- Offset printing
Part of a series on the History of printing Woodblock printing (200) Movable type (1040) Printing press (1454) Etching (ca. 1500) Mezzotint (1642) Aquatint (1768) Lithography (1796) Chromolithography (1837) Rotary press (1843) Offset printing (1875) Hectograph (19th century) Hot metal typesetting (1886) Mimeograph (1890) Screen printing (1907) Spirit duplicator (1923) Dye-sublimation (1957) Phototypesetting (1960s) Dot matrix printer (1964) Laser printing (1969) Thermal printing (ca. 1972) Inkjet printing (1976) Stereolithography (1986) Digital press (1993) 3D printing (ca. 2003)
Offset printing is a commonly used printing technique in which the inked image is transferred (or "offset") from a plate to a rubber blanket, 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 on which the image to be printed obtains ink from ink rollers, while the non-printing area attracts a water-based film (called "fountain solution"), keeping the non-printing areas ink-free.
Lithography was initially created to be a low cost method of reproducing artwork. This printing process was limited to use on flat, porous surfaces because the printing plates were produced from limestone. In fact, the word 'Lithograph' historically means "an image from stone." Tin cans were popular packaging materials in the 19th century, but transfer technologies were required before the lithographic process could be used to print on the tin.
The first rotary offset lithographic printing press was created in England and patented in 1875 by Robert Barclay. 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. 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, which is still the most commonly used material.
As the 19th century closed and photography captured favor, many lithographic firms went out of business. Photoengraving, a process that used halftone technology instead of illustration, became the leading 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. Rubel discovered in 1901—by forgetting to load a sheet—that when printing from the rubber roller, instead of the metal, the printed page was clearer and sharper. After further refinement, the Potter Press printing Company in New York produced a press in 1903. By 1907 the Rubel offset press was in use in San Francisco.
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,” (“Short History of Offset Printing”).
Compared to other printing methods, offset printing is best suited for cost-effectively producing large volumes of high quality prints in an economically sound manner that requires little maintenance.
Offset lithography is one of the most common ways of creating printed matter. A few of its common applications include: newspapers, magazines, brochures, stationery, and books.
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 letterpress printing 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 running in conjunction with optimized inks and fountain solution may exceed run lengths of a million impressions.
- Cost. Offset printing is the cheapest method to produce high quality printing in commercial printing quantities.
- A further advantage of offset printing is the possibility 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 trough to the fountain roller. By adjusting the screws the gap between the blade and the fountain roller is altered, leading to the amount of ink applied to the roller to be increased or decreased in certain areas. Consequently the density of the colour in the respective area of the image is modified. On older machines the screws are adjusted manually, but on modern machines the screw keys are operated electronically by the printer controlling the machine, enabling a much more precise result.
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 are now moving to digital offset machines.
The most common kind of offset printing is derived from the photo offset process, which involves using light-sensitive chemicals and photographic techniques to transfer images and type from original materials to printing plates.
In current use, original materials may be an actual photographic print and typeset text. However, it is more common — with the prevalence of computers and digital images — that the source material exists only as data in a digital publishing system.
Offset lithographic printing on to a web (reel) of paper is commonly used for printing of newspapers and magazines for high speed production.
Ink is transferred from the ink duct to the paper in several steps :-
- The ink duct roller delivers ink from the ink duct to the ink pyramid. Also called the 'Ink Train'.
- The ductor roller, sometimes called a vibrator roller due to its rapid back and forth motion, transfers ink from the duct roller to the first distribution roller. It is never in contact with both rollers at the same time.
- The distribution rollers evenly distribute the ink. The first distribution roller picks up the ink from driving rollers, and the last distribution rollers transfer the ink to the form rollers.
- The transfer rollers transfer ink between the ink-absorbing and ink-delivering driving rollers.
- Driving rollers roll against the distribution rollers and either absorb or deliver ink, depending on their placement.
- Ink form rollers transfer ink from the last distribution rollers on to the printing plate.
- The printing plate transfers the ink to the offset cylinder(typically called blanket cylinder) usually covered with a rubber 'blanket'.
- The paper is then pressed against the blanket cylinder by the impression cylinder, transferring the ink onto the paper to form the printed image.
Types of paper feed
"Sheet-fed" refers to individual sheets of paper or paperboard being fed into a press. 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.
"Web-fed" refers to the use of rolls (or "webs") of paper supplied to the printing press. Offset web printing is generally used for runs in excess of 5 or 10 thousand impressions. Typical examples of web printing include newspapers, newspaper inserts/ads, magazines, catalogs, and books. Web-fed presses are divided into two general classes: "Cold" or "Non-Heatset," and "Heatset" offset web presses, the difference being how the inks that are used dry. Cold web offset printing dries through absorption into the paper, while heatset utilizes drying lamps or heaters to cure or "set" the inks. Heatset presses can print on both coated (slick) and uncoated papers, while coldset presses are restricted to uncoated paper stock, such as newsprint. Some coldset web presses can be fitted with heat dryers, or ultraviolet lamps (for use with uv-curing inks). It is also possible to add a drier to a cold-set press. This can enable a newspaper press to print color pages heatset and black & white pages coldset.
