Abstract:
A heating mechanism includes a paper transmitting path for carrying a paper, a fixing roller provided on this paper transmitting path, a pressurizing roller disposed so as to oppose the fixing roller, a heating roller provided apart from the fixing roller for applying heat indirectly to the fixing roller, a belt which is wound around the heating roller and the fixing roller in order to transmit a rotation driving force from the fixing roller to the heating roller and thermal energy from the heating roller to the fixing roller and makes a contact with the other side face of the paper passing between the fixing roller and the pressurizing roller, and a pair of first restricting members disposed with a gap from both end faces of the heating roller to guard both sides of the belt on the heating roller thereby restricting a shifting motion of the belt in the width direction.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an image forming apparatus such as electro-photographic copier and laser printer and more particularly to a heating mechanism for heating, melting and fixing a toner image onto paper. 
     A copier employs a so-called belt-type heating mechanism. Features of the belt-type heating mechanism is low power consumption, short warming-up time, oilless, separation characteristic and the like. 
     The belt-type heating mechanism comprises a fixing roller, a heating roller, a fixing belt (hereinafter referred to as a heat transferring belt) stretched between the fixing roller and the heating roller and a pressurizing roller disposed so as to oppose the fixing roller through the heat transferring belt. The heat transferring belt is heated by the heating roller and fed to the fixing roller. A so-called nip portion is formed between the heat transferring belt and the pressurizing roller and toner of a toner image is melted under a pressure, so that the toner image is fixed on the paper. 
     In such a conventional belt-type heating mechanism, the heat transferring belt shifts in a width direction during a rotation of the fixing roller/heating roller, so that the heat transferring belt deviates from an image forming range, and consequently, a non-fixed image region may be sometimes generated. 
     BRIEF SUMMARY OF THE INVENTION 
     Jpn. Pat. Appln. KOKAI Publication No. 2000-338812(hereinafter referred to as a “preceding invention”) has proposed installation of a restricting member onto a roller in order to restrict the deviation of the heat transferring belt. However, when in the belt-type heating mechanism of the preceding invention, a side portion of the belt makes a sliding contact with the restricting member, overload is applied to the belt, so that buckling or swelling occurs in the belt. 
     Thus, in the heating mechanism of the preceding invention, a release agent roller is pressed to the belt so as to coat the belt with the release agent (oil) and apply an additional tension thereto, thereby preventing the belt from being buckled or swollen. Further, in the heating mechanism of the preceding invention, the release agent roller is set up shorter than the width of the fixing belt in order to prevent the release agent from invading to a rear side of the belt. 
     However, if the belt width is larger than the release agent roller for applying a tension, a tension shortage region in which no sufficient tension is applied to the belt by the release agent roller is generated in a belt side portion. Particularly, in a portion on the side in which the belt shifts in the width direction, this tension shortage region is expanded. If the belt side short of tension makes a sliding contact with the restricting member, a local buckling or swelling (out-of-plane deformation) is generated in the belt and finally, the belt may be broken. 
     An object of the present invention is to provide a heating mechanism for use in an image forming apparatus capable of fixing a toner image on a paper stably without generating a buckling or swelling (out-of-plane deformation) in the belt. 
     To achieve the above object, according to an aspect of the present invention, there is provided a heating mechanism for use in an image forming apparatus, comprising: a paper transporting path for carrying a paper having a toner image; a fixing roller disposed in the paper transporting path so that it is rotated; a pressurizing roller disposed so as to oppose the fixing roller across the paper transporting path so that it contacts one side face of a paper carried on the paper transporting path, the pressurizing roller being rotated synchronously with the fixing roller for applying a pressure on the paper in cooperation with the fixing roller; a heating roller provided apart from the fixing roller for applying heat indirectly to the fixing roller; a heat transferring belt which is wound around the heating roller and the fixing roller in order to transmit a rotation driving force from the fixing roller to the heating roller and thermal energy from the heating roller to the fixing roller, makes a contact with the other side face of the paper passing between the fixing roller and the pressurizing roller, and melts toner on the toner image by heat to fix the toner image on the paper; and a pair of first restricting members disposed with a predetermined gap from both end faces of the heating roller to guard both sides of the heat transferring belt on the heating roller thereby restricting a shifting motion of the heat transferring belt in a width direction. 
