Litho printing works on the principle that grease on the image area's attract ink, whilst water, on the non-image area's repels it.
The hardened image area's of the plate attract the oil based ink, and the non-image area's, acting like a very thin sponge attract water.
On the press, the plate, wrapped round a cylinder, comes into contact with three sets of rollers. The first one dampens the non-image areas on the plate, the second applies the ink. The ink is accepted by the image areas and repelled by the dampened non-image areas. The ink on the image areas is then offset onto the 'blanket' cylinder which is made from rubber. The image is then transferred from the blanket to the paper.
Offset lithography is a process used for printing on a flat surface, using printing plates. An image is transferred to a printing plate, which can be made of a variety of materials such as metal or paper. The plate is then chemically treated so that only image areas (such as type, colors, shapes and other elements) will accept ink. Water and ink is applied to the plate. Because of the chemical treatment, ink only "sticks" to the image areas, which reject the water. Areas without images reject the ink. The plate is then rolled onto a rubber cylinder applying the inked area, and in turn the rubber cylinder (or "blanket") applies the image to the paper. The system is "offset" because the plate does not come in direct contact with the paper, which preserves the quality of the plate. (about.com)
Web Offset
Web presses are mainly used for printing magazines and other similar publications. Paper is fed through the press from a continuous roll or 'web' of paper. Web presses can print up to 40,000, 32 page sections per hour. The paper is automatically cut folded and stacked into sections as it is printed.
Gravure
The plates have an indented image which holds the ink. The image is achieved by engraving or etching. Spirit inks are used which dry very quickly. This is ideal for fast running high volume print runs such as magazines, packaging, wall paper and stamps. It can also be used for the reproduction of fine art images.
Screen Printing
Stencils can be produced photographically or by hand. One colour is printed at a time and much of it is done on hand operated presses. The main advantage of screen printing is its versatility. It can be used to print on almost any material, including wood, glass, metal, plastics and fabrics.
Flexography
Flexography is a form of printing process which utilizes a flexible relief plate. It is essentially a modern version of letterpress which can be used for printing on almost any type of substrate, including plastic, metallic films, cellophane, and paper. It is widely used for printing on the non-porous substrates required for various types of food packaging (it is also well suited for printing large areas of solid color).
Pad Printing
Pad printing is a printing process that can transfer a 2-D image onto a 3-D object. This is accomplished using an indirect offset (gravure) printing process that involves an image being transferred from the cliché via a silicone pad onto a substrate. Pad printing is used for printing on otherwise impossible products in many industries including medical, automotive, promotional, apparel, and electronic objects, as well as appliances, sports equipment and toys.
Ref - The Spy Guide to Design and Print, 655.31 Leeds College of Art, Blenheim Walk
wikipedia
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