This basic overview of offset printing discusses sheetfed and web presses, prepress, inks, substrates, finishing, common applications, and trends.

1. What is offset printing?
This summer, we’re publishing a series of posts explaining different types of printing processes.
In this post, we review the ​basics of offset printing​ (also known as offset lithography).
Offset lithography is the primary method used to print longer runs of magazines, books,
manuals, brochures, direct-mail pieces, letterhead, posters, and paper-based labels and
packaging.
Because of the fixed costs involved with setting up each job for offset printing, the cost of each
copy in a 250-copy order will be higher than the cost of each piece in a 10,000-copy job.
How It Works
Offset lithography is a multi-step process that uses etched metal plates, water-repellent (e.g.
oil-based) inks, water, blanket cylinders, and impression cylinders.
The plates are created so that oily inks adhere only to the portions of the plate that contain the
images and text. A thin film of water over the blank areas of the plate repels the oil-based inks.
The ink on the plates is first transferred to a rubber-wrapped “blanket” cylinder. The ink
reaches the paper when the inked blanket cylinder rolls over the paper.
Many presses used impression cylinders to apply pressure to the paper as the ink is being
applied to the paper surface. Impression cylinders also help move the sheets through the
presses.
The offset printing process requires making separate plates and blanket rollers for each color of
ink that will be used.

2. Different variations of presses, plates, and inks are used for different run lengths and substrate
requirements. For example, some presses may print millions of copies of a magazine. Other
presses may print 250 large sheets that will be folded to make multi-page brochures.
Web Press​: These printers automatically feed wide rolls of paper from one ink station to
another until the appropriate about of ink is applied to each page. Web presses are typically
used to print 10,000 or more copies of multi-page documents such as magazines, books, and
catalogs.
In ​coldset web offset​, the ink is fixed by absorption into the paper.
In ​heatset web offse​t, ink is fixed by absorption into the paper plus evaporation at high
temperatures. Heatset web offset presses are four to five times faster than sheetfed presses
and typically include inline folding equipment.
Sheetfed Press​: This type of offset printing equipment feeds flat pre-cut sheets through the
press. Speeds range from 5,000 to 20,000 sheets per hour. Sheetfed presses can handle a wider
range of substrate types and thicknesses than web presses. Smaller sheetfed presses can
handle papers as small as 4 x 6 inches. Larger presses can handle sheets up to 40 in. x 26 in.

3. Larger sheet sizes are typically referred to as “parents” because each sheet holds multiple
pages of a book or manual. The page layouts on each parent sheet (“signatures”) may have 4, 8,
or 12 book pages on each side. The pages must be laid out in a proper “imposition” so that the
pages will appear in the proper sequence when the pages are trimmed, folded, and bound.
A ​Perfecting Press can print on both sides of the paper automatically. All web presses are
perfecting presses. But not all sheetfed presses are built to print on both sides automatically.
Prepress
Before a print job can go to an offset press the page designs created on a computer must be
separated by the types of inks that will be used to make up each page.
For years, a photo-engraving process exposed color-separated films onto photosensitive plates.
Today, ​computer-to-plate (CTP) systems use digitally controlled laser engraving systems to etch
the imaging data from digital files onto aluminum plates.
CTP systems have reduced the time and expense involved in setting up offset-printing jobs and
making last-minute revisions to the plates.
The prepress process for offset printing often includes more elaborate proofing methods, such
as imposition proofs that enable buyers to check that all of the pages of a published magazine
will appear in the desired sequence.
Offset Inks
A large variety of inks are available exclusively for use with different types of offset presses.
Colors​: In addition to the four process colors (cyan, magenta, yellow, and black), press
operators can use inks that match a client’s specific Pantone brand colors.
Formulations: Some offset inks use plant-derived oils such as soya oil or linseed oil to carry the
pigment. Other inks use water-repellant fluids made with synthetic chemical mixtures. .

4. Special Properties: Some offset inks are formulated to change the performance of the ink on
the paper. For instance, prints can be made with such as color-changing (thermochromatic)
inks, metallic inks, magnetic inks, or UV-curing inks. .
Polymer-based inks that cure when exposed to UV-LED light are popular because they enable
owners of sheetfed presses to make instantly dry prints. They don’t need to use heat or
spray-on powders to accelerate the on-press drying process.
With correctly formulated UV-LED inks and the proper UV-LED curing units, sheetfed offset
presses can print on paperboards for packaging, non-porous synthetic sheets, foil-laminated
sheets, plastic sheets, and a greater range of card and label stocks.
Substrates
Thanks to innovations in offset inks and substrates, offset printing can print on a wide range of
smooth and textured papers, paperboards, tear-resistant synthetic papers, and various plastics
used in gift cards, bags, packaging, and envelopes. Some of the specialty materials available for
offset printing include security papers, carbonless papers for business forms, and
metal-laminated papers with silver and gold foils.
Web offset presses primarily print on the inexpensive coated and uncoated papers needed to
control the costs of high volumes of books, manuals, magazines, and catalogs But these papers
must be strong enough to withstand roll-to-roll high-speed printing. Most of the specialty
materials are designed for use on sheetfed presses.
Finishing Processes
The type of finishing equipment used depends on the type and volumes of publications, direct
mail pieces, manuals, stationery, or packaging being produced.
Finishing equipment includes devices for trimming and cutting the paper, or folding, binding,
hole-punching, varnishing, laminating, or die-cutting printed sheets. Some printed sheets may
be foil stamped, perforated, embossed, glued, or inserted into envelopes for mailing. Some
finishing equipment makes it possible to apply clear UV-curable inks for added durability or
decorative effects.
Trends
As ​digital printing technologies have improved, so has the efficiency of offset printing.
Manufacturers of offset printing equipment have developed prepress processes, software, and
consumables that enable offset presses to produce more fast-turnaround, shorter-run print
jobs.
Press manufacturers such as Heidelberg are developing systems that allow more steps in the
offset printing and finishing process to be handled without any intervention by a human press
operator.
The next wave of major innovations for offset printing is likely to preview at the drupa global
print show June 16-26, 2020 in Dusseldorf, Germany.