A gravure print cylinder is typically chromed steel or a copper plate mounted on a cylinder. As it rotates in the ink bath or doctor blade chamber, ink is transferred to the engraved surface, with the excess removed with a metal or plastic doctor blade. Computerised control of the printing register gives high quality print.
In rotogravure, unlike offset, printing takes place by direct contact between the printing cylinder and the substrate. The web passes between the print cylinder and a counter pressure roller, so that ink is transferred from the print roller to the substrate.
For this purpose the used ink is generally very liquid and contains volatile organic carbons (solvents). In the case of water-based inks, the solvent content is largely reduced or eliminated.
Rotogravure printing in flexible packaging
The gravure process is suitable for a range of substrates used to make flexible packaging. In this type of application, a booster corona treatment before print can be beneficial for plastic films, and coated papers which have already been treated at extrusion. This is because they lose dyne rate over time and it is therefore desirable to restore an optimal wettability rate.
The corona treatment unit is usually mounted before the infeed to the printing section of the press to treat the material before it reaches the printing units.
The main effect is to increase the surface tension of the material being processed, allowing the ink to remain firmly anchored over time.
The choice of corona treatment
For the installation of a corona treater on gravure printing lines, the following factors should be considered:
- type of material to be treated (plastic or metallised)
- printing width
- dyne rate required
- one or two sided treatment
- suitable electrodes according to the substrate and printing process.