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Print Buyers Guide

Offset Lithography

The basic principle of offset printing, the dominant printing process today, is simple -- ink and water don't mix. Early lithographers created images on flat stones ("liths"). These images were made so that they would accept ink, while the non-image areas left the porous stone exposed, which would accept water. Once the stone was wet and ink was applied to the stone, it would only stick to the image area. When paper was laid on the surface and pressure applied, the inked image would clearly transfer.

Modern lithography uses the same concept but adds one important difference -- the image is transferred or "offset" from the inked surface (the plate) to a rubber blanket from which it is then transferred to the paper. Although there are many different kinds, sizes and qualities of offset presses, this basic process remains the same.

Following exposure of the offset lithography printing plate to intense light which passes through the clear areas of a film negative which correspond to the original image, an ink-receptive coating on its surface is activated during a chemical development process, precisely recreating the image. On the press, the plate first passes over a water roller which dampens the surface, then passes over an ink roller, where ink adheres to the image area only and to the dampened non-image area. As the plate continues on its path, it comes into contact with a blanket of rubber material, onto which the ink image transfers. The blanket cylinder then comes into contact with the paper onto which the ink image makes it final transfer.

The majority of offset lithography is sheet-fed followed by web. With a sheetfed press, cut paper is delivered to the blanket cylinder. In a web press, paper arrives at the blanket cylinder on a continuous roll and is cut after printing.

Sheetfed presses range in size and quality from 8.5" one-color duplicators to high quality eight-color presses with sheet sizes as large as 60". Web presses typically print at very high speeds, and are most appropriate for long commercial runs such as publications.

Waterless Lithography

Relatively new on the scene is waterless lithography promising superior quality. Special plates must be used so that the non-image area still repels the ink in the absence of water. Presses made for this process or retrofitted to accept it require a special cooling system for the ink, which will only perform properly at a reduced temperature.

Digital Printing

Recent developments in the output units of digital files include the use of toner technology, similar to color copiers, specially prepared inks, or plate imaging directly on special lithographic presses. Some of these processing units, generally referred to as digital printing, offer sheet to sheet personalization of images and text.

Another form of digital printing which is gaining popularity is large format plotters that can deliver printed sheets from continuous rolls for use in outdoor advertising and commercial display in small quantities.

Letterpress

Formerly the standard printing process, this is now used for specialty work such as numbering, imprinting, diecutting, hot stamping and embossing. The letterpress process is still used for fine art prints, limited edition books and posters. Images and type are raised above the surface of the non-printing areas on a metal plate, are inked and printed directly onto paper.

Engraving

Typically used for printing securities, currency, fine stationery and invitations. The image is acid-etched into a metal plate called a die. This plate is covered with ink, and the top plane, which is the non-printing area, is wiped clean. A handmade counter die pushes the substrate onto the remaining ink in the image area of the die, transferring the image leaving a telltale impression on the reverse side of the sheet and, along with the results of a quick-drying ink, a raised image on the front.

Flexography

Often called "flexo," this is a versatile process that uses molded rubber or photo-polymer plates which carry images and type, raised in a manner similar to letterpress, that are inked and printed directly on the substrate.

Advances in all parts of this process make flexo well suited to any substrate which can be supplied in rolls including paper, plastic films, box boards, and newsprint. Common uses are for labels, tags, envelopes, cartons, packaging and newspapers.

Gravure

In this process, tiny cells are etched into a printing cylinder, including a screen out of all images and type. These cells vary in depth and width and are on a plane below that of the non-printing surface. The cylinder rotates through a bath of ink and the non-printing areas are then cleaned of ink by a blade before the still-inked images are directly applied to the substrate. Gravure is used for long runs including publications and packaging.

Screen Printing

Formerly called silk screen printing, due to one of the fabrics originally used to hold the printing images in suspension, this process is generally used for relatively short runs. However, modern automatic equipment has significantly increased run lengths and consistency of the quality. Because the process prints a thick layer of ink, it is ideal for use in generating colors which are opaque and brilliant, including fluorescent inks which, for optimum appearance, require a lot of pigment to be deposited. The image is suspended within a screen of fabric or fine mesh wire stretched on a wooden or metal frame. Ink is placed on the screen away from the substrate and forced, by means of a rubber-edged squeegee, through the open image areas onto the substrate.

Some screen presses can print very large images on almost any material, often used for point-of-sale displays, exhibits, posters, and even dimensional objects such as glassware and containers.

Thermography

By way of an inline attachment to the offset lithography press, this process deposits a plastic resin on the wet ink and heats it, resulting in a raised image, originally developed to simulate the appearance of dried engraving ink.

If using thermography as a substitute for engraving, be aware of some pitfalls. Thermography is produced using heat. If your thermographed piece is going to be used in a laser printer, copier, etc., the ink will "run." There are special powders created to eliminate this problem.

You should advise your printer if your produce will be used in any heat based equipment.

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