Rotogravure Press

Overview

A rotogravure press prints from cylinders whose image is engraved as millions of tiny recessed cells, the opposite of the raised relief used in flexo and letterpress. Each cylinder turns through a pan of thin solvent ink, a steel blade wipes the smooth surface clean, and a rubber roller presses the web so ink lifts out of the cells onto the substrate. Because the image is cut into hard chrome over copper, a gravure cylinder survives tens of millions of impressions without losing detail, and the cell depth varies the ink film point by point to give smooth tones. That durability and tone quality make gravure the press of choice for very long runs of flexible packaging, decorative laminates, and publication printing, where a single design runs for hundreds of thousands of metres.

The web enters at the Unwind and Web Infeed, passes through a row of Gravure Print Unit decks each followed by a drying oven, then exits at the Rewind and Web Outfeed. The Solvent Ink System feeds solvent ink to every unit, the Drying and Solvent Recovery flashes the solvent off and recovers it, the Drive and Register System turns and registers each cylinder, and the Control System runs the line. Everything mounts on the rigid Machine Frame.

The gravure print unit

Each color is a separate Gravure Print Unit, and the chain is direct. The lower part of the Gravure Cylinder dips into the Ink Pan, so its cells fill with ink as it turns. A Doctor Blade Assembly holds a thin Doctor Blade at a set angle that wipes the smooth lands clean, leaving ink only in the recessed cells. The web then runs between the cylinder and a rubber Impression Roller under high nip load, which presses the substrate into contact so the ink transfers out of the cells. On smooth films an Electrostatic Assist charges the web to pull ink fully from each cell and stop the speckle that otherwise appears on shallow cells. The cylinder runs in cone Cylinder Chuck chucks and gets its own Servo Motor.

The engraved cylinder

The cylinder is the cost and the heart of gravure. A Cylinder Base of steel is electroplated with a Copper Engraving Layer soft enough to engrave, the image cells are cut into the copper by a diamond stylus or a laser, and the surface is then covered with a thin Chrome Wear Layer of hard chrome so it resists the constant wiping of the Doctor Blade and lasts a full run. Cone Cone Shaft ends chuck the cylinder into the unit. Because each cylinder is engraved per design and per color, makeready is slow and costly, which is why gravure pays off only on long runs: the cylinder cost is spread over hundreds of thousands of impressions. The Blade Oscillator on the doctor assembly shuffles the blade sideways so its wear, and any score in it, spreads out instead of leaving a fixed streak.

Ink, drying, and solvent recovery

Gravure inks are very low in viscosity so they flow into and out of the small cells. The Solvent Ink System gives each color a Ink Circulation Pump and Ink Supply Tank, with a Viscosity Controller that doses solvent to hold viscosity as it evaporates, and an Ink Filter on the return. Each unit has its own Drying Oven right after it, because the next color cannot print until the previous one is dry. The oven blows hot air from a Nozzle Box driven by a Recirculation Fan and Heating Element heaters. Solvent-laden exhaust goes to a Solvent Recovery Unit unit whose Carbon Bed adsorbs the solvent and is then steam stripped, a Solvent Condenser turns the vapor back to liquid, and the solvent is reused. A LEL Monitor keeps oven vapor below the explosive limit.

Materials and register

The substrate is usually a plastic film such as BOPP, PET, or PE, often later laminated, and also paper and aluminum foil for high-barrier packaging. Films run twelve microns and up and are corona treated upstream so the ink anchors. With each Gravure Cylinder on its own Servo Motor in the Drive and Register System system, a Register Camera reads register marks and the Register Controller shifts cylinder phase to hold color-to-color register within tens of microns, with an Cylinder Encoder closing each loop. Web tension is held end to end by the Dancer Roll and Tension Load Cell sensors, and the Chill Roll cools the web before the Rewind and Web Outfeed so the finished roll winds flat.

Variants and use

Packaging gravure presses run eight to twelve units at three hundred to nine hundred metres per minute and feed downstream lamination and slitting. Publication gravure, used for catalogs and magazines, runs even wider and faster on lighter paper with fewer colors. Where runs are short the engraved-cylinder cost cannot be justified, so converters use a Flexographic Press or a digital press instead, since those have no per-job cylinder to engrave. Gravure keeps its place where the run is long and the image quality matters: the Gravure Cylinder holds fine detail and smooth tone over millions of impressions, and the Solvent Recovery Unit plant keeps the high solvent throughput economical and within emission limits. The press splices rolls on the run at the Automatic Splicer and delivers a finished printed roll at the Rewind and Web Outfeed for the next converting step.