Areas of Application, Features and Printed Products

Gravure printing is a very good illustration printing

technology, achieving very high image quality.When

variable-depth and electronically engraved gravure

printing cylinders are used, the cells take up different

amounts of ink. The different layer thickness produced

on the substrate resulting from this corresponds to the tonal gradations of the original. Consequently, a gravure-printed image comes very close to the continuous tonal gradations of an original (e.g., photographic picture). The image effect is improved even more by the fact that, after the ink has been transferred, the liquid ink flows out somewhat in the areas of deeper shades on the substrate, as a result of which no sharply defined screen dots are produced and the cell walls of the printing cylinder are not visible.

The typical features of gravure printing are:

• serrated or jagged edge on letters and lines;

• very good image reproduction, similar to continuous

tone because the screen cells are of different depths (i. e., different volume of ink);

• with variable-depth gravure printing, square screen

dots of the same size are present in all tone values.

In light tone values, the cells often print out incorrectly;

• in variable-depth and variable-area gravure printing,

screen dots of different sizes and different color saturation levels are produced.

Typical gravure printed products are high-circulation,

high-quality printed products such as:

• periodicals, magazines, mail-order catalogs;

• plastic films;

• metal foils;

• transparent films, carrier bags;

• security papers, stamps, bank notes.

1.3.2.3 Lithography/Offset Printing

In lithography the printing and non-printing parts

are on the same level. The distinctive feature of the

printing areas is the fact that they are ink-accepting,

whereas the non-printing plate elements are ink-repellent.

This effect is produced by physical, interfacial surface phenomena. Lithographic printing can be subdivided

into:

• Stone lithography (direct printing process using a

stone printing plate),

• collotype (direct printing process),

• offset printing (indirect printing process), and

• di-litho (direct printing process with offset printing

plate).

Lithography was invented by Alois Senefelder in 1796.

The image to be printed was drawn on the stone with

a special ink. The stone was dampened before it was

inked up, after which the non-image areas of the stone

surface did not take on ink.

Collotype is another lithography technology. Its beginnings (1856) can be traced back to A.L.Poitevin. Continuous tones can be reproduced without screening.

Very high print quality can be achieved (e.g., moiré free).

A light-sensitive layer of gelatin is exposed on a

glass base over a negative and then developed. Areas having different swelling properties in relation to water are produced. After the printing plate has been dampened, differentiated color absorption properties are produced.

Like stone lithography, collotype is only used for artistic printed products (with very short print runs).

Offset printing is the major lithographic technology.

It is an indirect lithographic technology, in which the

ink is first transferred from the printing plate onto a

flexible intermediate carrier – the blanket – and then

onto the substrate.

To achieve an ink-repellent effect on the printing

plate (different interaction of printing plate surface

and ink), there are two commonly used systems:

• Conventional offset printing technology: Dampening

of the printing plate with dampening solution (water

with additives). The dampening solution is applied

to the plate in a very fine film by dampening rollers. The non-image areas of the plate are hydrophilic, that is, water-receptive and the inkaccepting surfaces are oleophilic, and are therefore almost totally unreceptive to water. The film of dampening solution prevents the transfer of ink.

Since this technology is by far the most widespread,

the repellent effect between ink and dampening solution is generally always associated with “offset

printing.” Consequently, inking and dampening units are needed for “offset printing.”

• Waterless offset printing technology: The printing

plate surface is basically ink-repellent, for instance,

due to an appropriate layer of silicone.

The ink-receptive base area is exposed by the deliberate

interruption of the layer of silicone (approximately

2 mm thick). This procedure is known as “waterless

offset” (quite frequently even as “dry offset”).

Corresponding printing plates and special inks must be

used for both systems.

In a conventional offset press, two different substance

flows have to be taken into consideration for the inking up of the printing plate:

• the ink supply,
• the dampening solution supply.

The ink supply over the surfaces is interlinked with the

dampening solution supply in a complex manner. Typically, the printing plate base material consists of aluminum or polyester. The layer producing the image is then applied to it.

The entire range of print media, from an individual

brochure to a high-quality catalog, can now be produced to a high standard with the offset printing technology.

The di-litho technology, a lithographic technology in

which the printing plate prints directly onto the printing

substrate, was specially developed for newspaper

printing. The advantage of this technology was that

conventional letterpress rotary printing presses could

be used. The printing units of these presses were modified by the installation of a dampening unit. Printing was done with conventional printing plates, although a special coating had to be applied to them on account of the very high stress caused by the direct contact with the paper and the high stability over the entire print run required in newspaper printing. This technology only played a temporary role. When new investment was made in newspaper printing companies, this technology was replaced by web offset presses, which print onto the substrate via a blanket cylinder.

Дата добавления: 2015-11-16