System for tailoring a transfer nip electric field for enhanced toner transfer in diverse environments

1. A system for tailoring a transfer nip electric field for enhanced toner image transfer in diverse environments, comprising:

a rotatable transfer roll having a first potential;

a rotatable backup roll having a second potential and forming a transfer nip between said rolls as said rolls counter-rotate relative to one another; and

a rotatable pre-nip roll having a third potential and being positioned upstream from said transfer and backup rolls and said transfer nip such that a toner image-supporting transfer belt moving past said pre-nip, transfer and backup rolls separately makes contact with, wraps partially around, and rotates each of said pre-nip, transfer and backup rolls as a media sheet is fed into said transfer nip after first passing through a gap defined between said pre-nip roll and said transfer roll such that by presetting the position, geometry and charge of said pre-nip roll relative to said transfer and backup rolls and the transfer belt an electric field at said transfer nip can be tailored for enhanced toner transfer from the transfer belt to the media sheet in diverse environments;

wherein in a relatively hot and humid environment, said first potential is approximately 800 volts above said second potential and said third potential is in a range of approximately 1500 volts to approximately 2000 volts above said second potential, and wherein said third potential is about zero volts above said second potential in a relatively cold and dry environment, the cold and dry environment being colder and dryer than the hot and humid environment.

2. The system of claim 1 wherein said second potential of said backup roll in the hot and humid environment is ground level.

3. The system of claim 1 further comprising media feed components for feeding media in proximity with said transfer roll, backup roll and pre-nip roll, wherein the media feed components are isolated from electrical ground by a resistance of at least 25 Mohms.

4. The system of claim 1 further comprising media feed components for feeding media in proximity with said transfer roll, backup roll and pre-nip roll, wherein the media feed components are isolated from electrical ground by a resistance of at least 10 Mohms.

5. The system of claim 1 wherein the potentials of the transfer roll, backup roll and pre-nip roll are chosen to keep a media potential close to ground.

6. The system of claim 1 wherein said transfer roll is in contact with said transfer belt and opposing said backup roll but off center relative to a vertical reference line through an axis of said backup roll in such a way as to allow for pre-wrapping said transfer belt partially around said transfer roll before said transfer belt engages said backup roll.

7. The system of claim 1 wherein said transfer belt has a pre-wrap on said transfer roll within a range being from approximately 0.5 to 4.0 mm.

8. The system of claim 1 wherein said transfer belt has a pre-wrap on said transfer roll of about 2.0 mm.

9. The system of claim 1 wherein said pre-nip roll is located in such a way as to reduce said gap to an angle formed between said transfer belt and incoming media sheet down to zero just before reaching said transfer nip.

10. The system of claim 1 wherein said transfer belt is composed of a polyimide material.

11. The system of claim 1 wherein said transfer belt has a surface resistivity from about 1E09 ohm-cm to 1E10 ohm-cm.

12. A system for tailoring a second transfer nip electric field at a second transfer station for enhanced toner transfer in diverse environments, comprising:

a plurality of image-forming first transfer stations;

a second transfer station having a second transfer nip; and

an endless transfer belt transported in an endless path passing, first, through a plurality of first transfer nips at said first transfer stations where toner forming an image is deposited on the transfer belt and, second, into and through said second transfer nip of said second transfer station where the toner is transferred from said transfer belt onto a media sheet;

said second transfer station including

a rotatable transfer roll having a first potential,

a rotatable backup roll having a second potential and forming said second transfer nip between said rolls as said rolls counter-rotate relative to one another, and

a rotatable pre-nip roll having a third potential and being positioned upstream from said transfer and backup rolls and said transfer nip such that said transfer belt moving past said pre-nip, transfer and backup rolls separately makes contact with, wraps partially around, and rotates with each of said pre-nip, transfer and backup rolls as a media sheet is fed into said second transfer nip after first passing through a gap defined between said pre-nip roll and said transfer roll such that by presetting the position, geometry and potential of said pre-nip roll relative to said transfer and backup rolls a voltage field at said second transfer nip can be tailored for enhanced toner transfer from the transfer belt to the media sheet in diverse environments;

wherein in a relatively hot and humid environment, said first potential is approximately 800 volts above said second potential and said third potential is in the range of approximately 1500 volts and approximately 2000 volts above said second potential, and wherein in a relatively cold and dry environment, said third potential is about zero volts above said second potential, the cold and dry environment being colder and dryer than the hot and humid environment.

13. The system of claim 12 , wherein said second potential of said backup roll in the hot and humid environment is a ground potential.

14. The system of claim 13 further comprising components which feed the media sheet to said transfer roll, said backup roll and said pre-nip roll, the components being isolated from electrical ground by a resistance of at least 25 Mohms.

15. The system of claim 13 further comprising components which feed the media sheet to said transfer roll, said backup roll and said pre-nip roll, the components being isolated from electrical ground by a resistance of at least 100 Mohms.

16. The system of claim 12 wherein said transfer roll is in contact with said transfer belt and opposing said backup roll but off center relative to a vertical reference line through an axis of said backup roll in such a way as to allow for pre-wrapping said transfer belt partially around said transfer roll before said transfer belt engages said backup roll.

17. The system of claim 12 wherein said transfer belt has a pre-wrap on said transfer roll within a range being from approximately 0.5 to 4.0 mm.

18. The system of claim 12 wherein said transfer belt has a pre-wrap on said transfer roll of about 2.0 mm.

19. The system of claim 12 wherein said pre-nip roll is located in such a way as to reduce said gap to an angle formed between said transfer belt and incoming media sheet down to zero just before reaching said transfer nip.

20. The system of claim 12 wherein said transfer belt is composed of a polyimide material.

21. The system of claim 12 wherein said transfer belt has a surface resistivity from about 1E09 ohm-cm to 1E10 ohm-cm.

22. The system of claim 1 wherein said third potential of said pre-nip roll is adjusted based in part on paper conductivity.

23. The system of claim 1 , wherein the hot and humid environment is at about 78 degrees F. and about 80 percent humidity.

24. The system of claim 12 wherein said third potential of said pre-nip roll is adjusted based in part on paper conductivity.

25. The system of claim 12 , wherein the hot and humid environment is at about 78 degrees F. and about 80 percent humidity.