Basic Principle of Offset technology

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.

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. 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 solutionaccepting

areas,

– surface roughness, especially of the ink-free/

non-image surface,

– capillary attraction, microstructure of the nonimage

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,

– viscoelastic 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.

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.