nip forming member and restrict an axial movement of the fixing belt, and first and second guide surfaces that are provided inside the fixing belt and guide rotation of the fixing belt. The first regulating surface may be provided on a first guide bush disposed at one end of the fixing belt and the second regulating surface may be provided on a second guide bush disposed at another end of the fixing belt. The first guide surface and the second guide surface may be formed adjacent to the first guide bush and the second guide bush on opposite sides of the nip forming member. 
     The nip forming member may include a plurality of guide ribs formed in a longitudinal direction, and the first guide surface and the second guide surface may be formed by two guide ribs provided at the opposite ends of the plurality of guide ribs. 
     Other objects, advantages and salient features of the disclosure will become apparent from the following detailed description, which, taken in conjunction with the annexed drawings, discloses preferred embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects and advantages of the disclosure will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is a partial view illustrating a state in which a guide bush used in a conventional belt type fixing apparatus guides a fixing belt; 
         FIG. 2  is an enlarged partial view illustrating an A portion of  FIG. 1 ; 
         FIG. 3  is a view illustrating a state in which the fixing belt is moved along a second surface of the guide bush in the A portion of  FIG. 1 ; 
         FIG. 4  is a perspective view illustrating a belt type fixing apparatus according to an embodiment of the disclosure; 
         FIG. 5  is an exploded perspective view illustrating the belt type fixing apparatus of  FIG. 4 ; 
         FIG. 6  is a cross-sectional view illustrating the belt type fixing apparatus of  FIG. 4  taken along a line  6 - 6 ; 
         FIG. 7  is a perspective view illustrating an example of a guide bush used in a belt type fixing apparatus according to an embodiment of the disclosure; 
         FIG. 8  is a partial perspective view illustrating a state in which the guide bush of  FIG. 7  guides the fixing belt; 
         FIG. 9  is a view illustrating a state in which the guide bush of  FIG. 7  guides the fixing belt; 
         FIG. 10  is a perspective view illustrating another example of a guide bush used in a belt type fixing apparatus according to an embodiment of the disclosure; 
         FIG. 11  is a partial perspective view illustrating a state in which the guide bush of  FIG. 10  guides the fixing belt; 
         FIG. 12  is a view illustrating a state in which the guide bush of  FIG. 10  guides the fixing belt; 
         FIG. 13  is a view illustrating another example of a guide bush used in a belt type fixing apparatus according to an embodiment of the disclosure; 
         FIG. 14  is a perspective view illustrating a state in which a nip forming member and a guide bush of a belt type fixing apparatus according to an embodiment of the disclosure are assembled; 
         FIG. 15  is a perspective view illustrating a state in which a nip forming member and a guide bush of a belt type fixing apparatus according to an embodiment of the disclosure are separated from each other; 
         FIG. 16  is a cross-sectional view illustrating a state in which the guide bush and the nip forming apparatus of  FIG. 14  guide the fixing belt; 
         FIG. 17  is a partial plan view illustrating a nip forming member of a belt type fixing apparatus according to an embodiment of the disclosure; 
         FIG. 18  is a view illustrating another example of a guide rib of a nip forming member used in a belt type fixing apparatus according to an embodiment of the disclosure; 
         FIG. 19  is a view illustrating an angle between a fixing belt and an entrance end of a guide bush used in a conventional belt type fixing apparatus; 
         FIG. 20  is a view illustrating a force applied to a fixing belt by a guide bush used in a belt type fixing apparatus; 
         FIG. 21  is a view illustrating a force applied to a fixing belt by a guide bush used in a belt type fixing apparatus according to an embodiment of the disclosure; 
         FIG. 22  is a view illustrating an angle between a fixing belt and an entrance end of a guide bush used in a belt type fixing apparatus according to an embodiment of the disclosure; and 
         FIG. 23  is a cross-sectional view schematically illustrating an image forming apparatus including a belt type fixing apparatus according to an embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to example embodiments which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. The embodiments are described below to explain the disclosure by referring to the figures. Throughout the drawings, like reference numerals will be understood to refer to like parts, components and structures. 
     Hereinafter, certain exemplary embodiments of the disclosure will be described in detail with reference to the accompanying drawings. 
     The matters defined herein, such as a detailed construction and elements thereof, are provided to assist in a comprehensive understanding of this description. Thus, it is apparent that exemplary embodiments may be carried out without those defined matters. Also, well-known functions or constructions are omitted to provide a clear and concise description of exemplary embodiments. Further, dimensions of various elements in the accompanying drawings may be arbitrarily increased or decreased for assisting in a comprehensive understanding. 
       FIG. 4  is a perspective view illustrating a belt type fixing apparatus according to an embodiment of the disclosure.  FIG. 5  is an exploded perspective view illustrating the belt type fixing apparatus of  FIG. 4 , and  FIG. 6  is a cross-sectional view illustrating the belt type fixing apparatus of  FIG. 4  taken along a line  6 - 6 . 
     Referring to  FIGS. 4, 5, and 6 , a belt type fixing apparatus  1  according to an embodiment of the disclosure may include a fixing roller  10 , a fixing belt  20 , a nip forming member  30 , a pair of regulating surfaces  40 , and a pair of guide surfaces  50 . 
     The fixing roller  10  is to apply a predetermined pressure to a printing medium P, and is formed in a roller shape. The fixing roller  10  may include a shaft  11  formed of a metallic material such as aluminum or steel and an elastic layer  13  elastically deformed to form a fixing nip N with the fixing belt  20 . The elastic layer  13  is generally formed of silicon rubber. The fixing roller  10  is rotatably supported by a pair of supporting brackets  90 . The pair of supporting brackets  90  are provided with bearings  91  capable of supporting the shaft  11 . 
