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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 metres, which then prints continuously as the paper is fed through.
Lithography was initially created to be an inexpensive 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" or "printed 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 became popular, many lithographic firms went out of business. 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. 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.
Modern offset printing
One of the most 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.
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. Many modern offset presses use computer-to-plate systems as opposed to the older computer-to-film work flows, which further increases their quality.
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 trough 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 colour 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.
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, 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.
Offset printing process
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 printing process consists of several parts:
- the inking system (ink fountain and ink rollers);
- the dampening system (water fountain and water rollers);
- the plate cylinder;
- the offset cylinder (or blanket cylinder);
- the impression cylinder.
In this process, ink is transferred from the ink fountain to the paper in several steps:
- The inking and dampening systems deliver ink and water onto the offset plate covering the plate cylinder.
- The plate cylinder transfers the ink onto the blanket covering the offset cylinder.
- The paper is then pressed against the offset cylinder by the impression cylinder, transferring the ink onto the paper to form the printed image.
The goal of any inking system is to place a uniform layer of ink across every dimension of the printing plate. The lithographic process is unique in that it requires the ink form rollers to pass in contact with the nonimage areas of the plate without transferring ink to them.
Inking systems are made up of several elements:
- the ink fountain;
- the ink fountain roller (or ink feed roller);
- the ink ductor roller;
- the ink distribution rollers;
- the ink form rollers.
The ink fountain stores a quantity of ink in a reservoir and feeds small quantities of ink to the distribution rollers from the ink fountain roller and the ink ductor roller. The ink ductor roller is a movable roller that moves back and forth between the ink fountain roller and an ink distribution roller. As the ductor contacts the ink fountain roller, both turn and the ductor is inked. The ductor then swings forward to contact an ink distribution roller and transfers ink to it. There are generally two types of ink distribution rollers: the ink rotating rollers (or ink transfer rollers), which rotate in one direction, and the ink oscillating rollers (or ink vibrating rollers), which rotate and move from side to side. The ink distribution rollers receive ink and work it into a semiliquid state that is uniformly delivered to the ink form rollers. A thin layer of ink is then transferred to the image portions of the lithographic plate by the ink form rollers.
The ink fountain holds a pool of ink and controls the amount of ink that enters the inking system. The most common type of fountain consists of a metal blade that is held in place near the fountain roller. The gap between the blade and the ink fountain roller can be controlled by adjusting screw keys to vary the amount of ink on the fountain roller. The printer adjusts the keys in or out as the ink fountain roller turns to obtain the desired quantity of ink. In simple presses, the printer must turn these screws by hand. In modern presses, the adjusting screws are moved by servomotors which are controlled by the printer at a press console. Thus the printer can make ink adjustments electronically. If the printer needs to increase or decrease ink in an area of the plate (print), he need only adjust the needed keys to allow more or less ink flow through the blade. The ink flow can also be controlled by the rotation velocity of the ink fountain roller.
A simple indication of the quality of a printing press is the number of distribution and form rollers. The greater the number of distribution rollers, the more accurate the control of ink uniformity. It is difficult to ink large solid areas on a plate with only one ink form roller. With three (generally the maximum), it is relatively easy to maintain consistent ink coverage of almost any image area on the plate. Business forms presses, which print very little coverage, usually only have one or two ink form rollers. Because of this, they cannot print large solid or screen images. Smaller, less sophisticated presses also have the same problem, however, many of the newer presses today are being equipped with larger, better inking systems to meet the growing print demands of the consumer.
Most lithographic plates function on the principle of water and ink receptive areas. In order for ink to adhere only to the image areas on the plate, a layer of moisture must be placed over the nonimage areas. The dampening system accomplishes this by moistening the plate consistently throughout the press run.
Dampening systems are made up of several elements:
- the water fountain;
- the water fountain roller (or water feed roller);
- the water ductor roller in intermittent-flow dampening systems and the water slip roller in continuous-flow dampening systems;
- the water distribution rollers;
- the water form rollers.
