The different printing techniques: roto-gravure process
Roto-gravure is a direct transfer method for printing on flexible supports, an ink or a varnish (lacquer); the most commonly used materials are:
- polyester films (PET), polypropylene (BOPP), nylon (BOPA) and polyethylene (PE).
- Plastic film and aluminum laminated together
- Two plastic films laminated together.
- Thin aluminum.
The current rotogravure printing machines use a printing cylinder on which are etched by laser or mechanical means or by a chemical process, tiny holes able to retain the ink. Their size and pattern depends on the image to print. These cells transfer the ink directly on the support under the combined action of pressure and capillarity of the support, in order to produce the printed image
The gravure process is used in the manufacture of food packaging and non-food labels, and various other applications in the industrial, and security printing.
Roto-gravure printing lines
On a gravure printing line, the printing units are installed in the required number on a horizontal axis, next to each other. In traditional gravure printing press, each printing unit consists of:
Printing cylinder: continuous tubular sleeve or full cylinder, made of steel, aluminum, plastic or composite material, on which is engraved the image to be printed,
Chamber doctor blade system: equipment for the removal of the ink non-etched areas of the printing cylinder, as well as removing the excess ink in etched areas,
pressure cylinder: rubber coated sleeve mounted on a steel mandrel. Its main function is to press the support against the printing cylinder,
Inking system: it consists of an inkwell, of an ink resevoir and an ink pump with pipes for feed and return system,
Drying system: it consists of a chamber where the ink dries after being deposited on the substrate and before reaching the next phase. The dryer capacity depends on the printing speed, the type of ink (aqueous or solvent based) and inking.
The printing cylinder rotates in the inkwell, its engraved (etched) cells being then filled with ink. A chamber doctor blade removes the excess. The cylinder comes into contact with the support, which is held against it by a pressure rubber cylinder.
Under the effect of the pressure cylinder and the capillary attraction of the support, the ink is transferred directly from the cells of the printing cylinder on the support surface. The printed support then goes into a dryer which leads to the next group, for the purpose of applying another color or another varnish.
Automatic control systems of lateral and longitudinal register also allows for precise registration between colors.
On the gravure press, the tape is rewinded after printing different colors, and after sometime applying varnish in the final phase, which allows a finished reel.
Benefits and applications
The roto-gravure process allows to transfer ink evenly, over a very large range of densities and at high speed. Therefore it is particularly suitable for applications that require high image quality, such as publishing, packaging, labels and security printing or decorative.
The resistance of the printing cylinders make rotogravure the ideal method for high quality printing during very long or repetitive prints, where it is more cost effective than other methods.