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.
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 offset, 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.
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.
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.
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 specified 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. .
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.
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.
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.
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.
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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.
What is Screen Printing?
What is Flexographic Printing?