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Methods of Printing

Modern lithography or litho as it is often referred to,depends on the photographic processes, flexible aluminum or plastic printing plates.

Modern printing plates have a brushed or roughened texture and are covered with a photosensitive emulsion. A photographic negative of the desired image is placed in contact with the emulsion and the plate is exposed to light.

LithographyAfter development, the emulsion shows a reverse of the negative image, which is thus a duplicate of the original (positive) image. The plate is then chemically treated so the positive image is receptive to printing inks. The plate is affixed to a drum on a printing press. Rollers apply water, which covers the blank portions of the plate, and ink, which adheres to the positive image areas where the type and pictures appear.

If this image were directly transferred to paper, it would create a positive image, but the paper would become too wet. Instead, the plate rolls against a drum covered with a rubber blanket, which squeezes away the water and picks up the ink. The paper rolls across the blanket drum and the image is transferred to the paper.  The image is first transferred, or offset to the rubber drum and is known as offset printing.

A different plate is required for each colour when printing from spot colours or four plates are required when using the four colour process or CMYK method.

Why Litho?

Litho printing is best suited to runs of a thousand copies or more. The cost associated with making plates and setting up a multiple unit press for multi-coloured prints is more cost effect the larger the prints run. Short runs of fewer than 1000 copies are best produced by digital printing as it is a plate-less system and quick to set.

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.

In the 1400s, Johann Gutenberg (among others) is credited with the Letterpress
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.

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.

Why Letterpress?

Letterpress is principally used in the stationery and social stationery market. A metal block needs to be photo-engraved instead of the modern flexible plate before the job can be produced which can be costly. It does without doubt give an excellent finish to a job particularly when using a good quality card but at a premium.

Digital Printer

Digital printing is a recent development that has entered the market. It works directly from electronic data and avoids the intermediate stage of films. Digital is very cost effective for short runs. The quality obtainable is not yet up to lithography standards but is improving steadily and is adequate for many purposes.  These systems use an inherently four colour process so there is no cost saving to be made from using one- or two-colour designs. Two popular digital machines are the Indigo E-Print and Heidelberg Quickmaster.

Why Digital?

No minimum quantity and print on demand. Set up time is minimal with no need for films or plates. One advance of the plate-less natural of this technology is that test prints or proofs can be produced on the same machine as the final job, something that would cost a great deal if attempted using lithography.

Artwork

Originally the physical art (sometimes referred to as Camera-Ready Artwork ) prepared by the designer including type, graphics and other originals, used by use to produce printing plates or converted into a suitable file format for digital printing.

Today the artwork exists almost wholly in electronic form. Photographs and illustrations are input to the computer using a scanner. All the elements are assembled using page layout software. Proofs can be made using colour laser or inkjet printers. The computer then separates the 'artwork' and, subject to the output system being used, produces high resolution films from which printing plates are made or the plates are imaged directly without film.

Can you tell the difference between litho and digital print on screen?

Digital Printing
 
Digital Print  
 
Lithographic Printing
Lithographic Printing

Colour on screen is transmitted light, whereas the printed image on paper is reflected light.