What is Offset Printing?
Offset printing is a common printing technique that works by transferring (or "offset") an inked image from a printing plate to a rubber blanket and then to the printing surface.
The offset technique uses a flat (planographic) image carrier in conjunction with the lithographic process that relies on the repulsion of oil and water. While a water roller applies a water-based film to the non-image areas of the image carrier, ink rollers transfer ink to the image areas of the image carrier.
The modern "web" process feeds a big reel of paper in sections, generally for several meters, through a large press machine, which then prints constantly as the paper is fed through.
Today, there are two different versions of the use of offset press. One is that Robert Barclay in England printed on the tin in 1875; the other version is that it was printed on paper by Ira Washington Rubell in the United States in 1904.
History of Printing
Lithography first emerged as an inexpensive way to reproduce artwork. This printing process was limited to flat, porous surfaces because the plates are made of limestone. In fact, the word "lithograph" comes from the Greek (λιθογραφία) meaning "image from stone" or "written on stone".
The first rotary lithographic printing press was born in England and patented by Robert Buckley in 1875. This development combined transfer printing techniques from the mid-19th century with Richard March Hoe's 1843 rotary printing press - a printing press that used metal cylinders instead of flat stones. The blanket cylinder was covered in specially treated cardboard, which allowed the printed image to be transferred from the stone to the metal surface. Later, the offset cylinder's cardboard cover was replaced with rubber, which is currently the most widely used material.
As the 19th century came to an end and photography became popular, many lithographic companies went into liquidation. Photoengraving, a craft that used halftone techniques rather than illustrations, became the dominant aesthetic of that era. Many printers, including Ira Washington Rubel of New Jersey, are using the low-cost lithographic process to duplicate images and books. In 1901, Rubell happened to find out that printing with rubber rollers instead of metal rollers made the prints sharper and clearer. After further advancements, 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.
Around the same time, the Harris Automatic Press Company developed a similar press. Charles and Albert Harris modeled their press "on a rotary letter press machine".
Newspaper publisher Staley T. McBrayer invented the Vanguard web offset press for newspaper printing, which he unveiled in 1954 in Fort Worth, Texas.
Modern Offset Printing
An important function in the printing process is prepress production. This stage ensures that all documents are processed correctly in preparation for printing. Converting to the right CMYK color model, finalizing the file, and producing plates for each color of the job that will be run on the press are all part of this process.
One of the most ordinary methods of producing printed materials is offset lithography. Some of its common applications include: newspapers, magazines, brochures, stationery, and books. Offset printing is the most cost-effective technique of printing when compared to other processes. It requires little maintenance and can produce a huge quantity of high-quality prints at a low cost.
Many modern offset presses use computer-to-plate systems instead of the old-fashioned computer-to-film workflow, which further improves the quality.
Wet offset printing and waterless offset printing are two types of offset printing. During wet offset printing, in order to control ink adherence and safeguard non-image areas, a blended dampening fluid (dampening fluid) is used. Waterless offset lithography uses a different process in which non-image areas of a plate are shielded by an ink-repellent silicon layer. Waterless offset lithography is a relatively modern technology.
Offset printing has several advantages over other printing techniques, including:
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higher image quality. Offset printing produces sharp and clean images and type more easily than, for example, letterpress printing;
Letterpress Printing
Offset Printing
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quick and easy production of printing plates;
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longer printing plate life than that of direct lithographic presses because there is no direct contact between the plate and the printing surface;
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cost-effective. Offset printing is the cheapest method for producing high-quality prints in commercial printing quantities;
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easier ink volume adjustment. On older machines, one adjusts the screws manually, but today's use of machines and equipment that control the screw keys through electronic operation enables more accurate results to be obtained.
Disadvantages of offset printing compared to other printing methods include:
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slightly inferior image quality compared to rotogravure or photogravure printing;
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sensitivity of anodized aluminum printing plates (caused by chemical oxidation) and tendency to print in regions other than the picture backdrop when developed plates are not handled properly;
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the setup of the printing press and the time and money spent making the plates. Digital offset printers can now be used for printing work in extremely small quantities.
Offset printing is no different from other printing technologies in having unique identifying marks. The font edges and outlines are sharp and distinct in text reproduction. Usually, the paper around the ink dots is blank. Various screening techniques allow for the possibility of hexagonal halftone dots.
