Fixing device and electro-photographic recording apparatus using the same

1. A fixing device for an electro-photographic recording apparatus, comprising:

a back-up roller having an elastic layer on a surface of the back-up roller; and

a heating roller, wherein a nip part is formed by a pressing mechanism where the back-up roller is pressed against the heating roller,

wherein an axis-to-axis distance between the heating roller and the back-up roller is set to be a first value under a first fixing condition where an offset due to a rush/exhaust impact does not have adverse effect on the recording medium in case that a thin paper as the recording medium is transported through the nip part, while the back-up roller comes into contact with the heating roller to receive a driving force from the heating roller when the recording medium is not nipped at the nip part,

wherein, when a thick paper as the recording medium is fixed, the axis-to-axis distance between the heating roller and the back-up roller is set to be a second value larger than the first value under the first fixing condition, and is set to be the second value under a second fixing condition where the back-up roller comes into contact with the heating roller to receive a driving force from the heating roller.

2. The fixing device according to claim 1 , when the recording medium thicker than Do under the first fixing condition is nipped and fixed, the axis-to-axis distance L between the heating roller and the back-up roller is set in a range satisfying an expression (1) Lo+(D−Do)≦L<Lo+D and an expression (2) Lo+D<r H +r B ,

wherein the axis-to-axis distance under the first fixing condition is Lo,

a radius of the heating roller is r H ,

a radius of the back-up roller is r B ,

a thickness of the recording medium under the first fixing condition is Do, and

a thickness of the recording medium thicker than Do is D.

3. The fixing device according to claim 1 , wherein when the recording medium thicker than Do is nipped and fixed, the axis-to-axis distance L is set to satisfy an expression (3) L=Lo+(D−Do),

wherein the axis-to-axis distance under the first fixing condition is Lo,

a thickness of the recording medium under the first fixing condition is Do, and

a thickness of the recording medium thicker than Do is D.

4. The fixing device according to claim 2 , comprising:

an axis-to-axis distance adjusting member for changing the axis-to-axis distance in accordance with the thickness of the recording medium,

wherein, when the recording medium thicker than the Do is nipped at the nip part of the fixing device, the axis-to-axis distance is set so at not to apply force on the axis-to-axis distance adjusting member.

5. The fixing device according to claim 1 , comprising:

a paper thickness detection unit detecting the thickness of the recording medium before the recording medium reaches the fixing device; and

an axis-to-axis distance calculation unit calculating the axis-to-axis distance based on the detected paper thickness,

wherein the axis-to-axis distance under the first fixing condition is controlled to a value calculated by the axis-to-axis distance calculation unit.

6. An electrophotographic recording apparatus comprising: a fixing device according to claim 1 .