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Offset printing or web 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. 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.

 

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. In this process, ink is transferred from the ink duct to the paper in several steps:

1.     The ink duct roller delivers ink from the ink duct to the ink pyramid, also called the Ink Train.

2.     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.

3.     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.

4. The transfer rollers transfer ink between the ink-absorbing and ink-delivering driving rollers.

5.Driving rollers roll against the distribution rollers and either absorb or deliver ink, depending on their placement.

6. Lnk form rollers transfer ink from the last distribution rollers on to the printing plate.

7. The printing plate transfers the ink to the offset cylinder (typically called the blanket cylinder) usually covered with a rubber “blanket.”

8. The paper is then pressed against the blanket cylinder by the impression cylinder, transferring the ink onto the paper to form the printed image.

Process 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 color model, finalizing the files, and creating plates for each color of the job to be run on the press.

Every printing technology has its own identifying marks, as does offset printing. In text reproduction, the type edges are sharp and have clear outlines. The paper surrounding the ink dots is usually unprinted. The halftone dots are always hexagonal though there are different screening methods.

Variations

Blanket-to-blanket: A printing method in which there are two blanket cylinders through which a sheet of paper is passed and printed on both sides.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.

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 .005"; as is the squeeze or pressure between the blanket cylinder and the substrate. 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. 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.

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.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.

Metal plates: the plates used in offset printing are thin, flexible, and usually larger than the paper size to be printed, and are usually made of aluminum, although sometimes they are made of multimetal, paper, or plastic.

Polyester plates: These are much cheaper and can be used in place of aluminum plates for smaller formats or medium quality jobs, as their dimensional stability is lower.

Computer-to-plate (CTP) / direct-to-plate (DTP)

Main article: Computer to plate

Computer-to-plate (CTP) is a newer technology that allows the imaging of metal or polyester plates without the use of film. By eliminating the stripping, compositing, and traditional plate making processes, CTP altered the printing industry, which 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 higher quality, but violet CTP does cost significantly less. Thermal plates are generally used for shorter runs, while violet CTP is employed for longer runs, and is popular with two-up and four-up applications.Thermal CTP has the additional benefit of using binary exposure, which limits the risk of under or overexposure, and makes it possible to work under yellow light.

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.” 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 in which conventional offset plates can be exposed, making it an economical option.

 

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.

Perfecting press: A perfecting press, also known as a duplex press, is one that can print on both sides of the paper at the same time. 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.

Offset duplicators: Small offset lithographic presses that are used for fast, good quality reproduction of one- and two-color copies in sizes up to 12” by 18”. 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; 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.

Feeder system: The feeder system is responsible for making sure paper runs through the press correctly.This is where the substrate is loaded and then the system is correctly set up to the certain specifications of the substrate to the press.

Printing/inking system: 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.

Delivery system: 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 the step in which sheets are inspected to make sure they have proper ink density and registration.

Slur: Production or impact of double image in printing is known as 'slur'.

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 five or ten thousand impressions. Typical examples of web printing include newspapers, newspaper inserts/ads, magazines, direct mail, 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 and white pages coldset.

Web offset presses are beneficial in long run printing jobs, typically press runs that exceed ten or twenty thousand impressions. Speed is a determining factor when considering the completion 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 quick completion, 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 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 (ultraviolet) based inks which "cure" on the surface by polymerisation rather than by evaporation or absorption.

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.

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. (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.

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 uses imaging plates that have a raised image.

·    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.

·    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.

Ink/water balance

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 (the water overpowering and mixing with the ink). This 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 for minor ones. An example would be 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

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 lower the surface tension 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 improved 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 become 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, lowerdot 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. However, 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).

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.

Today, offset lithography is "responsible for over half of all printing using printing plates".^[16] The consistent high quality of the prints 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.