     Although not illustrated in  FIGS. 4 to 6 , the fixing roller  10  is configured to rotate by receiving power from a driving source such as a motor. The structure in which the fixing roller  10  is rotated by the driving source is the same as or similar to the driving structure of the conventional fixing roller; therefore, a detailed description thereof is omitted. 
     The fixing belt  20  is to apply predetermined heat to the printing medium P, and is formed of a substantially cylindrical endless belt. The fixing belt  20  is heated by a heat source in the same manner as a heating roller according to the prior art, thereby transferring heat to the printing medium P passing through the fixing nip N. Accordingly, the fixing belt  20  is disposed to face the fixing roller  10 , and forms the fixing nip N through which the printing medium P passes with the fixing roller  10 . When the fixing roller  10  rotates, the fixing belt  20  is rotated by a friction force between the fixing belt  20  and the fixing roller  10 . The axial length of the fixing belt  20  may be longer than the axial length of the fixing roller  10 . 
     The fixing belt  20  may be formed in a variety of structures. For example, the fixing belt  20  may be composed of a single layer formed of a metal sleeve, a heat-resistant resin film or the like. Alternatively, the fixing belt  20  may be composed of a base layer formed of a metal sleeve, a heat-resistant resin film or the like, and release layers formed on opposite side surfaces of the base layer. At this time, the release layer may be formed only on one surface facing the fixing roller  10 . Alternatively, the fixing belt  20  may include an elastic layer provided between the base layer and the release layer. The fixing belt  20  may be the same as or similar to the fixing belt used in the conventional belt type fixing apparatus; therefore, a detailed description of the structure of the fixing belt  20  is omitted. 
     The nip forming member  30  is provided inside the fixing belt  20 , and supports an inner surface of the fixing belt  20  so that the fixing belt  20  is in contact with the fixing roller  10  to form the fixing nip N. The nip forming member  30  has a length longer than the length of the fixing roller  10 . In detail, the nip forming member  30  may include a guiding member  31  that is in contact with the inner surface of the fixing belt  20  to guide and press the fixing belt  20 , and a supporting member  32  that is disposed on an upper side of the guiding member  31  and supports the guiding member  31 . 
     The guiding member  31  is in contact with the inner surface of the fixing belt  20  to form the fixing nip N, and guides the fixing belt  20  so that the fixing belt  20  can move smoothly in the vicinity of the fixing nip N. The guiding member  31  may be formed in a channel shape whose cross-section has a substantially U shape with a flat bottom, and the supporting member  32  is provided inside the guiding member  31 . A plurality of guide ribs may be provided in the longitudinal direction on both side surfaces of the guiding member  31 . 
     The supporting member  32  reinforces the guiding member  31  so as to minimize the bending deformation of the guiding member  31 . The supporting member  32  may be formed in a channel shape whose cross-section has a substantially U shape with a flat bottom, and is disposed inside the guiding member  31 . The supporting member  32  may be formed in a structure having a large cross-sectional moment of inertial such as an I-beam, an H-beam, etc., in addition to the U shape having a flat bottom. 
     As illustrated in  FIG. 6 , the bottom surface of the nip forming member  30 , that is, the bottom surface  31 a of the guiding member  31  is in contact with the inner surface of the fixing belt  20 , and an upper portion of the fixing roller  10  that is in contact with a portion of the fixing belt  20  supported by the bottom surface  31 a of the guiding member  31  forms the fixing nip N. Accordingly, when the fixing roller  10  rotates, the fixing belt  20  is rotated by friction with the fixing roller  10 . 
     Opposite ends of the guiding member  31  are supported by the pair of supporting brackets  90 . The pair of supporting brackets  90  are fixed to a frame of an image forming apparatus that is not illustrated. The pair of supporting brackets  90  are provided with a pair of guide bushes  60  and  60 ′. The pair of guide bushes  60  and  60 ′ are inserted into guide grooves  92  provided in the pair of supporting brackets  90 , and can slide up and down along the side walls  92   a  of the guide groove  92 . Accordingly, opposite side surfaces of each of the guide bushes  60  and  60 ′ are provided with insertion slots  65  in the longitudinal direction. When the side walls  92   a  of the guide groove  92  of each of the supporting brackets  90  are inserted into the insertion slots  65  provided in the opposite side walls  92   a  of each of the guide bushes  60  and  60 ′, the guide bushes  60  and  60 ′ may slide up and down with respect to the supporting brackets  90 . In other words, the side walls  92   a  of the guide groove  92  of the supporting bracket  90  may function as a guide rail for guiding the movement of the guide bushes  60  and  60 ′. 
     A pressing member  94  for pressing the guide bush  60  and  60 ′ is provided on one surface of the supporting bracket  90 . The pressing member  94  may include a pressing link  94   a  rotatably disposed in the supporting bracket  90  and a coil spring  94   b  connected to the pressing link  94   a . The pressing link  94   a  is provided to be in contact with the top of each of the guide bushes  60  and  60 ′. One end of the coil spring  94   b  is fixed to the pressing link  94   a , and the other end of the coil spring  94   b  is fixed to the frame (not illustrated) of the image forming apparatus, thereby applying a force pulling the pressing link  94   a  downward. Accordingly, when the coil spring  94   b  applies the force pulling the pressing link  94   a  downward, the guide bushes  60  and  60 ′ are urged in a downward direction. When the guide bushes  60  and  60 ′ are urged downward, the guiding member  31  is also urged in the downward direction so that the fixing nip N is formed between the fixing belt  20  and the fixing roller  10 . 
