(1) Field of the Invention
The present invention relates to an electromagnetic induction-heating type fixing device and an image forming apparatus including such a fixing device, and in particular to a technology for improving a speed for fixing toner images on recording sheets of a small size (hereinafter, referred to as “small-sized recording sheets”) in a fixing device that fixes toner images on recording sheets of various sizes.
(2) Related Art
In recent years, in the field of an image forming apparatus, saving of energy has been increasingly required as a part of global warming countermeasures, and accordingly, an induction-heating type fixing device that realizes high energy efficiency has been attracting attention.
An induction-heating type fixing device passes recording sheets through a fixing nip portion that is formed by a heating roller heated by induction heating and a pressurizing roller pressed against the heating roller so as to fuse and fix toner images thereon, for example.
In the fixing device that fixes toner images on recording sheets of various sizes, not only an excitation coil but also a demagnetization coil is used. The excitation coil heats the heating roller by induction heating over a width of the maximum sheet-passing region of the fixing device. The demagnetization coil prevents overheating of a part of a fixing roller in a region where, while small-sized recording sheets are passing through in the sheet-passing region, these sheets do not pass through (hereinafter, referred to as “non sheet-passing region”).
FIG. 9 is an appearance perspective view showing an excitation coil and demagnetization coils pertaining to a conventional art. As FIG. 9 shows, demagnetization coils 901 are provided at positions corresponding to non sheet-passing regions that are aligned with both ends of fed small-sized recording sheets in a width direction thereof.
When small-sized recording sheets are passed through, a circuit of the fixing device for applying current is closed and accordingly current inducted by a magnetic flux generated by an excitation coil 902 flows through the demagnetization coils 901. This causes the demagnetization coils 901 to generate a reversed polarity magnetic flux, which cancels the magnetic flux generated by the excitation coil 902. On the other hand, when recording sheets of a large size are passed through, the circuit is opened and accordingly demagnetization is stopped.
An image forming apparatus has been always required to improve a printing speed. It is thus necessary to increase a process speed (hereinafter, referred to as “fixing speed”) of a fixing device, that is, the number of recording sheets on which fixing is performed in units of time. In order to improve the fixing speed, more heat is naturally required, and accordingly output of an excitation coil is required to be increased.
However, there is a problem that, if output of the excitation coil is increased, overheating of a heating roller in the non sheet-passing region becomes too extreme for practical use. FIG. 10 shows a temperature of the non sheet-passing region when recording sheets of an A6T size (105 [mm]×148.5 [mm]) are passed through in an image forming apparatus that can fix recording sheets of up to an A3 size. A horizontal axis of FIG. 10 represents a position (distance from the center of a sheet-passing region) in a direction perpendicular to a direction in which the recording sheets pass through, and a longitudinal axis of FIG. 10 represents a temperature of a fixing roller. Besides, a solid line 2101 and a dashed line 2102 indicate temperature distributions in the cases where speeds of passing through the recording sheets are 75 [ppm] and 65 [ppm], respectively. Note that ppm (papers per minute) represents the number of recording sheets that are passed for one minute.
As FIG. 10 shows, in the sheet-passing region, the fixing roller is deprived of heat by the recording sheets and heated by an excitation coil by electromagnetic induction at the same time, and accordingly a fixing temperature of the sheet-passing region remains at an appropriate temperature. On the other hand, since the recording sheets do not perform cooling of the fixing roller in the non sheet-passing region, a temperature of the fixing roller in the non sheet-passing region becomes higher than in the sheet-passing region.
Also, as the dashed line 2102 shows, when the speed of passing through the recording sheets is 65 [ppm], the highest temperature does not exceed 240° C., which is a general heat resistant temperature of silicone rubber. However, as the solid line graph 2101 shows, when the speed is increased to 75 [ppm], it turns out that the temperature exceeds 240° C. in some positions.