Web-fed vs. Sheet-fed (photo offset)
Sheet-fed presses offer several advantages. Because individual sheets are fed though, 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 allows for lower cost makereadies, so that good paper is not wasted while setting up the press, for plates & inks (waste sheets do bring some disadvantages as often there are dust, offset powder particles that transfer on to the blankets and plate cylinders, thereby creating imperfections on the printed sheet in the form of "hickies").
Web-fed presses, on the other hand, are much faster than sheet-fed presses, with speeds in excess of 20,000 cut-offs per hour. Their speed makes them ideal for large runs such as newspapers or magazines. However, web-fed presses have a fixed cut-off, unlike rotogravure or flexographic presses.
Types of commercial offset processes
A perfecting press, also known as a duplex press, is one that can print on both sides of the paper at the same time (Bruno, Romano, and Riordan 137). 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.
Small offset lithographic presses that are used for fast, good quality reproduction of 1- and 2-color copies in sizes up to 12”X18”. (Romano, & Riordan 139–141) Popular models were made by A.B. Dick, Multilith, and the Chief and Davidson lines made by A.T.F./Davidson.
Offset duplicators are made for fast and quick printing jobs; therefore have faster make-readies and turn-around time, printing up to 12,000 impressions per hour.
They are able to print business forms, letterheads, labels, bulletins, postcards, envelopes, folders, reports, and sales literature.
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. (“What is Offset Printing”)
The actual process of printing is quite involved. One of the most important functions in the process is Pre-press Production. This stage makes sure that all files are correctly processed in preparation for printing. This includes converting to the proper CMYK, finalizing the files, and creating plates for each color of the job to be run on the press. The sheet fed press consists of different systems that complete the actual process; feeder system, printing system, inking/dampening system, and the delivery system.
The feeder system is responsible for making sure paper runs through the press correctly. This is where you load the substrate and then correctly set up the system to the certain specifications of the substrate to the press. (DeJidas & Destree, 2005, p. [55-57])
The Printing Unit consists of many different systems. The dampening system is used to apply dampening solution to the plates with water rollers. The inking system uses rollers to deliver ink to the plate and blanket cylinders to be transferred to the substrate. The plate cylinder is where the plates containing all of the imaging are mounted. Finally the blanket and impression cylinders are used to transfer the image to the substrate running through the press. (DeJidas & Destree, 2005, p. )
The delivery system is the final destination in the printing process while the paper runs through the press. Once the paper reaches delivery, it is stacked for the ink to cure in a proper manner. This is also where you can check on sheets to make sure they have proper ink density and registration.
Production or impact of double image in printing is known as slur (AMAN-2008)
A high run, speed printing press that uses rolls of paper rather than individual sheets (Spectrum Printers).
Web Offset Presses are beneficial in long run printing jobs, typically press runs that exceed 10 or 20 thousand impressions. Speed is a huge factor when considering turn around time for press production; some web presses print at speeds of 3,000 feet per minute or faster. In addition to the benefits of speed and possible faster turn around times, some web presses have the inline ability to cut, perforate, and fold.
Heatset Web Offset
This subset of Web offset printing uses inks which dry by evaporation in a dryer typically positioned just after the printing units. This is typically done on coated papers, where the ink stays largely on the surface, and gives a glossy high contrast print image after the drying. As the paper leaves the dryer too hot for the folding and cutting that are typically downstream procedures, a set of "chill rolls" positioned after the dryer lowers the paper temperature and sets the ink. The speed at which the ink dries is a function of dryer temperature and length of time the paper is exposed to this temperature. This is one of the limits on print speed in a Heatset press. This type of printing is typically used for magazines, catalogs, inserts and other medium-to-high volume, medium-to-high quality production runs.
Coldset Web Offset
This is also a subset of Web offset printing, typically used for lower quality print output. It is typical of Newspaper production. In this process, the ink dries by absorption into the underlying paper. A typical Coldset configuration is often a series of vertically arranged print units and peripherals.
As newspapers seek new markets, which often imply higher quality (more gloss, more contrast) , they may add a heatset tower (with a dryer) or use UV (Ultra Violet) based inks which "cure" on the surface by polymerisation rather than by evaporation or absorption.
A printing method in which there are two blanket cylinders through which a sheet of paper is passed and printed on both sides. (Commercial color offset printing – a compendium of commercial printing terminology)
Blanket-to-blanket presses are considered a perfecting press because they print on both sides of the sheet at the same time. Since the blanket-to-blanket press has two blanket cylinders, making it possible to print on both sides of a sheet, there is no impression cylinder. The opposite blanket cylinders act as an impression cylinder to each other when print production occurs. There are also two plate cylinders on the press.
A printing method similar to a sheet offset press; except that the plate and cylinder gaps are very narrow. (Romano, & Riordan 139–141)
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. (Romano, & Riordan 139–141)
The method can be used to print business forms, computer letters, and direct mail advertising.
A printing process that uses removable printing units, inserts, or cassettes for one-sided and blanket-to-blanket two-sided printing. (Romano, & Riordan 139–141)
Keyless offset is 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. (Romano, & Riordan 139–141) It is suitable for printing newspapers.
