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Applications - Process Overview - Image Preparation - Equipment Design - Flat-Bed Cylinder - Rotary
Typical products printed with letterpress printing processes include business cards, letterhead, proofs, billheads, forms, posters, announcements, imprinting, embossing and hot-leaf Stamping
Offset Letterpress Printing Process Overview
Letterpress is the oldest method of printing with equipment and images printed by the "relief" type printing plates where the image or printing areas are raised above the non-printing areas. The use of letterpresses is on the decline being replaced with faster and more efficient printing presses such as the offset lithographic press or the flexographic press. The amount of setup required to prepare the equipment to print a job is significant. For example, the image must be metal cast prior to print versus offset printing plates which are comparatively cheaper and require less time to make.
How letterpress works: Letterpress printing exerts variable amounts of pressure on the substrate dependent on the size and image elements in the printing. The amount of pressure per square inch or "squeeze" is greater on some highlight dots than it is on larger shadow dots. Expensive, time consuming adjustments must be made throughout the press run to make sure the impression pressure is just right. Major chemicals used in letterpress printing, very similar to those used in lithography, include film developers and fixers, inks, and blanket and roller washes (GATF 1992b).
Image Preparation of Letterpress Printing Plates
Letterpress printing uses type that is raised above (relief) the non-printing areas. In traditional letterpress work, letters were assembled into copy, explanatory cuts were placed nearby, line drawings were etched or engraved into plates, and all these were placed (composed) on a flat marble stone, within a rigid frame (chase) spaced artistically with blocks (furniture) tightened up (locked-up) with toothed angular blocks (quoins).More Information...
Letterpress Equipment Design
There are three different types of letterpress printing devices in use today: platen, flat-bed, and rotary presses. The two most common types of letterpress presses, the unit-design perfecting rotary press and the rotary letterpress typically used for magazine printing.
Rotary Letterpress Printing
There are two types of rotary letterpresses, sheet-fed and web-fed. Web-fed rotary presses are the most popular type of letter press printing. Sheetfed rotary presses are also declining in use; in fact these sheetfed rotary presses are no longer manufactured in the U.S. Like all rotary presses, rotary letterpress requires curved image carrying plates. The most popular types of plates used are stereotype, electrotype, and molded plastic or rubber. When printing on coated papers, rotary presses use heat-set inks and are equipped with dryers, usually the high-velocity hot air type.
Web-fed rotary letterpress presses are used primarily for printing newspapers. These presses are designed to print both sides of the web simultaneously. Typically, they can print up to four pages across the web; however, some of the new presses can print up to six pages across a 90-inch web. Rotary letterpress is also used for long-run commercial, packaging, book, and magazine printing.
Platen-type Letterpress Printing
A platen press is made up of two flat surfaces called the bed and the platen. The platen provides a smooth backing for the paper or other substrate that is to be printed. The raised plate (image to be printed) is locked onto a flat surface. The plate is inked, the substrate is then placed on another flat surface called the bed and pressed against the inked plate producing the impression.
The platen and bed carry both the paper and the type form. The press then opens and closes like a clam shell. Platen printing is typically used for short runs such as invitations, name cards, and stationary. Larger platen presses are used for die-cutting and embossing. Some platen presses are arranged with the bed and platen in the vertical plane.
The plate is inked with an inking roller that transfers ink from an inking plate to the image carrier. Ink is placed on the inking plate by an ink fountain roller. The platen style press has been widely used in printing small-town newspapers since the late 1800s. The printing area is usually limited to a maximum of 18 inches by 24 inches. These presses are also used to print letterhead, billheads, forms, posters, announcements, and many other types of printed products, as well as for imprinting, embossing, and hot-leaf stamping.
Flat-Bed Cylinder Letterpress Printing
Flat-bed cylinder presses use either vertical or horizontal beds. The plate is locked to a bed which passes over an inking roller and then against the substrate. The substrate passes around an impression cylinder on its way from the feed stack to the delivery stack. Another way of describing this is that a single revolution of the cylinder moves over the bed while in a vertical position so that both the bed holding the substrate and cylinder move up and down in a reciprocating motion. Ink is supplied to the plate cylinder by an inking roller and an ink fountain. The presses can print either one or two-color impressions. Flat-bed cylinder presses, which operate in a manner similar to the platen press, will print stock as large as 42 inches by 56 inches.
Flat-bed cylinder presses operate very slowly, having a production rate of not more than 5,000 impressions per hour. As a result, much of the printing formerly done on this type of press is now done using rotary letterpress or lithography. The horizontal bed press, the slower of the two types of flat-bed cylinder press, is no longer manufactured in the United States.