Screen, relief, intaglio and thermographic printing. The various types of printing. Screen printing. Screen printing uses a stencil — which is mounted to a screen — to form images.
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Relief printing processes have raised image areas and sunken non-image areas.
Inked rollers touch the top of the raised image. The non-image does not receive ink because it is too far below the surface for the ink rollers to touch.
The raised and inked image is then pressed directly onto paper or other substrate. The ink transfers (splits) from the raised surface to the substrate.
There are two relief printing processes…one uses a hard raised plate and the other uses a softer raised plate.
“Letterpress” uses a hard relief plate (metal or hard polyester). The process is the same as students used to print on the Columbian and Gutenberg presses at the museum of printing history. The process is very seldom used nowadays except for fine arts and historical applications.
“Flexography” uses a softer relief plate. The process is used extensively for the production of all sorts of packaging materials from labels to food boxes to plastic bags.
Neither letterpress nor flexography can print extremely fine text or halftone dots.
Flexo plates stretch when they’re mounted to a press cylinder. The stretch is around the cylinder. So, the prepress process must shorten the around-the-cylinder length by an amount determined by a special formula. The across-the-cylinder dimension does not change.
Since flexo is often used for packaging, the specifications for bar codes and RFID tags must be followed carefully.
Documents to be printed by Flexography need to adhere to the specifications published in FIRST (Flexographic Image Reproduction Specifications and Tolerances—available through the bookstore on www.flexography.org)
Traditionally, both letterpress and flexography began with metal type characters (show type in type stick and linotype slugs)
When letterpress was used, the type itself, or a curved metal “stereotype” made from the type, was used to directly print onto the substrate. This hard plate-against-hard paper resulted in a somewhat broken appearance in type (especially headlines), solids, and halftone photos.
The softer flexo plates compress slightly when they print on rough stocks and can press the ink deeper into the crevices than can letterpress. Thus, flexo images are smoother than images printed by letterpress.
Gravure: the image is created using a stylus to “peck” cells into a metal cylinder. The cells, which can vary in circumference and depth, are filled with a liquid ink, the excess ink is squeegeed off of the raised non-image area (with a doctor blade), and the paper is pressed against the cylinder. (Refer to the Intaglio Printing Process PDF and/or pg 146 of Kenly.
Ink rollers press ink into the engraved image…and cover the non-image, too.
The ink is wiped off of the non-image area and then a large amount of force is applied to press the paper into the engraved image. This results in a characteristic “raised” image on the front of the sheet and sunken image on the back of the sheet.
This is a time consuming, difficult, and costly process used primarily for the production of currency, stamps, and other security items.
Engraving is also a symbol of status…as in the old saying, “What are you waiting for? An engraved invitation?” Thus, engraved stationery and business cards are associated with wealth or rank within an organization.
Prepress and platemaking vary considerably from one intaglio process to another and differ from lithography and screen-printing.
Gravure: prepress uses digital illustration, photographic, and page layout files like lithography.
Completed forms are sent to a RIP that converts the entire image, not just photographs, to dots. Every image…including “solids,” text, and illustrations printed by gravure is composed of dots (pass around samples of gravure printing and a loupe).
A HelioKlischograph follows the instructions provided by the RIP and engraves wells of varying circumference and depth into a copper cylinder.
Engraving: Engraving plates for security purposes are generally cut by hand by a “master engraver” and then duplicated for multiple-up printing. This is a fundamental part of the security of money and other negotiables because engraved plates are hard to counterfeit and few people have the skill and training to engrave.
Engraving plates can be cut by laser for more “mundane” purposes such as business cards, wedding invitations, and “classy” stationery.
Engraving plates can also be prepared photographically by exposing an image to a light sensitive coating. The image area is washed away and then an etch is used to “cut into” the metal plate.