Image forming apparatus

1. An image forming apparatus comprising:

an image bearing member;

toner image forming means for forming a toner image on said image bearing member;

a transfer member for transferring the toner image from said image bearing member onto a recording material at a transfer position;

an executing portion for executing a first cleaning mode in which said transfer member is cleaned by being supplied with a voltage of an identical polarity to a normal charge polarity of toner during a period in which there is no recording material at the transfer position and executing a second cleaning mode in which said transfer member is cleaned by being supplied with a voltage of an opposite polarity to the normal charge polarity of the toner during a period in which there is no recording material at the transfer position; and

a control portion for changing a ratio between a first cleaning mode execution period and a second cleaning mode execution period depending on an operation environment of said image forming apparatus.

2. An apparatus according to claim 1 , wherein during continuous image formation for successively forming an image on the recording material, said transfer member is cleaned by being supplied with a voltage during an interval between an image forming operation and a subsequent image forming operation, and

wherein said control portion changes the ratio between the first cleaning mode execution period and the second execution period corresponding to the first cleaning mode and the second cleaning mode executed during the interval.

3. An apparatus according to claim 1 , further comprising a developing device for developing with the toner an electrostatic latent image formed on said image bearing member,

wherein the second cleaning mode execution period is increased with an increase in number of occurrences of image formation by said image forming apparatus.

4. An apparatus according to claim 1 , further comprising an environment detecting portion for detecting an environment,

wherein the first cleaning mode execution period is increased when a detected humidity is lowered.

5. An apparatus according to claim 1 , wherein said image bearing member is an intermediary transfer belt for carrying the toner image.

6. An image forming apparatus comprising:

an image bearing member;

toner image forming means for forming a toner image on said image bearing member;

a transfer member for transferring the toner image from said image bearing member onto a recording material at a transfer position;

an executing portion for executing a first cleaning mode in which said transfer member is cleaned by being supplied with a voltage of an identical polarity to a normal charge polarity of toner during a period in which there is no recording material at the transfer position and executing a second cleaning mode in which said transfer member is cleaned by being supplied with a voltage of an opposite polarity to the normal charge polarity of the toner during a period in which there is no recording material at the transfer position; and

a control portion for changing a ratio between a first cleaning mode execution period and a second cleaning mode execution period depending on an operation state of said image forming apparatus.

7. An apparatus according to claim 6 , wherein during continuous image formation for successively forming an image on the recording material, said transfer member is cleaned by being supplied with a voltage during an interval between an image forming operation and a subsequent image forming operation, and

wherein said control portion changes the ratio between the first cleaning mode execution period and the second execution period corresponding to the first cleaning mode and the second cleaning mode executed during the interval.

8. An apparatus according to claim 6 , further comprising a developing device for developing with the toner an electrostatic latent image formed on said image bearing member,

wherein the second cleaning mode execution period is increased with an increase in number of occurrences of image formation by said image forming apparatus.

9. An apparatus according to claim 6 , wherein said control portion increases the first cleaning mode execution period when an image ratio of an image to be formed is decreased.

10. An apparatus according to claim 6 , wherein said control portion increases the second cleaning mode execution time period when an image ratio of an image to be formed is increased.

11. An apparatus according to claim 6 , wherein said image bearing member is an intermediary transfer belt for carrying the toner image.