Digital printing

Electrophotographic method

Digitized data is mainly printed using the electrophotographic method, which works in a similar way to color laser copying. A corona discharge first creates a homogeneous charge image on the surface of a drum with a photoconductive layer. A controlled light source – usually a laser – then writes the image to be printed onto the drum, the light discharging the relevant areas of the drum. The positioning of the light signals on the photoconductor drum thus corresponds to the print image. Next, the drum is inked up. The charged areas accept toner, creating the latent image. The toner is then transferred to the printing stock with the help of electrostatic forces – directly or via a carrier – and fixed by heat. Finally, the drum is cleaned and prepared for the next printing cycle. With four-color printing, this process is run through four times until the print image is ready.

Hewlett Packard’s Indigo method is one type of electrophotography that produces particularly good results with films. This method uses liquid inks instead of the more common powder toner. The print image’s individual ink layers are not transferred onto the film in turn but first collected on a rubber blanket and then applied to the film in a single step. This has the advantage that the electrostatic forces removing the ink from the drum do not have to work through the film. As a result, even thicker films can be printed cleanly using this method.