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The
most simple explanation of the offset printing process that
I have come across is from Understanding Desktop Color
by Michael Kieran, published by Peachpit Press © in 1994.
Offset
lithography is a printing process whereby the image that
is to be printed is chemically etched
into a paper or metal plate,
inked and offset from the plate
to a rubber blanket, and then printed
from the blanket to the paper. This is done because
paper will mar the surface of the plate after a few thousand
impressions.
The image areas on the litho plate attract grease and repel
water, while the non-printing areas attract water and repel
grease. On press, the plate contacts the rollers that have
been wet by dampening solution, then the rollers wet by ink.
The non-image areas pick up the dampening solution and repel
ink. The image areas are are coated with ink, which is offset
onto the rubber impression blanket cylinder. The ink is transferred
to the paper as it passes between the blanket cylinder and
the impression cylinder.
The result—a major advantage of
the offset method—is that the soft rubber surface of
the blanket creates a crisp impression on a variety of papers
and other surfaces, both rough and smooth.
Each color that is to be printed requires a separate plate
and impression pass through the press. Because the
process is offset, the original image must be first output
as a negative (film), then etched onto the plate as a positive
and offset as a negative to the blanket cylinder, then printed
onto the paper as a positive.
There
are two different ways in which paper is fed into a press:
sheet-fed and web.
Sheet-fed:
Paper is cut into individual sheets prior to printing. Most
sheet-fed presses can print only one side of the sheet at
a time, although perfecting presses, or perfectors,
can print on both sides simultaneously.
The most popular sheet-fed presses have five or six inking
stations that can print up to five or six colors and varnishes
in one pass. Sheet-fed presses tend to make for crisper
images and are used for everything from one-color business
cards to fancy annual reports.
Web: Paper is supplied to the press in large reels,
and both sides are usually printed at the same time. Web
printing takes longer for make-ready (press preparation)
and is typically used only on larger jobs (50,000 impressions
or more), such as newspapers, magazines, catalogs, and direct
mail pieces. Some presses even have sophisticated binding
and trimming modules at the back end, capable of taking
12 or more 32-page signatures and folding, trimming, and
gluing or stapling them into finished magazines.
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