Offset lithography is the most widely used print process. About 40% of all print jobs are produced with offset printing. It is an indirect printing process which means that an image is transferred, or offset, from one surface to another. A printing plate mounted on a cylinder transfers the image to a rubber blanket mounted on another cylinder. The image is then transferred from the blanket cylinder to the substrate as the substrate passes between the blanket cylinder and an impression cylinder. The image on the plate is “right reading” and when the image is transferred to the blanket it becomes “wrong reading”. When the image is transferred to the printing surface it becomes right reading again.
The image area and non-image area of the offset plate are on the same plane and work on the principle that oil and water do not mix. The non-image areas of the plate attract a wetting agent (fountain solution) and repel ink made from an oil base. The image areas attract the ink and repel the fountain solution.
The types of printed materials that can be produced with offset lithography are numerous and varied. Some of the items include: newspapers, magazines, books, continuous business forms, unit sets, advertising pieces, brochures, posters, greeting cards, business cards, folders, mailers, laser sheets, integrated products, coupons, and art reproductions.
Offset presses can be put into two categories: sheet-fed and web-fed.
Sheet-fed: A sheet-fed press prints an image on single sheets of paper as they are fed individually into the press. The print quality and sheet to sheet registration is often better than web-fed printing, but it is often more economical to produce very large runs on web presses because of their higher running speeds.
Sheet-fed presses can be divided into three categories: small, medium, and large sheet presses.
- Small Sheet-fed: The small sheet-fed press can print sheets up to 14″ x 17″. They are used primarily for short runs of one or two colors for such items as business forms, letterheads, and business cards and are popular for instant print companies.
- Medium Sheet-fed: Sheet sizes of up to 25″ x 38″ can be printed on a medium sheet-fed press. The presses are used for runs up to 20,000 and are common equipment for many medium and large printers. Products such as brochures, business forms, medium press runs of color work are produced with the mid-size press.
- Large Sheet-fed: The largest runs (usually 100,000 or more) and the most complex jobs are reserved for the large format sheet-fed presses. They can accommodate a paper size of up to 49″ x 74″ and they may have several printing towers so that multiple colors can be printed with one pass.
Web-fed: A web-fed press prints images on a continuous web of paper fed into the press from a large roll of paper. The web of paper is then cut into individual sheets after printing or as with continuous business form applications, it is left in web form and is perforated for later separation into individual sheets.
Like sheet-fed presses, web-fed presses come in many types and sizes. Some smaller web presses are capable of printing only on narrow width paper rolls and can only print one or two colors on the front side of the paper. Other web presses can handle large width webs and can print on the front and the back side of the paper in one pass through the press. There may be 8 or more printing units so that applications requiring full color on the front and back can be printed.