In the inkjet process, the ink can be transferred directly on to the paper. Inkjet technologies can be classified as continuous inkjet and drop-on-demand inkjet. The ink used for inkjet printing is usually liquid. An alternative, however, is hot-melt inks which are liquified by heating. The ink is sprayed onto the substrate where it solidifies after cooling.

The continuous inkjet technology generates a constant stream of small ink droplets, which are charged according to the image and controlled electronically. The charged droplets are deflected by a subsequent electric field, while the uncharged ones flow on to the paper. This means that the imaging signal for charging the droplets corresponds to a negative print image.

Continuous inkjet printing usually feeds only a small proportion of the stream of droplets to the substrate. With continuous inkjet, generally, only a small part of the drop volume cover individual nozzles.

To do so, the paper is fastened to a drum and the individual inkjet systems transfer the individual colour separations to the substrate.

This occurs due to the axial scanning motion of the imaging head and rapid rotation of the drum. An A3 multicolour print is produced in approximately five minutes. This type of system is, therefore, mainly employed to produce the proofs necessary in digital pre-press before the computer to plate process produces a printing plate. Such a proof allows the quality of the data file and the content and visual quality of the subsequent impression to be checked at a preliminary stage.

Sachin Shardul is assistant editor at PrintWeek India

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