Gravure is a high quality process that is capable of producing printed images which have a continuous tone effect. Unlike other processes which utilize a photosensitive plate, the gravure process utilizes a metal printing cylinder onto which the image 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 many as 22,500 depressions per square inch. The various sizes and depths of the depressions create the different densities of the image. A larger or deeper depression, transfers more ink to the printing surface creating a larger and/or darker area. The areas of the cylinder that are not etched become the non-image areas.
During the printing process, the gravure cylinder is coated with a very fluid ink and a stainless steel blade (doctor blade) clears the ink from the unwanted 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 method so there is no need to utilize fountain solution to keep the non-image areas clean. Eliminating the variable of keeping the non-image areas clean allows for better print quality control and jobs can be run at higher speeds. The microscopic depressions on the gravure cylinder create an almost continuous tone image on the printed surface.
Gravure is largely printed as a rotary web process (rotogravure) and is capable of very long press runs. It is used for packaging, magazines, pressure sensitive labels, gift wrap, and wallpaper. A gravure sheet-fed process is used for smaller runs for such items as limited edition prints and other artwork, photographic books, high denomination postage stamps, and some advertising 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 four layer film. A special screen is used with the rotofilm during the exposure to create a square dot pattern of the image. After processing, the carbon tissue or rotofilm is applied to the cylinder and the image is etched chemically into the cylinder. The screen used during the exposure helps to create the walls of the depressions made during the etching process. The diffusion-etch process produces an image 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 process. The photopolymer method is less expensive than conventional gravure, so it can be competitive with offset printing and flexography for runs under 100,000. The plates are made of stainless steel and are mounted on magnetic cylinders. They are much like photopolymer relief plates in that film is contacted with the plate and given a measured light exposure. The exposed plates are then chemically etched to produce the image on the plate. The main difference between the photopolymer plate and a relief plate is that the image on the gravure plate is below the plate surface rather than being above the surface as with a relief plate.