     The pair of regulating surfaces  40  and  40 ′, that is, a first regulating surface  40  and a second regulating surface  40 ′ are provided at the opposite ends of the fixing belt  20 , and restrict axial movement of the fixing belt  20 . A pair of guide surfaces  50  and  50 ′, that is, a first guide surface  50  and a second guide surface  50 ′ are provided to abut or contact with the pair of regulating surfaces  40  and  40 ′, and support inner surfaces of the opposite end portions of the fixing belt  20  so that the fixing belt  20  can rotate. In other words, the first and second guide surfaces  50  and  50 ′ are provided inside the fixing belt  20  and are formed to guide the rotation of the fixing belt  20 . 
     The pair of regulating surfaces  40  and  40 ′ and the pair of guide surfaces  50  and  50 ′ are provided to be perpendicular to each other. In detail, the first regulating surface  40  is provided at one end of the fixing belt  20  to be perpendicular to the first guide surface  50 , and the second regulating surface  40 ′ is provided at the other end of the fixing belt  20  to be perpendicular to the second guide surface  50 ′. 
     The first guide surface  50  is provided in the axial direction of the fixing belt  20  such that the entire surface of the first guide surface  50  is perpendicular to the first regulating surface  40 . In other words, the entire width of the first guide surface  50  is formed to be perpendicular to the first regulating surface  40 . Accordingly, unlike the guide bush  200  (see  FIG. 1 ) according to the prior art, there is no round shape  205  at a portion where the first regulating surface  40  and the first guide surface  50  are connected or contacted with each other. Accordingly, in the belt type fixing apparatus  1  according to an embodiment of the disclosure, even when the fixing belt  20  moves in the axial direction to be close to the first regulating surface  40 , the force that causes the end portion of the fixing belt  20  to be spread like a morning glory is not applied to the fixing belt  20 . 
     Further, the first guide surface  50  is provided to be able to contact and support at least a portion of the edge of the bottom surface of the fixing belt  20  connected to one end of the fixing belt  20  which is in contact with or adjacent to the first regulating surface  40 . 
     The second regulating surface  40 ′ and the second guide surface  50 ′ may be formed in the same manner as the first regulating surface  40  and the first guide surface  50  as described above; therefore, detailed descriptions thereof are omitted. 
     The first and second regulating surfaces  40  and  40 ′ and the first and second guide surfaces  50  and  50 ′ as described above may be formed in a variety of manners. 
     For example, the regulating surfaces  40  and  40 ′ and the guide surfaces  50  and  50 ′ may be provided in a single guide bush  60  and  60 ′. In detail, the first regulating surface  40  and the first guide surface  50  may be provided integrally with a first guide bush  60  disposed at one end of the fixing belt  20 , and the second regulating surface  40 ′ and the second guide surface  50 ′ may be provided integrally with a second guide bush  60 ′ disposed at the other end of the fixing belt  20 . 
     In the following description, the first and second regulating surfaces  40  and  40 ′ are collectively referred to as a regulating surface  40 , and the first and second guide surfaces  50  and  50 ′ are collectively referred to as a guide surface  50 . Also, the first and second guide bushes  60  and  60 ′ are collectively referred to as a guide bush  60 . However, if necessary, the first and second regulating surfaces  40  and  40 ′, the first and second guide surfaces  50  and  50 ′, and the first and second guide bushes  60  and  60 ′ may be used separately. 
     Hereinafter, a guide bush provided with a regulating surface and a guide surface usable in a belt type fixing apparatus according to an embodiment of the disclosure will be described in detail with reference to  FIG. 7 . 
       FIG. 7  is a perspective view illustrating an example of a guide bush used in a belt type fixing apparatus according to an embodiment of the disclosure. 
     Referring to  FIG. 7 , a guide bush  60  may include a fixed body  62  and a rotary support part  61 . The fixed body  62  is formed to be slidable up and down with respect to the supporting bracket  90  of the fixing apparatus. The fixed body  62  is formed in a substantially octagonal shape, the rotary support part  61  is provided on the front surface of the fixed body  62 , and the insertion slots  65  into which the opposite side walls  92   a  of the supporting bracket  90  are inserted are formed on opposite side surfaces of the fixed body  62 . 
     The front surface of the fixed body  62  is provided with the regulating surface  40  for restricting the axial movement of the fixing belt  20 . The regulating surface  40  may include at least one regulating step portion  41  with which one end of the fixing belt  20  is not in contact. Since the regulating step portion  41  is formed to be lower in height than the regulating surface  40 , when one end of the fixing belt  20  is in contact with the regulating surface  40 , the regulating step portion  41  is not in contact with the one end of the fixing belt  20 . In other words, the fixed body  62  is formed so that the entire front surface  40  does not restrict the one end of the fixing belt  20  but only a portion of the front surface  40  restricts the one end of the fixing belt  20 . 
     Also, the regulating surface  40  may include at least one sub-regulating surface  43  in contact with the one end of the fixing belt  20 . Accordingly, the regulating surface  40  may include at least one sub-regulating surface  43  and at least one regulating step portion  41  that are formed alternately. When the one end of the fixing belt  20  is in contact with the regulating surface  40 , the sub-regulating surface  43  is in contact with the one end of the fixing belt  20 , and the regulating step portion  41  is not in contact with the one end of the fixing belt  20 . 
     A portion  41 a of the regulating step portion  41  connected to the sub-regulating surface  43  is formed to be inclined upward in the rotational direction of the fixing belt  20 . Accordingly, when the fixing belt  20  rotates, the one end of the fixing belt  20  may easily enter the sub-regulating surface  43  of the guide bush  60 . 
