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Web-fed offset lithographic press at speed


Offset printing is a commonly used printing technique where 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 Englandmarker for printing on tin, and in 1903 by Ira Washington Rubel of the United Statesmarker for printing on paper.

History

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. Tin cans were popular packaging materials in the 1800s, 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 Englandmarker and patented in 1875 by Robert Barclay. This development combined mid-1800s 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 Jerseymarker, 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 Yorkmarker produced a press in 1903. By 1907 the Rubel offset press was in use in San Franciscomarker.

Present day

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 are using computer to plate systems as opposed to the older computer to film workflows, which further increases their quality.

Advantages

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 to produce high quality printing in commercial printing quantities.


Disadvantages

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.


Types

Photo offset

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 litho printing on to a web (reel) of paper is commonly used for printing of newspapers and magazines for high speed production.

Types of paper feed

Sheet-fed litho

"Sheet-fed" refers to individual sheets of paper or paperboard being fed into a press. 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.

Web-fed litho

"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 10 or 20 thousand impressions. Typical examples of web printing include newspapers, newspaper inserts/ads, magazines, 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 is air dried, 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). There is also another possibility of adding a drier in a cold-set press and making it as a semi-commercial press. It is a concept where, a newspaper can print colour pages in heatset and BW pages in coldset

Types of chemicals used

Paste inks for offset litho

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 employment 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 cost-conscious option. Energy-curable inks are the highest-quality offset litho inks and are set by application of light energy. They require specialized equipment and are usually the most expensive type of offset litho ink.

Fountain solution

Fountain solution is the water-based (or "aqueous") component in the lithographic process that cleans the background area of the plate in order to keep ink from depositing (and thus printing) in the non-image (or "white") areas of the paper. Historically, fountain solutions were acid-based and made of gum arabic, chromates and/or phosphates, and magnesium nitrate.

While the acid fountain solution has come a long way 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 started becoming less common due to the inherent health hazards of high pH and the objectionable odor of the necessary microbiogical 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. That said, 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).

References

  • Hird, Kenneth F. Offset Lithographic Technology. Tinley Park, Ill: Goodheart-Willcox Co, 2000. ISBN 9781566376211.
  • "Offset Printing". Encyclopædia Britannica. Retrieved March 22, 2004, from Encyclopædia Britannica Premium Service.[60411]


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