Transfer device, image forming apparatus and recording medium

1. A transfer device comprising:

an image bearing member on which to bear a toner image;

a transfer member used for forming a transfer nip at the image bearing member;

a power source configured to apply to the transfer member a transfer bias of one polarity for transferring a toner image on the image bearing member to a sheet passing through the transfer nip; and

a controller configured to control the power source to apply the transfer bias to the transfer member when a first sheet having a small surface smoothness passes through the transfer nip, the transfer bias including a first application period for applying a first transfer bias greater than a reference transfer bias in absolute value and a second application period for applying a second transfer bias equal to or smaller than the reference transfer bias in absolute value, the first application period and the second application period being alternately provided,

wherein the controller is further configured to control the power source to apply the transfer bias to the transfer member when a second sheet passes through the transfer nip, the transfer bias is equal to the reference transfer bias when all portions of the second sheet passes through the transfer nip, and the second sheet having a surface smoothness greater than that of the first sheet.

2. The transfer device according to claim 1 , wherein a period during which all positions on the first sheet pass through the transfer nip includes the first application period and the second application period.

3. The transfer device according to claim 1 , wherein the second transfer bias is smaller than the reference transfer bias in absolute value.

4. The transfer device according to claim 3 , wherein a time integration value over the first application period of a difference between the reference transfer bias and the first transfer bias is equal to a time integration value over the second application period of a difference between the reference transfer bias and the second transfer bias.

5. The transfer device according to claim 1 , wherein the power source includes a direct current power source configured to apply and switch the first transfer bias and the second transfer bias.

6. The transfer device according to claim 5 , wherein

the direct current power source includes:

a first direct current power source configured to apply the first transfer bias; and

a second direct current power source configured to apply the second transfer bias.

7. The transfer device according to claim 1 , wherein, in the transfer bias, the number of the first application periods and the second application periods is eight or ten.

8. An image forming apparatus comprising:

an image bearing member on which to bear a toner image;

a transfer member used for forming a transfer nip at the image bearing member;

a power source configured to apply to the transfer member a transfer bias of one polarity for transferring a toner image on the image bearing member to a sheet passing through the transfer nip; and

a controller configured to control the power source to apply the transfer bias to the transfer member when a first sheet having a small surface smoothness passes through the transfer nip, the transfer bias including a first application period for applying a first transfer bias greater than a reference transfer bias in absolute value and a second application period for applying a second transfer bias equal to or smaller than the reference transfer bias in absolute value, the first application period and the second application period being alternately provided,

wherein the controller is further configured to control the power source to apply the transfer bias to the transfer member when a second sheet passes through the transfer nip, the transfer bias is equal to the reference transfer bias when all portions of the second sheet passes through the transfer nip, and the second sheet having a surface smoothness greater than that of the first sheet.

9. The image forming apparatus according to claim 8 , wherein a period during which all positions on the first sheet pass through the transfer nip includes the first application period and the second application period.

10. The image forming apparatus according to claim 8 , wherein the second transfer bias is smaller than the reference transfer bias in absolute value.

11. The image forming apparatus according to claim 10 , wherein a time integration value over the first application period of a difference between the reference transfer bias and the first transfer bias is equal to a time integration value over the second application period of a difference between the reference transfer bias and the second transfer bias.

12. The image forming apparatus according to claim 8 , wherein the power source includes a direct current power source configured to apply and switch the first transfer bias and the second transfer bias.

13. The image forming apparatus according to claim 12 , wherein

the direct current power source includes:

a first direct current power source configured to apply the first transfer bias; and

a second direct current power source configured to apply the second transfer bias.

14. The image forming apparatus according to claim 8 , wherein, in the transfer bias, the number of the first application periods and the second application periods is eight or ten.

15. A computer-readable recording medium storing a program of a transfer device, the transfer device comprising:

an image bearing member on which to bear a toner image;

a transfer member used for forming a transfer nip at the image bearing member; and

a power source configured to apply to the transfer member a transfer bias of one polarity for transferring a toner image on the image bearing member to a sheet passing through the transfer nip, wherein

the program causes a computer of the transfer device to execute a process of controlling the power source to apply the transfer bias to the transfer member when a first sheet having a small surface smoothness passes through the transfer nip, the transfer bias including a first application period for applying a first transfer bias greater than a reference transfer bias in absolute value and a second application period for applying a second transfer bias equal to or smaller than the reference transfer bias in absolute value, the first application period and the second application period being alternately provided,

wherein the process of controlling the power source causes the power source to apply the transfer bias to the transfer member when a second sheet passes through the transfer nip, the transfer bias is equal to the reference transfer bias when all portions of the second sheet passes through the transfer nip, and the second sheet having a surface smoothness greater than that of the first sheet.

16. The recording medium according to claim 15 , wherein a period during which all positions on the first sheet pass through the transfer nip includes the first application period and the second application period.

17. The recording medium according to claim 15 , wherein the second transfer bias is smaller than the reference transfer bias in absolute value.

18. The recording medium according to claim 17 , wherein a time integration value over the first application period of a difference between the reference transfer bias and the first transfer bias is equal to a time integration value over the second application period of a difference between the reference transfer bias and the second transfer bias.

19. The recording medium according to claim 15 , wherein the power source includes a direct current power source configured to apply and switch the first transfer bias and the second transfer bias.

20. The recording medium according to claim 19 , wherein

the direct current power source includes:

a first direct current power source configured to apply the first transfer bias; and

a second direct current power source configured to apply the second transfer bias.