Personalized Invitations & Stationery; Offset Lithographic Printing

Alois Senefelder of Bohemia invented the ‘offset’ printing technique in 1798. This ‘offset’ printing technique is commonly known as lithography. The technology is used to print text or graphical images on paper or another suitable material used for printing. In Lithography, the ink is not applied straight from the printing plate or cylinder to the substrate, as it is in flexography and letterpressing. Ink is applied to the printing plate to form the ‘image’, such as text or artwork and then relocated or ‘offset’ to a rubber mantle. The figure on the mantle is then transferred to the substrate that is typically paper or paperboard, to produce the printed product.

Lithography follows the principle that oil and water do not combine, the hydrophilic and hydrophobic process. The plates are chemically treated and this results in the image area of the plate becoming oleophilic and ink-receptive and the non-image area becoming hydrophilic or water loving.
 
During the printing process, the dampening solution comprises of water, small quantities of isopropyl alcohol and other additives, to lower surface tension and to control pH. This solution is applied in a thin layer to the printing plate and migrates to the hydrophilic non-image areas of the plate. Ink is then applied to the plate that later migrates to the oleophilic image areas. Since ink and water do not blend, the fountain solution prevents the ink from moving towards the non-image areas of the plate.

Diazo, Photopolymer and Electrostatic are different types of litho plates. Most printers use one or two types of plates.
 
Diazo Plates

The Diazo plates are layered with an organic amalgam and are developed with a special solvent. They have a shelf life of a year. These are used for print process to create over 150,000 impressions.

Photopolymer plates

Photopolymer plates are coated with an organic amalgam that is very inert and resists abrasion and damage. This provides a longer shelf life than the diazo plates. They are used for print runs of over 250,000 impressions.

Electrostatic plates

These plates are based on the principle of the electrostatic copier. There are two types of plates, the inorganic photoconductors on a drum and the organic photo conductors on substrates. These are used mostly in quick printing jobs of 100,000 impressions or less.

There are three basic lithographic press designs, the unit-design, common impression cylinder design and the 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 plates and blankets sharing a common impression cylinder. A blanket-to-blanket press consists of two sets of plates and blanket cylinders, without an impression cylinder. The paper is printed on both sides simultaneously, as it passes between the two blanket cylinders. The inks used in Lithography are very viscous, like glue. To accelerate drying and control ink flow characteristics, litho inks contain solvents that result in some emissions from the ink. Linseed and rapeseed oil have been added to litho inks for years, however, vegetable oils like soybean oil are more frequently being used because of their lower soluble content that helps eliminate smudging.

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