     When the belt shifts in the width direction on the heating roller, a side portion of the belt is intercepted by the first restricting member so that a further shift in the width direction of the belt is restricted. Consequently, an excessive deviation of the belt from the heating roller is eliminated, so that a buckling or swelling (out-of-plane deformation) of the side portion of the belt is suppressed. In this case, the belt side is permitted to deviate from a peripheral face of the heating roller if it is slight. The reason is that unless the deviation amount of the belt side from the peripheral face of the heating roller exceeds a predetermined value, the tension applied to the belt side does not drop excessively, and therefore, substantially the belt side portion is not deformed. Therefore, even when the first restricting member is provided apart from an end face of the heating roller, if a gap G 1  is set to a predetermined value or less, the buckling or swelling of the belt side portion is never generated. In this case, it is desirable that the gap G 1  is not more than 1.5 mm. 
     The aforementioned first restricting member is desired to be disposed on the side in which the belt is fed from the heating roller. On the side (loose side) in which the belt is fed from the heating roller, the belt tension drops so that the buckling or swelling (out-of-plane deformation) is likely to occur on the belt side. Thus, by disposing the first restricting member on the belt loose side of the heating roller, the buckling or swelling (out-of-plane deformation) of the belt side can be prevented effectively. 
     Preferably, the heating mechanism in an image forming apparatus further comprises a pair of second restricting members disposed with a predetermined gap from both end faces of the fixing roller to guard both sides of the heat transferring belt. Because the second restricting member restricts a shift motion in the width direction of the belt on the fixing roller, synergistic effect is exerted with the restriction by the first restricting member on the heating roller side. That is, if the second restricting member is provided on the side of the fixing roller, the total length of the first and second restricting members which the belt side makes a sliding contact with is increased and therefore, a local stress concentration on the belt side is relaxed, so that the belt side is not buckled. 
     Preferably, the second restricting member is disposed on the side (loose side) in which the heat transferring belt is fed from said fixing roller. The belt tension drops on the side (loose side) in which the belt is fed from the fixing roller, so that the buckling or swelling (out-of-plane deformation) is likely to occur on the belt side. Thus, by disposing the second restricting member on the belt loose side of the fixing roller, the buckling or swelling (out-of-plane deformation) on the belt side can be prevented effectively. 
     Meanwhile, the width L B  of the belt needs to be smaller than the length L H  of the heating roller and the length of the fixing roller. The reason is that if the width L B  of the belt is larger than the roller length, the belt side deviates from the peripheral rolling face peripheral circumferential surface of the heating roller or the fixing roller even when the belt does not shift in the width direction. 
     Preferably, the heating mechanism further comprises a release agent roller which coats the belt with release agent and is pressed on the entire width L B  of the belt so as to apply a tension to the heat transferring belt. In this case, the length L oil  of the release agent roller is substantially the same as a clearance between the pair of the first restricting members or a gap G 2  between the end face of the release agent roller and the first restricting member is not more than 12 mm. 
     To apply a tension to the entire belt from the release agent roller, the length L oil  of the release agent roller needs to be larger than the width L B  of the heat transferring belt. 
     Preferably, the gap G 2  in plan view from an end face of the release agent roller to the first restricting member is not more than 7 mm. If the gap G 2  is not more than 7 mm, no deformation occurs in the belt side portion as shown in FIG.  9 B. However, if the gap G 2  is, for example, 12 mm, the belt side portion is deformed as shown in FIG. 9C, so that the buckling or swelling occurs and finally, the belt is broken. This reason is estimated to be that the tension applied from the release agent roller to the belt acts on a deviated belt side portion from the heating roller. Therefore, by providing with the release agent roller, it is possible to mount the first restricting member apart from the end face of the heating roller. 
