Digital printing process
A printing process is disclosed which comprises directing droplets of an ink onto an intermediate transfer member to form an ink image, the ink including an organic polymeric resin and a coloring agent in an aqueous carrier, and the transfer member having a hydrophobic outer surface so that each ink droplet in the ink image spreads on impinging upon the intermediate transfer member to form an ink film. The ink is dried while the ink image is being transported by the intermediate transfer member by evaporating the aqueous carrier from the ink image to leave a residue film of resin and coloring agent. The residue film is then transferred to a substrate. The chemical compositions of the ink and of the surface of the intermediate transfer member are selected such that attractive intermolecular forces between molecules in the outer skin of each droplet and on the surface of the intermediate transfer member counteract the tendency of the ink film produced by each droplet to bead under the action of the surface tension of the aqueous carrier, without causing each droplet to spread by wetting the surface of the intermediate transfer member.
1. A method of operating a printing system comprising: (i) an intermediate transfer member (ITM) comprising an endless flexible belt having upper and lower runs, the belt passing over guide and drive rollers, (ii) an image forming station comprising a plurality of print bars and a plurality of gas-blowers wherein each gas-blower respectively follows a different associated print bar, and (iii) a main drying station which is disposed both downstream of the image forming station and above an upper run of the belt, the method comprising:
at the image forming station, depositing droplets of ink onto an outer surface of the ITM to form ink images thereon;
at the image forming station, respectively operating each gas-blower of the plurality of print-bar-associated gas-blowers to blow a respective gas-stream onto the ITM outer surface so as to only partially dry the deposited ink-droplets;
at the main drying station which is disposed both downstream of the image forming station and above the upper run of the belt, drying the ink images sufficiently to produce therefrom a residue film of ink solids, sufficient heat being delivered at the main drying station to render the residue film tacky; and
at an impression station disposed downstream of the main drying station, transferring the tacky residue film from the ITM outer surface to substrate, wherein the method is performed such that:
(i) a maximum temperature of the ITM outer surface as it passes through the image forming station is defined as an imaging forming station maximum temperature (IFS-Temp MAX ); and
(ii) as the ITM outer surface passes through the main drying station, the ITM outer surface reaches a temperature exceeding the imaging forming station maximum temperature (IFS-Temp MAX ); and
(iii) the printing system which performs the method is configured such that a plurality of said guide rollers is disposed at the image forming station, each guide roller of said plurality of guide rollers being both (A) in contact with the upper run of the belt; and (B) disposed adjacent to a respective print bar of the image-forming station,
wherein the method is performed such that at least one of a first condition, a second condition and a third condition is true, and wherein the first, second and third conditions are defined as follows:
A. according to the first condition, the intermediate transfer member (ITM), of the printing system which performs the method, further comprises lateral formations on the side edges of the member, the lateral formations compatible with guiding channels positioned at least at the image forming station to maintain the transfer member taut in its width ways direction;
B. according to the second condition, the printing system which performs the method is configured such that there is no gas-blower disposed immediately upstream of the most upstream print-bar of the image forming station; and
C. according to the third condition, a hot gas is blown by the most upstream gas-blower of th he plurality of gas-blowers, said most upstream gas-blower following the most upstream print-bar of the plurality of print bars.
2. The method of claim 1 wherein a temperature of the ITM outer surface upon transfer of the tacky residue film therefrom to substrate at the impression station exceeds, by at least 10 degrees Celsius, the imaging forming station maximum temperature (IFS-Temp MAX ).
3. The method of claim 1 wherein the ink-droplet-depositing comprises depositing different ink colors sequentially and respectively to the ITM outer surface as the ITM outer surface passes through the image forming station, each ink color being applied by a different respective one of the print bars.
4. The method of claim 3 wherein for first and second of the print bars which respectively apply first and second colors, the gas blower associated with the first print bar blows gas onto the ITM outer surface: (i) after deposition thereon of ink of the first color by the first after and (ii) before reposition on the ITM outer surface of ink of the second color by the second print bars.
5. The method of claim 1 , wherein the impression station of the printing system comprises a pressure cylinder having a compressible outer surface or carrying a compressible blanket and an impression cylinder and wherein the image transfer member (ITM) comprises an endless belt of greater length than the circumference of the pressure cylinder and passing through a nip between the impression and pressure cylinders and contacting the pressure cylinder over only a portion of the length of the endless belt.
6. The method of claim 1 , wherein the intermediate transfer member is an endless blanket incorporating a compressible layer guided over guide rollers or supporting surfaces and having a run or a region thereof that is selectively deflectable by a movable nip roller to be urged against an impression cylinder.
7. The method of claim 1 wherein at least the first condition is true.
8. The method of claim 1 further comprising: before the depositing of the droplets of ink, applying a dilute solution of a charged polymer to the outer surface of the ITM.
9. The method of claim 1 wherein: before the depositing of the droplets of ink, applying a treatment solution to the outer surface of the ITM, and wherein at a time when the ink droplets are deposited on the ITM outer surface, a thickness of the treatment solution on the ITM outer surface is at most a few molecules.
