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colour printing

Colour Printing
Offset colour-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 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.”

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 cost-effectively producing large volumes of high quality prints in an economically sound manner that requires little maintenance. Many modern offset presses use computer to plate systems as opposed to the older computer to film workflows, which further increases 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 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 for producing high quality prints in commercial printing.

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.

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 offset printing process
Side view of the offset printing process. Multiple ink rollers are used to distribute and homogenize the ink.

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 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.

Process printing identification
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 hexagonal though there are different screening methods (AM and FM).

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.

A printing method similar to a sheet offset press; except that the plate and cylinder pressures are very 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
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. 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.

Polyester plates
Polyester plates 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)
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 shorter runs, while violet CTP is employed for longer runs, and popular with two-up and four-up applications.

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.

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 wherein you can expose conventional offset plates, which is very economical.

Sheet-fed offset
"Sheet-fed" refers to individual sheets of paper or paperboard 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”.[12] 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.

Feeder 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.

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 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'.
(extract taken from wikipedia)