Offset printing - basic

Offset printing is an indirect lithographic printing technology. It is so accepted among printing service agencies and shoppers because for big quantities, it turns out being more economical and still have much greater quality in comparison with alternative techniques. The offset printing technology is now the major printing technology. Inside the offset printing process the printing and non-printing areas of the plate are basically on one level.The printing areas of the printing plate are oleophilic / ink-accepting and water-repellent, that is, hydrophobic.The non-printing fields of the printing plate are hydrophilic, thence oleophobic in behavior. This effect is produced by physical phenomena at the contact surfaces. The dampening system spreads across the non-printing areas of the printing plate with a thin film of dampening solution.This dampening solution (water plus additives) distributes across the non-printing fields. To obtain good wetting, surface tension should be decreased by means of dampening solution additives. In severe situations, reducing the surface tension of the dampening solution too much could result in too great an emulsification of printing ink as well as dampening solution, leading to a situation where an explicit split of printing and non-printing areas at the plate is not realized when inking.The perfect offset printing process depends on many chemical and physical details of the materials and components involved with the process. The most important are :
• 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,– production processes in making the offset plate (mechanical or electrolytic graining, etc.);
• Influence of the inking rollers – qualities of the roller covers,– surface tension of the roller material,– surface roughness,– concentric running;
• Influence of the blanket– surface tension of the blanket,– surface roughness,– compressibility,– ink acceptance and ink transport performance,
• Influence of the ink– surface tension, contact surface tension in relation to the dampening solution,– rheological features (viscosity, tack, etc.),– dampening solution absorption / emulsification – running clean behavior during start-up,– ink composition,– drying behavior;
• Influence of the dampening solution– water hardness / impurities,– dampening solution additives and preservatives (alcohol, detergents,buffer agents),– pH value, surface tension,– rheological characteristics (viscosity, tack),– reliance on temperature of the rheological characteristics;
• Influence of the substrate – print-ability properties (smoothness, absorption capacity, wett-ability), – 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 (effective, steady,vibration absorbent, etc.),– design of the inking equipment (front-heavy, back-heavy, free surface, little retro action),– design of the dampening unit (contact dampening,non-contact dampening),– construction of the ink feed system (ink metering)