Digital printing system

1. A digital printing system having two independently operable printing towers each having an endless intermediate transfer member, an image forming system serving under digital control to deposit ink onto the intermediate transfer member to form an ink image, and an 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 same side of each substrate sheet as the first tower or on the opposite side of each substrate sheet, wherein, when the perfecting system acts to reverse the substrate sheets during transfer between the two towers, each tower is operative to impress a complete image onto a respective side of the substrate, and when the perfecting system is inoperative, the on first printing tower serves to impress at least one selected separation of an image onto each substrate sheet to form a partial image and the second printing tower is operative to impress the remaining separations of the same image onto the same side of the substrate sheet in register with the partial image formed by the first printing tower wherein:

i. 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; and

ii. the digital printing system further comprises 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.

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 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.

6. 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.

7. A digital printing system as claimed in claim 1 , wherein the lifting arms of the carriage are capable of raising two print bars as a pair.

8. The digital printing system as claimed in claim 1 wherein each printing tower comprises a respective drier for drying the ink image while it is being transported by the respective intermediate transfer member from the respective image-forming station to the respective impression station.

9. 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.

10. 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.

11. A printing system having two independently operable printing towers each having an endless intermediate transfer member, an image forming system serving under digital control to deposit ink onto the intermediate transfer member to form an ink image, and an 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 same side of each substrate sheet as the first tower or on the opposite side of each substrate sheet, wherein, when the perfecting system acts to reverse the substrate sheets during transfer between the two towers, each tower is operative to impress a complete image onto a respective side of the substrate, and when the perfecting system is inoperative, the on first printing tower serves to impress at least one selected separation of an image onto each substrate sheet to form a partial image and the second printing tower is operative to impress the remaining separations of the same image onto the same side of the substrate sheet in register with the partial image formed by the first printing tower, wherein the first and second towers comprise respective sets of print bars, each set of print bars defining a respective inner pair of print bars and a respective outer pair of print bars.

12. A method of operating the printing system of claim 11 comprising:

a. operating the printing system in one of simplex and duplex modes;

b. subsequently, interchanging only inner pairs of print bars between the first and second towers while retaining the outer pairs of print bars in their positions;

c. subsequently, operating the printing system in the other of simplex and duplex modes.