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Other Printing Methods

Letterpress

Letterpress is a direct printing method where inked plates apply images directly to a substrate. It is referred to as a “relief” process because printed images are produced from plates on which image areas are slightly raised above non-image surfaces. Letterpress is one of the printing processes and was the most widely used until the mid-20th century.

Types of letterpress printing:

Rotary press - The plate is mounted onto a cylinder where a roller system applies ink to the raised area of the plate. The paper passes between the plate cylinder and an impression cylinder where the resulting squeeze between the two cylinders produces the printed impression on paper.

Flatbed Cylinder Press - The type or plate is locked into a chase that is then mounted on the press flatbed. Grippers on a rotating impression cylinder pick up a sheet of paper it is pulled around the cylinder as it revolves. The inked flatbed with the letterpress plate then moves under the cylinder. The compression between the impression cylinder and the flatbed creates the printed image on paper. When the impression is complete the flatbed returns to its original position and is inked for the next impression.

Platen Press - Movable metal type is locked into a frame called a chase, which is then placed in the press bed and locked into position. During the printing process, grippers move paper sheets from the feedboard to the platen, which is the surface where the print impression is made. A set of rollers applies ink to the type on the press bed, then the press bed and platen are pressed together to produce the image on paper. When the impression is complete, the platen and press bed spread apart and the grippers remove the paper to a delivery tray.

Flexography

Flexography is an updated version of letterpress printing that is more versatile because it can print on almost any type of substrate, including plastic, metallic films, cellophane and paper. The process uses a flexible relief plate that can be adhered to a printing cylinder. An inked roller called the “anilox roller” applies ink to the raised portions of the plate, which is then transferred to the substrate. The anilox roller has cells that carry a specific amount of ink to the plate. The density of cells can vary depending on type of print job and quality required.

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Gravure

Gravure is a high-quality process capable of producing printed images that have continuous tone effects similar to photographs. The process uses a metal printing cylinder etched with images. The gravure cylinder revolves in an ink fountain where it is coated with low viscosity ink. A stainless steel blade clears the ink from unwanted areas, leaving ink in the depressions of the cylinder. The substrate passes between the gravure cylinder and a rubber coated impression cylinder, and the ink from the cells is deposited on the substrate. Because of cylinder expense, gravure is most often used for very long runs performed as a rotary web process (rotogravure).

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Screen Printing

Formally known as silk-screen printing, this method employs a porous screen such as fine silk, polyester or metal mesh mounted on a frame. A stencil is produced on the screen either manually or photomechanically. Printing is done on a substrate under the screen by applying ink to the screen. Ink is forced through the fine mesh openings with a rubber squeegee. The stencil protects non-printing areas. Screen printing is usually slower but has lower equipment costs than other printing processes.

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Thermography

Thermography uses powdered resins that are applied to a printed surface on which the ink is still wet, enabling the powder to stick to print areas. Any excess powder or powder on non-image areas is suctioned off before heating takes place. The powdered substrate is passed through a heat tunnel where the powder melts onto the heated substrate and fuses with the wet ink. When the heating process is complete, the sheet is cooled and the melted powder hardens into the raised thermographic image.

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Engraving

Engraving involves a metal die or plate with an impression cut into the surface, which is filled with opaque engraving ink and is then pressed against paper. The paper is usually slightly moist and the pressure of another plate or cylinder forces the ink from the recessed area onto the paper. The pressure also leaves the imprinted area slightly raised and the backside slightly indented, giving the piece its engraved appearance. The raised effect of engraving is similar in appearance to thermography, without the glossy look. Engraving is generally more expensive than thermography.

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Stochastic

Stochastic printing uses a special screening technology that makes continuous tone images that look more like photographs than printed images. The printer takes a finite number of dots and varies their size and placement to create the illusion of images. In first order stochastic printing, all the dots are the same size but very small. The dots are arranged in a random pattern and vary in density to create crisp images. Second order stochastic printing uses dots of various sizes in a random pattern. By removing the barriers of screen angle interference from the printing process, stochastic printing makes it possible for the use of more than the basic four ink colors and allows the accurate reproduction of pastel colors.

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Collotype

Collotype, or photogelatin, is the only process that can print a full tonal range without screening. It’s relatively rare and is a slow process capable of only small print runs. Collotype is used for limited-edition art prints of exceptional quality. To produce a collotype, a right reading, unscreened negative is made from an original image. This is contact-printed to an aluminum plate coated with light-sensitive gelatin. The gelatin hardens in proportion to the amounts of light hitting it. Highlights remain soft while darker areas are harder. Collotype plates are prepared for printing by flooding them with a solution containing glycerin. Soft areas absorb more than harder ones. Ink is then applied, adhering to the hardened areas and progressively less to the softer parts. An impression is made, producing a continuous tone image with a mottled grain.

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