Image forming apparatus

1. An image forming apparatus comprising:

an image forming station configured to form an unfixed image on a recording material;

a fixing portion configured to heat and fix an unfixed image formed on the recording material, said fixing portion including a heater, a fixing rotatable member, and a pressing roller cooperative with said fixing rotatable member to form a nip for nipping and feeding the recording material;

an ambient condition sensor configured to detect an ambient temperature and an ambient humidity; and

a controller configured to control said fixing portion and to control a conveyance of the recording material,

wherein said controller starts, upon receiving print instructions, electric power supply to said heater so as to raise a temperature of said fixing portion to a control target temperature for a fixing operation, and

wherein said controller sets a time period from the timing at which the temperature of the fixing portion reaches the control target temperature to the timing at which the recording material enters the nip, on the basis of the ambient temperature and the ambient humidity.

2. An apparatus according to claim 1 , wherein said controller corrects the time period in accordance with the control target temperature of said fixing portion at the time of a fixing operation of a first recording material by the nip.

3. An apparatus according to claim 2 , wherein the time period decreases with a decrease of the control target temperature.

4. An apparatus according to claim 1 , wherein said controller corrects the time period in accordance with a size of the recording material.

5. An apparatus according to claim 4 , wherein the time period decreases with a decrease of the size of the recording material.

6. An apparatus according to claim 1 , wherein said fixing rotatable member includes a belt.

7. An apparatus according to claim 6 , wherein said fixing portion includes a heater contacted to an inner surface of said endless belt.

8. An image forming apparatus comprising:

an image forming station configured to form an unfixed image on a recording material;

a fixing portion configured to heat and fix an unfixed image formed on the recording material, said fixing portion including a fixing rotatable member, and a pressing roller cooperative with said fixing rotatable member to form a nip for nipping and feeding the recording material;

an ambient condition sensor configured to detect an ambient humidity; and

a controller configured to control said fixing portion and to control a conveyance of the recording material,

wherein said controller sets a start timing of feeding of a first recording material on the basis of the ambient humidity, and

wherein said controller corrects the sheet feeding start timing in accordance with a control target temperature of said fixing portion at the time of a fixing operation of the first recording material by the nip.

9. An apparatus according to claim 8 , wherein the start timing advances with a decrease of the control target temperature.

10. An apparatus according to claim 8 , wherein said controller corrects the sheet feeding start timing in accordance with a size of the recording material.

11. An apparatus according to claim 10 , wherein the start timing advances with a decrease of the size of the recording material.

12. An apparatus according to claim 8 , wherein said fixing rotatable member includes a belt.

13. An apparatus according to claim 12 , wherein said fixing portion includes a heater contacted to an inner surface of said endless belt.

14. An apparatus according to claim 1 , wherein the time period increases with an increase of the ambient humidity.

15. An image forming apparatus comprising:

an image forming station configured to form an unfixed image on a recording material;

a fixing portion configured to heat and fix an unfixed image formed on the recording material, said fixing portion including a heater, a fixing rotatable member, and a pressing roller cooperative with said fixing rotatable member to form a nip for nipping and feeding the recording material;

an ambient condition sensor configured to detect an ambient humidity; and

a controller configured to control said fixing portion and to control a conveyance of the recording material,

wherein said controller sets a time period from the timing at which the temperature of the fixing portion reaches a control target temperature to the timing at which the recording material enters the nip, on the basis of the ambient humidity.

16. An apparatus according to claim 15 , wherein the time period increases with an increase of the ambient humidity.

17. An apparatus according to claim 16 , wherein said controller corrects the time period in accordance with the control target temperature of said fixing portion at the time of a fixing operation of a first recording material by the nip.

18. An apparatus according to claim 17 , wherein the time period decreases with a decrease of the control target temperature.

19. An apparatus according to claim 15 , wherein said controller corrects the time period in accordance with a size of the recording material.

20. An apparatus according to claim 19 , wherein the time period decreases with a decrease of the size of the recording material.

21. An apparatus according to claim 15 , wherein said fixing rotatable member includes a belt.

22. An apparatus according to claim 21 , wherein said fixing portion includes a heater contacted to an inner surface of said endless belt.