Method and system for reduction of caking

1. A method for the reduction of caking of excess liquid developer dispersion that remains present on the surface of a member after transfer of liquid developer dispersion from one member to another member in a digital printing apparatus, comprising the step of adding a dispersing composition to the excess liquid developer dispersion by a dispersing agent adding means,

wherein the dispersing composition comprises a free dispersing agent;

wherein liquid developer is stored in a liquid developer dispersion reservoir for feeding a toner supply member of the digital printing apparatus; and

wherein the excess liquid developer dispersion is collected in a reservoir, and

wherein the adding means are arranged for adding an amount of dispersing agent to said reservoir and/or wherein the adding means are arranged for adding an amount of dispersing agent to the excess liquid developer dispersion before or after it is removed from the member; and

wherein the adding by the dispersing agent adding means is controlled by a controller which determines a suitable amount of dispersion composition to be added; and

wherein the excess liquid developer dispersion with the added dispersing composition is returned to the liquid developer dispersion reservoir.

2. Method according to claim 1 , wherein the step of adding free dispersion agent is performed during printing.

3. Method according to claim 1 , wherein the excess liquid developer dispersion is removed from the member before adding the dispersing composition.

4. Method according to claim 1 , wherein the free dispersing agent is added with an amount that is sufficient to obtain a concentration between 0.005 wt % and 0.5 wt %, preferably between 0.01 wt % and 0.3 wt %, and most preferably between 0.02 wt % and 0.2 wt % of free added dispersing agent with regard to the total weight of excess liquid developer dispersion.

5. Method according to claim 1 , wherein the dispersing agent is selected from the group consisting of polymeric dispersing agents with a polyethylenimine or polyallylamine backbone and polyhydroxystearate and/or polycaprolactone grafts; statistical copolymer of vinylpyrrolidone and long chain olefins.

6. Method according to claim 1 , wherein the amount of the dispersing agent in the dispersing composition that is added is sufficient to provide a lower viscosity to the excess liquid developer dispersion compared to the viscosity of the excess liquid developer dispersion where no dispersing agent is added.

7. Method according to claim 6 , wherein the viscosity is between 1.1 and 25 times lower, preferably between 1.5 and 15 times lower and most preferably between 2 and 10 times lower compared to the viscosity of untreated excess liquid developer dispersion when the viscosity is measured at 25° C. at a shear rate of 0.88 l/s.

8. Method according to claim 1 , comprising the step of collecting excess liquid developer dispersion and the step of mixing the collected excess liquid developer dispersion with the added dispersing composition.

9. A method for the reduction of caking of excess liquid developer dispersion that remains present on the surface of a member after transfer of liquid developer dispersion from one member to another member in a digital printing apparatus, comprising the step of adding a dispersing composition to the excess liquid developer dispersion, wherein the dispersing composition comprises a free dispersing agent;

wherein the method further comprises the step of determining an amount of dispersing composition to be added to reduce caking of excess liquid developer dispersion, wherein the adding comprises adding the determined amount;

wherein a viscosity of the excess liquid developer dispersion is measured, and determining an amount of dispersing composition to be added is based on the measured viscosity.

10. A method for the reduction of caking of excess liquid developer dispersion that remains present on the surface of a member after transfer of liquid developer dispersion from one member to another member in a digital printing apparatus, comprising the step of adding a dispersing composition to the excess liquid developer dispersion,

wherein the dispersing composition comprises a free dispersing agent;

wherein the method further comprises collecting image information about an image that is being printed in the digital printing apparatus; the image information being representative for the fraction of liquid developer dispersion that is being developed; and determining an amount of dispersion composition to be added based on the collected image information;

wherein the adding of the dispersion composition comprises adding the determined amount of dispersion composition.

11. A method for the reduction of caking of excess liquid developer dispersion that remains present on the surface of a member after transfer of liquid developer dispersion from one member to another member in a digital printing apparatus, comprising the step of adding a dispersing composition to the excess liquid developer dispersion during printing, wherein the dispersing composition comprises a free dispersing agent, and

wherein the excess liquid developer dispersion is removed from the member before adding the dispersing composition.

12. Digital printing system comprising:

a first member in rotational contact with a second member; the first and second member being configured for transferring liquid developer dispersion from the first member to the second member, such that an excess liquid developer dispersion remains present on the surface of the first member after transfer of liquid developer dispersion to the second member;

adding means configured for adding an amount of free dispersing agent to the excess liquid developer dispersion; and

control means for controlling the adding means such that the amount added is sufficient to reduce caking in the excess liquid developer dispersion;

a liquid developer dispersion reservoir for storing liquid developer; wherein the system is configured to return the excess liquid developer dispersion with the added dispersing composition to said liquid developer dispersion reservoir;

a toner supply member for feeding the first member with liquid developer from the liquid developer dispersion reservoir;

a reservoir adapted for collecting the excess liquid developer dispersion; and

wherein the adding means are configured to add an amount of dispersing agent to said reservoir and/or wherein the adding means are arranged for adding an amount of dispersing agent to the excess liquid developer dispersion before or after it is removed from the member.

13. Digital printing system of claim 12 , wherein the adding means are configured for adding an amount of free dispersing agent during printing.

14. Digital printing system of claim 12 , wherein the control means are configured for controlling the adding means such that an amount of free dispersing agent added results in the viscosity of the excess liquid developer dispersion being between 1.1 and 25 times lower, preferably between 1.5 and 15 times lower, and most preferably between 2 and 10 times lower, when the viscosity is measured at 25° C. at a shear rate of 0.88 l/s.

15. Digital printing system of claim 12 , further comprising collecting and determining means configured for collecting image information about an image that is being printed in the digital printing system; the image information being representative for the fraction of liquid developer dispersion that is being developed on the development roller; and for determining an amount of dispersion agent to be added based on the collected image information; wherein the control means are configured for controlling the adding means using the determined amount of dispersion agent to be added.

16. Digital printing system of claim 12 , wherein the second member is an imaging member adapted to sustain a pattern of electric charge forming a latent image on its surface,

wherein the first member is a development member arranged to receive a quantity of liquid developer dispersion, and to develop the latent image by transferring a portion of the quantity of liquid developer dispersion onto the imaging member in accordance with the pattern.

17. Digital printing system of claim 12 , further comprising a viscosity sensor for measuring the viscosity of the excess liquid developer dispersion, wherein the control means are configured for controlling the adding means on the basis of the measured viscosity.

18. Digital printing system of claim 12 , wherein the adding means are arranged for adding an amount of dispersing agent in the first reservoir.

19. Digital printing system of claim 18 , further comprising:

a second reservoir adapted for collecting the excess liquid developer solution in which the amount of dispersing agent is added;

carrier liquid adding means arranged for adding an amount of carrier liquid in the second reservoir;

toner concentrate adding means arranged for adding an amount of toner concentrate in the second reservoir;

the second reservoir being connected to the liquid developer dispersion reservoir for returning recycled developer dispersion to the liquid developer dispersion reservoir.