10. The method of claim 1 wherein the printing system which performs the method is configured such that each of the plurality of the guide rollers disposed at the image-forming station is vertically aligned with a respective print bar in the image-forming station or disposed at most a few millimeters downstream therefrom.
11. The method of claim 1 wherein at least the second condition is true.
12. The method of claim 1 wherein at least the third condition is true.
13. A method of operating a printing system comprising: (i) an intermediate transfer member (ITM) comprising an endless flexible belt having upper and lower runs, the belt passing over guide and drive rollers, (ii) an image forming station comprising a plurality of print bars and a plurality of gas-blowers wherein each gas-blower respectively follows a different associated print bar, and (iii) a main drying station which is disposed both downstream of the image forming station and above an upper run of the belt, the method comprising:
at the image forming station, depositing droplets of ink onto an outer surface of the ITM to form ink images thereon;
at the image forming station, respectively operating each gas-blower of the plurality of print-bar-associated gas-blowers to blow a respective gas-stream onto the ITM outer surface so as to only partially dry the deposited ink-droplets;
at the main drying station which is disposed both downstream of the image forming station and above the upper run of the belt, drying the ink images sufficiently to produce therefrom a residue film of ink solids, sufficient heat being delivered at the main drying station to render the residue film tacky; and
at an impression station disposed downstream of the main drying station, transferring the tacky residue film from the ITM outer surface to substrate, wherein the intermediate transfer member (ITM), of the printing system which performs the method, further comprises lateral formations on the side edges of the member, the lateral formations compatible with guiding channels positioned at least at the image forming station to maintain the transfer member taut in its width ways direction.
14. The method of claim 13 wherein the printing system which performs the method is configured such that a plurality of said guide rollers is disposed at the image forming station, each guide roller of said plurality of guide rollers being both: (i) in contact with the upper run of the belt; and (ii) disposed adjacent to a respective print bar of the image-forming station.
15. The method of claim 14 wherein the printing system which performs the method is configured such that there is no gas-blower disposed immediately upstream of the most upstream print-bar of the image forming station.
16. The method of claim 13 wherein the printing system which performs the method is configured such that there is no gas-blower disposed immediately upstream of the most upstream print-bar of the image forming station.
17. The method of claim 16 wherein a hot gas is blown by the most upstream gas-blower of the plurality of gas-blowers, said most upstream gas-blower following the most upstream print-bar of the plurality of print bars.
18. The method of claim 13 wherein a hot gas is blown by the most upstream gas-blower of the plurality of gas-blowers, said most upstream gas-blower following the most upstream print-bar of the plurality of print bars.
19. A method of operating a printing system comprising: (i) an intermediate transfer member (ITM) comprising an endless flexible belt having upper and lower runs, the belt passing over guide and drive rollers, (ii) an image forming station comprising a plurality of print bars and a plurality of gas-blowers wherein each gas-blower respectively follows a different associated print bar, and (iii) a main drying station which is disposed both downstream of the image forming station and above an upper run of the belt, the method comprising:
at the image forming station, depositing droplets of ink onto an outer surface of the ITM to form ink images thereon;
at the image forming station, respectively operating each gas-blower of the plurality of print-bar-associated gas-blowers to blow a respective stream of gas onto the ITM outer surface so as to only partially dry the deposited ink-droplets, wherein a hot gas is blown by the most upstream gas-blower of the plurality of gas-blowers, said most upstream gas-blower following the most upstream print-bar of the plurality of print bars;
at the main drying station which is disposed both downstream of the image forming station and above the upper run of the belt, drying the ink images sufficiently to produce therefrom a residue film of ink solids, sufficient heat being delivered at the main drying station to render the residue film tacky; and
at an impression station disposed downstream of the main drying station, transferring the tacky residue film from the ITM outer surface to substrate, wherein the printing system which performs the method is configured such that a plurality of said guide rollers is disposed at the image forming station, each guide roller of said plurality of guide rollers being both: (i) in contact with the upper run of the belt and (ii) disposed adjacent to a respective print bar of the image-forming station.
20. The method of claim 19 wherein the printing system which performs the method is configured such that there is no gas-blower disposed immediately upstream of the most upstream print-bar of the image forming station.
21. The method of claim 20 wherein a hot gas is blown by the most upstream gas-blower of the plurality of gas-blowers, said most upstream gas-blower following the most upstream print-bar of the plurality of print bars.
22. The method of claim 19 wherein a hot gas is blown by the most upstream gas-blower of the plurality of gas-blowers, said most upstream gas-blower following the most upstream print-bar of the plurality of print bars.
23. The method of claim 22 wherein the printing system which performs the method is configured such that each of the plurality of the guide rollers disposed at the image-forming station is vertically aligned with a respective print bar in the image-forming station or disposed at most a few millimeters downstream therefrom.
24. The method of claim 19 wherein the printing system which performs the method is configured such that each of the plurality of the guide rollers disposed at the image-forming station is vertically aligned with a respective print bar in the image-forming station or disposed at most a few millimeters downstream therefrom.