Lithography

Lithography is a method for printing on a smooth surface, as well as a method of manufacturing semiconductor and MEMS devices.

Printing

The principle

Lithography as a manual process is based on the repulsion of oil and water.

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The image is placed on the surface with an oil-based medium; acid is then used to 'burn' the oil into the surface.

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When printing, the surface is covered in water, which remains on the non-oily surface and avoids the oily parts; a roller can then apply an oil-based ink that adheres only to the oily portion of the surface.

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The early process

Lithography was invented by Alois Senefelder in Bohemia in 1798, and it was the first new printing process since the invention of relief printing in the fifteenth century. In the early days of lithography, a smooth piece of limestone was used (hence the name "lithography"—"lithos" is the ancient Greek word for stone). After the oil-based image was put on the surface, acid burned the image onto the surface; gum arabic, a water soluble solution, was then applied, sticking only to the non-oily surface and sealing it.

Related Topics:
Alois Senefelder - Bohemia - 1798 - Relief print - Limestone - Gum arabic

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During printing, water adhered to the gum arabic surfaces and avoided the oily parts, while the oily ink used for printing did the opposite.

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Within a few years of its invention, the lithographic process was used to create multi-color printed images, a process known by the middle of the 19th Century as Chromolithography. Many fine works of chromolithographic printing were produced in America and Europe during this period. A separate stone was used for each colour, and a print went through the press separately for each stone. The main challenge was of course to keep the images aligned (in register).

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See: THE INVENTION OF LITHOGRAPHY, by Alois Senefelder, (Eng. trans. 1911)

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The modern process

In modern high-volume lithography, which depends on photographic processes, flexible aluminum or plastic printing plates are used in place of stone tablets. Modern printing plates have a brushed or roughened texture and are covered with a photosensitive emulsion. A photographic negative of the desired image is placed in contact with the emulsion and the plate is exposed to light. After development, the emulsion shows a reverse of the negative image, which is thus a duplicate of the original (positive) image. The plate is then chemically treated so the positive image is receptive to printing inks. The plate is affixed to a drum on a printing press. Rollers apply water, which covers the blank portions of the plate, and ink, which adheres to the positive image areas--such as the type and photographs on a newspaper page.

Related Topics:
Aluminum - Printing plate - Photosensitive - Emulsion

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If this image were directly transferred to paper, it would create a positive image, but the paper would become too wet. Instead, the plate rolls against a drum covered with a rubber blanket, which squeezes away the water and picks up the ink. The paper rolls across the blanket drum and the image is transferred to the paper. Because the image is first transferred, or offset to the rubber drum, this reproduction method is known as offset lithography or offset printing.

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Many innovations and technical refinements have been made in printing process and presses over the years, including the development of presses with multiple units (each containing one printing plate) that can print multi-color images in one pass, and presses that accommodate continuous rolls (webs) of paper, known as web presses. Another innovation was the Dahlgren inking system, which combined the wetting and inking steps into one.

Related Topics:
Presses - Dahlgren inking system

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The advent of desktop publishing made it possible for type and images to be manipulated easily on personal computers for eventual printing on desktop or commercial presses. The development of digital imagesetters enabled print shops to produce negatives for platemaking directly from digital input, skipping the intermediate step of photographing an actual page layout. The development of the digital platesetter in the late Twentieth century eliminated film negatives altogether by exposing printing plates directly from digital input.

Related Topics:
Desktop publishing - Imagesetters - Platesetter

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