Offset printing
Offset printing is a common printing technique in which the inked image is transferred (or "offset") from a plate to a rubber blanket and then to the printing surface. When used in combination with the lithographic process, which is based on the repulsion of oil and water, the offset technique employs a flat (planographic) image carrier. Ink rollers transfer ink to the image areas of the image carrier, while a water roller applies a water-based film to the non-image areas.
The modern "web" process feeds a large reel of paper through a large press machine in several parts, typically for several meters, which then prints continuously as the paper is fed through.
Development of the offset press came in two versions: in 1875 by Robert Barclay of England for printing on tin and in 1904 by Ira Washington Rubel of the United States for printing on paper.[1] Rubel's contemporary in Continental Europe was Kašpar Hermann, the author of the offset machine prototype (1904), holder of a patent for an offset disc machine (two rubber transfer rollers facing each other) – rolling-press. In 1907, he successfully started printing in Germany on his Triumph sheetfed offset press.[2]
History[edit]
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:
Disadvantages of offset printing compared to other printing methods include:
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]
Sheet-fed vs. web-fed[edit]
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.
In industry[edit]
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".[27]