Offset Lithography
Offset Lithography is by far the
most common form of commercial printing.
The basic principle on which it works is
that oil and water do not mix. A litho printing plate has non-image
areas which absorb water. During printing the plate is kept
wet so that the ink, which is inherently greasy, is rejected
by the wet areas and adheres to the image areas.
Artwork is produced digitally with graphic
design software. An imagesetter is then used to produce films
(either positive or negative). When printing with more than
one colour there is a separated film for
each ink used. (See Four
Colour Process Printing). Each film is used to make a printing
plate by a photochemical process. The plate surface has non-image
areas which absorb moisture and repel ink.
The flexible plates, which can be made of
a variety of materials, are attached to the plate cylinder.
The plate is kept moist throughout so that ink only adheres
to image areas.
During every cycle of the press the ink
image is first transferred to a rubber surfaced blanket cylinder
and from there to the paper. This indirect method is the 'offset'
after which the process is named. The blanket cylinder's flexibility
both preserves the delicate plate and conforms to the surface
of textured papers.
After printing the sheets are taken for
finishing — trimming, folding and binding.
The press can either be fed with paper one
sheet at a time (Sheet fed) or from a large roll of paper (Web).
Web printing is normally reserved for large scale, long run
work such as magazines and catalogues.
The basic stages of the modern production
process are:
1 Original
artwork
— photographs, illustrations and text — are scanned
and entered into a computer.
2 These
elements are combined into a document using page makeup software.
3 Full
size films are output using a high-resolution imagesetter.
These could be either positives or negatives.
4 Printing
plates are made from the films using a photochemical process.
The plates are exposed to high-intensity light through the
films and then chemically treated so that non-image areas are
water absorbent.
5 The
flexible plates are attached to the plate cylinders of a litho
press and the job is printed.
This diagram shows the production process
for a colour job. See Four
Colour Process Printing for a more detailed explanation. |