     The rotary support part  61  extends perpendicularly from the front surface of the fixed body  62 , and supports the fixing belt  20  to rotate. The rotary support part  61  may be formed in a variety of shapes as long as it can support the rotation of the fixing belt  20 . In  FIG. 7 , the rotary support part  61  formed in an arc shape or an arch shape for providing a space below the rotary support part  61  is illustrated. Accordingly, a predetermined space is provided below the rotary support part  61 . Further, the rotary support part  61  may be formed in an arc shape that is larger or smaller than a semi-circle. In the embodiment, the rotary support part  61  is formed in an arc shape substantially larger than the semi-circle. The guide surface  50  is formed on the top surface of the rotary support part  61 . 
     The guide surface  50  may include at least one guide step portion  51  that is not in contact with the inner surface of the fixing belt  20 . The guide step portion  51  is formed to be lower in height than the guide surface  50 , and when the inner surface of the fixing belt  20  is in contact with the guide surface  50 , the guide step portion  51  is not in contact with the inner surface of the fixing belt  20 . In other words, the guide surface  50  is formed so that the entire portion of the guide surface  50  does not support the inner surface of the fixing belt  20  but only a portion of the guide surface  50  supports the inner surface of the fixing belt  20 . Since a point where the portion of the guide surface  50  that supports the inner surface of the fixing belt  20  meets the front surface of the fixed body  62  is positioned at the inner side than the point where the regulating surface  40  meets the guide surface  50 , the one end of the fixing belt  20  is not in contact with the point where the portion of the guide surface  50  supporting the inner surface of the fixing belt  20  meets the front surface of the fixed body  62 . 
     Further, the guide surface  50  may include at least one sub-guide surface  53  in contact with the inner surface of the fixing belt  20 . Accordingly, the guide surface  50  may include at least one sub-guide surface  53  and at least one guide step portion  51  that are formed alternately. When the inner surface of the fixing belt  20  is in contact with the guide surface  50 , the sub-guide surface  53  is in contact with the inner surface of the fixing belt  20 , and the guide step portion  51  is not in contact with the inner surface of the fixing belt  20 . 
     Further, the at least one guide step portion  51  and the at least one regulating step portion  41  are formed to be staggered from each other. In other words, the at least one sub-guide surface  53  and the at least one sub-regulating surface  43  are provided to be staggered from each other. Accordingly, the at least one sub-regulating surface  43  and the at least one sub-guide surface  53  do not intersect each other, the at least one sub-regulating surface  43  intersects with the at least one guide step portion  51 , and the at least one sub-guide surface  53  intersects with the at least one regulating step portion  41 . 
     For example, referring to  FIG. 7 , the guide surface  50  may include three sub-guide surfaces  53  and two guide step portions  51  provided therebetween. Also, the regulating surface  40  may include two sub-regulating surfaces  43  and three regulating step portions  41 . One regulating step portion  41  is provided between the two sub-regulating surface  43 , and two regulating step portions  41  are provided outside the two sub-regulating surfaces  43 . Accordingly, the three sub-guide surfaces  53  are formed to intersect at right angles with the three regulating step portions  41 , respectively. Accordingly, a rounded tool shape is formed at each portion where the three sub-guide surfaces  53  and the three regulating step portions  41  are connected. However, since the height of the regulating step portion  41  is lower than that of the sub-regulating surface  43 , the connecting portion between the sub-guide surface  53  and the regulating step portion  41  is located inside than the sub-regulating surface  43 . Accordingly, when the fixing belt  20  guided by the three sub-guide surface  53  moves in the axial direction, the one end of the fixing belt  20  is in contact with only the sub-regulating surface  43  and is not in contact with the round shape of the connecting portion of the sub-guide surface  53 . 
     At this time, the number of the at least one regulating step portion  41  may be formed to be larger than that of the at least one guide step portion  51 . In other words, the number of the at least one sub-guide surface  53  may be formed to be larger than the number of the at least one sub-regulating surface  43 . Since the guide bush  60  as illustrated in  FIG. 7  is provided with two sub-regulating surfaces  43  and three sub-guide surfaces  53 , the number of the sub-guide surfaces  53  is one more than the number of the sub-regulating surfaces  43 . 
       FIG. 8  is a partial perspective view illustrating a state in which the guide bush of  FIG. 7  guides the fixing belt, and  FIG. 9  is a view illustrating a state in which the guide bush of  FIG. 7  guides the fixing belt. 
     As illustrated in  FIGS. 8 and 9 , when the fixing belt  20  rotates, the inner surface adjacent to one end  20   a  of the fixing belt  20  is supported by the three sub-guide surfaces  53 , and the one end  20   a  of the fixing belt  20  is restricted by the two sub-regulating surfaces  43 . At this time, since the sub-regulating surface  43  protrudes toward the guide surface  50  more than the connecting portion between the sub-guide surface  53  and the regulating step portion  41 , the one end  20   a  of the fixing belt  20  is not in contact with the connecting portion between the guide surface  50  and the regulating surface  40 . Also, when the one end  20   a  of the fixing belt  20  is in contact with the sub-regulating surface  43 , the one end  20   a  of the fixing belt  20  is not in contact with the regulating step portion  41 . Accordingly, the one end  20   a  of the fixing belt  20  does not receive the force acting from the inside to the outside by the round shape of the connecting portion. 
     In the above description, the guide surface  50  includes three sub-guide surfaces  53 , and the regulating surface  40  includes two sub-regulating surfaces  43 . However, the number of the sub-guide surfaces  53  constituting the guide surface  50  and the number of the sub-regulating surfaces  43  constituting the regulating surface  40  are not limited thereto. For example, the regulating surface  40  may include one sub-regulating surface  43  or three or more sub-regulating surfaces  43 , and the guide surface  50  may include one sub-guide surface  53  or four or more sub-guide surfaces  53 . 
