Offset printing

Lithography was initially created to be an inexpensive method of reproducing artwork.[3][4] This printing process was limited to use on flat, porous surfaces because the printing plates were produced from limestone.[3] In fact the word "lithograph", which comes from Greek (λιθογραφία), means "an image from stone" or "written in stone".

The first rotary offset lithographic printing press was created in England and patented in 1875 by Robert Barclay.[3] This development combined mid-19th century transfer printing technologies and Richard March Hoe's 1843 rotary printing press—a press that used a metal cylinder instead of a flat stone.[3] The offset cylinder was covered with specially treated cardboard that transferred the printed image from the stone to the surface of the metal. Later, the cardboard covering of the offset cylinder was changed to rubber,[3] which is still the most commonly used material.

As the 19th century closed and photography became popular, many lithographic firms went out of business.[3] Photoengraving, a process that used halftone technology instead of illustration, became the primary aesthetic of the era. Many printers, including Ira Washington Rubel of New Jersey, were using the low-cost lithograph process to produce copies of photographs and books.[5] Rubel discovered in 1901—by forgetting to load a sheet—that printing from the rubber roller, instead of the metal, made the printed page clearer and sharper.[5] After further refinement, the Potter Press printing Company in New York produced a press in 1903.[5] By 1907 the Rubel offset press was in use in San Francisco.[6]

The Harris Automatic Press Company also created a similar press around the same time. Charles and Albert Harris modeled their press "on a rotary letter press machine".[7]

Newspaper publisher Staley T. McBrayer invented the Vanguard web offset press for newspaper printing, which he unveiled in 1954 in Fort Worth, Texas.[8]

One of the important functions in the printing process is prepress production. This stage makes sure that all files are correctly processed in preparation for printing. This includes converting to the proper CMYK color model, finalizing the files, and creating plates for each color of the job to be run on the press.[9]

Offset lithography is one of the most common ways of creating printed materials. A few of its common applications include: newspapers, magazines, brochures, stationery, and books. Compared to other printing methods, offset printing is best suited for economically producing large volumes of high quality prints in a manner that requires little maintenance.[10] Many modern offset presses use computer-to-plate systems as opposed to the older computer-to-film work flows, which further increases their quality.

There are two types of offset printing: wet offset and waterless offset. Wet offset lithography uses a mix of wetting fluids (dampening solutions) to manage ink adhesion and to protect non-image areas. Waterless offset lithography employs a different method where a plate's non-image areas are protected via a layer of ink-repellent silicon. Waterless offset lithography is newer, invented in the 1960s by 3M. It was later sold and commercialized by Toray.[11]

Advantages of offset printing compared to other printing methods include:

• consistent high image quality. Offset printing produces sharp and clean images and type more easily than, for example, letterpress printing; this is because the rubber blanket conforms to the texture of the printing surface;
• quick and easy production of printing plates;
• longer printing plate life than on direct litho presses because there is no direct contact between the plate and the printing surface. Properly developed plates used with optimized inks and fountain solution may achieve run lengths of more than a million impressions;
• cost. Offset printing is the cheapest method for producing high quality prints in commercial printing quantities;
• ability to adjust the amount of ink on the fountain roller with screw keys. Most commonly, a metal blade controls the amount of ink transferred from the ink duct to the fountain roller. By adjusting the screws, the operator alters the gap between the blade and the fountain roller, increasing or decreasing the amount of ink applied to the roller in certain areas. This consequently modifies the density of the color in the respective area of the image. On older machines one adjusts the screws manually, but on modern machines the screw keys are operated electronically by the printer controlling the machine, enabling a much more precise result.[12]

Disadvantages of offset printing compared to other printing methods include:

• slightly inferior image quality compared to rotogravure or photogravure printing;
• propensity for anodized aluminum printing plates to become sensitive (due to chemical oxidation) and print in non-image–background areas when developed plates are not cared for properly;
• time and cost associated with producing plates and printing press setup. As a result, very small quantity printing jobs may now use digital offset machines.

Every printing technology has its own identifying marks, and offset printing is no exception. In text reproduction, the type edges are sharp and have clear outlines. The paper surrounding the ink dots is usually unprinted. The halftone dots can be hexagonal though there are different screening methods.[13]

Side view of the offset printing process. Multiple ink rollers are used to distribute and homogenize the ink.[14]

Several variations of the printing process exist:

Blanket-to-blanket

A printing method in which both sides of a sheet of paper are printed simultaneously, with two blanket cylinders per colour; a sheet of paper is passed between them, with each cylinder printing on one side of it.[15]

Blanket-to-blanket presses are also called perfecting or duplex presses because they print on both sides of the sheet at the same time.[16] There is no impression cylinder because the opposite blanket cylinders act as impression cylinders to each other during print production. This method is most used on offset presses designed for envelope printing. There are also two plate cylinders per colour on the press. Web and sheet-fed offset presses are similar in that many of them can also print on both sides of the paper in one pass, making it easier and faster to print duplex.

Blanket-to-steel

A printing method similar to a sheet offset press; except that the plate and cylinder pressures are quite precise. Actual squeeze between plate and blanket cylinder is optimal at 0.005″; as is the squeeze or pressure between the blanket cylinder and the substrate.[17] Blanket-to-steel presses are considered one-color presses. In order to print the reverse side, the web is turned over between printing units by means of turning bars.[17] The method can be used to print business forms, computer letters and direct mail advertising.

