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Commercial
Colour Printing: Part
1 | Part 2
Contributor:
Bob Swinbank
Bob Swinbank of Cardinal Publicity continues
his overview of the printing process by explaining
the fundamentals of Offset Lithography,
and The Five Stages of Print Production.
To view the first part of this extended article
go to The
Four Colour Process.
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
Five Stages of Print Production
The basic stages of the modern print 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.
Graphical Representation
of the Print Production Process
The following images represents the five stages
of print production:
Commercial
Colour Printing: Part
1 | Part 2
This article has been reproduced under permission
and may not be copied without explicit consent from
the author Bob Swingbank © 2003.
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