Letterpress printing
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Letterpress printing is the oldest printing technique, in which a raised surface is inked and then pressed against a smooth substance to obtain an image in reverse.
Early Chinese wood-blocks used characters or images carved in relief, and this form of image printing was known in Europe in the 13th century. In the 1400s, Johann Gutenberg (among others) is credited with the invention of printing from individually-cast, reusable letters (moveable type) set together in a forme (frame). He used a wooden press where the type surface was inked and paper laid carefully on top by hand, then slid under a padded surface and pressure applied from above by a huge threaded screw. Later metal presses used a knuckle and lever arrangement instead of the screw, but the principle was the same.
Letterpress.png
A letterpress
With the advent of industrial mechanisation, the inking was carried out by rollers which would pass over the face of the type and move out of the way onto a separate ink-bed where they would pick up a fresh film of ink for the following sheet. Meanwhile a sheet of paper was slid against a hinged platen (see image) which was then rapidly pressed onto the type and swung back again to have the sheet removed and the next sheet inserted (during which operation the now freshly-inked rollers would run over the type again). In a fully-automated 20th century press, the paper was fed and removed by vacuum sucker grips.
Rotary presses were used for high-speed work. In the oscillating press, the forme slid under a drum around which each sheet of paper got wrapped for the impression, sliding back under the inking rollers while the paper was removed and a new sheet inserted. In a newspaper press, a papier-mâché mixture (flong) was used to make a mould of the entire forme of type, then dried and bent, and a curved metal plate cast against it. The plates were clipped to a rotating drum, and could thus print against a continuous reel of paper at the enormously high speeds required for overnight newspaper production.
As computerised typesetting and imaging replaced cast metal types, letterpress began to die out, as photographic imaging onto smooth flexible plates (lithography) became more economical (see Offset printing). A small amount of high-quality art and hobby metal letterpress printing remains -- fine letterpress work is crisper than offset litho because a microscopic border of ink is squashed out around each letter, giving greater visual definition. There is however still a large amount of flexographic printing, which uses rubber plates to print on curved or awkward surfaces, and a lesser amount of relief printing from huge wooden letters for lower-quality poster work.ja:活版印刷 pl:Druk wypukły