15-12-2012, 05:58 PM
OFFSET PRINTING
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HISTORY
The word 'lithograph' historically means "an image from stone.“
The first rotary offset lithographic printing press was created in England and patented in 1875 by Robert Barclay.
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.
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 image carrier on which the image to be printed obtains ink from ink rollers, while the non-printing area attracts a water-based film, keeping the non-printing areas ink-free.
Offset Lithography
Based on the principle that ink and water don’t mix.
Images are photographically reproduced on printing plates which are dampened first with water, then with ink.
Ink adheres to the image area, the water to the non-image area
The image is transferred to a rubber blanket (offset) and then to the paper.
TYPES
Photo offset
the photo offset process, 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.
Offset lithographic 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 via a suction bar that lifts and drops each sheet onto place.
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 of paper supplied to the printing press.
Offset web printing is generally used for runs in excess of five or ten thousand impressions.
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.
COMMERCIAL OFFSET PROCESSES
Sheet-fed offset
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.
PROCESS
Pre-press production stage makes sure that all files are correctly processed in preparation for printing.
The sheet fed press consists of different systems that complete the actual process;
feeder system
printing system
inking/dampening system
delivery system.
COMMERCIAL OFFSET PROCESSES
Web offset
A high run, speed printing press that uses rolls of paper rather than individual sheets.
Web offset presses are beneficial in long run printing jobs, typically press runs that exceed ten or twenty thousand impressions.
Some web presses print at speeds of 3,000 feet per minute or faster.