Types of platemaking
Generally, “the plates used in offset printing are thin,” and are mostly made of aluminum, though sometimes can be made of multimetal, paper, or plastic (Kipphan 209).
Polyester plates can be used in place of aluminum plates for smaller formats or medium quality jobs, “as their dimensional stability is lower” (Kipphan 209).
Computer to Plate (CTP) / Direct to Plate (DTP)
Computer to Plate (CTP) is a newer technology that allows the imaging of metal or polyester plates without the use of film. Eliminating the stripping, compositing, and traditional plate making processes, CTP revolutionized the printing industry and led to reduced prepress times, lower costs of labor, and improved print quality.
Most CTP systems used thermal CTP as opposed to violet CTP, though both systems are effective, depending on the needs of the printing job. Thermal CTP does have the advantage of extremely high quality, but Violet CTP does cost significantly less. Thermal plates are generally used for longer runs, while Violet CTP is employed for shorter runs, and popular with 2-up and 4-up applications (Bruno, Romano, and Riordan 126).
Thermal CTP has the added bonus of utilizing binary exposure, which limits the risk of under or overexposure, and makes it possible to work under yellow light (Bruno, Romano, and Riordan 126).
Thermal CTP involves the use of thermal lasers to expose and/or remove areas of coating while the plate is being imaged. This depends on whether the plate is negative, or positive working. These lasers are generally at a wavelength of 830 nanometers, but vary in their energy usage depending on whether they are used to expose or ablate material. Violet CTP lasers have a much lower wavelength, 405–410 nanometers. Violet CTP is “based on emulsion tuned to visible light exposure,” (Bruno, Romano and Riordan 126). The general trend of platesetters has been to move toward coatings whose success on press is independent of post imaging chemical bath processing.
Another process is CTCP (Computer to conventional plate) system where you can use conventional offset ps plates to expose, which is very economical.
Types of chemicals used
Paste inks for offset litho
There are many types of paste inks available for employment 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 cost-conscious 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 uses imaging plates that have a raised image. (Romano, & Riordan 160)
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. (Romano, & Riordan 160)
Single Fluid Inks are newer ink that uses a process allowing lithographic plates on a lithographic press without using a dampening system during the process. (Romano, & Riordan 160)
Ink and water balance is an extremely important part of offset printing. If ink and water are not properly balanced, the press operator may end up with many different problems affecting the quality of the finished product, such as emulsification. This means the water overpowering and mixing with the ink.It leads to scumming, catchup, trapping problems, ink density issues and in extreme cases the ink not properly drying on the paper, resulting in the job being unfit for delivery to the client. With the proper balance, the job will have the correct ink density and should need little further adjustment except minor ones. Such as when the press heats up during normal operation, thus evaporating water at a faster rate. In this case the machinist will gradually increase the water as the press heats up to compensate for the increased evaporation of water. Printing machinists generally try to use as little water as possible to avoid these problems.
Fountain solution is the water-based (or "aqueous") component in the lithographic process that moistens the non image area of the plate in order to keep ink from depositing (and thus printing). Historically, fountain solutions were acid-based and made with gum arabic, chromates and/or phosphates, and magnesium nitrate. Alcohol is added to the water to make the water wetter (as we say) and help cool the press a bit so the ink stays stable so it can set and dry fast.
While the acid fountain solution has come a long way in the last several decades, neutral and alkaline fountain solutions have also been developed. Both of these chemistries rely heavily on surfactants/emulsifiers and phosphates and/or silicates to provide adequate cleaning and desensitizing, respectively. Since about 2000, alkaline-based fountain solutions have started becoming less common due to the inherent health hazards of high pH and the objectionable odor of the necessary microbiological additives.
Acid-based fountain solutions are still the most common variety and yield the best quality results by means of superior protection of the printing plate, lower dot gains, and longer plate life. Acids are also the most versatile, capable of running with all types of offset litho inks. However, because these products require more active ingredients to run well than do neutrals and alkalines, they are also the most expensive to produce. That said, neutrals and, to a lesser degree, alkalines are still an industry staple and will continue to be used for most newspapers and many lower-quality inserts.
In recent years alternatives have been developed which do not use fountain solutions at all (waterless printing).
Every printing technology has its own identifying marks, as does offset printing. In text reproduction the type edges are sharp and have clear outlines (see following picture). The paper surrounding the ink dots is usually unprinted. The halftone dots are always irregular and blurry though there are different screening methods (AM and FM).
Offset lithography became the most popular form of commercial printing in the 1950s ("Offset Printing"). Subsequent improvements in plates, inks, and paper enhanced the technology and maximized its superior production speed and plate durability. Today, lithography is the dominant printing technology in the US, and most lithography is printed as offset lithography.
Today, offset lithography is "responsible for over half of all printing using printing plates" (Bruno, Romano and Riordan 137). The quality of the prints made is consistently high, and the volume of prints created for their respective cost makes commercial offset lithography very efficient for businesses, especially when many prints must be created quickly.
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