     Meanwhile, it is permissible to employ only a tension roller which applies only a tension to the belt without coating the belt with the release agent, which substitutes the aforementioned release agent roller. 
     Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention. 
     FIG. 1 is a perspective sectional view showing an entire internal structure of a copier; 
     FIG. 2 is a perspective sectional view showing internal structures of a heating mechanism and peripheral units; 
     FIG. 3 is a sectional view of major parts of a heating mechanism according to a first embodiment of the present invention; 
     FIG. 4 is a plan view showing the heating mechanism of the first embodiment; 
     FIG. 5 is an enlarged view showing part of the heating mechanism of the first embodiment; 
     FIG. 6 is a sectional view of major parts of a heating mechanism according to a second embodiment of the present invention; 
     FIG. 7 is a plan view showing the heating mechanism of the second embodiment; 
     FIG. 8A and 8B are enlarged views showing part of the heating mechanism of the second embodiment; and 
     FIG. 9A and 9B are partial sectional views showing the major parts of the heating mechanism according to the second embodiment and 
     FIG. 9C is a partial sectional view showing a heating mechanism as a comparative example. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, the preferred embodiments of the present invention will be described with reference to the accompanying drawings. 
     As shown in FIG. 1, a digital copier (image forming apparatus)  51  comprises an image reading unit (scanner)  52 , which catches an image of an object as brightness and darkness of light and converts photoelectricaly to generate an image signal and an image forming portion  53 , which forms an image corresponding to the image signal supplied from the scanner  52  or outside and fixes it onto a paper P which is a fixing object member (transfer member). Meanwhile, the scanner  52  is provided with an automatic draft paper feeding unit (ADF)  54  integratedly, which changes over draft papers successively interlockingly with image reading operation of the scanner  52 , when the copying object is a sheet. 
     The image forming portion  53  comprises an exposure unit  55 , a photosensitive drum  56 , a developing unit  57  and a heating mechanism  10 . The exposure unit  55  has a light source for irradiating with laser beam corresponding to image information supplied from the scanner  52  or an external unit. The photosensitive drum  56  has an outer circumferential face for holding a latent image as image information corresponding to laser beam from the exposure unit  55 . The developing unit  57  is provided with a developing roller for developing an image by supplying toner to an image formed on the photosensitive drum  56 . 
     The heating mechanism  10  has three rollers  12 ,  14  and  18  and a belt  16  for transferring a toner image developed by the developing unit  57  onto a paper P, melting it by heat and further applying a pressure. 
     In such image forming apparatus  51 , a draft image is copied in a following manner. If an image signal is supplied from the scanner  52  or an external apparatus (not shown), the photosensitive drum  56  charged with electric charge is irradiated with laser beam (not shown) whose intensity is modulated corresponding to an image signal from the exposure unit  55 . Consequently, an electrostatic latent image corresponding to an image to be copied (outputted) is formed on the photosensitive drum  56 . Then, toner is supplied from the developing unit  57  to this electrostatic latent image so as to form a toner image. 
     The paper P is picked up one by one from a cassette  59  by a pick-up roller  60  and carried along a transportation aisle  61  between the photosensitive drum  56  and the cassette  59 . Then, after timing between the toner image on the photosensitive drum  56  and the paper P is matched by an aligning roller  62 , the toner image is transferred to the paper P by a transfer unit. 
     Further, the paper P is carried vertically from the transfer unit to the heating mechanism  10  along a paper transporting path  71 . The heating mechanism  10  melts toner on the toner image on the paper P by heat and applies a pressure thereon so as to fix it on the paper P. The paper P is discharged to a discharged paper tray  64  from the heating mechanism  10  by a discharge roller  63 . 
     First Embodiment 
     Next, a first embodiment of the present invention will be described with reference to FIGS. 2 to  5 . 
     The heating mechanism  10  is provided along a paper transporting path  71  between a photosensitive drum  56  and a discharge roller  63  and comprises a fixing roller  12 , a heating roller  14 , an endless heat transferring belt  16 , a pressurizing roller  18  and first/second restricting members  15  and  13 . The fixing roller  12  and the pressurizing roller  18  are provided so as to oppose each other across the paper transporting path  71 . 
