Offset Printing - Basic Introduction

in Printing

 The offset printing technology is now the major printing technology. In the offset printing process the printing and nonprinting areas of the plate are basically on one level.The printing areas of the printing plate are oleophilic/ink-accepting and water-repellent, that's, hydrophobic.The non-printing regions in the printing plate are hydrophilic,thus oleophobic in behavior. This effect is created by physical phenomena at the contact surfaces. The dampening system spreads accross 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 zones. To achieve good wetting, surface tension has to be decreased by means of dampening solution additives. In extraordinary cases, reducing the surface tension of the dampening solution too much could result in too great an emulsification of printing ink and dampening solution, leading to a situation where an exact split of printing and non-printing areas on the plate is not accomplished when inking.A perfect offset printing process depends on many chemical as well as physical specifics of the materials and components included in the process. The most important are :

• Influence of the printing plate– surface tension of the ink-accepting places,– surface tension of the dampening solution acceptingreas,– surface roughness, primarily of the ink-free/non-image surface,– production technologies in producing 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 behavior,

• Influence of the ink– surface tension, contact surface tension in relation to the dampening solution,– rheological characteristics (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 properties (viscosity, tack),– dependence on temperature of the rheological characteristics;

• Influence of the substrate– printability features (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, stablised,vibration absorbent, etc.),– creation of the inking equipment (front-heavy, backheavy,free surface, little retroaction),– design of the dampening equipment (contact dampening,non-contact dampening),– model of the ink feed system (ink metering).
 

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Gjorevski Saso has 11 articles online and 1 fans

Saso is a graduated printing engineer specialized in offset printing(also prepress and postpress technics) with 15 years working experience who likes to share and upgrade his knowledge with anybody who is interested or need advice about this printing technology

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Offset Printing - Basic Introduction

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Offset Printing - Basic Introduction

This article was published on 2011/11/18