Description

Gravure is a top quality technique that can producing printed images which have a continuous tone effect. Unlike other processes which utilize a photosensitive plate, the gravure technique utilizes a metal printing cylinder onto which the picture is etched. The printing impression is made by the transfer of ink from depressions (cells) of various sizes and depths that are etched onto a copper sheet which is plated onto a steel cylinder. The cylinder can contain as lots of as 22,500 depressions per square inch. The various sizes and depths of the depressions generate the different densities of the picture. A bigger or deeper depression, transfers more ink to the printing surface making a bigger and/or darker area. The areas of the cylinder that are not etched become the non-image areas.

In the coursework of the printing technique, the gravure cylinder is coated with a liquid ink as well as a stainless steel blade (doctor blade) clears the ink from the undesirable areas, leaving the ink in the depressions of the cylinder. The ink in depressed areas is then transferred to the printing surface. Gravure is a direct printing process so there is no require to utilize fountain solution to keep the non-image areas neat. Eliminating the variable of keeping the non-image areas neat allows for better print quality control and jobs can be run at higher speeds. The microscopic depressions on the gravure cylinder generate an  continuous tone picture on the printed surface.
Gravure is largely printed as a rotary web method (rotogravure) & can very long press runs. It is used for packaging, magazines, pressure sensitive labels, gift wrap, & wallpaper. A gravure sheet-fed method is used for smaller runs for such items as limited edition prints & other artwork, photographic books, high denomination postage stamps, & some promotion pieces.

Gravure Plating Methods

Diffusion-etch: Diffusion-etch is a method that utilizes a carbon tissue or rotofilm onto which positive images are exposed. Rotofilm is a special presensitized layer film. A special screen is used with the rotofilm in the work of the exposure to generate a square dot pattern of the picture. After processing, the carbon tissue or rotofilm is applied to the cylinder & the picture is etched chemically in to the cylinder. The screen used in the work of the exposure helps to generate the walls of the depressions made in the work of the etching method. The diffusion-etch method produces an picture that is of high quality but the plate/cylinder may only last for 50,000 impressions.

Photopolymer: A photopolymer plate is also utilized with the gravure method. The photopolymer method is less pricey than conventional gravure, so it can be competitive with offset printing & flexography for runs under 100,000. The plates are made of stainless steel & are mounted on magnetic cylinders. They are much like photopolymer relief plates in that film is contacted with the plate & given a measured light exposure. The exposed plates are then chemically etched to produce the picture on the plate. The main difference between the photopolymer plate & a relief plate is that the picture on the gravure plate is below the plate surface than being above the surface as with a relief plate.