Process Variations
Several variations of the printing process exist:
Blanket-to-blanket
A type of perfecting press used most commonly in web offset lithography in which the impression cylinder in the printing unit has been replaced with a second blanket cylinder, so that as a press sheet or paper web passes between the two blankets it is printed on both sides simultaneously. This method is most used on offset presses designed for envelope printing. Successive printing units can print additional colors.
Blanket-to-steel
A printing technology comparable to a sheet offset press, with the exception that the plate and cylinder pressures are extremely exact. The squeeze or pressure between the blanket cylinder and the substrate is ideal at 0.005′′, as is the squeeze or pressure between the plate and the blanket cylinder. Blanket-to-steel presses are one-color machines. The web is flipped over between printing units by turning bars in order to print the opposite side. Business forms, electronic letters, and direct mail advertising may all be printed using this technology.
Variable-size printing
Variable printing, also known as variable data printing (VDP), is a type of digital printing that allows you to modify specific elements of the piece, such as the headline, photos, name, or color, without stopping the printing process. With the correct technology, this can be done swiftly and with great quality. This is a great approach to tailor each content to your audience. It is ideal for direct mail pieces that benefit from personalization.
Keyless offset
A printing method that uses fresh ink for each revolution by removing the remaining ink from the inking drum after each revolution. It is appropriate for printing newspapers.
Dry offset printing
Also called Letterset, or Indirect Relief Printing. The offset printing process combines the characteristics of letterpress and offset. A special plate prints directly onto the blanket of an offset press, and the blanket then offsets the image onto the paper. The process is called dry offset because the plate is not dampened as it would be in the offset lithography process. This method is used for printing on injection molded rigid plastic buckets, tubs, cups and flowerpots.
Plates
Materials
The plates used in offset printing are thin, flexible, and usually larger than the paper size to be printed. Two main materials are used:
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Metal plates, usually aluminum, although sometimes they are made of multi-metal, paper, or plastic.
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Polyester plates, which 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
Main article: Computer-to-plate
Computer-to-plate (CTP) is an imaging technology that helps to transfer a digital image generated in a computer directly to a printing plate. Before CTP, the technology used was computer-to-film (CTF), where the image output was passed to a photographic film, and the output film was then used to make the printing plate. This process is similar to darkroom photography. The CTP technology helps to eliminate all the darkroom processes, and hence is cost-effective. CTP is a much faster process compared to film-based printing, so the productivity of printing increases greatly.
The majority of CTP systems employ thermal CTP or violet technology. In terms of plate durability and quality, both technologies are comparable (for longer runs). However, violet CTP systems are frequently less expensive than thermal CTP systems, because thermal CTP systems do not require the use of yellow light.
Thermal CTP utilizes the use of thermal lasers to expose or remove regions of coating while imaging the plate. This is determined by whether the plate is negative or positive functioning. These lasers have a wavelength of 830 nm but utilize different amounts of energy depending on whether they are used to expose or ablate material. Violet CTP lasers have a much shorter wavelength, 405 nm-410 nm. Violet CTP is "based on emulsion."
Another process is the computer-to-conventional plate (CTCP) system, which exposes traditional offset plates and is a cost-effective choice.
Sheet-fed offset
Roland Press Machines
4 Color Ryobi Offset Printing Machine
Sheet-fed offset printing is a method in which individual pages of paper are fed into the machine. 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. This printing method is popular for small and medium-sized fixed jobs such as limited-edition books. In another method, Web offset printing , a continuous roll of paper is fed through the printing press.
Offset duplicators
In the United States, an offset press with a paper size of up to 12 in 18 in (300 mm 460 mm) is referred to as a duplicator rather than a press. Offset duplicators are used to produce one-color and two-color duplicates in sizes ranging from 12 in to 18 in (300 mm 460 mm). [15] A. B. Dick Company, Multilith, and A.T.F.-Davidson's Chief and Davidson lines were all popular. Offset duplicators are designed for high-volume printing projects, capable of producing up to 12,000 impressions per hour. Business forms, letterheads, labels, bulletins, postcards, envelopes, folders, reports, and sales brochures can all be printed by them.
Feeder system
The feeder system is in charge of ensuring that paper is properly fed through the press. This is where the substrate is loaded and the system is properly set up to the substrate's unique criteria for the press.
