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Offset printing What is it and how does it work?

La impresión offset

Offset printing, also called offset lithography, in commercial printing, is a widely used printing technique. The inked image on a printing plate is printed on a rubber cylinder and then transferred (i.e. offset) to paper or other material. The rubber cylinder offers great flexibility, allowing printing on wood, cloth, metal, leather and rough paper

An American printer, Ira W. Rubel, of Nutley, N.J., accidentally discovered the process in 1904 and soon built a press to exploit it.

Curiosity

how does offset printing work?

In offset printing, the material to be printed neither rises above the surface of the printing plate nor sinks below it. It is flush with the surface of the plate; therefore, offset is classified as a planographic printing method.

Offset printing, as a development of lithography, is based on the principle that water and grease do not mix, so that a greasy ink can be deposited on the grease-treated areas of the plate, while the unprinted areas, which contain water, reject the ink.
The offset plate is usually made of zinc or aluminum or a combination of metals, with the surface treated to make it porous and then coated with a photosensitive material. Exposure to an image hardens the coating on the printing areas; the coating on the non-printing areas is washed away, leaving wet metal that will reject ink.

Modern offset printing is done on a press consisting basically of three rotating cylinders: a plate cylinder, to which the metal plate is attached; a blanket cylinder covered by a rubber sheet; and a printing cylinder that presses the paper in contact with the blanket cylinder

It works very distinctly from digital printing or photographic printing.

The plate cylinder first comes in contact with a series of dampening rollers that deposit moisture on the metal granulations. A series of inking rollers then pass over the plate, and the ink is rejected by the water retention zones and accepted by the greasy image. The inked image is transferred to the rubber blanket and then offset by the paper moving around the printing cylinder.

Techniques in the offset printing process

The offset printing process requires the manufacture of separate blanket plates and rollers for each color of ink to be used.

Different variations of presses, plates and inks are used for different run lengths and substrate requirements. For example, some presses can print millions of copies of a magazine. Other presses can print 250 large sheets that will be folded to make multi-page booklets.

Web-fed presses

These printers automatically feed wide rolls of paper from one ink station to another until the proper amount 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.

  • Cold web offset: ink is fixed by absorption into the paper.
  • Hot web offset: ink is set by absorption into the paper plus evaporation at high temperatures. Heatset web offset presses are four to five times faster than sheetfed presses and usually include in-line folding equipment.

Sheetfed press

This type of offset printing equipment feeds pre-cut flat 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 paper as small as 4 x 6 inches. Larger ones can handle sheets up to 40 x 26 inches.

Perfecting press

A perfecting press can print on both sides of the paper automatically. All presses are perfecting presses. But not all sheetfed presses are built to print on both sides automatically.

Colors

In addition to the four process colors (cyan, magenta, yellow and black), press operators can use inks that match the Pantone brand colors specified by the customer.

Formulations

Some offset inks use vegetable-based oils such as soybean oil or linseed oil to carry the pigment. Other inks use water repellent fluids made from synthetic chemical mixtures.

Finishing processes in offset printing

The type of finishing equipment used depends on the type and volume of publications, advertising items, manuals, stationery or packaging being produced.

The type of finishing equipment includes devices for trimming and cutting paper, or for folding, binding, perforating, 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 allows the application of transparent UV-curable inks for increased durability or decorative effects.

Special properties

Some offset inks are formulated to change the performance of the ink on paper. For example, prints can be made with color-changing (thermochromatic) inks, metallic inks, magnetic inks or UV-curable inks.

Polymer-based inks that cure when exposed to UV-LED light are popular because they allow sheetfed press owners to make instant dry prints. They do not need to use heat or spray powders to speed up the drying process on press.

With properly formulated UV-LED inks and the right UV-LED curing units, sheetfed offset presses can print on packaging cartons, non-porous synthetic sheets, laminated sheets, plastic sheets and a wider range of paperboards and labels.

Differences between offset printing and digital printing

Offset printing technology uses plates, usually aluminum, which are used to transfer an image onto a rubber “blanket,” and then roll that image onto a sheet of paper. It is called offset because the ink is not transferred directly to the paper. Because offset presses work so efficiently once they are set up, offset printing is the best choice when large quantities are needed, and it provides accurate color reproduction, and sharp, professional-looking printing.

Digital printing is more useful when small quantities are required.
While offset printing is a fantastic way to produce great looking print projects, many companies or individuals do not need large print runs of 500 or more, and digital printing is the best solution.

What are the advantages of offset printing over digital?

  • Large quantities can be printed cost-effectively
  • The more you print, the cheaper the price per piece.
  • A wide variety of paper types can be used with customized finishes
  • Specialty custom inks such as metallic and Pantone colors are availableThe
    highest possible print quality, with greater detail and color fidelity

Other printing

As you know offset printing is used when we want to print on some materials such as paper, fabric and even wood. But each product has a recommended printing. For example, for textile products such as T-shirts, caps or sweatshirts, screen printing and sublimation are used. And for wood, woodcut printing is used.

If you liked our article share it and continue reading other types of printing in our main category of printing.

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