Electric heating – Heating devices – With heating unit structure
Patent
1996-10-16
2000-09-19
Walberg, Teresa
Electric heating
Heating devices
With heating unit structure
219216, B65D 124, B65D 8834
Patent
active
061215899
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a heating device for heating a sheet material such as paper for a copying machine, a material sheet for a film laminating machine and the like.
BACKGROUND ART
Heating devices used for the above purposes are disclosed in Japanese Patent Application Laid-open No. 2-59356 and in Japanese Patent Application Laid-open No. 2-65086 for example. Such a heating device includes a strip-like heating resistor layer formed on a substrate made of a heat-resistant insulating material such as ceramic for example, and a protective layer formed on the substrate to cover the heating resistor layer. Typically, the protective layer is made of a glass material and arranged to withstand the heat generated at the heating resistor layer while insuring electrical insulation from the exterior and avoiding wear due to contacting with a sheet material fed relative to the heating device.
In such a heating device, it is necessary to insure a sufficient electrical insulation, since a considerably large current is passed through the heating resistor layer to generate Joule heat for heating the sheet material. However, generally, a conventional glass material used for the protective layer has a dielectric strength of only about 14-15 volts per thickness of 1 .mu.m. Thus, it is necessary to make the thickness of the protective layer considerably large for insuring a sufficient electric insulation. As a result, in the conventional heating device, the heat capacity of the protective layer becomes large, so that the thermal response at the surface of the protective layer is likely to deteriorate (the temperature rises slowly). If, to compensate for this, the amount of the heat generated at the heating resistor is increased, a problem of wasting energy will occur due to low thermal efficiency.
DISCLOSURE OF THE INVENTION
An object of the present invention is to provide a heating device having a rapid thermal response and a high thermal efficiency.
For attaining the above object, according to the present invention, there is provided a heating device for a sheet material comprising a substrate made of a heat-resistant insulating material, a heating resistor layer formed on the substrate, and a protective layer formed on the substrate to cover the heating resistor layer, wherein the protective layer is formed of glass containing 3-30 Wt % of alumina powder as an additive.
With such an arrangement, the addition of alumina powder remarkably increases the dielectric strength per unit thickness of the protective layer in comparison with a glass protective layer containing no additional alumina powder. Thus, since a sufficient dielectric strength can be obtained even with a thin protective layer, thermal transmission from the heating resistor layer to the sheet material can be prevented from being unduly hindered due to the presence of the protective layer.
It is for the purpose of sufficiently enjoying the advantage of the improved dielectric strength that the proportion of added alumina powder is set to be no less than 3 Wt %.
On the other hand, it is for the purpose of preventing the surface of the protective layer from becoming unduly rough that the addition proportion of added alumina powder is set to be no greater than 30 Wt %. If the surface of the protective layer is rough, there will occur inconveniences such as damage caused to the surface of the sheet material in contact with the protective layer, and deterioration of the fixing quality of toner onto a paper sheet in a copying machine and the like. For the same reason, the grain size of the alumina powder is preferably no greater than 5 .mu.m.
The experiments conducted by the inventor have shown that the proportion of alumina powder added to the glass is advantageously 3-22 Wt % and particularly 10-22 Wt % for increasing remarkably the dielectric strength while providing a smooth surface at the protective layer.
According to a preferred embodiment of the present invention, the heating resistor layer is made in a strip-like form. Further
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Rhom Co., Ltd.
Robinson Daniel
Walberg Teresa
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