Print Production - Offset Lithography

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:

• General commercial printing Quick printing
• Newspapers Books
• Business Forms Financial and Legal Documents
• 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.

Before the job can be printed, the document must be converted to film and "plates." There are different materials for plates, including paper (which produces a lower-quality product). The best plate material is aluminum, which is more costly.

Each of the primary colors -- black, cyan (blue), magenta (red), and yellow -- has a separate plate. Even though you see many, many colors in the finished product, only these four colors are used (you'll also hear this called the four-color printing process

The image area of the plate picks up ink from the ink rollers. The water rollers keep the ink off of the non-image areas of the plate. Each plate then transfers its image to a rubber blanket that in turn transfers the image to the paper. The plate itself does not actually touch the paper -- thus the term "offset" lithography. All of this occurs at an extremely high speed.

The paper is left slightly wet by all of the ink and water being applied. Obviously, there is a risk of the ink smudging. The smudging is avoided by having the paper pass through an oven. The oven is gas fired, and the temperature inside runs at 350 to 400 degrees Fahrenheit (176 to 206 degrees Celsius).

Color and Registration Control

Immediately after leaving the oven, the paper is run through a short series of large metal rollers that have refrigerated water flowing through them. These chill rollerscool the paper down instantly and set the ink into the paper. If this were not done, the ink would rub off on your fingers.

Color and registration control is a process that is aided by the use of computers. Registration is the alignment of the printing plates as they apply their respective color portion of the image that is being printed. If the plates do not line up perfectly, the image will appear out of focus and the color will be wrong. A computer takes a video image of registration marks that have been placed on the press sheet. Each plate has its own individual mark. The computer reads each of these marks and makes adjustments to the position of each plate in order to achieve perfect alignment. All of this occurs many times per second while the press is running at full speed.

Color control is a process that involves the way in which the ink blends together, and is tied closely to the plate registration. The amount of ink that is released into the units depends on how much ink is needed to achieve a desired look. The ink is adjusted via the control panel that is part of the overall control console. Prior to being placed on the press, the plates are scanned and the data is then transferred to a micro cassette. This serves as the "master" that directs the release of ink to pre-set values.

Inks used

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.

sources http://computer.howstuffworks.com/offset-printing3.htm
http://www.pneac.org/printprocesses/lithography/