Variable-size printing

A printing process that uses removable printing units, inserts, or cassettes for one-sided and blanket-to-blanket two-sided printing.[17]

Keyless offset

A printing process that is based on the concept of using fresh ink for each revolution by removing residual inks on the inking drum after each revolution.[17] It is suitable for printing newspapers.

Dry offset printing

A printing process which uses a metal backed photopolymer relief plate, similar to a letterpress plate, but, unlike letterpress printing where the ink is transferred directly from the plate to the substrate, in dry offset printing the ink is transferred to a rubber blanket before being transferred to the substrate. This method is used for printing on injection moulded rigid plastic buckets, tubs, cups and flowerpots.

Roland Favorit RF01 sheet-fed offset press

Ryobi 4 color offset press

Sheet-fed refers to individual sheets of paper or rolls being fed into a press via a suction bar that lifts and drops each sheet onto place. A lithographic ("litho" for short) press uses principles of lithography to apply ink to a printing plate, as explained previously. Sheet-fed litho is commonly used for printing of short-run magazines, brochures, letter headings, and general commercial (jobbing) printing. In sheet-fed offset, "the printing is carried out on single sheets of paper as they are fed to the press one at a time". Sheet-fed presses use mechanical registration to relate each sheet to one another to ensure that they are reproduced with the same imagery in the same position on every sheet running through the press.[20]

Web-fed refers to the use of rolls (or "webs") of paper supplied to the printing press.[24] Offset web printing is generally used for runs in excess of five or ten thousand impressions. Typical examples of web printing include newspapers, newspaper inserts or ads, magazines, direct mail, catalogs, and books. Web-fed presses are divided into two general classes: cold-set (or non-heat-set) and heat-set offset web presses, the difference being how the inks dry. Cold web offset printing dries through absorption into the paper, while heat-set utilizes drying lamps or heaters to cure or "set" the inks. Heat-set presses can print on both coated (slick) and uncoated papers, while cold-set presses are restricted to uncoated paper stock, such as newsprint. Some cold-set web presses can be fitted with heat dryers, or ultraviolet lamps (for use with UV-curing inks), thus enabling a newspaper press to print color pages heat-set and black & white pages cold-set.

Web offset presses are beneficial in long run printing jobs, typically press runs that exceed 10,000 or 20,000 impressions. Speed is a determining factor when considering the completion time for press production; some web presses print at speeds of 3,000 feet (910 meters) per minute or faster. In addition to the benefits of speed and quick completion, some web presses have the inline ability to cut, perforate, and fold.

Sheet-fed presses offer several advantages. Because individual sheets are fed through, a large number of sheet sizes and format sizes can be run through the same press. In addition, waste sheets can be used for make-ready (which is the testing process to ensure a quality print run). This allows for lower cost preparation so that good paper is not wasted while setting up the press, for plates and inks. Waste sheets do bring some disadvantages as often there are dust and offset powder particles that transfer on to the blankets and plate cylinders, creating imperfections on the printed sheet. This method produces the highest quality images.

Web-fed presses, on the other hand, are much faster than sheet-fed presses, with speeds up to 80,000 cut-offs per hour (a cut-off is the paper that has been cut off a reel or web on the press; the length of each sheet is equal to the cylinder's circumference). The speed of web-fed presses makes them ideal for large runs such as newspapers, magazines, and comic books. However, web-fed presses have a fixed cut-off, unlike rotogravure or flexographic presses, which are variable.

Offset printing uses inks that, compared to other printing methods, are highly viscous. Typical inks have a dynamic viscosity of 40–100 Pa·s.[25]

There are many types of paste inks available for utilization in offset lithographic printing and each have their own advantages and disadvantages. These include heat-set, cold-set, and energy-curable (or EC), such as ultraviolet- (or UV-) curable, and electron beam- (or EB-) curable. Heat-set inks are the most common variety and are "set" by applying heat and then rapid cooling to catalyze the curing process. They are used in magazines, catalogs, and inserts. Cold-set inks are set simply by absorption into non-coated stocks and are generally used for newspapers and books but are also found in insert printing and are the most economical option. Energy-curable inks are the highest-quality offset litho inks and are set by application of light energy. They require specialized equipment such as inter-station curing lamps, and are usually the most expensive type of offset litho ink.

• Letterset inks are mainly used with offset presses that do not have dampening systems and use imaging plates that have a raised image.[26]
• Waterless inks are heat-resistant and are used to keep silicone-based plates from showing toning in non-image areas. These inks are typically used on waterless Direct Imaging presses.[26]
• Single Fluid Inks are newer inks that use a process allowing lithographic plates on a lithographic press without using a dampening system during the process.[26]

Offset lithography became the most popular form of commercial printing from the 1950s ("offset printing"). Substantial investment in the larger presses required for offset lithography was needed, and had an effect on the shape of the printing industry, leading to fewer, larger, printers. The change made a greatly increased use of colour printing possible, as this had previously been much more expensive. Subsequent improvements in plates, inks, and paper have further refined the technology of its superior production speed and plate durability. Today, lithography is the primary printing technology used in the U.S. and most often as offset lithography, which is "responsible for over half of all printing using printing plates".[16]

• Variable data printing
• Spot color

• Hird, Kenneth F. (2000). Offset Lithographic Technology. Tinley Park, Illinois: Goodheart - Willcox. ISBN 978-1-56637-621-1.
• "Offset Printing". Encyclopædia Britannica. Retrieved March 22, 2004, from Encyclopædia Britannica Premium Service.