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All offset presses have three printing cylinders, as well as the inking and dampening systems. The plate cylinder, the blanket cylinder and the impression cylinder.
Lithography uses a planographic plate, a type of plate on which the image areas are neither raised nor indented (depressed) in relation to the non-image areas. Instead the image and non-image areas, both on essentially the same plane of the printing plate, are defined by deferring physiochemical properties.
Lithography is based on the principal that oil and water do not mix
(hydrophilic and hydrophobic process). Lithographic plates undergo
chemical treatment that render the image area of the plate oleophilic
(oil-loving) and therefore ink-receptive and the non-image area
hydrophilic (water-loving). During printing, fountain (dampening)
solution, which consists primarily of water with small quantities of
isopropyl alcohol and other additives to lower surface tension and control
pH, is first applied in a thin layer to the printing plate and migrates to
the hydrophilic non-image areas of the printing plate. Ink is then applied
to the plate and migrates to the oleophilic image areas. Since the ink and
water essentially do not mix, the fountain solution prevents ink from
migrating to the non-image areas of the plate.
Traditionally, isopropyl alcohol was used to control surface tension in
the fountain solution, but in recent years it’s use has been reduced,
and in many cases eliminated by using alcohol substitutes. The reason for
this shift is due to the VOC emissions attributed to the evaporation of
isopropyl alcohol and the level of environmental regulation this lead to.
Alcohol substitutes may use glycol ethers such as butyl cellosolve
(2-butoxy ethanol) or other glycols to control surface tension.