     Hereinafter, as another example of the guide bush, with reference to  FIGS. 10 to 12 , a guide bush  60   a  will be described in which the guide surface  50  includes two sub-guide surfaces  53 , and the regulating surface  40  includes one sub-regulating surface  43 . 
       FIG. 10  is a perspective view illustrating another example of a guide bush used in a belt type fixing apparatus according to an embodiment of the disclosure.  FIG. 11  is a partial perspective view illustrating a state in which the guide bush of  FIG. 10  guides the fixing belt, and  FIG. 12  is a view illustrating a state in which the guide bush of  FIG. 10  guides the fixing belt. 
     Referring to  FIG. 10 , the guide surface  50  may include two sub-guide surfaces  53  and one guide step portion  51  provided between the two sub-guide surfaces  53 . Also, the regulating surface  40  may include one sub-regulating surface  43  and two regulating step portions  41 . The two regulating step portions  41  are provided on opposite sides of the one sub-regulating surface  43 . Accordingly, the two sub-guide surfaces  53  are formed to intersect at right angles with the two regulating step portions  41 , respectively. Accordingly, a rounded tool shape is formed at each connecting portion where the two sub-guide surfaces  53  and the two regulating step portions  41  are connected. At this time, since the regulating step portion  41  is lower than the sub-regulating surface  43 , the connecting portion between the sub-guide surface  53  and the regulating step portion  41  is located inside than the sub-regulating surface  43 . Accordingly, when the fixing belt  20  guided by the two sub-guide surfaces  53  moves in the axial direction, the one end of the fixing belt  20  is only in contact with the sub-regulating surface  43 , and does not move along the round shape of the connecting portion between the sub-guide surface  53  and the regulating step portion  41 . 
     In detail, as illustrated in  FIGS. 11 and 12 , when the fixing belt  20  rotates, the inner surface adjacent to the one end  20   a  of the fixing belt  20  is supported by the two sub-guide surfaces  53 , and the one end  20   a  of the fixing belt  20  is restricted by the one sub-regulating surface  43 . At this time, since the sub-regulating surface  43  protrudes toward the guide surface  50  more than the connecting portion between the sub-guide surface  53  and the regulating step portion  41 , the one end  20   a  of the fixing belt  20  is not in contact with the connecting portion. Accordingly, the one end  20   a  of the fixing belt  20  does not receive the force acting from the inside to the outside by the round shape of the connecting portion, so that cracking of the one end  20   a  of the fixing belt  20  may be prevented. 
       FIG. 13  is a front view illustrating another example of a guide bush used in a belt type fixing apparatus according to an embodiment of the disclosure. 
     Referring to  FIG. 13 , a guide bush  60   b  may include a guide surface  50  provided with four sub-guide surfaces  53  and three guide step portions  51 , and a regulating surface  40  provided with three sub-regulating surfaces  43  and four regulating step portions  41 . At this time, since connecting portions between the four sub-guide surfaces  53  and the four regulating step portions  41  are positioned behind the three sub-regulating surfaces  43 , when the fixing belt  20  rotates along the guide surface  50  of the guide bush  60   b , one end of the fixing belt  20  is not in contact with the connecting portions between the sub-guide surfaces  53  and the regulating step portions  41 . 
     The above-described guide bush  60  may be formed of a material having high heat resistance. For example, the guide bush  60  may be formed of poly phenylene sulfide (PPS) or the like. 
     The heat source  70  is provided inside the fixing belt  20 , and generates heat to heat the fixing belt  20  to the fixing temperature. As illustrated in  FIG. 6 , the heat source  70  is disposed on the bottom surface of the nip forming member  30  to directly heat the fixing belt  20 . As the heat source  70 , a ceramic heater or the like may be used. An electric wire for supplying electricity is connected to the heat source  70 . However, the electric wire connected to the heat source  70  is omitted for the convenience of illustration. The heat source  70  may use a heat source used in a conventional fixing apparatus; therefore, a detailed description thereof is omitted. 
     In the above description, the heat source  70  is provided on the bottom surface of the nip forming member  30  to directly heat the fixing belt  20 . However, the heat source  70  may be disposed above the nip forming member  30  to heat the fixing belt  20  by radiation. For example, a halogen lamp may be provided as the heat source  70  above the nip forming member  30  so that the halogen lamp radiates heat onto the inner surface of the fixing belt  20 . 
     In the above description, the guide surface  50  and the regulating surface  40  for guiding the rotation of the fixing belt  20  are integrally formed on the guide bush  60 , but the guide surface  50  and the regulating surface  40  may be formed as separate parts. 
     Hereinafter, a case where the guide surface and the regulating surface are formed as separate parts will be described with reference to  FIGS. 14 to 17 . 
       FIG. 14  is a perspective view illustrating a state in which a nip forming member and a guide bush of a belt type fixing apparatus according to an embodiment of the disclosure are assembled.  FIG. 15  is a perspective view illustrating a state in which a nip forming member and a guide bush of a belt type fixing apparatus according to an embodiment of the disclosure are separated from each other.  FIG. 16  is a cross-sectional view illustrating a state in which the guide bush and the nip forming apparatus of  FIG. 14  guide the fixing belt. 