Direct dampening systems employ a water fountain roller which picks up the water from the water fountain. The water is then passed to a water distribution roller. From here the water is transferred to the offset plate via one or two water form rollers.
Indirect dampening systems (or integrated dampening systems) feed the water directly into one of the ink form rollers (ink rollers that touch the offset plate) via a water form roller in contact with it. These systems are known as "indirect" since the water travels to the offset plate passing through the inking system and not directly to the offset plate as direct systems do. Some indirect systems will have the ability to feed the water into the inking system as well as to the offset plate. A fine emulsion of ink and water is then developed on the ink form roller. This is one reason printers need to know about "water pickup" or what percentage of water can be taken up by the ink. These systems are also known as "integrated" dampening systems as they are integrated into the inking system. One of the benefits of these systems, is that they do not use covers thus they react quicker when dampening changes are made. One generally finds this type of dampening systems on newer and faster press equipment today.
Intermittent-flow dampening systems (direct or indirect) use a water ductor roller to pick up the water and transfer it to a water distribution roller. A drawback of these systems is the slow reaction time in making adjustments due to the back and forth action of the ductor.
Continuous-flow dampening systems (direct or indirect), are used by most newer presses today because they do not have the slow reaction time of intermittent-flow dampening systems. They do not employ the water ductor roller but use the water slip roller (a roller in contact with both the water fountain roller and a distribution roller, contrary to the water ductor roller that moves back and forth between the two) for a continuous flow. The speed of the water slip roller controls the supply. The use of alcohol on these type of dampeners was standard for years. Alcohol (isopropyl alcohol) was used as it increased the water viscosity and made it "more wettable" so that transfer was easier from one roller to the other. However, alcohol substitutes such as glycol ethers, butyl cellusolve, etc., are being used today to accomplish the same task because alcohol contains volatile organic compounds. Roller hardness is also being changed to help accomplish the same job—easy transfer of the water.
Several variations of the printing process exist:
- blanket-to-blanket, a printing method in which there are two blanket cylinders per colour 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 per colour, 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. This method is most utilized on offset presses designed for envelope printing. There are also two plate cylinders per colour 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 0.005″; 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.
Quality control steps
- The paper is visually inspected to make sure there are no rips or damaged pieces of paper.
- The chemical plate is checked so that there is a right amount of chemical solution applied to the plate so that when the offset cylinder touches the paper that there aren't any smudges.
- During the printing process an operator has to check that there is no smudging between two or more sheets, where the ink is just coming off. If this does happen then quicker drying ink or a higher quality paper is required.
- After the printing has taken place there has to be a check for quality on colour, image, shapes and type and other preference.
- Quality control of the registration marks ensures that any colours produced beyond the edges of the bar are corrected immediately. A printing colour for each colour means that this could easily go wrong if a plate is not set up to a precision, meaning it will look out of focus and blurry.
The plates used in offset printing are thin, flexible, and usually larger than the paper size to be printed. Two main materials are used:
- metal plates, usually 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) is a newer technology which replaced computer-to-film (CTF) technology, and 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 or violet technologies. Both technologies have the same characteristics in term of quality and plate durability (longer runs). However often the violet CTP systems are cheaper than thermal ones, and thermal CTP systems do not need to be operated 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 nm, 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 nm–410 nm. Violet CTP is "based on emulsion tuned to visible light exposure".
Another process is computer-to-conventional plate (CTCP) 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.
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.
Small offset lithographic presses that are used for fast, good quality reproduction of one-color and two-color copies in sizes up to 12″ by 18″. Popular models were made by A. B. Dick Company, 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.
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.
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.
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.
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 or ads, magazines, direct mail, catalogs, and books. Web-fed presses are divided into two general classes: coldset (or non-heatset) and heatset offset web presses, the difference being how the inks 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), thus enabling a newspaper press to print color pages heatset and black & 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; it 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 vs. web-fed
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.
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 inks that uses a process allowing lithographic plates on a lithographic press without using a dampening system during the process.
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 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, 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. 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, which is "responsible for over half of all printing using printing plates". 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.
Odor free offset printing is the newest technology.
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