Printing Technology
Lithography
Lithography is a printing process that uses chemical processes to create an image. For instance, the positive part of an image would be a hydrophobic chemical, while the negative image would be water. Thus, when the plate is introduced to a compatible ink and water mixture, the ink will adhere to the positive image and the water will clean the negative image. This allows for a relatively flat print plate which allows for much longer runs than the older physical methods of imaging (e.g., embossing or engraving).
The chemical process
Lithography works because of the repulsion of oil and water. The image is drawn on the surface of the print plate with an oil-based medium (hydrophobic). The range of oil-based mediums is endless, but the dexterity of the image relies on the lipid content of the material being used--its ability to withstand water and acid. Following the placement of the image is the application of an acid emulsified with gum arabic. The function of this emulsion is to create a salt layer directly around the image area. The salt layer seeps into the pores of the stone, completely enveloping the original image. This process is called etching. Using lithographic turpentine, the printer then removes the greasy drawing material, leaving only the salt layer; it is this salt layer which holds the skeleton of the image's original form. When printing, the stone or plate is kept wet with water. Naturally the water is attracted to the layer of salt created by the acid wash. Ink that bears a high lipid content is then rolled over the surface. The water repels the grease in the ink and the only place for it to go is the cavity left by the original drawing material. When the cavity is sufficiently full, the stone and paper are run through a press which applies even pressure over the surface, transferring the ink to the paper and off the stone.
The modern process
Modern high-volume lithography is used to produce posters, maps, books, newspapers, and packaging —just about any smooth, mass-produced item with print on it.
In this form of 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 image on the plate emulsion can also be created through direct laser imaging in a CTP (Computer-To-Plate) device called a platesetter. The positive image is the emulsion that remains after imaging. For many years, chemicals have been used to remove the non-image emulsion, but now plates are available that do not require chemical processing.
The plate is affixed to a drum on a printing press. Rollers apply water, which covers the blank portions of the plate but is repelled by the emulsion of the image area. Ink, applied by other rollers, is repelled by the water and only adheres to the emulsion of the image area--such as the type and photographs on a newspaper page.
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.
Many innovations and technical refinements have been made in printing processes 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 on both sides of the sheet, and presses that accommodate continuous rolls (webs) of paper, known as web presses. Another innovation was the continuous dampening system first introduced by Dahlgren. This increased control over the water flow to the plate and allowed for better ink and water balance. Current dampening systems include a "delta effect" which slows the roller in contact with the plate, thus creating a sweeping movement over the ink image to clean impurities known as "hickies".
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, a process known as computer to plate printing.
Microlithography and nanolithography
Microlithography and nanolithography refer specifically to lithographic patterning methods capable of structuring material on a fine scale. Typically features smaller than 10 micrometers are considered microlithographic, and features smaller than 100 nanometers are considered nanolithographic. Photolithography is one of these methods, often applied to semiconductor manufacturing of microchips. Photolithography is also commonly used in fabricating MEMS devices. Photolithography generally uses a pre-fabricated photomask or reticle as a master from which the final pattern is derived.
Although photolithographic technology is the most commercially advanced form of nanolithography, other techniques are also used. Some, for example electron beam lithography, are capable of much higher patterning resolution (sometime as small as a few nanometers). Electron beam lithography is also commercially important, primarily for its use in the manufacture of photomasks. Electron beam lithography as it is usually practiced is a form of maskless lithography, in that no mask is required to generate the final pattern. Instead the final pattern is created directly from a digital representation on a computer, by controlling an electron beam as it scans across a resist-coated substrate.
In addition to these commercially well-established techniques, a large number of promising microlithographic and nanolithographic technologies exist or are emerging, including nanoimprint lithography, interference lithography, X-ray lithography, extreme ultraviolet lithography, and scanning probe lithography. Some of these emerging techniques have been used successfully in small-scale commercial and important research applications.
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the negative lithographic stone