     Referring to  FIGS. 14 to 16 , a front surface of each of a pair of guide bushes  600  provided at opposite ends of a nip forming member  300  forms a regulating surface  40  for restricting axial movement of the fixing belt  20 , and a pair of guide surfaces  310  for guiding rotation of the fixing belt  20  are provided at portions of the nip forming member  300  adjacent to the front surfaces of the pair of guide bushes  600 . 
     In detail, a first regulating surface  40  is provided in a first guide bush  600  disposed at one end of the fixing belt  20  to restrict the axial movement of the fixing belt  20 , and a second regulating surface (not illustrated) is provided in a second guide bush (not illustrated) disposed at the other end of the fixing belt  20  to restrict the axial movement of the fixing belt  20 . In other words, the front surface of the first guide bush  600  provided in the vicinity of one end of the nip forming member  300  that is disposed inside the fixing belt  20  forms the first regulating surface  40 , and the front surface of the second guide bush (not illustrated) provided in the vicinity of the other end of the nip forming member  300  forms the second regulating surface. 
     The pair of guide surfaces  310  for supporting the inner surfaces of the opposite ends of the fixing belt  20 , that is, a first guide surface  310  and a second guide surface are provided in the nip forming member  300 . In detail, the first guide surface  310  is provided adjacent to the first guide bush  600  disposed at one side of the nip forming member  300 , and the second guide surface is provided adjacent to the second guide bush disposed at the other side of the nip forming member  300 . The first and second guide surfaces  310  may be formed by two guide ribs  320 - 1  which are provided near the opposite ends of the nip forming member  300  among a plurality of guide ribs  320  provided in the nip forming member  300 . 
     For example, referring to  FIG. 14 , a guiding member  301  of the nip forming member  300  may include the plurality of guide ribs  320  formed in the longitudinal direction. The plurality of guide ribs  320  support the inner surface of the fixing belt  20  so that the fixing belt  20  can smoothly rotate. The guide surface  310  may be formed on each of the two guide ribs  320 - 1  that are provided at both ends of the plurality of guide ribs  320  and support the inner surfaces of the opposite ends of the fixing belt  20 . In other words, a first guide surface  310  may be formed on the first guide rib  320 - 1  for supporting the inner surface adjacent to the first guide bush  600 , and a second guide surface may be formed on the second guide rib (not illustrated) adjacent to the second guide bush (not illustrated). The first guide surface  310  may be formed by the top surface of the first guide rib  320 - 1  in contact with the inner surface of the one end of the fixing belt  20 , and the second guide surface may be formed by the top surface of the second guide rib in contact with the inner surface of the other end of the fixing belt  20 . Accordingly, the first guide surface  310  may support the inner surface of the fixing belt  20  in contact with the one end of the fixing belt  20 , and the second guide surface may support the inner surface of the fixing belt  20  in contact with the other end of the fixing belt  20 . 
     The first guide bush  600  is provided in the guide member  301  so that the front surface of the first guide bush  600  is in contact with or adjacent to the side surface of the first guide rib  320 - 1  and is perpendicular to the top surface of the first guide rib  320 - 1 , that is, the first guide surface  310 . Further, the second guide surface is provided in the guide member  301  so that the front surface of the second guide bush is in contact with or adjacent to the side surface of the second guide rib and is perpendicular to the top surface of the second guide rib, that is, the second guide surface. 
     Referring to  FIG. 14 , each of the plurality of guide ribs  320  provided in the guide member  301  may include two sub-guide ribs  321  and  322  facing each other in the width direction of the guide member  301 . In detail, the guide rib  320  may include an entry side sub guide rib  321  extending from an entry end of the guide member  301  into which the fixing belt  20  enters the fixing nip N and an exit side sub guide rib  322  extending from an exit end of the guide member  301  through which the fixing belt  20  exits the fixing nip N, the above-described first guide rib  320 - 1  may be formed by connecting the two sub-guide ribs  321  and  322  provided at the one end of the guide member  301 . Also, the second guide rib may be formed by connecting two sub-guide ribs provided at the other end of the guide member  301 . At this time, the first guide surface  310  of the first guide rib  320 - 1  and the second guide surface of the second guide rib may be formed in an arch shape corresponding to the shape of the fixing belt  20 . 
     Further, the width W 1  of each of the two guide ribs  320 - 1  provided at both ends of the plurality of guide ribs  320  provided in the guide member  301 , that is, the first guide rib  320 - 1  and the second guide rib may be formed wider than the width W 2  of each of the remaining guide ribs  320  located between the first guide rib  320 - 1  and the second guide rib. When increasing the widths W 1  of the first guide rib  320 - 1  and the second guide rib, the opposite end portions of the fixing belt  20  may be stably supported so that the opposite ends of the fixing belt  20  may be prevented from being wrinkled or damaged by external force. 
     On the other hand, the first guide bush  600  and the second guide bush may be detachably provided in the nip forming member  300 . 
       FIG. 17  is a partial plan view illustrating a nip forming member of a belt type fixing apparatus according to an embodiment of the disclosure. 
     Referring to  FIG. 15 , a pair of coupling ribs  603  facing each other are formed on opposite side surfaces of an opening  601  provided in the middle of the lower portion of the first guide bush  600 . Also, referring to  FIG. 17 , one end of the guide member  301  of the nip forming member  300  is provided with a pair of coupling slots  303  into which the pair of coupling ribs  603  of the first guide bush  600  are inserted. Accordingly, when the pair of coupling ribs  603  of the first guide bush  600  are inserted into the pair of coupling slots  303  of the guide member  301 , the first guide bush  600  is firmly fixed to the guide member  301 . Accordingly, movement of the first guide bush  600  for restricting the axial movement of the fixing belt  20  with respect to the guide member  301  may be minimized. 
