This invention relates to a fixing device for fixing a toner image to a recording sheet in a printer or the like, and relates to an image forming apparatus having the fixing device.
A fixing device is used to fix a toner image to a recording sheet in an image forming apparatus such as a printer. There is a type of fixing device having an endless belt as disclosed in Laid-Open Patent Publication Nos. 2001-249569 (Page 5, FIG. 1), 2001-194946 (Pages 6-7, FIG. 1) and HEI 7-36306 (Page 4, FIG. 4). Such a fixing device is called a belt-type fixing device.
The belt-type fixing device includes a fixing roller such as a rubber covered roller, a heating roller having an internal heat source, and an endless belt stretched around the fixing roller and the heating roller. A pressure roller faces the fixing roller via the endless belt. The pressure roller is pressed against the fixing roller via the endless belt, so that a nip portion is formed between the pressure roller and the endless belt.
In the belt-type fixing device, if the endless belt is pressed by the fixing roller and the pressure roller for a long time, an impression may be formed on the endless belt by the fixing roller or the pressure roller. Such an impression may shorten the lifetime of the endless belt. Moreover, such an impression may cause a so-called offset phenomenon that a part of the toner image is not fixed to the recording sheet, and may cause a fluctuation of the running speed of the endless belt, which may result in non-uniformity of the fixed image. Furthermore, an impression (a so-called nip mark) may be formed on the fixing roller or the pressure roller by the nip pressure. Such an impression may be transferred to the endless belt, and may shorten the lifetime of the endless belt. In order to solve these problems, Japanese Laid-Open Patent Publication No. 2001-249569 discloses a fixing device in which the fixing roller is able to separate from the fixing roller.
However, in the conventional fixing device, the pressure roller has no heat source and is heated by the indirect heat transmitted through the endless belt. Therefore, it is necessary to heat the endless belt and the rollers (i.e., the fixing roller, the heating roller and the pressure roller) while the endless belt is pressed by the fixing roller and the pressure roller. In order to uniformly heat the endless belt and the rollers, it is preferable to move the endless belt during the heating operation. However, if the endless belt moves while the endless belt is pressed by the fixing roller and the pressure roller, the endless belt may be damaged by a solidified toner that remains on the endless belt or the pressure roller. Thus, it is not possible to start moving the endless belt until the solidified toner is molten. Therefore, it takes a long time to uniformly heat the endless belt and the rollers.
In addition, the temperatures of the endless belt and the rollers affect one another, and therefore the controlling of the temperatures of the endless belt and the rollers is complicated. As a result, it takes a long time for the endless belt and the rollers to reach the predetermined temperatures.
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