OFFSET
Printing
Offset printing is a widely used printing technique where the inked
image is transferred (or "offset") from a plate first 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 film of water,
keeping the nonprinting areas ink-free.
The advantages of offset printing include:
Consistent high image quality — sharper and cleaner than letterpress
printing because the rubber blanket conforms to the texture of the
printing surface
Usability on a wide range of printing surfaces in addition to smooth
paper (e.g., wood, cloth, metal, leather, rough paper)
Quick and easy production of printing plates
Longer plate life than on direct litho presses — because there is no
direct contact between the plate and the printing surface.
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History
The first lithographic offset printing press was created in England
around 1875 and was designed for printing on metal. The offset
cylinder was covered with specially treated cardboard that
transferred the printed image from the litho stone to the surface of
the metal. About five years later, the cardboard covering of the
offset cylinder was changed to rubber, which is still the most
commonly used material.
The first person to use an offset press to print on paper was most
likely American Ira Washington Rubel in 1903. He got the idea
accidentally by noticing that whenever a sheet of paper was not fed
into his lithographic press during operation, the stone printed its
image to the rubber-covered impression cylinder, and the next
impression had an image on both sides: direct litho on the front and
an image from the rubber blanket on the back. Rubel then noticed
that the image on the back of the sheet was much sharper and clearer
than the direct litho image because the soft rubber was able to
press the image onto the paper better than the hard stone. He soon
decided to build a press which printed every image from the plate to
the blanket and then to the paper. Brothers Charles and Albert
Harris independently observed this process at about the same time
and developed an offset press for the Harris Automatic Press Company
soon after.
Harris designed his offset press around a rotary letterpress
machine. It used a metal plate bent around a cylinder at the top of
the machine that pressed against ink and water rollers. A blanket
cylinder was positioned directly below, and in contact with, the
plate cylinder. The impression cylinder below pressed the paper to
the blanket in order to transfer the image to the sheet (see
diagram). While this basic process is still used today, refinements
include two-sided printing and web feeding (using rolls of paper
rather than sheets).
During the 1950s, offset printing became the most popular form of
commercial printing as improvements were made in plates, inks and
paper, maximizing the technique's superior production speed and
plate durability. Today, the majority of printing, including
newspapers, is done by the offset process, although digital printing
has greatly increased in popularity due to demand and cost
advantages for low quantity runs.
Present day
Offset printing is by far the dominant form of commercial printing
due to its quality in respect of volume and paper costs, with this
market being split between sheet-fed offset for low to medium volume
(any job too large to be economic for laser printer or digital
press, but too small for web offset) and web offset for medium
volume up to the 1-2 million copies market. (For high volume, a
rotogravure press is often used.) The principal difference here is
that sheet-fed litho machines are fed with sheets of paper whereas
web offset machines (which are larger) are fed with reels of paper
and run at higher speeds; the basic offset technology remains the
same. Modern offset presses increasingly use computer to plate
systems.
Private or hobby presses, engaged in patient production of limited
editions of fine quality books, often use letterpress as well as
offset methods, some "purists" preferring the slightly embossed look
resulting from the direct impression of inked type upon fine paper.
These books are sometimes printed from hand-set foundry type
(individual pieces of movable, lead-alloy type). Flexography, a form
of letterpress, is still used in the printing of high-quality
premium labels, in ticket printing, and in envelope
manufacturing/printing, though is now no longer the dominant
technology.
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