A brief explanation into the basic mechanics of Litho Printing can help you choose the right printing method as well as to ensure that you are making the right choices at planning and design stages of your projects.
Lithographic (or litho) printing has been the most effective way to print in volume onto paper or card since it overtook the outdated Letterpress printing. The basic mechanics of the process have changed very little since these days although the control systems and pre-press areas around it have become much more sophisticated.
Essentially for litho printing, a metal plate is made from the original imposed artwork files. Prior to imaging, the plate carries an emulsion coating which is then exposed to lasers from a platesetter and all non-imaging areas are then washed away.
There are three main cylinders on each unit of a litho printing press.
The plate is clamped to the topmost of these – the Plate Cylinder – and water is applied during its rotation cycle. As it continues round it comes into contact with ink rollers and the ink adheres to the emulsion areas of the plate. At the bottom of its cycle it comes into contact with a rubber ‘Blanket’ (or ‘Offset’) Cylinder onto which the positive image from the plate is transferred to become a negative image.
The blanket is then basically a large rubber stamp and as it continues its rotation, the paper is pressed between it and an impression cylinder whereupon the ink is transferred to the paper as a positive image.
Modern printing presses are then able to scan a set of ‘Colour Bars’ running along the foot of the sheet to check the density for each of the inks used and then the press will automatically adjust the ink allocation and water balance for each area across the sheet. These scans are taken periodically during a print run to ensure that colour integrity is consistent and accurate.