Printing–inking system
The Printing Unit is made up of numerous systems. The dampening system employs water rollers to apply dampening solutions to the plates. Rollers are used to supply ink to the plate, and blanket cylinders are used to transfer ink to the substrate. The plate cylinder is where all of the imaging plates are mounted. Finally, the picture is transferred to the substrate passing through the press using the blanket and imprint cylinders.
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
The production or impact of the double image in printing is known as a slur.
Web-fed offset
Web-fed offset press
Web offset is a form of offset printing in which a continuous roll of paper is fed through the printing press. Pages are separated and cut to size after they have been printed. Web offset printing is used for high-volume publications such as mass-market books, magazines, newspapers, catalogs and brochures. Web-fed presses fall into two categories: cold-set (or non-heat-set) and heat-set offset web presses, the difference being the way the ink dries. Cold web offset printing dries by absorption into the paper, while heat setting uses a drying lamp or heater to cure or "cure" the ink.
Web offset presses are good for long runs, usually in excess of 10,000 or 20,000 impressions. Speed is a decisive factor when considering press production finish times; some web presses print at speeds of 3,000 feet per minute (915 meters per minute) or faster. In addition to the benefits of speed and quick turnaround, some web presses have in-line cutting, perforating and folding capabilities.
Heat-set web offset
The ink used in this subset of web offset printing is dried by evaporation in a dryer, which is normally positioned after the printing unit; it is commonly done on coated paper, where the ink stays primarily on the surface and gives a glossy, high-contrast printed picture after drying. Because the paper exits the dryer too hot for folding and cutting, which are normally downstream procedures, a set of "cooling rollers" situated behind the dryer reduces the paper temperature and solidifies the ink. The rate of ink drying is controlled by the drier temperature and the length of time the paper is exposed to that temperature. This printing method is commonly used for magazines, catalogs, inserts, and other medium to high volume, medium-format publications.
Cold-set web offset
This is a subset of web offset printing that is commonly used for lower quality printouts. This is common in newspaper production. The ink dries by absorbing into the underlying paper throughout this process. A typical cold curing configuration consists of a series of print units and peripherals that are vertically oriented. As newspapers seek new markets, they may add a heat-setting tower (with a dryer) or employ UV (ultraviolet) based inks that evaporate or absorb through polymerization rather than "curing" on the surface through polymerization.
Sheet-fed vs. Web-fed
Sheet-fed presses have a number of advantages. Because the sheets are fed separately, a wide range of sheet sizes and format sizes can be run through the same press. Furthermore, waste paper can be used for preparation (which is a testing process to ensure print quality). This lowers preparation expenses by preventing good paper from being wasted when setting up the press for plates and inks. Scrap paper does have some drawbacks, since dust and offset powder particles are frequently transmitted to the blanket and plate cylinder, causing faults on the sheet. The photographs produced by this technology are of the greatest quality.
Yet web presses are significantly faster than sheetfed presses, with speeds of up to 80,000 cuts per hour (a cut is a sheet cut from a roll or reel on the press; the length of each sheet is equal to the circumference of the cylinder). Because of its speed, web presses are perfect for high-volume printing such as newspapers, periodicals, and comic books. However, unlike rotogravure or flexographic presses, web presses have predetermined cutoff times.
Inks
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.
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Letterset inks are mostly utilized in offset presses that lack damping mechanisms and use imaging plates with raised images.
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Waterless inks are heat resistant and used to prevent tinting in non-image areas on silicon-based plates. Typically, these inks are used in waterless direct imaging presses.
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Single-fluid inks are newer inks that use technology that allows lithographic printing on lithographic presses without the need for a wetting system.
In industry
The offset printing method revolutionized the printing industry when it was first used by professionals. Before the development of the offset press, reproducing images that needed to be clear and crisp required a lot of effort, expense, and time.
Offset lithography rose to prominence as the most widely used type of commercial printing in the 1950s ("offset"). The shape of the printing industry was impacted by the need for significant investment in the large presses needed for offset printing, which resulted in a decrease in the quantity and size of presses. The change made a greatly increased use of colour printing possible, as this had previously been much more expensive. The technology's superior production speed and plate durability were further refined through later advancements in plates, inks, and paper. Today, offset lithography, which is "responsible for more than half of all printing using printing plates," is the most widely used type of lithography in the United States.