Offset Printing
Offset printing is a process of stampa on rollers of the indirect type, that is, the image is not transferred directly from the plate to the paper but through a complex system of rollers, which allows high-definition prints and on supports with an irregular surface. The system adopted is planographic (referring to the particular printing system that uses flat matrices, typical of phototyping and lithography) indirect which is based on the phenomenon of chemical / physical repulsion between water and ink (fat-based ink that does not adhere to the thin layer of water spread on the slab).
Aluminum foil itself is hydrophilic (accepts water) and is treated so that graphisms appear lipophilic (accept fat). Following the writing of the graphisms through a machine called CTP (Computer to Plate), this machine by means of a laser beam "writes" the graphisms on the plate making them stand out.
As in lithography, it is a "planographic" process because graphisms and countergraphisms are on the same plane.
Unlike lithography: a) instead of being of stone (lithography), the matrix consists of a thin sheet of aluminum which, properly treated, behaves like a lithographic stone; moreover, b) it is a method defined as "indirect" as the printing does not take place directly from the metal matrix on the sheet of paper (ie the plate mounted on the plate holder cylinder does not come into direct contact with the support), but through a rubberized fabric, called rubber, which collects the inked image from the matrix to transfer it in turn on the paper.
The process was invented in 1875 by Robert Barclay with regard to printing on tin; in 1904 it was adapted to printing on paper by Ira Washington Rubel[1]. Subsequently, it was perfected above all by the Germans and the English, to the point of gradually making obsolete the process, previously in use, of zinctype.
