Lithography is an “offset” printing technique. Ink is not applied directly from the printing plate (or cylinder) to the substrate as it is in gravure, flexography, and letterpress. Rather, it is applied to the printing plate to form the “image” (such as text or artwork to be printed), then transferred or “offset" to a rubber “blanket". The image on the blanket is then transferred to the substrate (typically paper or paperboard) to produce the printed product.
There are three types of offset printing: non-heatset sheetfed, heatset, and non-heatset web offset. The difference between heatset and non-heatset is primarily dependent on the type of ink and how it is dried. On sheet-fed presses, the substrate is fed into the press one sheet at a time at a very high speed. Web fed presses print on a continuous roll of substrate, or web, which is later cut to size.
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 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 principle 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 ink-receptive, The non-image area is 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. It 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. Because the ink and water 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 its use has been reduced, and in many cases eliminated, by using alcohol substitutes. The reason for this shift is due to regulation of the VOC emissions attributed to the evaporation of isopropyl alcohol. Alcohol substitutes may include glycol ethers such as butyl cellosolve (2-butoxy ethanol) or other lo glycols to control surface tension.
The major unit operations in a lithographic printing operation include:
The sheet-fed offset process is used mainly for relatively short runs in the production of commercial and packaging products. In sheet-fed lithography, the paper or paperboard substrate is normally delivered to the facility in sheets. If rolls are supplied, the paper or paperboard must be sheeted (cut into sheets and trimmed before printing).
The heatset web offset process is used primarily for long jobs at high speed (up to 40,000 impressions per hour) for the production of magazines, other periodicals, and catalogs.
In heatset web lithography, the paper substrate is delivered to the facility in rolls. The paper is fed directly into the press and is termed a “web” because it is a continuous feed of paper, rather than individual sheets. After printing, the paper is folded and/or cut "in-line” with the printing units.
Heatset web lithographic inks are pastes that dry through evaporation of the ink oils. This is usually accomplished with a recirculating hot air system normally fueled by natural gas although direct flame impingement and infrared drying systems are in limited use (Buonicore).
Ink oil that evaoporates and is released through dryer stacks is a potentially significant source of VOC emissions. Because of this, many heatset web lithographic presses require an emission control device, such as a catalytic or thermal oxidizer, to reduce VOC concentrations in the dryer exhaust air stream.
VOC emissions also occur from isopropyl alcohol used in fountain solution and cleanup solvents used to clean ink fountains (trays that hold ink), rollers, blankets, and other press components. Major chemicals used are similar to those used in sheetfed offset.
The non-heatset web offset printing is a high-speed process most often used for the production of newspapers, journals, directories, and forms.
In non-heatset web lithography, the paper or paperboard substrate is delivered to the facility in rolls. The paper is fed directly into the press from the roll and is termed a “web” because it is a continuous feed of paper, rather than individual sheets. After printing, the paper is folded and/or cut “in-line” with the printing units.
Non-heatset lithographic inks are pastes that dry by oxidative polymerization and adsorption into the substrate, rather than evaporation. They typically do not require mechanical drying, so there are fewer VOC emissions generated from this process than from heatset web offset.
Dampening and inking systems, which include dampening chemistry and ink formulations, differ significantly from heatset web offset. The other major chemicals used in this process, such as fountain solution cleaning solvents, etc. are quite similar to those used in heatset web offset.
After printing, the substrate may run through a number of operations to be “finished and ready for shipment to the customer. Finishing may include operations such as coating, cutting, folding, binding, stitching, embossing, and die cutting.