Digital printing system
Embodiments of the invention relate to a digital printing system comprising two independently operable printing towers, each tower having a respective endless intermediate transfer member, a respective image forming system serving under digital control to deposit ink onto the intermediate transfer member to form an ink image on the respective endless intermediate transfer member, a drier for drying the ink image while it is being transported by the intermediate transfer member to form a residue film and a respective impression station at which the residue film is impressed onto a sheet substrate.
1. A digital printing system comprising two independently operable printing towers, each tower having a respective endless intermediate transfer member, a respective image forming system serving under digital control to deposit ink onto the intermediate transfer member to form an ink image on the respective endless intermediate transfer member, a drier for drying the ink image while it is being transported by the intermediate transfer member to form a residue film, and a respective impression station at which the residue film is impressed onto a sheet substrate, wherein the two printing towers are arranged in series such that each substrate sheet passes sequentially through both printing towers, and wherein a selectively operable perfecting mechanism is provided between the two towers to reverse each substrate sheet during transfer from the first printing tower to the second printing tower, the perfecting mechanism selectively enabling the second tower to print either on the opposite side of each substrate sheet as the first tower when the digital printing system operates in duplex mode or only on the same side of each substrate sheet when the digital printing system operates in simplex mode, wherein:
i. when the perfecting mechanism is operative, the digital printing system operates in duplex mode so that the perfecting mechanism acts to reverse the substrate sheets during transfer between the two towers so that a first of the towers impresses a complete image onto a first side of the substrate and a second of the towers impresses a complete image onto a second side of the substrate, the first and second sides facing away from each other; and
ii. when the perfecting system is inoperative, the digital printing system operates in simplex mode so that (A) images or portions thereof are impressed by the printing towers only onto one side of the substrates, and (B) the first printing tower serves to impress at least one selected separation of an image onto each substrate sheet to form a partial image onto the one side of substrate and the second printing tower is operative to impress the remaining separations of the same image onto the same one side of the substrate sheet in register with the partial image formed on the one side of substrate by the first printing tower.
2. A digital printing system as claimed in claim 1 , wherein each intermediate transfer member is a rigid drum.
3. A digital printing system as claimed in claim 1 , wherein each intermediate transfer member is an endless flexible blanket guided over at least two rollers.
4. A digital printing system as claimed in claim 1 , wherein the image forming system direct droplets of a water-based ink onto said intermediate transfer member.
5. A digital printing system as claimed in claim 1 , wherein each intermediate transfer member has a hydrophobic outer surface.
6. A digital printing system as claimed in claim 5 , wherein the hydrophobic outer surface comprises a molecule selected from silanol-, sylyl- or silane-modified or terminated polydialkylsiloxane curable silicone polymers, hybrids and/or mixtures thereof; and from amino silicones and mixtures thereof.
7. A digital printing system as claimed in claim 1 , wherein each printing tower includes a pre-treatment station preceding the image forming system.
8. A digital printing system as claimed in claim 7 , wherein the pre-treatment station is operative to apply to the surface of the intermediate transfer member a weak aqueous solution of a chemical agent having a high charge density.
9. A digital printing system as claimed in claim 8 , wherein the chemical agent applied at the pre-treatment station is a charged or chargeable polymer having an average molecular weight of 10,000 g/mole or more and comprising amine nitrogen groups selected from linear, branched and cyclic, primary amines, secondary amines, tertiary amines, quaternized ammonium groups, and combinations thereof.
10. A digital printing system as claimed in claim 1 , wherein the ink comprises particles of an organic polymeric resin and coloring agent pigment suspended or dissolved in an aqueous carrier.
11. A digital printing system as claimed in claim 1 , wherein the film residue is softened prior to reaching the impression station, and is substantially entirely transferred at the impression station onto the substrate without significant change to the size nor the thickness of the film.
12. A digital printing system as claimed in claim 1 , wherein any tower serving to print one or more selected separation(s) of an image, includes a plurality of print bars of the same color circumferentially spaced from one another along the surface of the intermediate transfer member.
13. A digital printing system as claimed in claim 1 , wherein each printing tower comprises four sequentially disposed print bars and the colors of the print bars are arranged in different sequences in the two printing towers, the colors of the two inner print bars in each printing tower being matched to the colors of the two outer print bars in the second printing tower.
14. A digital printing system as claimed in claim 13 , further comprising a print bar positioning system for transferring print bars from one tower to the other, the print bar positioning system including a carriage movable between the two towers and having lifting arms for engaging and raising the print bars.
15. A digital printing system as claimed in claim 14 , wherein the lifting arms of the carriage are capable of raising two print bars as a pair.
16. The digital printing system as in claim 1 wherein:
i. when the selectively operable perfecting mechanism is inoperative, substrate passes from the first printing tower to the second printing tower without being reversed by the perfecting mechanism; and
ii. when the selectively operable perfecting mechanism is operative, substrate passes is reversed by the perfecting mechanism en route from the first printing tower to the second printing tower.
17. The digital printing system of claim 1 wherein when the selectively operable perfecting mechanism is operative, after substrate-reversing by the perfecting mechanism, substrate is subsequently transported to the second printing tower without passing by the first printing tower in the interim.