Image formation apparatus having cleaning mode to clean charging device configured to charge image carrier

1. An image formation apparatus, comprising:

an image carrier;

a charge device configured to uniformly charge a surface of the image carrier;

a development device configured to pass a developer to an electrostatic latent image on the image carrier formed by exposure;

a transfer device configured to transfer a developer image formed on the image carrier onto a medium;

a remover device configured to remove the developer on the image carrier; and

a controller configured to control a cleaning mode for removing extraneous matter adhered to a surface of the charge device at a time other than a time for a process of exposing the image carrier, and to change an execution frequency of the cleaning mode in accordance with an environmental condition.

2. The image formation apparatus according to claim 1 , wherein the controller is configured to set the execution frequency of the cleaning mode high when the environmental condition comprises a low-humidity environment.

3. The image formation apparatus according to claim 1 , wherein the controller is configured to set the execution frequency of the cleaning mode low when the environmental condition comprises a high-temperature environment.

4. The image formation apparatus according to claim 1 , wherein when the environmental condition is not of a normal-temperature and normal-humidity environment, but is of a low-temperature and low-humidity environment or a high-temperature and high-humidity environment, the controller is configured to set the execution frequency of the cleaning mode lower than that in the normal-temperature and normal-humidity environment.

5. The image formation apparatus according to claim 4 , wherein when the environmental condition is not of a normal-temperature and normal-humidity environment, but is of a low-temperature and low-humidity environment, the controller is configured to set the execution frequency of the cleaning mode lower than that in the high-temperature and high-humidity environment.

6. The image formation apparatus according to claim 1 , wherein the controller holds history information of the number of prints counted at printing and is configured to change the execution frequency of the cleaning mode based on the history information of the number of prints counted.

7. The image formation apparatus according to claim 1 , wherein the controller holds history information of the number of jobs counted at printing and is configured to change the execution frequency of the cleaning mode based on the history information of the number of jobs counted.

8. The image formation apparatus according to claim 1 , wherein

a pressing force of the remover device against the image carrier is 10.1 gf/cm to 29.1 gf/cm, both inclusive, and

the developer includes an external additive having an opposite polarity to a bias applied to the charge device during printing.

9. The image formation apparatus according to claim 1 , wherein in the cleaning mode controlled by the controller, extraneous matter adhered to the charge device has a surface potential of not lower than +1 V and not higher than +10 V on the charge device.

10. The image formation apparatus according to claim 1 , wherein

the controller holds a job number-of-print counter value representing the number of pages continuously printed from a start of a print job, a number-of-print counter value representing the number of pages printed after an end of a previous cleaning mode, and a total number-of-print counter value representing the number of pages printed after the image formation apparatus is powered on or is awoken from a sleep mode, and

the controller comprises:

a threshold determination portion configured to calculate a threshold to determine the execution frequency of the cleaning mode in accordance with an environmental value determined based on temperature and humidity; and

an execution portion configured to execute the cleaning mode at a frequency determined based on results of a comparison of the job number-of-print counter value, the number-of-print counter value, and the total number-of-print counter value with the threshold determined by the threshold determination portion.

11. The image formation apparatus according to claim 10 , wherein

the execution portion of the controller is configured to:

execute the cleaning mode based on a comparison of a first execution frequency value with the number-of-print counter value when the job number-of-print counter value is not more than the threshold, and

execute the cleaning mode based on a comparison of a second execution frequency value with the number-of-print counter value when the job number-of-print counter value is more than the threshold, the second execution frequency value being less than the first execution frequency value.

12. The image formation apparatus according to claim 11 , wherein

the first and second execution frequency values are larger when the temperature is between predetermined high and low temperatures and the humidity is between predetermined high and low humidities, than when the temperature is below the predetermined low temperature and the humidity is below the predetermined low humidity or when the temperature is equal to or above the predetermined high temperature and the humidity is equal to or above the predetermined high humidity.

13. The image formation apparatus according to claim 11 , wherein

when the job number-of-print counter value is not more than the threshold, the execution portion of the controller executes the cleaning mode based on a comparison of a third execution frequency value with the number-of-print counter value, and

when the job number-of-print counter value is more than the threshold, the execution portion compares a fourth execution frequency value with the number-of-print counter value and executes the cleaning mode if the number-of-print counter value is not less than the fourth execution frequency value and the number-of-print counter value is not less than a fifth execution frequency value.

14. The image formation apparatus according to claim 13 , wherein the third, fourth, and fifth execution frequency values are larger when the temperature is between predetermined high and low temperatures and the humidity is between predetermined high and low humidities than when the temperature is below the predetermined low temperature and the humidity is below the predetermined low humidity or when the temperature is equal to or above the predetermined high temperature and the humidity is equal to or above the predetermined high humidity.

15. The image formation apparatus according to claim 10 , wherein

the threshold determination portion of the controller holds environmental value determination information based on combinations of temperature and humidity and is configured to determine the environmental value based on a detected temperature and humidity with reference to the environmental value determination information.

16. The image formation apparatus according to claim 10 , wherein the threshold determination portion of the controller holds threshold determination information to determine the threshold in accordance with the environmental value and is configured to determine the threshold value in accordance with the environmental value with use of the threshold determination information.