Lithography Printing – Basic Principles
In lithography the printing and non-printing elements are on the very same level. The sophistication of the printing zones is the fact that they are ink-accepting, where the non-printing plate areas are ink-repellent. This effect is created by physical, inter-facial surface phenomena.
Lithographic printing can easily be subdivided to:
– Stone lithography (direct printing technique utilizing a stone printing plate),
– collotype (direct printing method),
– offset printing (indirect printing process), and
– di-litho (direct printing process with offset printing plate).
Lithography was created by Alois Senefelder in 1796. The image to be printed was gripped on the stone with the use of a special ink. The stone was moistened just before it was up, after which the non-image fields of the stone surface didn’t take on ink.
Collotype is actually another lithography technology. A light-sensitive level of gelatin is exposed on a glass base over the negative and then developed. Places having diverse swelling properties relative to water are produced. After the printing plate has been moistened, differentiated color shade absorption characteristics are generated. Just like stone lithography, collotype is only utilized for artistic goods (and having very short print runs).
Offset printing is the leading lithographic technology. It is an indirect lithographic technology, a situation where the ink is first transferred from the printing plate into a adaptable midpoint carrier – the blanket – and after that onto the substrate.
To obtain an ink-repellent effect on the plate (distinctive interaction of printing plate surface and ink), there are two widely used techniques:
– Conventional offset printing technology: Dampening of the printing plate by using dampening solution (water along with additives). The dampening solution is put on the plate in a very fine film by dampening rollers. The non-image fields of the plate are hydrophobic, that’s, water-receptive and also the ink accepting surfaces are oleophilic, and are just about 100 % unreceptive to water. The film of dampening solution inhibits the transfer of ink. Because this technology is by far essentially the most widespread, the repellent outcome among the ink and dampening solution is regularly always associated with “offset printing.” For that reason, inking and dampening units are required for “offset printing.”
-Waterless offset printing technology: The plate surface is mainly ink-repellent, for instance, due to an appropriate layer of silicone. The ink-receptive base zone is exposed by the deliberate interruption of the layer of silicone (just about 2 microns thick). This treatment is known as “waterless offset” (very frequently as “dry offset”). Suitable printing plates and special inks needs to be used for both of those techniques.
The di-litho technology, a lithographic technology in which the printing plate prints directly onto the substrate, was uniquely formed for newspaper printing. The benefit from this technology was that conventional letterpress rotary printing presses could be used. The printing units of these presses have been modified by the installing of a dampening unit. Printing was done with conventional printing plates, nevertheless a special coating needed to be applied to them due to the very high stress because of the direct contact with the paper and the high stability during the total print run essential in newspaper printing. This technology only played short-time role. When new investment was made in newspaper printing firms, this kind of technology was exchanged by web offset presses, which print onto the substrate via a blanket cylinder.
Advantages of offset lithography
– constantly high-image quality.
– much quicker and much easier production of plates.
– extended life of plates because they only come in touch with the printing blanket, which happens to be softer and less abrasive than paper
– most inexpensive production on excellent quality images in large quantities.
– high cost for small quantities
– the image quality is excellent for commercial purpose however not as good as rotogravure or photogravure printing.