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Offset Lithographic Printing
Lithographic printing is well suited for printing both text and illustrations in short to medium length runs of up to 1,000,000 impressions. Typical products printed with offset printing processes include:
Offset Lithographic Printing Process Overview
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. Ink is applied to the printing plate to form the "image" (such as text or artwork to be printed) and 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.
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. There is a total of 3 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.
Offset Lithographic Printing Process
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.
As the plate cylinder rotates, the plate comes in contact with the dampening rollers first. The dampening rollers wet the plate so the non-printing areas repel ink. Then the inking rollers transfer ink to the dampened plate, where ink only adheres to the image areas. The inked image is transferred to the rubber blanket, and the substrate is printed as it passes between the blanket and impression cylinder.
There are three basic lithographic press designs: unit-design, common impression cylinder design, and blanket-to-blanket design. The unit-design press is a self-contained printing station consisting of a plate cylinder, a blanket cylinder, and an impression cylinder. Two or more stations may be joined to perform multi-color printing. A common impression cylinder press consists of two or more sets of plate and blanket cylinders sharing a common impression cylinder. This allows two or more colors to be printed at a single station. A blanket-to-blanket press consists of two sets of plate and blanket cylinders without an impression cylinder. The paper is printed on both sides simultaneously as it passes between the two blanket cylinders (Field).
The major unit operations in a lithographic printing operation include:
Image preparation begins with camera-ready (mechanical) art/copy or electronically produced art supplied by the customer. Images are captured for printing by camera, scanner or computer. Components of the image are manually assembled and positioned in a printing flat when a camera is used. This process is called stripping. When art/copy is scanned or digitally captured the image is assembled by the computer with special software. A simple proof (brown print) is prepared to check for position and accuracy. When color is involved, a color proof is submitted to the customer for approval.
Processing of Lithographic Printing Plates
There are eight different types of litho plates common to the commercial printing industry: Diazo, Photopolymer, Silver Halide, Electrophotographic (Electrostatic), Bimetal, Waterless, Thermal, and Ablation. The predominant surface plate in use today is termed a "presensitized" plate. Most printers will primarily use one or two types of plates. It is highly unlikely that you would encounter a printer that could use a few of each type of plate nor is it easy for them to switch to a different type of plate due to equipment, expense and application reasons.
Diazo plates are coated with organic compounds and are developed with a special solvent. They have a shelf life of about one year. These are used for print runs of about 150,000 impressions.
Photopolymer plates are coated with organic compounds which are very inert and abrasion resistant. This makes them last much longer than diazo plates. They are used for print runs of up to 250,000 impressions.
Silver halide plates use photosensitive coatings similar to photographic film, except that the silver halide emulsions are slower and for color reproduction are coated on anodized aluminum. The processing solutions contain silver which must be recovered with the proper equipment before being discharged to the sewer. Film based silver halide plates are used for single color printing and metal based silver halide plates are used in computer-to-plate systems.
Electrostatic plates are based on the principle of the electrostatic copier. There are two types, inorganic photoconductors on a drum and the second is organic photo conductor on a substrate. These are used mostly in quick printing jobs of 100,000 impressions or less.
Bimetal plates use presensitized polymer coatings. There are two types of bimetal plates; copper plated on stainless steel or aluminum and chromium plated on copper. These are the most expensive, but rugged plates and are used for very long print runs. In fact they are capable of print runs in the millions.
Waterless plates, used on waterless presses only, consist of ink on aluminum for the printing areas and a silicone rubber for the non-image areas. These systems require special inks and high grade paper to avoid debris accumulating on the blanket.
Ablation plates are imaged by digital data and requires no chemical processing. These plates are digitally imaged by selectively burning tiny holes in thin coatings of a polyester or metal base. These types of plate are used on the new computer to plate imaging systems and the brand new computer to press system. The cost of equipment and materials is high and the technology is relatively new.
Heat sensitive plates are exposed by infrared diodes in special imagesetter and processed in water based chemistry. This a relatively new technology and requires the printer to invest in new equipment that can be quite costly.
Offset Lithographic Inks:
There are four common types Lithographic inks, unlike Gravure, Flexo, and Screen are very viscous to the point they are paste-like. Litho inks are generally very strong in color value to compensate for the lesser amount applied. Sheetfed litho inks are similar to oxidizing types of letterpress inks. To accelerate drying and control ink flow characteristics litho inks contain solvents (or drying oils) which result in some VOC emissions from the ink.
Linseed and rapeseed (canola) oil have been added to litho inks for years, but other vegetable oils like soybean oil are more frequently being used because because of their lower VOC content, which helps eliminate smudging.
Heatset Inks are completely different from non-heatset inks and cannot be interchanged between the two types of presses. Heatset inks are quick drying inks for web printing. The solvents are vaporized as they pass through resins fixed to the paper in such a way that there is no chance for the ink to spread, smear, or soak into the paper. Heatset presses are equipped with a drier, and a chilling system to cool the heated resins and set the image. Heatset inks emit a significantly greater amount of VOC as compared to non-heatset lithographic inks. Therefore most heatset presses are also equipped with pollution control equipment such as a thermal oxidizer or after burner to destroy the high volumes of VOCs that are being emitted from these inks.
Ultraviolet (UV) and Electron Beam (EB) curable inks are also available for litho printing, but the press must be properly equipped to run these types of inks. The use of UV curable inks is on the rise, particularly for the application of overprint coatings.
One advantage of low VOC content is the ability to operate presses at comparable speeds to conventional inks, versus the slow drying and slow press speeds associated with water-based coatings.
One disadvantage is equipment can be costly and is still in the development stage, and the inks and coatings may cost as much as three times the price of conventional coatings.
Electron beam curing inks make a good alternative to U.V. inks because they are less costly and less reactive materials can be used. They also require less energy than U.V. curing inks. The down side of E.B. curing inks is the capital costs to outfit a press. Additionally, EB inks, like UV inks, can be a skin irritant. The inks, if exposed to sensitive skin or left on skin, may cause dermatitis and could even cause chemical burns.
Reprinted with permission from pneac.org/.