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Letterpress Printing
Applications - Process Overview
- Image Preparation - Equipment Design
- Flat-Bed Cylinder - Rotary
Applications
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: Letter press 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.
More Information...
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.
Rotary Letterpress Printing
There are two type of rotary letterpresses, sheet-fed and web-fed.
Sheetfed rotary presses are also declining in use; in fact these presses
are no longer manufactured in the U.S. Web-fed rotary presses are the most
popular type of letter press printing.
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. |
