Print & Copy Bureau
pcb@leeds.ac.uk
FACT SHEET SERIES
OFFSET PRINTING
HIGH QUALITY PRINTING = OFFSET PRINTING
Offset printing is the most common form of high-volume commercial printing due to advantages in quality
and efficiency in high-volume jobs. The more you print, the less you pay per page because most of the price
goes into the `set up` of the printing press, before the first sheet of paper is printed. Any ongoing printing
will only cost the client the substrate and ink price, which is very minimal. While modern digital presses are
getting closer to the cost/benefit of offset for high-quality work, they have not yet been able to compete
with the sheer volume and efficiency of product that an offset press can produce. Traditional printing
plates required the preparation of a negative film, however, many modern offset presses use computer to
plate systems which requires no intermediate process between computer design and the preparation of
printing plates.
THE OFFSET LITHOGRAPHIC PRESS
Printing ink is supplied to the inking rollers via a reservoir (duct). Greater and lesser amounts of ink can be
fed to the inking rollers by opening and closing duct keys. The inking rollers transfer the required amount of
ink to the printing plate. The printing image is then transferred (or offset) from the plate to the `blanket`
and then to the paper on the impression cylinder. The printed sheet is then passed into the next printing
unit via the transfer cylinder. Each colour must have a separate printing unit.
The printing plate is made from aluminium and backed with a sensitive coating. The plate has two distinct
areas; the image area and non image area. Ink is accepted by the image areas of the plate while the nonimage areas accept a water based mix called a damper solution. When applied to the plate the solution
repels ink from the non-image area.
Offset Printing Unit
Side view of the offset printing process
The most common kind of offset printing is derived from the photo offset process, which involves using
light-sensitive chemicals and photographic techniques to transfer images and type from original materials
to printing plates.
In current use, original materials may be an actual photographic print and typeset text. However, it is more
common — with the prevalence of computers and digital images — that the source material exists only as
data in a digital publishing system.
Offset litho printing on to a web (reel) of printing substrate is commonly used for the printing of
newspapers and magazines in high speed production.
TYPES OF PAPER FEED
Sheet-fed litho
"Sheet-fed" refers to individual sheets of paper or paperboard being fed into a press. A lithographic ("litho"
for short) press uses the principles of lithography to apply ink to a printing plate. Sheet-fed litho is
commonly used for printing of short-run magazines, brochures, letter headings, and general commercial
(jobbing) printing.
Web-fed litho
"Web-fed" refers to the use of rolls (or "webs") of paper supplied to the printing press. Offset web printing
is generally used for runs in excess of 50 thousand impressions. Typical examples of web printing include
newspapers, newspaper inserts/ads, magazines, catalogs, and books. Web-fed presses are divided into two
general classes: "Cold" or "Non-Heatset," and "Heatset" offset web presses, the difference being how the
inks that are used dry. Cold web offset printing is air dried, while heatset utilizes drying lamps or heaters to
cure or "set" the inks. Heatset presses can print on both coated (slick) and uncoated papers, while coldset
presses are restricted to uncoated paper stock, such as newsprint. Some coldset web presses can be fitted
with heat dryers, or ultraviolet lamps (for use with uv-curing inks).
THE PRINTING PROCESS
Full Colour Images
A full colour halftone image requires four colours to be printed, Cyan, Magenta, Yellow and Black. In
addition there may be a requirement for special colours and sealer varnish.
Duotone Images
Duotone are images that are produced using 2 colours and provide greater subtlety than can be achieved
by single colour halftone printing.
Process & Spot Colour Working:
There are two methods of colour reproduction commonly used: process colour and spot colour. The
difference between these two methods is the number of colour separations required to reproduce each
colour.
Process Colour Printing
Process colour printing uses four inks: cyan (blue), magenta (red), yellow and black. With these four colours
it is possible to reproduce the whole colour spectrum.
Spot Colour
Spot colour printing uses a different ink for each colour – with each one given its own individual printing
plate. This process is most often used when process colour does not give a precise colour match. This
method also reduces colour variation when printing over a large area.
However, for each additional colour there is the additional cost of the extra printing plate, ink and making
the job ready to run. Spot colours are usually identified as a Pantone colour.
Print & Copy Bureau Roger Stevens Building University of Leeds LS2 9J T 0113 343 2668