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Description | Applications
| Press Types
Description
The letterpress process is referred to as a "relief"
process because the printed image is produced from a plate in which the image
area is slightly raised above the non-image surface of the plate. It is a direct
printing method in that the inked plate applies the image directly to the substrate.
Letterpress is one of the oldest printing processes and was the most widely
used process until the middle of the 20th century when advances in other printing
processes made it obsolete. Flexography,
which is an updated version of letterpress, is now the dominate relief printing
process.
The letterpress process utilizes an ink that is thick in consistency
and is well suited for relief printing. A set of rollers deposits the ink on
the raised image area of the type or plate, but ink is not deposited on the
non-image areas. For this reason,
letterpress plates do not require any dampening in order to keep the
non-image areas free of ink. This makes the process a simple one and allows
for consistent results, but the process still cannot match the quality of more
sophisticated print processes.
Applications
Because of the popularity of other printing processes, letterpress
is a fast diminishing printing method. It is still used for publishing a few
small town newspapers, several types of labels, packaging materials, and some
narrow web printing.
An area where letterpress has greater usage than it does with
actual printing is with some finishing operations. Old letterpress equipment
is used for procedures such as embossing, die-cutting, numbering, perforating,
and foil stamping. The types of products that can be finished using letterpress
equipment include embossed business cards and government documents, die-cut
labels and folders, numbered tickets and membership cards, perforated mailers,
and foil stamped letterheads and invitations.
Press Types
There are four types of relief presses which are described
below:
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Platen Press: With platen press, movable metal type is
locked into a frame called a chase. The chase is then placed in
the press bed and it is also locked into position. During the
printing process, grippers move a sheet of paper from the feedboard,
which contains a stack of paper, to the platen, which is the surface
where the print impression is made. A set of rollers applies ink
to the type on the press bed and then the press bed and the platen
are pressed together like a clamshell which produces the image
on the paper. When the impression is complete, the platen and
the press bed spread apart and the grippers remove the paper,
placing it on a delivery tray.
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Flatbed Cylinder Press: The type or plate is locked in
a chase which is then mounted on the flatbed of the press. Grippers
on a rotating impression cylinder pick up a sheet of paper and
as the cylinder revolves, the paper is pulled around it. The inked
flatbed containing the letterpress plate then moves under the
impression cylinder. The squeeze between the impression cylinder
and the flatbed creates the printed impression on the paper. When
the impression is complete, the flatbed returns to its original
position and is inked for the next impression.
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- Rotary Sheet-Fed Press: The plate is mounted on a
cylinder where a roller system applies ink to the raised area of
the plate. The paper passes between the plate cylinder and an impression
cylinder where the resulting squeeze between the two cylinders produces
the printed impression on the paper.
- Rotary Web-Fed Press: The web-fed system also utilizes
a plate cylinder and impression cylinder, but instead of individual
sheets passing between the two cylinders, the paper is a continuous
web unwound from a large roll. After printing, the web is cut into
individual sheets. Web-fed presses are used for larger print runs.
Like other printing processes, the letterpress web-fed press usually
contains several printing units so that multiple colors can be printed
from a single pass.
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