Greenprint Guide to offset lithography
Litho printing was invented in 1798 and is based on the principle that ink and water do not mix. Artists would paint onto flat stone (lithos is the ancient Greek word for stone) using an oily paint or ink. Water was applied to the stone, followed by a coating of oily ink which was attracted to the oily image area and repelled by the water-covered non-image area. Paper was then pressed onto the surface to create a print before wetting and inking the stone for the next print.
Lithography is still the most common form of printing today, but the term "offset" has been added to denote the way that the inked image is now transferred from the printing plate to the material via an intermediate rubberised blanket, which allows very fine images to be transferred at high speeds. The printing plate carries the image which works on the same principle as the original stone, with the image area attracting ink and repelling water (oleophilic and hydrophobic) and the non-image area attracting water and repelling ink (oleophobic and hydrophilic). Each colour is printed separately on one printing unit and multi-colour images are created by overlaying inks one after the other, building up to full colour by the time the sheet reaches the last printing unit as it passes through the press.
Modern presses can run incredibly fast - up to 70,000 impressions per hour, made possible by highly computerised presses which can print up to 6 colours on both sides of the paper in one revolution. Web offset litho presses use large reels of paper and most can deliver a folded, printed section ready to be finished immediately. Coldset web offset presses are used to print uncoated papers, usually newspapers and puzzle books. Heatset web offset presses use high-powered ovens to dry the inks inline before folding. Modern web offset presses can print up to 72 pages of A4 in full colour in one revolution, but there is a wide range of press sizes from 8 pages through to 72 pages of A4. Web offset is typically used to print medium to long run magazines, brochures and catalogues.
Sheetfed litho is used for small to medium size runs, running at speeds up to 18,000 sheets per hour and in sizes from B3 up to B0 (click here for ISO size guide). It is also used for papers which are too heavy to print on a web offset press which can handle a maximum of 200gsm. Most towns have at least one sheetfed litho printer who can print short to medium run brochures, leaflets, posters and booklets, as well as company stationery such as letterheads and business cards.
Litho uses very viscous (thick) inks which take a long time to dry unless the press is fitted with assisted drying (Infra red, hot air knife or instant UV curing on sheetfed presses and long gas-powered ovens on heatset web offset presses).