Offset printing is a form of lithography that has been around since the 1950s. The offset process advanced traditional lithography by providing better image quality, extending the useful life of printing plates, and providing more consistent results on a wide range of papers and other substrates.
Offset and traditional lithography both use printing plates that capitalize on the fact that oil and water repel each other. After the images and text are etched onto a printing plate, the oil-based ink adheres to the imaged areas, while a film of water covers the non-imaged areas.
The main difference between traditional lithography and offset printing is how the etched images on the plates reach the paper.
The traditional lithography process transfers images from the printing plates directly to the paper.
The offset printing process transfers the images from the inked plate to a rubber blanket on a rotating cylinder. The ink reaches the paper when the blanket cylinder rolls over the paper. Before a print job goes to an offset press, the computer-generated page designs must be separated by the types and colors of ink 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.
The prepress process for offset printing often includes more elaborate proofing methods, such as imposition proofs that enable buyers to check that all pages of a published magazine will appear in the desired sequence.
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 multipage 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 dries primarily by penetration into the paper.
Heatset web offset presses are up to five times faster than coldset because heat helps evaporate the liquid from the ink.
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.
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.
Perfecting Press: This type of 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.
A large variety of inks are available for offset presses. In addition to the four process colors (cyan, magenta, yellow, and black), press operators can use inks that match a client’s specified Pantone brand colors.
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.
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 (thermochromic) inks, metallic inks, magnetic inks, or UV-curing inks.
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. High-speed/high-volume heatset web offset presses typically include inline finishing equipment.
Owners of digital sheetfed presses can add decorative or tactile effects to their prints by running them through a digital embellishment press that applies clear UV-curable inks to the printed materials.
Offset printing continues to be a preferred choice for many printing projects. It is costeffective for high-volume jobs because once the fixed costs of platemaking are covered, the unit cost of producing each print drops as the print volume increases.
The ability to apply spot colors is also important for major brands that need color accuracy and consistency in their packaging and labels.
The indirect ink transfer minimizes image damage of delicate or textured materials and can produce consistent quality, color, and fine details throughout the press run.
While digital presses may have reduced the volume of short-run or personalized jobs that go to offset presses, offset printing isn’t disappearing anytime soon.
While digital printing technologies have improved in terms of speed and print quality, the capabilities of offset printing have also advanced.
When printing companies upgrade from offset presses that may have lasted 20+ years, they take capitalize on new offset-printing technologies such as UV-LED inks and prepress, production, and finishing automation.
UV-LED Inks: Sheetfed offset presses equipped with UV-LED curing units can print on a wider variety of materials to serve a broader base of customers. When correctly formulated UV-LED inks are exposed to controlled intensities of UV light, the inks cure instantly on the surface of substrate.
This enables sheetfed offset printers to print non-porous materials such as tear-resistant plastic films and synthetic papers as well as on textured papers, folding-carton paperboards, and various plastics used in gift cards, bags, packaging, and envelopes.
Automated Press Functions: Heidelberg’s Speedmaster series of “smart” sheetfed offset presses feature automated systems that minimize the need for operator intervention in prepress, printing, and finishing process.
RMGT sheetfed offset presses can include automatic plate-changing systems that also wash away the ink from the previous job.
Sentinel’s iPro ink-management system automatically monitors and dispenses ink on offset presses to reduce waste, control color, and accurately track ink costs.
Because many of the biggest printing companies today use a combination of digital and offset press, Heidelberg’s Prinect software can integrate digital press production with offset press production. Prinect can route incoming orders to the device or facility with the capacity to best handle the job.
In upcoming posts in this series, we’ll discuss the basics of screen printing, flexography, and other types of analog processes. Then, we’ll dive into the different methods of digital printing.
To wrap up this series, we’ll review the strengths and limitations of these different types of printing, provide useful glossaries, and talk about pricing models. Follow Ordant on LinkedIn, Facebook, or Twitter to learn when the next post in this series is published.
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