Fixing apparatus and image forming apparatus using it

1. A fixing apparatus comprising:

a heat roller that comprises:

a heat source that generates heat to be supplied on a toner image formed on a recording medium;

a cylindrical core;

a rubber layer that is formed on the cylindrical core and comprises a thermal-conductivity-enhanced rubber, the rubber layer having a thickness of Tg that satisfies 200 μm ≦Tg≦600 μm and having a rubber hardness defined by JIS-A hardness of Hg that satisfies 40 degrees≦Hg≦80 degrees; and

a fluororesin layer that is formed on the rubber layer, the fluororesin layer having a thickness of Tj that satisfies 80 μm≦Tj≦300 μm and 2 Tj≦Tg ≦10 Tj; and

a pressure roller that presses the recording medium against the heat roller.

2. The fixing apparatus according to claim 1 , wherein the heat roller has an outer diameter of 70 mm or more, and

wherein the rubber layer has a thermal conductivity in a range of from 0.6 W/m×K to 1.5 W/m×K and a thermal conductivity variation of within 0.1 W/m×K.

3. The fixing apparatus according to claim 1 , wherein the cylindrical core is formed in a hollow cylindrical shape, and

wherein the heat source is disposed in a hollow space of the cylindrical core.

4. The fixing apparatus according to claim 3 , wherein the cylindrical core has a thickness in a range of 4 mm to 8 mm.

5. The fixing apparatus according to claim 3 , wherein the cylindrical core has a thickness in a range of 5 mm to 7 mm.

6. The fixing apparatus according to claim 1 , wherein the thickness of the rubber layer Tg further satisfies 300 μm≦Tg≦500 μm.

7. The fixing apparatus according to claim 1 , wherein the pressure roller has an outer diameter of 70 mm or more, and

wherein the pressure roller comprises:

a second cylindrical core;

a second rubber layer that is formed on the second cylindrical core, the second rubber layer having a thickness in a range of 4 mm to 8 mm; and

a second fluororesin layer that is formed on the second rubber layer, the second fluororesin layer having a thickness in a range of 80 μm to 300 μm.

8. The fixing apparatus according to claim 1 , wherein the thermal-conductivity-enhanced rubber is made of a silicone rubber and a thermal conductivity enhancer, and

wherein a ratio of parts by weight of the thermal conductivity enhancer to the silicone rubber is in a range of 1 to 5.

9. The fixing apparatus according to claim 8 , wherein the thermal conductivity enhancer is made of alumina fine powder, silica, or metal silicon.

10. The fixing apparatus according to claim 1 , wherein a thermal conductivity of the rubber layer is 2 to 4 times higher than a thermal conductivity of silicon rubber.

11. The fixing apparatus according to claim 1 , wherein the pressure roller comprises:

a second cylindrical core; and

a second rubber layer that is formed on the second cylindrical core, the second rubber layer having a thickness greater than the thickness of the rubber layer of the heat roller.

12. The fixing apparatus according to claim 1 , wherein the heat roller has an outer diameter of 70 mm or more.

13. An image forming apparatus comprising:

an image forming unit that forms a toner image on a recording medium; and

a fixing unit that fixes the toner image on the recording medium conveyed thereto, the fixing unit comprising:

a heat roller that comprises:

a heat source that generates heat to be supplied on the toner image formed on the recording medium;

a cylindrical core;

a rubber layer that is formed on the cylindrical core and comprises a thermal-conductivity-enhanced rubber, the rubber layer having a thickness of Tg that satisfies 200 μm≦Tg≦600 μm and having a rubber hardness defined by JIS-A hardness of Hg that satisfies 40 degrees≦Hg≦80 degrees; and

a fluororesin layer that is formed on the rubber layer, the fluororesin layer having a thickness of Tj that satisfies 80 μm≦Tj≦300 μm and 2 Tj≦Tg≦10 Tj; and

a pressure roller that presses the recording medium against the heat roller.