For long runs (over 100,000)
it is often more cost-efficient to use web offset presses. Web offset presses,
as the name suggests transfer the image onto a moving web of paper. The
fastest modern sheet fed presses cannot print more than 17,000 impressions
per hour. Although this figure is reduced because when the pile of paper
reaches around 10,000 sheets the press has to be stopped whilst it is removed.
Printers often talk about presses being able to put 10,000 sheets per hour
"on the floor" for this reason.
Web presses can run in excess
of 30,000 impressions per hour. The term impression means each cycle of
the press cylinders, which on a B1 (28") sheetfed press can mean 8 pages
on both sides of the paper. On a web press this can mean 16 pages
at 30,000 per hour. Most newspapers are now printed web offset rather than
letterpress, as the durability of printing plates has increased dramatically
over recent years.
Other common printing processes
include Gravure, which is used for very long print runs of colour magazines.
The Gravure process uses large copper cylinders, which are etched from
photographic masters. Unlike letterpress the image is recessed into the
cylinder as a series of holes called cells. The larger the cell, the more
ink it contains and consequently the more ink (darker colour) is placed
on the paper. Because of the expense of manufacturing etched copper cylinders
Gravure tends only to be used for long run magazines or packaging. Gravure
is in a constant battle with web offset for long run magazine work. Gravure
cylinders last much longer than web offset litho plates.
Flexography is a kind of
letterpress that uses a raised image photographically created on hard wearing
photopolymer plates. It is used for packaging and newspaper applications.
Screen printing is used for
printing on fabrics and point of sale materials. Although it creates vivid,
solid colours the mechanics of the process mean that it is usually too
slow to compete with the other processes on anything other than short-run.
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