Printing

One of the most widely used printing processes to apply decorative and functional coating on substrates is rotogravure printing. From the name of this process, “roto” indicates the use of a rotary press and “gravure” indicates the use of a gravure cylinder where the design is engraved or etched into copper and is chrome plated. The cylinder is a hollow shell and may come in steel or centrifugal cast iron. The thickness of the cylinder’s wall varies from 3/8” to 7/8”, depending on the kind of substrate it will be used on. Lighter substrates like cellophane require light pressure thus thinner cylinders are used. Rougher ones like cardboard require thicker cylinders. The cylinder is prepared by grinding it until there are no signs of scratches. It is then plated with 10” of copper, which is polished down to 0.007”. The design or pattern is engraved into the copper plating after which it is chrome-flashed to achieve a hard and scratch-resistant surface.

The coating material or ink is held in the engraved areas of the cylinder, called cells. The depth of the cells determines the amount of ink that will be transferred to the substrate. Deep cells are used to apply more coating and shallow cells are used to apply less. The portions of the cylinder that are not engraved do not carry any ink.

Because the ink is contained in the cells of the cylinder, it must have the correct consistency so that it can be pulled out of the cells and transferred onto the substrate through capillary action when the cylinder and the substrate are pressed together. The ink should also have good affinity for the substrate to be properly transferred.

There are two types of rotogravure presses: the direct gravure coater and the offset gravure coater. In both processes, the ink is loaded into the cells in the same way. The main difference between them is in the direct process, the ink is transferred directly to the substrate as it is pressed against the engraved cylinder, while in the offset process the ink is transferred to a rubber covered transfer roll before it is transferred to the substrate. The offset process is more suitable for printing on rougher substrates. In gravure presses, the engraved cylinder is the only power-driven roller; all others are driven by the substrate’s movement.

After printing, the coated substrate may be sent to a drying tunnel to evaporate the solvents contained in the ink.