Wet null cycle printing

1. A liquid electro-photographic printing device comprising:

a photoreceptor;

an intermediate transfer member to receive each of multiple successive color separations individually from the photoreceptor;

an impression drum to receive each of multiple successive color separations individually from the intermediate transfer member for transfer individually to a print substrate;

multiple binary ink developers each to print a different color separation to the photoreceptor;

a voltage source that includes printing voltages and wet null voltages;

a forecast one of the binary ink developers to print a next color separation to the photoreceptor;

a non-forecast one of the binary ink developers; and

a controller to apply the printing voltages to the forecast binary ink developer in preparation for printing the next color separation and to apply the wet null voltages to the non-forecast binary ink developer in response to a null cycle trigger.

2. A printing device as in claim 1 , wherein the wet null voltages have the same polarity as the printing voltages and a magnitude that reduces the strength of the corresponding electric field compared to the printing voltages.

3. A printing device as in claim 1 , wherein the wet null voltages have the same polarity as the printing voltages and a magnitude of 500V-800V.

4. A printing device as in claim 1 , wherein the forecast binary ink developer is a single one of the binary ink developers and the non-forecast one of the binary ink developers includes multiple non-forecast ones of the binary ink developers.

5. A printing device as in claim 1 , wherein:

the non-forecast one of the binary ink developers comprises a first non-forecast one of the binary ink developers for a first null cycle trigger and a second non-forecast one of the binary ink developers different from the first one for a second null cycle trigger; and

the controller is to apply to the wet null voltages to the first non-forecast binary ink developer in response to receiving the first null cycle trigger and to the second non-forecast binary ink developer in response to receiving the second null cycle trigger.

6. A printing device as in claim 1 , further comprising a printing subsystem to generate the null cycle trigger when the subsystem senses it is not ready to perform a print cycle.

7. A method of wetting a bare print blanket, comprising:

triggering a null cycle during a printing session after transferring a color separation from the print blanket to a print substrate and before transferring a next color separation to the print blanket;

maintaining printing voltages on a forecast binary ink developer that has been prepared to print the next color separation onto a photoreceptor;

applying wet null voltages to a non-forecast binary ink developer; and

during the null cycle, transferring fluid other than ink from the non-forecast binary ink developer to the photoreceptor and from the photoreceptor to the print blanket.

8. A method as in claim 7 , wherein applying wet null voltages comprises applying voltages that have the same polarity as the printing voltages and reduce an electric field between electrified surfaces in the non-forecast binary ink developer compared to the printing voltages.

9. A method as in claim 7 , further comprising:

before printing the next color separation, triggering a subsequent null cycle;

maintaining the printing voltages on the forecast binary ink developer and the wet null voltages on the non-forecast binary ink developer; and

during the subsequent null cycle, transferring fluid other than ink from the non-forecast binary ink developer to the photoreceptor and from the photoreceptor to the print blanket.

10. A method as in claim 7 , wherein triggering a null cycle comprises triggering a sequence of null cycle and the method further comprising, after a final null cycle in the sequence of null cycles, printing the next color separation.

11. A method as in claim 7 , wherein applying wet null voltages comprises applying wet null voltages to each of a plurality of non-forecast binary ink developers.

12. A method as in claim 7 , wherein triggering a null cycle comprises receiving an interrupt signal from a printing subsystem indicating the subsystem is not ready to continue the printing.

13. A non-transitory machine-readable storage medium storing instructions that when executed by a processor of a printing device, cause the printing device to:

trigger a null cycle during a printing session after transferring a color separation from the print blanket to a print substrate and before transferring a next color separation to the print blanket;

maintain printing voltages on a forecast binary ink developer that has been prepared to print the next color separation onto a photoreceptor;

apply wet null voltages to a non-forecast binary ink developer; and

during the null cycle, transfer fluid other than ink from the non-forecast binary ink developer to the photoreceptor and from the photoreceptor to the print blanket.

14. A non-transitory machine-readable storage medium as in claim 13 , wherein the wet null voltages have the same polarity as the printing voltages and a magnitude that reduces the strength of the corresponding electric field compared to the printing voltages.

15. A non-transitory machine-readable storage medium as in claim 13 , storing instructions to select the non-forecast binary ink developer based on the last color separation printed.

16. A non-transitory machine-readable storage medium as in claim 13 , storing instructions to:

determine a color of the last color separation printed; and

select the non-forecast binary ink developer whose color does not immediately follow the color of the last color separation printed in the normal order for printing color separations.