Transfer device and image forming apparatus incorporating same

1. An image forming apparatus comprising:

a plurality of image forming units to form a process color toner image and a special color toner image;

a plurality of image bearers to bear the process color toner image and the special color toner image; and

a transfer device including an intermediate transfer member, to transfer the process color toner image and the special color toner image from the plurality of image bearers onto the intermediate transfer member; and

a controller configured to correct a transfer bias applied to transfer the special color toner image onto the intermediate transfer member,

the controller being further configured to decrease the transfer bias in a special color mode, in which image formation is performed with only a special color, and increase the transfer bias in a full color mode, in which image formation is performed with the special color and a process color, according to a degree of degradation of a developer used to form the special color toner image.

2. The image forming apparatus according to claim 1 ,

wherein the special color toner image includes metallic luster toner.

3. The image forming apparatus according to claim 1 ,

wherein the special color toner image includes colorless toner.

4. The image forming apparatus according to claim 1 ,

wherein the controller is configured to decrease the transfer bias according to a calculation value calculated with a conveyance distance of the developer used to form the special color toner image and a consumption of toner.

5. The image forming apparatus according to claim 1 ,

wherein the controller is configured to decrease a transfer bias applied to transfer the process color toner image, according to a calculation value calculated with a conveyance distance of a developer used to form the process color toner image and a consumption of toner.

6. The image forming apparatus according to claim 1 ,

wherein the controller is configured to perform process control to change an image forming condition according to a detection value of a sensor to detect, on the intermediate transfer member, a density of a toner adhesion pattern for image adjustment formed on a non-image formation area of the plurality of image bearers,

wherein the toner adhesion pattern includes at least a patched pattern and a lateral band pattern extending in a main scanning direction, and

wherein the controller is configured to calculate a difference in density between a toner image of the patched pattern and a toner image of the lateral band pattern for each of the process color and the special color and decrease the transfer bias according to a calculated value of the difference of the special color.

7. The image forming apparatus according to claim 6 ,

wherein the controller is configured to decrease a transfer bias applied to transfer the process color toner image, according to a calculated value of the difference of the process color.

8. The image forming apparatus according to claim 1 , further comprising:

a primary transfer unit to transfer the process color toner image and the special color toner image from the plurality of image bearers onto the intermediate transfer member by a primary transfer bias; and

a secondary transfer unit to transfer the process color toner image and the special color toner image from the intermediate transfer member onto a transfer target by a secondary transfer bias,

wherein the controller is configured to correct the primary transfer bias with a correction amount and also correct the secondary transfer bias with the correction amount with which the controller corrects the primary transfer bias.

9. The image forming apparatus according to claim 8 ,

wherein the controller is configured to decrease the secondary transfer bias according to the correction amount when the controller corrects the primary transfer bias to decrease the primary transfer bias.

10. The image forming apparatus according to claim 8 ,

wherein the controller is configured to more greatly change the correction amount according to the degree of degradation of the developer as temperature and humidity of use environment are higher, when the controller corrects the primary transfer bias to decrease the primary transfer bias.

11. The image forming apparatus according to claim 8 ,

wherein the controller is configured to more greatly change the correction amount of the primary transfer bias according to the degree of degradation of the developer as an amount of change in resistance of a transfer member of the transfer device from a reference resistance value is greater, when the controller corrects the primary transfer bias to decrease the primary transfer bias.

12. The image forming apparatus according to claim 1 ,

wherein the controller is configured to change the transfer bias according to a change in use environment.

13. The image forming apparatus according to claim 1 ,

wherein the controller is configured to change a correction amount of the transfer bias according to a change in resistance of a transfer member of the transfer device.

14. The image forming apparatus according to claim 1 ,

wherein the controller is configured to change the transfer bias in a stepwise manner according to the degree of degradation of the developer.