This section covers the main elements of current digital printing technology. It should be noted that this does not include presses where the printing plate is imaged on the press (computer-to-press), only those machines capable of changing successive images.
Currently the majority of digital presses rely on electrostatic methods of imaging. This is very similar to the way that a photocopier or laser printer works. An image is painted on to a uniformly charged photosensitive drum using a laser(s) or Light Emitting Diodes (LED's).
The light from the laser or LED hits the drum and dissipates the electrical charge, creating a latent image (ink attracting). The drum continues to rotate through a trough containing the toner (ink) and developer. The toner is attracted to the latent image, but not the white areas.
As the drum revolves the ink is transferred to the surface of the paper using electrical charges. The drum continues to revolve and is electrically cleaned to remove the image from the previous exposure. As each time the drum revolves and entirely new image is created, it is possible for each successive print to be different from the last.
In general, for four colour work, the process is repeated four times for the four process colours yellow, magenta, cyan and black. In the illustration above the paper is fed into the IBM InfoColor 70 from the large roll within the paper supply. The paper is conditioned using heated rollers to create the correct moisture content. It then passes through the developer units. There are eight developer units in all, four for each side of the paper. These image the main process colours (yellow, magenta, cyan and black).
When the image has been printed in all four colours, it is fused onto the paper at high temperatures. The paper with the fused image is then cooled and cut to the required page length. Printed sheets are stacked in the receive unit.