     A shaft  12   a  of the fixing roller  12  is connected to a driving shaft of a motor (not shown) and a shaft  18 a of the pressurizing roller  18  is connected to a driving shaft of another motor (not shown). Both the driving motors are controlled by a control unit (not shown) so that the rollers  12  and  18  rotationally drive synchronously. Further, a shaft  14   a  of the heating roller  14  is rotatably supported Consequently, the heating roller  14  is rotated according to the rotation of the fixing roller  12  through the heat transferring belt  16 . 
     The heat transferring belt  16  is stretched between the fixing roller  12  and the heating roller  14 . The heat transferring belt  16  is provided with heat energy from the heating roller  14  and makes contact with a paper P passing between the fixing roller  12  and the pressurizing roller  18  so as to melt toner on the toner image by heat. The heat transferring belt  16  is made of laminated composite material in which heat resistant silicone rubber and fluoroethylene based resin are layered successively on a nickel substrate. As the fluoroethylene based resin, for example, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA) is employed. The width L B  of the belt  16  is preferred to be in a range of from 280 to 320 mm while the circumference length of the belt  16  is 70±5 mm. 
     In the fixing roller  12 , its core is made of metal and its outer circumferential face is made of sponge-like soft heat resistant resin. An elastic outer circumferential face of the roller  12  is coated with silicone rubber or flour rubber. The diameter of the roller  12  is 37 to 38 mm. 
     In the heating roller  14 , both its core and outer circumferential face are made of heat resistant and abrasion resistant metal. The heating roller  14  is made of, for example, pure iron, stainless steel, aluminum, an alloy of stainless steel and aluminum or the like. This roller  14  all made of metal incorporates a heater (its maximum heating capacity is 250° C.). The heat transferring belt  16  is heated at temperatures of 170° C. to 220° C. by this heater. The diameter of the heating roller  14  is 30 to 40 mm. The temperature of at least one of the heat transferring belt  16  and the rollers  12  and  14  is detected by a temperature sensor (not shown) and a power supply of the heater is feed-back controlled by a controller unit (not shown) based on that detected temperature. 
     In the pressurizing roller  18 , its core is made of metal and its outer circumferential face is made of fluoroethylene based resin, for example, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA). The diameter of the pressurizing roller  18  is 38 to 42 mm. 
     As shown in FIG. 3, the first restricting members  15  are mounted on both sides of the heating roller  14  so as to restrict the belt  16  from shifting in the width direction on the heating roller  14 . Further, the second restricting members  13  are mounted on both sides of the fixing roller  12  so as to restrict the belt  16  from shifting in the width direction on the fixing roller  12 . Meanwhile the first restricting member  15  and the second restricting member  13  need to be disposed in the same phase. 
     The fixing roller  12  is rotationally driven by a driving motor (not shown) and the heating roller  14  is rotated according to the rotation of the heating roller through the belt  16 . For the reason, a shift force hereinafter referred to as a belt shifting force in the width direction of the belt  16  is relatively small on the side of the heating roller  14 , while the belt shifting force is larger on the side of the fixing roller  12 . 
     Next, such an advantage by providing not only the first restricting member  15  on the side of the heating roller  14  but also the second restricting member  13  on the side of the fixing roller  12  will be described. 
     In the heating mechanism having only the first restricting member  15 , when the belt shifting force increases, a belt side portion  16   a  is pressed against the restricting member  15  strongly, so that the belt side portion  16   a  may be buckled. However, by providing the second restricting member  13  on the side of the fixing roller  12 , the total length of the first and second restricting members  13  and  15  which the belt side portion  16   a  makes a sliding contact increases, so that a local stress concentration at the belt side portion  16   a  is relaxed, thereby the belt side portion  16   a  is not buckled. 
