Offset printing is a widely 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.
Offset printing 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.
- The more you print, the less you pay per page, because most of the price goes into the preparation undergone before the first sheet of paper is printing and ready for distribution. Any additional paper print will only cost the client paper price (and ink), which is very minimal.
Offset printing 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.
Photo offset
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.
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.
Paste inks for offset litho
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 are usually the most expensive type of offset litho ink.
Fountain solution
As mentioned previously, 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 comprised 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 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.
Present day
Offset printing is the most common form of high-volume commercial printing, due to advantages in quality and efficiency in high-volume jobs. While modern digital presses (Xerox iGen3 Digital Production Press or the family of HP Indigo solutions or Kodak Nexpress solutions, for example) are getting closer to the cost/benefit of offset for high-quality work, they have not yet been able to compete with the sheer volume of product that an offset press can produce. Furthermore, many modern offset presses are using computer to plate systems as opposed to the older computer to film workflows, which further increases their quality.
In the last two decades, flexography has become the dominant form of printing in packaging due to lower quality expectations and the significantly lower costs in comparison to other forms of printing.
References
See also
www.dougrose.co.uk for a dissertation on the process of printing, binding and manufacturing a large-format case-bound book with slip case. See the section "How the Book Was Made".
External links
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