1. Field of the Invention
This invention relates to a fixing device which fixes an image on a recording medium and to an image forming apparatus incorporating the fixing device.
2. Description of the Related Art
As a fixing device used for a variety of image forming apparatuses such as copiers, printers, facsimiles, multifunction apparatuses that print, fax, copy, and so on, a device which includes a thin fixing belt consisting of a metal substrate and an elastic rubber layer is known. Use of such a thin fixing belt which has a low heat capacity makes it possible to drastically reduce the amount of energy required to heat the fixing belt to required temperatures. Accordingly, it is possible to shorten a warm-up time (e.g., at power-up, a time required to go from a room temperature to a predetermined temperature (reload temperature) for printing), and a time to first print (i.e., a time to completion of the paper output after performing printing operation including preparation for printing after receiving a print request). Conventionally, as shown in FIG. 1, such a fixing device includes an endless belt (fixing belt) 100 formed into a loop, a pipe-shaped metal heat conduction member 200 disposed within the loop formed by the endless belt 100, a heat source 300 disposed inside the metal heat conductor 200, and a pressure roller 400 to form a nip portion N by contacting the metal heat conductor 200 via endless belt 100 (See JP-2007-334205-A).
In this case, the endless belt 100 is rotated by the rotation of the pressure roller 400, and at this time, the metal heat conductor 200 guides the movement of the endless belt 100. Further, since the endless belt 100 is heated by the heat source 300 disposed inside the metal heat conductor 200 via the metal heat conductor 200, it becomes possible to warm the entire endless belt 100. Accordingly, it is possible to shorten the time to first print from the heating wait state and overcome the shortage of heat during high speed operation.
In order to achieve further improvement of the energy efficiency and time to first print, a fixing device which heats the endless belt directly (without heating through the metal heat conductor) has been proposed (See JP-2007-233011-A).
In the example shown in FIG. 2, the pipe-shaped metal heat conductor is not provided inside the endless belt 100. Instead, a planar nip forming member 500 is provided at a position facing a pressure roller 400. In this case, since it is possible to heat the endless belt 100 directly by the heat source 300 at a portion other than the portion where the nip forming member 500 is disposed, heat transfer efficiency is significantly improved and power consumption can be reduced. Accordingly, it is possible to further shorten time to first print. Further, since the metal heat conductor is not provided, cost reduction can be also expected.
A variety of fixing devices which heats the endless belt directly is known.
FIG. 3 is another example of a fixing device which heats the endless belt directly. The fixing device shown in FIG. 3 includes a nip forming member 500 and a shielding member 700 that shields heat from a heat source 300 to a support member 600 that supports the nip forming member 500 (See JP-2010-20248-A). In this device, in the cross-sectional view perpendicular to the axial direction of the endless belt 100, the shielding member 700 has a convex shape toward the heat source 300. The shielding member 700 is formed in this way so as to increase the area of the endless belt 100 to be heated directly.
FIG. 4 is another example of a fixing device. The fixing device shown in FIG. 4 includes a reflective member (reflector) 800 which reflects the radiation light emitted from the heat source 300 to the endless belt 100. The reflective member 800 is formed of a support portion 800b disposed in substantially vertical direction, and a pressure receiving portion 800a projecting in substantially horizontal direction from the lower end of the support portion 800b (side end portion of the pressure roller 400), and a radiation adjusting section 800c projecting in substantially horizontal direction from the upper end portion of the support portion 800b (end portion opposite to the pressure roller 400) (See JP-2010-78839-A). In the radiation adjusting section 800c, a plurality of cutouts are formed in the width direction of the endless belt 100. Therefore, the occurrence of unevenness of the temperature of the belt surface is prevented by varying the radiation time of the radiation light for the endless belt 100 in the belt width direction.
As described above, by heating the endless belt directly, it becomes possible to achieve high energy efficiency and shorten the time to first print. However, there are drawbacks. One of them is the thermal deformation of the endless belt called kinking. Kinking is a phenomenon in which localized thermal expansion occurs when a part of the belt in the circumferential direction is heated rapidly so that the endless belt is deformed due to the expansion difference between the part being heated and the part that is not heated directly. Particularly in the configuration using an extremely thin endless belt to improve energy efficiency and time to first print which is popular in the recent years, the possibility of kinking occurring is increased because the endless belt is likely to be heated.
As a way to avoid kinking, a method in which a broad area of the endless belt is heated may be used. However, when the area of the endless belt to be heated is too broad, components other than the fixing belt which do not need to be heated may be heated up, resulting in a new problem, for example, heating efficiency deteriorates.