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.

Ink Transfer

Letterpress and Planographic Printing.

The transfer of ink to the printing substrate or intermediate carrier through splitting the ink layer is mainly affected by the following parameters:

• the thickness of the ink layer on the printing plate

(ink supply),

• the period of contact (printing speed and geometry

of the printing components),

• the contact pressure (also called printing pressure),

• the rheological properties of the ink,

• the temperature ratios (temperature has a marked

effect on the ink’s rheological properties),

• the surface properties of the printing substrate and

printing plate or intermediate carrier (wettability,

absorbency, roughness, etc.).

A further factor influencing the layer of ink on the substrate is the absorption properties of the ink in the

printing substrate.

With both absorbent and non-absorbent printing

substrates, the ink transfer is reduced if the printing

speed is increased, as the period of contact then becomes shorter. In indirect printing, ink transfer takes

place in two stages. The layer of ink on the printing

plate is brought into contact with a rubber blanket and

then part of the ink layer adheres to the blanket. Ink is

then transferred to the substrate.

Ink splitting processes and factors are still beset by

many unresolved questions and corresponding research

is underway worldwide.

Gravure Printing. With gravure printing the ink remaining in the cells after doctoring is brought into contact with the printing substrate, at which point part of this ink is transferred to the substrate.

The emptying of the cells is incomplete because

of the ink splitting. This transfer of ink is affected by

• the wetting properties of the printing substrate,

• the surface properties of the materials involved,

• the properties of the paper,

• the viscosity of the ink,

• the printing pressure,

• the printing speed, and

• the shape of the cells and their filling level.

In all printing technologies, a multicolor print is produced by the superimposed printing of several color

separations using the appropriate printing plates.

The printing plates for the primary color system of cyan, magenta, yellow, and black are produced in

prepress. In a printing press with four printing units

these four separated colors can be printed consecutively

in one sheet pass, the multicolor print being produced

corresponding to the original.

1.3.2.1 Letterpress/Flexographic Printing

A common feature of all letterpress processes is the fact that the printing elements of the plate are raised above the nonprinting elements. The printing elements that are at the same height (image areas) are coated with a layer of ink of constant thickness by the application rollers.

This is followed by the transfer of the ink to the substrate.

The letterpress printing technique is used with the

following printing systems:

• book printing,

• flexographic printing,

• letterset.

Дата добавления: 2015-11-16