     Although not illustrated, the second guide bush also has a pair of coupling ribs like the first guide bush  600 , and the guide member  301  is provided with a pair of coupling slots into which the pair of coupling ribs of the second guide bush are inserted. The coupling ribs of the second guide bush and the coupling slots of the guide member  301  are the same as the coupling ribs  603  of the first guide bush  600  and the coupling slots  303  of the guide member  301  as described above; therefore, detailed descriptions thereof are omitted. 
     In the above description, the guide surface  310  provided in the nip forming member  300  is formed as a continuous curved surface. However, the structure of the guide surface  310  is not limited thereto. The guide surface may be formed as separate curved surfaces. 
       FIG. 18  is a view illustrating another example of a guide rib of a nip forming member used in a belt type fixing apparatus according to an embodiment of the disclosure. 
     For example, as illustrated in  FIG. 18 , an opening  311  may be provided at the top of the first guide rib  320 - 1  adjacent to the first guide bush  600 . In detail, the first guide rib  320 - 1  may be formed of two cut curved surfaces instead of a continuous curved surface. In other words, the entry side sub guide rib  321  and the exit side sub guide rib  322  of the guide rib  320 - 1  may be formed not to be connected to each other. Accordingly, the guide surface  310 ′ may be formed as two separate curved surfaces. 
     On the other hand, the regulating surface  400  of the guide bush  600  for restricting the axial movement of the fixing belt  20  may be formed in a partially cut shape in order to avoid interference with the counterpart part. 
       FIG. 19  is a view illustrating an angle between a fixing belt and an entrance end of a guide bush used in a conventional belt type fixing apparatus. 
     For example, as illustrated in  FIG. 19 , the guide bush  600  is provided with the opening  601  at the lower portion of the guide bush  600  to avoid interference with the guide member  301 . Accordingly, when the fixing belt  20  rotates, the one end of the fixing belt  20  enters the regulating surface  400  of the guide bush  600  from the opening  601 , and then comes into contact with the regulating surface  400 . At this time, a portion of the guide bush  600  where contact with the fixing belt  20  starts is provided with an inclined surface  611  so that the fixing belt  20  may smoothly enter the regulating surface  400 . 
     In  FIG. 19 , the fixing belt  20  rotating in the clockwise direction is brought into contact with the regulating surface  400  through the opening  601  and the inclined surface  611 . In other words, the one end of the fixing belt  20  is brought into contact with the regulating surface  400  through an entrance end  610  where the inclined surface  611  and the regulating surface  400  abut. When the one end of the fixing belt  20  passes through the entrance end  610 , a force F is applied to the fixing belt  20 . At this time, an angle between the entrance end  610  and a tangent line of the fixing belt  20  drawn at a point where the fixing belt  20  enters the entrance end  610  is  8 . 
       FIG. 20  shows the force (i.e., reaction force) applied to the fixing belt  20  at the time when the fixing belt  20  passes through the entrance end  610  of the guide bush  600 . In  FIG. 20 , the reaction force F applied to the fixing belt  20  may be divided into a radial force component F 1  and a circumferential force component F 2 . At this time, the larger the radial force component F 1  is, the more the one end of the fixing belt  20  spread, so that the fixing belt  20  may be easily broken. Accordingly, it is desirable to minimize the radial force component F 1 . 
     When the fixing belt  20  begins to contact the regulating surface  400  at the entrance end  610  of the guide bush  600 , the radial force component F 1  of the force applied to the fixing belt  20  is zero when the angle θ between the entrance end  610  and the tangent line of the fixing belt  20  drawn at the entering point is 90 degrees. That is, F 1 =0. At this time, F 2 =F. 
     Accordingly, as illustrated in  FIG. 21 , the entrance end  610  may be formed at an angle of 90 degrees with the tangent line of the fixing belt  20 . However, the angle θ between the entrance end  610  of the guide bush  600  and the tangent line of the fixing belt  20  may be formed within a range of 90±5 degrees in consideration of the machining tolerance of the parts, the assembly deviation between the parts, and the like. Here,  FIG. 21  is a view illustrating a force applied to a fixing belt by a guide bush used in a belt type fixing apparatus according to an embodiment of the disclosure. 
     As another embodiment, as illustrated in  FIG. 7 , in the case in which the regulating surface  400  may include a plurality of sub-regulating surfaces  43  and a plurality of regulating step portions  41 , when the fixing belt  20  enters the sub-regulating surface  43  from the regulating step portion  41 , a reaction force is applied to the fixing belt  20 . Accordingly, if the angle θ between an entrance end  48  connecting the regulating step portion  41  and the sub-regulating surface  43  and the tangent line of the fixing belt  20  is caused to be 90 degrees, the radial force component of the force applied to the fixing belt  20  may be made zero. 
       FIG. 22  is a view illustrating an angle between a fixing belt and an entrance end of a guide bush used in a belt type fixing apparatus according to an embodiment of the disclosure. 
     As illustrated in  FIG. 22 , when the fixing belt  20  rotates in the clockwise direction, the fixing belt  20  enters the first sub-regulating surface  43 - 1  through the inclined surface  41 - 1  a of the first regulating step portion  41 - 1  adjacent to the opening  60   a . At this time, when the fixing belt  20  passes through the entrance end  48  where the first regulating step portion  41 - 1  is in contact with the first sub-regulating surface  43 - 1 , a force is applied to the fixing belt  20 , and when the angle θ between the entrance end  48  and the tangent line of the fixing belt  20  is 90 degrees, the radial force component applied to the fixing belt  20  becomes zero. In this case as well, the angle θ between the entrance end  48  and the tangent line of the fixing belt  20  may be a range of 90±5 degrees in consideration of the machining tolerance of the parts, the assembly deviation between the parts, and the like. 
