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Offset Lithography

The basic principle of offset printing, the dominant printing process, is this simple: ink and water don't mix. Early lithographers etched images onto flat stone. These images would accept ink, while the porous stone accepted water. When ink was applied, it stayed on the greasy image area and avoided the rest of the stone.

Modern lithography uses the same concept but adds one important element. In modern presses, the image is transferred from the printing plate to a rubber blanket and then to the paper. Hence the name "offset."

Although there are many different kinds, sizes and qualities of offset presses, the basic configuration remains the same. When the printing plate is exposed, an ink receptive coating is activated at the image area. On the press, the plate is dampened, first by water rollers, then by ink rollers. Ink adheres to the image area and water to the non-image area. As the cylinders rotate, the image is transferred to the blanket. Paper passes between the blanket cylinder and the image is transferred to the paper.

The major types of offset presses are sheetfed and web. In a sheetfed press, cut paper is delivered into the machine. In a web press, paper comes on a large roll and is cut after printing.

Sheetfed presses run the gamut in size and quality from 8-1/2" x 11" or 11" x 17" one-color duplicators found in the majority of quick printers and small commercial shops to high-quality eight-unit presses with sheet sizes as large as 55". Because web presses can print at very high speeds and large sheet sizes, they are generally used for forms, publications and very long runs.

Letterpress

Formerly the standard printing process, letterpress is now largely used for specialty work. Letterpress is often used for fine art prints, books and posters. Letterpress is also used for imprinting business cards and forms. In the process, the image area is actually raised above the rest of the plate, so the image makes a physical impression on the paper. Modern letterpress uses a photo-etching technique to remove the non-image area from the plate.

Flexo

Modern flexography, often called flexo, is a versatile process that uses photo-etched plates. The non-image areas on these plates are etched away, leaving only the printing surface that carries the ink directly to the substrate.

Technological advances in platemaking, inks and presses make flexo well-suited for any job that can be printed on a substrate that can be supplied in a roll form. In addition to paper, these materials include box boards, foils, plastics, newsprint and more. Common applications for flexo include labels, tabs, corrugated boxes, cartons and newspapers.

Gravure

Basically, gravure turns everything in the image into halftone dots. The plate cylinder consists of tiny cells, varying in depth and width, that hold the ink. As the press runs, a doctor blade scrapes excess ink off the surface of the plate, leaving ink only in cells. As the paper contacts the plate, the ink is transferred, reproducing type, rules, graphics, and photographs as composites of very fine dots. Gravure is used only in very long runs, usually for publications and packaging printing.

Screen Printing

Screen printing is the most versatile form of print reproduction. Although once thought of as being oriented to short production runs, modern high-speed technology allows for volume production where brilliant, accurate colors, and close tolerance are necessary. Ink is expressed through a stretched fabric mesh by a squeegee blade to reproduce the original image onto the substrate below. Screen printing is not limited to press size or the same of any substrate. A variety of materials such as paper, plastic, metal, fabric and glass can be screen printed.

Waterless Lithography

Print buyers are hearing more and more about waterless offset as the process becomes more commonly used in the United States. Almost any offset printing press can be converted to run waterless if it is retrofitted with a cooling system that, among other things, helps maintain the ink's temperature. Newer presses are available with the conventional dampening system and a cooling system so that they can run waterless. Special plates must be used so that the non-image area still repels the ink.

Key Concepts

Printing is more than reproducing words and images on paper. It is the physical experience itself - holding the piece, feeling the paper, the interplay of ink and paper - that printing is concerned with. As such, paper - the vehicle of the message, as well as the biggest cost item in a printing job - is of prime importance. Understanding paper and picking the right sheet for your job can make or break your job.

Papers are defined by grade and basic weight. Understanding these two concepts is the key to specifying the right paper.

This information was prepared by Printing Industries of Texas, whose website is www.piatexas.org. All copyrights reserved.