The present invention relates to a fixing device, for fixing an unfixed toner image formed on a sheet medium, comprising a fuser roller having an outer surface coated with an elastic member and a built-in heat source, a pressure roller to be pressed against the fuser roller, a heat-resistant belt which Is wound around the periphery of the pressure roller and is sandwiched between the pressure roller and the fuser roller so as to travel, and a belt tensioning member for tensioning the heat-resistant belt. Further, the present invention relates to an image forming apparatus.
In an image forming apparatus such as a copying machine, a printer, and a facsimile machine, a fixing device of fuser roller type in which an unfixed toner image on a receiving medium is fused by contact beating has been proposed (Japanese Patent No. 3084692) which comprises a rotatable fuser roller having an outer surface coated with an elastic member and a built-in heat source, a heat-resistant belt which is tensioned by a plurality of supporting rollers, and pressurizing means which brings the heat-resistant belt to be wrapped around the fuser roller partially for a predetermined angle to have a nip area and applies pressure locally such that the pressure on an outlet of the nip area is larger than the pressure on the other portion to create a deflection in the elastic member for facilitating the ejection of a sheet medium from the nip portion.
In this conventional fixing device, the fuser roller has a deflection beforehand in the surface thereof because of the existence of the pressurizing means. At the outlet of the nip area, the deflection is instantaneously cancelled from a state that toner is in contact with the surface of the fuser roller. Therefore, when ejecting the sheet medium from the nip portion, the adherence between the toner and the fuser roller is reduced to prevent the sheet medium from adhering to the fuser roller, whereby even a weak recording medium can be easily peeled off at the outlet of the belt nip portion. Therefore, this device achieves the elimination of a peeling pawl which has been used in a prior technique.
Further, a fixing device has been proposed (Japanese Patent Publication No. H06-40235) in which rollers have a preset pressure therebetween to deform the roller(s) to form a nip and a sheet medium having an unfixed toner image thereon passes the nip, thereby fixing the toner image. Depending on the characteristics of the sheet medium, the driving speed of the rollers can be selected from a first speed and a second speed.
Furthermore, a fixing device has been proposed (Japanese Patent Unexamined Publication No. H08-262903) comprising an endless belt which is tensioned in such a manner as to travel while being in contact with a rotating fuser roller which has an outer surface coated with an elastic member and a built-in heat source and a pressure pad which is non-rotatably arranged inside of the endless belt to press the endless belt to the fuser roller to form a nip and to deform the elastic member as the outer layer of the fuser roller, wherein a sheet medium having an unfixed toner image thereon passes between the fuser roller and the endless belt, thereby fixing the toner on the sheet medium. This device has an advantage that as the pressure pad is arranged as a non-rotatable member, the heat transmitted from the fuser roller is hardly emanated so that the heat drawn from the fuser roller can be minimized.
However, in the structure of the aforementioned fixing device of Japanese Patent No. 3084692, the heat-resistant belt which is tensioned and supported by the supporting rollers in such a manner as to allow its traveling is wrapped around the fuser roller only partially for such an angle enabling the nip formation by pressurizing means and is driven while applying a large pressure locally at the outlet of the nip area, thus requiring plural supporting rollers and their bearings. Further, a long peripheral length of the heat-resistant belt is required. Accordingly, the fixing device becomes not only complex and large but also expensive. The complexity, large size, and expensiveness of the fixing device inevitably lead to the complexity, large size, and expensiveness of an image forming apparatus in which the fixing device is mounted.
There is another disadvantage. That is, the heat-resistant belt is heated at the nip relative to the rotatable fuser roller with the built-in heat source. During this, the heat energy is drawn by the plural supporting rollers since the heat-resistant belt has the long peripheral length because the belt is supported by the plural supporting rollers. In addition, the natural heat release is increased according to the peripheral length. Accordingly, a long time is necessary to reach a predetermined temperature, thus unfortunately requiring a long warm-up time from a time point at which the power is ON to a time point at which fixing is enabled.
Though the structure, in which the heat-resistant belt is wrapped around the fuser roller only partially for such an angle enabling the nip formation and a pressure is locally applied such that the pressure on the outlet of the nip area is larger than the pressure on the other portion to create a deflection in the elastic member, is preferable to prevent a sheet medium from adhering to the fuser roller, but curls the sheet medium because it is ejected along the deflection of the elastic member or wrinkles because of the local high pressure.
The device of Japanese Patent Publication No. H06-40235, in which the driving speed of the roller can be selected from the first speed and the second speed depending on the characteristics of a sheet medium, is not preferable because the beat capacity of the roller is so large as to require a long warm-up time. In addition, the sheet medium which passes the long nip formed by deforming the roller with pressure may be deformed similarly to the former device, that is, curled or wrinkled due to large stress by the pressure.
In the device of Japanese Patent Unexamined Publication No. H08-262903, the heat transmitted from the fuser roller is hardly emanated by the arrangement of the pressure pad not allowing its rotation. However, there is a problem that heat is transmitted from the fuser roller to the pressure pad through the endless belt during the warm-up time, thus requiring a long warm-up time. In addition, three rollers or more are required to move the belt, thus making the device larger.