Course:LIBR548F/2010WT1/Offset Printing

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Offset printing, alternatively known as offset lithography or offset photolithography, is a form of printing derived from lithography and rotary printing. Images are applied to a lithographic cylinder and then transferred, or offset, to an intermediate rubber blanketed cylinder which then prints to a substrate. Efficient, high quality, and fast, this has become the most common method of producing high-volume books, newspapers, and periodicals.[1]

Press room, The Topeka Capital-Journal, September 18, 2008.



Description

 

Contributing Technologies

 

Lithography

Main article: Course:LIBR548F/Lithography

Lithography is a planographic printing method based on resistance. A lithographic surface (historically limestone, but now more commonly a metal or synthetic sheet) is printed with an image (text, graphics, or both) using a hydrophobic, or water-repelling, medium. The void spaces are coated with a hydrophilic, or water-attracting, solution. When the surface is inked, the ink clings to the hydrophobic image and is repelled by the hydrophilic solution, keeping the voids free of ink. The printing substrate is pressed against the surface, transferring the image. [2]

Rotary printing

Rotary printing employs a cylindrical printing surface. A substrate is pressed between this and either a platen or counter-rotating impression cylinder, with each rotation producing an image. Rotary presses can be fed by sheets or continuous roll (web fed printing).[3]

Process

Offset printing schematic.

Offset printing is a two step process using three cylinders.

  • A lithographic surface is a metal or synthetic sheet which has been photo-printed from a negative to produce a right-reading image. The sheet is then applied to a cylinder, damped with water, and inked. The inked image is then transferred to a rubber blanketed offset cylinder, reversing the image.
  • The printing substrate is then pressed between the offset and impression cylinders, producing a right-reading final product. The substrate is then dried and trimmed.

Variations

  • Dry, or waterless, offset printing is achieved using an inherently ink resistant surface, such as metal with a silicone coating, on which an image is photo-printed. This technique allows the use of water soluble inks.
  • Duplex, or perfected, offset printing is achieved with four cylinders rather than three: two lithographic surfaces transfer to two rubber blanketed offset cylinders, each of which acts as the impression cylinder for the other. This can also be achieved by passing a substrate through a series of rollers to present the reverse side to a second set of cylinders.
  • Colour offset printing is achieved cumulatively by offset printing in sequence with multiple cylinder and inking sets, and requires precise registration to be effective.

Comparisons

  • Strengths:
    • Offset printed images are sharper than letterpress text.
    • Offset printing is an order of magnitude quicker than letterpress printing.
    • Thin layers of ink used in offset printing dry very quickly.
    • The forgiving surface of the rubber blanket creates a smoother image on most types of paper and many other media. [4]
    • The more forgiving rubber blanket causes less wear on lithographic plates, slowing the erosion that would otherwise result from contact with the substrate. This allows for hundreds of thousands of impressions from a single plate, and tens of thousands of impressions from a single blanket.[5]
    • Metal or synthetic lithographic plates store compactly and, if carefully preserved, can be used for many subsequent print runs.[6]
    • Offset printing is not constrained to cast type and relief images, as with letterpress.
  • Weaknesses:
    • Offset printed graphics are not as brilliant as those produced by gravure.
    • The production of original lithographic plates is too expensive and time consuming to be economical for short runs.[7]

Development

In England, offset printing was patented in 1853 by John Strather, but it wasn't until 1875 that Robert Barclay used rubber blanketed cylinders on a flatbed press to print on metal.

In America, offset printing to paper was independently developed by Charles and Albert Harris, Caspar Hermann, and Ira Rubel. In 1904, Rubel was operating a lithographic press with a rubber blanketed impression cylinder when a misfed sheet led to a serendipitous discovery. He found that the image transferred by the impression cylinder was clearer and sharper than that made by the metal cylinder.[8] He then developed a press to take advantage of this.

Timeline of offset printing and related technologies.

Like many innovations in printing, implementation was resisted, and the mechanized letterpress remained the most common form of book production until the mid-20th century, because of the cost and labour involved in production of lithographic plates as well as textural objections. [9] In addition, it was resisted by organized labour amid fears of job loss. Also, like other innovations in printing, profit margins prevail, aesthetic expectations adjust, and jobs evolve to meet the requirements of offset printing maintenance and preparation, which demand human skill and intervention.[10]


The photo transfer process, patented in 1860, allows a photographic image to be projected onto specially treated paper, which is then inked and the image transferred to a lithographic plate. Images of type could be transferred to lithographic plates, but type still had to be laboriously set to be photographed. Photocomposition evolved over the first half of the 20th century, and by the 1970s phototypsetting had become the predominant means of producing text for offset printing.[11] As a result, production by offset printing has become competitive enough to replace letterpress printing for large print runs.[12]

With the advent of desktop publishing, digital images can be transferred either photographically or directly onto specially prepared lithographic plates, further increasing production efficiency. Electronic image files can be stored and recalled efficiently.

Impact

The industrialization of printing has been driven by a growing demand for newspapers and periodicals by an increasingly educated and literate populace. The industrial revolution dramatically changed industries and economies. For example, Henry Bessemer's development for steelmaking allowed for steel to be made in large quantities more cheaply than the lolder processes for blister or crucible steel. As a result, machines for all types of industry, including pressess and machinery in paper mills, became less expensive to produce, in turn making their products less expensive.

With the cost of reading materials decreasing, and more cost-effective distribution (rail, air, etc.), reading materials became more readily available. Political, economic, and social trends were communicated more quickly, fostering greater education and literacy opportunity, cyclically driving the printing industry.[13]

Offset printing represents a convergence of technologies, and is an example of "how profoundly information and media technology affect culture and history, and how vital the technology of the book has been to cultural and intellectual change." [14]

Notes

  1. ↑ Banham 2007, p.273
  2. ↑ Howard 2009, pp. 140-141.
  3. ↑ Twyman 1998, pp. 70-71.
  4. ↑ Twyman 1998, p.78.
  5. ↑ Howard 2009, p. 142-143.
  6. ↑ Howard 2009, p. 142.
  7. ↑ Banham 2007, p. 285.
  8. ↑ Howard 2009, p. 141.
  9. ↑ Banham 2009, p. 285.
  10. ↑ Howard 2009, p. 143
  11. ↑ Banham 2007, p. 288.
  12. ↑ Twyman 1998, p. 78.
  13. ↑ Banham 2007, p. 273.
  14. ↑ Howard 2009, p. ix.

References

1. Banham, R. (2007). The industrialization of the book 1800-1970. In S. Eliot & J. Rose (Eds.), A companion to the history of the book (pp. 273-290). Chichester, West Sussex, UK: Wiley-Blackwell. ISBN 13: 978 1 4051 9278 1

An essay focusing on the technologies that contributed to the mechanization and automation of printing, with brief social commentary.


2. Howard, N. (2009). The book: The life story of a technology. Baltimore: Johns Hopkins University Press. ISBN 10: 0 8018 9311 9

A succinct, illustrated, easily read story introducing the development of printing technology, from papyrus to e-book, in the context of social, political, and economic change.


3. Twyman, M. (1998). The British Library guide to printing. London: The British Library. ISBN 10: 0 7123 4588 4

A concise, illustrated treatment of printing technologies from early Chinese examples to digital technologies by a scholar noted for his contributions to the study of the printing and lithography.

Further reading

Kipphan, H. (2001). Handbook of print media: Technologies and production methods. New York: Springer. ISBN 13: 3 540 673261

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