1.3.2 Printing Technologies with

a Printing Master

Printing technologies with a master are also referred to as conventional printing technologies. The printing plate is the information carrying medium for all the procedures Information is generated on the printing

substrate by the partial surface transfer of ink. All information is denoted by image elements (transferred

ink) and non-image elements (no ink).

To enable continuous color tone values to be reproduced in print, such as from photographic originals, the original has to be broken up into extremely small dots (screen dots) that vary in size or are at various distances from each other. This process is called screening.

The main function of screening is to generate halftone

values, which are intended to simulate the continuous

tonal gradation, ending with the conversion of gray images into binary images.

Such screening is necessary because most printing

technologies operate on a binary system and can therefore only perform one of two actions, namely to transfer ink (in an evenly distributed layer) or not to transfer ink.

With the exception of the variable-depth gravure

printing technology, printing technologies requiring a

printing plate transfer layers of ink to printing plate elements that are basically of the same thickness throughout (the printing plates are generally set up in this way, the printing units in the printing press being designed accordingly). It is only the area and configuration of the print elements on the surface of the printing substrate that vary, and it is by using this that different tone values can be reproduced. The observer receives the impression of a continuous gradation of tone if the human eye can no longer break down the image into its individual print elements. This phenomenon occurs if, for example, a 60 lines/cm (150lpi) screen structure has been used (i.e., dot spacing of 0.16 mm) and it is observed from a distance of approximately 30 cm (1 ft).

Variable-depth gravure printing generates quantitative

ink metering corresponding to the tone value during

ink transfer. The printing elements of equal size

(halftone dots) only vary in the thickness of their layers

depending on the tone values of the original, although

in principle the dot size of the individual halftone dots

can vary in gravure printing as well as the quantity of

ink.

All printing technologies requiring a printing plate

therefore have in common that information is generated

by the surface of the substrate being partially coated

with ink. Ink is transferred in the contact zone (nip,

printing zone of the surfaces involved) and sufficient

process-dependent contact pressure must be applied

between the printing plate and the printing substrate

or an intermediate carrier.When the layer of ink on the

printing plate or intermediate carrier is brought into

contact with the printing substrate,only part of the layer

of ink is transferred to the substrate. A residual layer

remains on the printing plate. Thus the ink is not

transferred in full, the ink layer is split.