Research into different commercial print processes
http://en.wikipedia.org/wiki/Offset_press
http://www.facemediagroup.co.uk/?page=what-is-litho-printing
Offset Printing
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.
Development of the offset press came in two versions: In 1875 by Robert Barclay of England for printing on tin, and in 1903 by Ira Washington Rubel of the United States for printing on paper.
Lithography was initially created to be an inexpensive method of reproducing artwork.[1][2] This printing process was limited to use on flat, porous surfaces because the printing plates were produced from limestone.[1] 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.[1]
The first rotary offset lithographic printing press was created in England and patented in 1875 by Robert Barclay.[1] 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.[1] 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,[1] which is still the most commonly used material.
As the 19th century closed and photography became popular, many lithographic firms went out of business.[1] Photoengraving, a process that used halftonetechnology 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.[3] 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.[3] After further refinement, the Potter Press printing Company in New York produced a press in 1903.[3]By 1907 the Rubel offset press was in use in San Francisco.[4]
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.”[5]
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. 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.[6] 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.
- A further advantage of offset printing is the possibility of adjusting 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.[7]
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.
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:
- 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 the 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.
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 (see this example picture). The paper surrounding the ink dots is usually unprinted. The halftone dots are always hexagonal though there are different screening methods (AM and FM).
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.[11] 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.[12] 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.[12] 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.[12]
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..[12] It is suitable for printing newspapers.
Metal plates: Generally, 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.[13]
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.
Offset printing uses inks that, compared to other printing methods, are highly viscous. Typical inks have a dynamic viscosity of 40–100 Pa·s.[21]
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.[22]
- 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.[22]
- 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.
Offset lithography became the most popular form of commercial printing in the 1950s ("offset printing"). 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.
Lithographic Printing Explained
If you are a designer, printer, or just want to print your company newsletter there are a few terms you need to know. One of them is Lithography.
The name lithography comes from lithos, stone, and graphia and was invented in Prague byAlois Senefelder around 1796. Lithography is best described as a planographic process; this is nothing more than a process for printing from a smooth surface, called a plate, to a substrate, generally paper.
In Mr. Senefelder's day lithography transferred the ink directly from the stone or plate to the substrate. Today lithography is generally called Offset or Offset Lithography; both of these terms refer to the same process. In this printing process, offset lithography, the image to be printed is rendered or etched onto a flat surface, (the plane) such as a sheet of aluminum, plastic, or zinc. Next the image is transferred to a rubber roller, and then finally to the substrate.
How does Lithographic printing work?
How does this work you may be wondering. Well the process is simple and it is based on the elementary principle that oil and water do not mix. There are a few steps in converting your image/text into a printed page so let us discuss the plates first.
In lithography the plates have a roughened texture and are coated with a photosensitive (light sensitive) emulsion. This emulsion is a suspension of two chemicals that cannot be mixed together, a common household example of an emulsion is butter or margarine.
Plates are made one of two ways: Exposure from light source with film on top of the plate, or using a machine that exposes the plate, using lasers directly from the computer. Either way creates a photographic negative of the desired image, releases the emulsion, and transfers a positive image to the emulsion. The emulsion is then chemically treated to remove the unexposed portions of the emulsion. This final step is similar to developing film at a photo lab.
When the printing plate is made, the printing image is rendered grease receptive and hydrophobic, or water repelling. The non-printing areas are rendered hydrophilic, or water attracting, and ink repelling. On the press the plate is mounted on the plate cylinder which as it rotates, comes into contact first with the rollers wet by a dampening solution or water, which adheres to the rough, or negative portions of the image. Then the plate comes in contact with the roller coated with ink, which adheres to the smooth, or positive portions of the image.
If this image were directly transferred to paper, it would create a positive image, but the paper would be moistened. Instead, a cylinder covered with a rubber surface, called a blanket, is rolled over the plate. The blanket squeezes away the water, and picks up the ink. The cylinder is then rolled over the paper, transferring the ink. Because the image is first transferred to the blanket cylinder, we call this process “offset lithography” because the image is offset to the drum before being applied to the paper.
The Advantages of Lithographic Printing
One major advantage of the lithography is that the soft rubber surface of the blanket creates a clear impression on a wide variety of paper surfaces and materials. Lithography printing is easily recognized by its smooth print, as well as by the lack on any impression or ring of ink or serrated edges that are characteristic of letterpress or gravure printing.
Lithography has equipment for short, medium, and long press runs. Sheet-fed and web presses are both used in lithography. Sheet-fed lithography is used for printing advertising, books, catalogs, greeting cards, posters, packaging, direct mail inserts, coupons, and art reproduction.
Many sheet-fed lithography presses can ‘perfect’, print on both sides of the paper, in one pass through the press. Web lithography is used for newspapers, preprinted newspaper inserts, advertising literature, catalogs, books, and magazines.
