PRINTING TECHNOLOGY

In the conventional offset printing process the dampening solution is used to separate the image and nonimage areas, that is, to prevent the transfer of ink onto non-image areas of the printing plate. The dampening solution consists mainly of water. Experience has shown that in conventional offset printing the dampening solution should have a pH value between 4.8 and 5.5 and the water used in the dampening solution should have a hardness level of between 8 and 12° dH (1.43 and 2.14 mmol/liter; 1 mmol/litre = 100 mg CaCO3 per 1 liter water). Dampening solution usually contains plate preservative agents, wetting agent, isopropyl alcohol (IPA), buffer substances, and anti-microbe additives.

Printing Ink The ink used in offset printing is usually a highly viscous mixture having the basic components of ink pigment, vehicle (binder), additives, and carrier substance. Ink pigments can have either an organic or inorganic nature. The pigments determine the hue of the printing ink. They consist of solid, irregularly-shaped particles that are about 0.1–2 μm in size. Vehicles (binders) are needed to bind the pigment, which is supplied in powder form, to the substrate. Vehicles also form a protective film around the pigments so that they are protected from mechanical abrasion. The composition of the vehicles depends on the printing technologies and substrate to be used. The vehicles used in the production of printing ink are also called “varnishes”. The real know-how of the printing ink manufacturer lies in the creation of the formula and the preparation and combination of the individual raw materials which make up the vehicle. Additives can be mixed into the ink to achieve specifi...

Thermal Plates Alongside conventional printing plate systems, thermal plates have been developed for digital imaging.

The plates used in offset printing are thin (up to about 0.3 mm), and easy to mount on the plate cylinder, and they mostly have a monometal (aluminum) or, less often, multimetal, plastic or paper construction. Aluminum has been gaining ground for a long time among the metal-based plates over zinc and steel. The necessary graining of the aluminum surface is done mechanically either by sand-blasting, ball graining, or by wet or dry brushing.Nowadays, practically all printing plates are grained in an electrolytic process (anodizing), that is, electrochemical graining with subsequent oxidation. The imaging, ink-accepting coating (light-sensitive coating, thickness around 1μm) is applied to the base material. This material is usually a polymer, or copper in the case of multimetal plates (bimetal plates). Lightsensitive, diazo (photopolymer) pre-coated aluminum printing plates are now the predominant plates in printshops. The image transfer is produced via the different properties on the sur...

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 p...