     The inventors measured the number of printing media in which breakage occurred in the fixing belt in accordance with the change in the angle between the entrance end and the tangent line of the fixing belt. 
     In the case in which the angle between tangent line of the fixing belt and the entrance end was 75 degrees, the flaring of the end of the fixing belt started when approximately 90,000 sheets of the printing media were printed. However, in the case in which the angle between tangent line of the fixing belt and the entrance end was 85 degrees, the end of the fixing belt was not damaged until approximately 170,000 sheets of the printing media were printed. 
     The conditions of the durability test of the above-described fixing belt are as follows. 
     One side pressing force of the fixing apparatus; 10 Kgf 
     Fixing nip size; 9.5 mm 
     Axial diagonal force of the fixing belt (Fz); 500 gf 
     Inclination angle of the guide bush; 165 degrees 
     Reaction force applied to the fixing belt; F=Fz×Tan(180°−165°)=133.98 gf 
     When an angle between a tangent line of the fixing belt and the entrance end is 75 degrees, the radial force component of the reaction force: F 1 =F×Sin(90°−75°)=45.82 gf 
     When an angle between a tangent line of the fixing belt and the entrance end is 85 degrees, the radial force component of the reaction force: F 1 =F×Sin(90°−8°)=11.68 gf 
     As described above, when the angle between the tangent line of the fixing belt and the entrance end is set to be close to 90 degrees, the force applied to the fixing belt is reduced, so that the lifetime of the fixing belt may be prolonged. 
     Hereinafter, an image forming apparatus  100  provided with a belt type fixing apparatus  1  according to an embodiment of the disclosure will be described with reference to  FIG. 23 . 
       FIG. 23  is a cross-sectional view schematically illustrating an image forming apparatus including a belt type fixing apparatus according to an embodiment of the disclosure. 
     Referring to  FIG. 23 , the image forming apparatus  100  may include a main body  101 , a printing medium feeding unit  110 , an image forming unit  120 , a belt type fixing apparatus  1 , and a printing medium discharging unit  150 . 
     The main body  101  forms an appearance of the image forming apparatus  100 , accommodates the printing medium feeding unit  110 , the image forming unit  120 , the belt type fixing apparatus  1 , and the printing medium discharging unit  150  therein, and fixes and supports them. 
     The printing medium feeding unit  110  is disposed inside the main body  101  to supply the printing medium P to the image forming unit  120 , and may include a printing medium feeding cassette  111  and a pickup roller  112 . The printing medium feeding cassette  111  accommodates a predetermined number of printing media, and the pickup roller  112  picks up the printing medium P accommodated in the printing medium feeding cassette  111  one by one, and supplies the printing medium P to the image forming unit  120 . 
     A plurality of conveying rollers  115  for conveying the printing medium P picked up by the pickup roller  112  are provided between the pickup roller  112  and the image forming unit  120 . 
     The image forming unit  120  forms a predetermined image on the printing medium P supplied from the printing medium feeding unit  110 , and may include an exposure unit  121 , a developing cartridge  130 , and a transfer roller  140 . The exposure unit  121  emits a predetermined light corresponding to the print data depending to the printing command. The developing cartridge  130  may include an image carrier  131  on which an electrostatic latent image is formed by the light emitted from the exposure unit  121 , and a developing roller  132  which is disposed at a side of the image carrier  131 , and supplies developer to the image carrier  120 , thereby developing the electrostatic latent image formed on the image carrier  131  into a developer image. In addition, the developing cartridge  130  stores a predetermined amount of developer, and may include a developer supply roller  133  for supplying the developer to the developing roller  132 , an agitator  134  for agitating the developer, a cleaning blade  135  for cleaning the surface of the image carrier  131 , and the like. The transfer roller  140  is rotatably disposed to face the image carrier  131  of the developing cartridge  130 , and transfers the developer image formed on the image carrier  120  onto the printing medium P. 
     The belt type fixing apparatus  1  fixes the developer image onto the printing medium P by applying heat and pressure while the printing medium P on which the developer image is transferred in the image forming unit  120  passes through the belt type fixing apparatus  1 , and may include a fixing roller  10  and a fixing belt  20 . The structure and operation of the belt type fixing apparatus  1  are described above; therefore, a detailed description thereof will not be repeated for the sake of brevity. 
     The printing medium discharging unit  150  discharges the printing medium P on which the image is fixed while passing through the belt type fixing apparatus  1 , to the outside of the image forming apparatus  100 . The printing medium discharging unit  150  may include a pair of discharging rollers that face each other and rotate. 
     As described above, the belt type fixing apparatus  1  according to one or more embodiments of the disclosure may fix the developer image transferred to the printing medium P onto the printing medium P. 
     Also, the belt type fixing apparatus according to one or more embodiments of the disclosure does not have a round shape that connects the guide surface and the regulating surface of the guide bush in the regions through which the opposite ends of the fixing belt pass. Therefore, the fatigue cracks at the opposite ends of the fixing belt, which are generated when the fixing belt climbs up the round shape, may be minimized. 
     While various embodiments of the disclosure have been described, additional variations and modifications of the embodiments may occur to those skilled in the art once they learn of the basic inventive concepts. Therefore, it is intended that the appended claims shall be construed to include both the above embodiments and all such variations and modifications that fall within the spirit and scope of the inventive concepts. 
     Although example embodiments have been shown and described, it would be appreciated by those skilled in the art that changes may be made to these embodiments without departing from the principles and spirit of the disclosure, the scope of which is defined in the claims and their equivalents.