Задание 2.7 Write a short summary of the text.

Методические рекомендации:

Студентам следует прочитать текст очень внимательно, затем готовится к занятиям в определенной последовательности. Подготовится к устному пересказу и выполнить все упражнения в письменной форме.

Рекомендуемая литература: 1 осн. [18-25], 4 осн. [34-41]

Контрольные вопросы:

1. What is the first step in producing a printing plate?

2. Which parts the process of producing printed products is divided?

3. Influence of Gutenberg’s invention of movable metal type on development of printing production

4. What are advantages and disadvantages of digital prepress?

5. Where are photomechanical processes used for?

Тема 3: Printing Technologies

Задание 3.1 Read the text:

In the offset printing process the printing and nonprinting areas of the plate are practically on one level. The printing areas of the printing plate are oleophilic ink-accepting and water-repellent, that is, hydrophobic. The non-printing areas of the printing plate are hydrophilic, consequently oleophobic in behavior. This effect is created by physical phenomena at the contact surfaces (figs. 2 and 3).

The dampening system covers the non-printing areas of the printing plate with a thin film of dampening solution. This dampening solution (water plus additives) spreads over the non-printing areas. To achieve good wetting, surface tension has to be reduced by means of dampening solution additives. In extreme cases, reducing the surface tension of the dampening solution too much can result in too great an emulsification of printing ink and dampening solution, leading to a situation where an exact separation of printing and non-printing areas on the plate is not achieved when inking.

The perfect offset printing process depends on many chemical and physical specifics of the materials and components involved in the process.

Figure 2 - Offset printing (lithography).

a. Components of a printing unit; b. Basic principle

Figure 3 - Wettability of surfaces and wetting angles

The most important are given in the following list:

• Influence of the printing plate

– surface tension of the ink-accepting areas, – surface tension of the dampening solution-accepting areas,

– surface roughness, especially of the ink-free/non-image surface,

– capillary attraction, microstructure of the non-image surface,

– type of materials,

– production methods in making the offset plate (mechanical or electrolytic graining, etc.);

• Influence of the inking rollers

– characteristics of the roller coverings, – surface tension of the roller material,

– surface roughness,

– visco-elastic properties of the rubber coverings,

– throw-on (pressure in the nip), adjustment,

– concentric running;

• Influence of the blanket

– surface tension of the blanket,

– surface roughness,

– compressibility,

– ink-acceptance and ink transfer behavior,

– tone value transfer behavior,

– setting/swelling, release behavior, hardness,

dimensional stability;

• Influence of the ink

– surface tension, contact surface tension in relation to the dampening solution,

– rheological properties (viscosity, tack, etc.),

– temperature behavior,

– dampening solution absorption/emulsification behavior (leeway between smearing limit and water vanes),

– running clean behavior during start-up,

– ink composition,

– drying behavior;

• Influence of the dampening solution

– water hardness/impurities,

– dampening solution additives (alcohol, detergents, buffer agents),

– pH value, surface tension,

– rheological properties (viscosity, tack),

– dependence on temperature of the rheological characteristics;

• Influence of the substrate

– printability properties (smoothness, absorption capacity, wettability),

– pH value of the substrate,

– workability properties (tension/stretch behavior, picking, tearing);

• Influence of the printing press (on print quality and process stability)

– design of the printing unit (accurate, stable, vibration absorbent, etc.),

– design of the inking unit (front-heavy, backheavy, free surface, little retroaction),

– design of the dampening unit (contact dampening, non-contact dampening),

– design of the ink feed system (ink metering),

– temperature control.

This short overview shows that the offset printing technology must be viewed as a multiparameter system. Changing just one of the parameters can have an immediate effect on the printing process. Offset printing is a technology that is clearly determined by interfacial processes, of both a physical and a chemical nature. The fact that homogeneous phases (e.g., pure water) are hardly ever involved in this process, and that more often than not it is a matter of mixed phases (e.g., water in which other substances have been dissolved) or even compound phases (e.g., printing ink, a dispersion of solid and fluid content) makes the understanding of how the various “partners” involved in this process interact more difficult.

To understand the actual mechanism of ink transfer in offset printing it must be taken into account that the contacting liquid films are always split in addition to the wetting process. Therefore, if a film of ink and a film of water come into contact with each other, the decisive factor in the transfer of ink is not whether there is some sort of repulsion, but in which liquid cross-section splitting occurs. Splitting depends to a great extent on the cohesion of the liquid film.

Offset inks have higher cohesion properties compared to water, which in turn means that splitting always takes place in the water film and not in the ink film. Since it is always the water film that splits, any contact between printing ink and water has the effect that water remains on the ink film (and may consequently also penetrate the ink in an emulsified form). The spread coefficient determines whether the water spreads over the ink surface or not. So that water is not repelled by the ink, the contact surface tension between ink and water must not be too high.

Studies have shown that the contact surface tension primarily affects the water adsorbed on the ink surface, whereas the proportion of emulsified water depends on the cohesion of the ink. As there is water on the ink film surface of the printing plate, the ink must also be able to displace the water from the image areas during inking (the water film reaches the image area first via the dampening unit). This does not cause problems, provided that the image area has already been inked. Both the printing plate with its special properties and the ink and dampening solutions play a fundamental role.

In conventional offset printing the interaction of surface tension between printing plate and ink is achieved by the addition of dampening solution. The same basic principle is applied in waterless offset printing, but with different surface/materials combinations. Consequently the surface of the waterless printing plate is made up of a highly ink-repellent silicone coating. Silicone oil is mixed into the printing ink, so that a separation layer forms when the ink comes into contact with the plate. Ink is only accepted on areas of the printing plate in which the silicone coating has been removed.

Waterless offset printing requires ink of comparatively high viscosity. Due to the milling operation in the inking unit, the inking form rollers get too hot (up to 50 °C); and the cooling effect offered by the dampening solution is also not present, which causes scumming effects. The temperature of the inking unit must therefore be stabilized. This may be done either by means of distributor rollers with water flowing through them or by an air current (very often in conjunction with plate cylinder cooling). The temperature of the inking form roller should not exceed 28–30 °C. Waterless offset printing is particularly suitable for fine and very fine screens (low dot gain). The absence of dampening solution in this process simplifies the printing unit set-up, meaning that the desired production run quality is achieved more quickly. The absence of the cleaning effect provided by the dampening solution is a disadvantage and hickeys and paper dust deposit easily on the blanket and printing plate.

Задание 3.2 Find the proper Russian equivalents for the following terms:

Задание 3.3 Find the following word combinations in the text:

1. увлажняющий раствор

2. пробельные элементы

3. шероховатая поверхность

4. поверхностное натяжение

5. красочные валики

6. офсет без увлажнения

7. бумажная пыль

8. реологические свойства

9. печатная форма

10. красочный цилиндр

11. краевой угол смачивания

Задание 3.4 Find the definitions of the words:

Задание 3.5 Fill in the gaps and complete the sentences:

1. ________________have higher cohesion properties compared to water, which in turn means that splitting always takes place in the water film and not in the ink film.

2. Offset printing is a technology that is clearly determined by_____________________, of both a physical and a chemical nature.

3. To achieve good wetting, surface tension has to be reduced by means of _______________________ additives.

4. In the offset printing process the ____________ and ______________areas of the plate are practically on one level.

5. ____________________ requires ink of comparatively high viscosity.