     In this case, while the first restricting member  15  is provided on substantially a half circumference (πD/2) of the roller  14 , the second restricting member  13  is provided on substantially a ⅓ the entire circumference (πD/3) of the roller  12 . The second restricting member  13  is disposed on the side in which the belt  16  is fed out of the roller  12 . Because at this place, the tension of the belt  16  drops and the belt shifting force increases, the belt side portion  16   a  is likely to slip out of the roller  12 . Such slip-out of the belt side portion  16   a  is blocked by the second restricting member  13  so as to prevent the belt side portion  16   a  from being buckled. 
     The inventors examined whether or not the buckling occurs in the belt side portion  16   a  by changing a gap G 1  between an end face of the heating roller  14  and the first restricting member  15 . As a result, it is found that if the first restricting member  15  is disposed so that the gap G 1  is not more than 1.5 mm, the belt side portion  16   a  is not buckled. 
     Additionally, the inventors have noticed that when the width L B  of the belt  16  is larger than the length L H  of the fixing roller  12  and the heating roller  14 , the local stress concentration occurs in the belt  16  at end portions of the rollers  12  and  14 , thereby the belt side portion  16   a  is buckled. Thus, by making the width L B  of the belt  16  shorter than the length L H  of the fixing roller  12  and the heating roller  14 , occurrence of the buckling of the belt side portion  16   a  is eliminated. 
     Second Embodiment 
     Next, the second embodiment of the present invention will be described with reference to FIGS. 6,  7 ,  8 A,  8 B,  9 A,  9 B and  9 C. Meanwhile, a description of the same components as the above-described FIG. 1 is omitted. 
     A heating mechanism  20  comprises a fixing roller  12 , a heating roller  14 , an endless heat transferring belt  16 , a pressurizing roller  18 , a first restricting member  15  and a release agent roller  22 . 
     The release agent roller  22  is constituted so as to be supplied with oil from a supply source (not shown) in order to coat the heat transferring belt  16  with oil. Further, the release agent roller  22  is pressed against the heat transferring belt  16  across the entire belt width L B  of the belt by pressing means (not shown) in order to apply an additional tension to the heat transferring belt  16  so that it is freely rotatable. The release agent roller  22  is disposed on the side in which the belt  16  is fed out of the fixing roller  12  (loose side). Further, the release agent roller  22  is located nearer the heating roller  14  than the fixing roller  12 . 
     The length L oil  of the release agent roller  22  is longer than the width L B  of the belt  16 . If the length L oil  of the release agent roller  22  is shorter than the width L B  of the belt  16 , a tension short region is generated in the belt side portion  16   a , so that the belt  16  is likely to occur. Particularly, when the heat transferring belt  16  shifts in the width direction, deviation of the heat transferring belt  16  from the rollers  12  and  14  increases and then, if a paper P is passed with this condition, a belt side portion  16   a  in a sliding contact with the restricting member  15  is swollen, thereby the belt  16  being damaged. 
     The inventors examined whether or not the buckling occurred in the belt side portion  16   a  by changing a gap G 2  between an end face of the release agent roller  22  and the first restricting member  15 . As a result, it has been confirmed that no buckling occurs in the belt side portion  16   a  when the gap G 2  is 7 mm while it is unlikely that the buckling occurs in the belt side portion  16   a  when the gap G 2  is not more than 12 mm. That is, even if the belt  16  shifts from its normal position shown in FIG. 9A to an abnormal position shown in FIG. 9B, no deformation occurs in the belt side portion  16   a  and the belt side portion  16   a  comes into a sliding contact with the first restricting member  15 . Thus, the belt side portion  16   a  is never buckled and a motion in the width direction of the belt  16  is restricted. 
     On the other hand, if the gap G 2  exceeds 12 mm, the belt side portion  16   a  is swollen a s shown in FIG. 9C, so that the belt side portion  16   a  is buckled and consequently, the belt  16  is broken in a short period. 
     Because according to the present invention, the belt does not shift largely in the width direction, no unfixed image is generated and all toner images in the image forming region can be fixed securely on the paper. 
     Further, because according to the present invention, the buckling or swelling (out-of-plane deformation) of the belt is eliminated, the rotational driving of the belt is stabilized, thereby substantially extending the service life of the belt. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.