Gravure Printing Mechanism
Source:LPC Date:11/23/2010

Gravure Process
Gravure is a mechanically simple and direct printing process. The ink is transferred directly to the substrate from small cells that are precisely engraved into the surface of the printing cylinder (image carrier). The cylinder rotates in the ink pan; the ink is wiped from the printing cylinder surface by a doctor blade. Each engraved cell holds the ink in place as the cylinder turns past the doctor blade. The substrate is pressed onto the printing cylinder surface by the rubber covered impression roller, resulting in a direct ink transfer on the substrate.

The printing group and the dryer are the two main sections of a Rotogravure printing unit. The printing group of each rotogravure printing unit consists of:
Printing cylinder
Doctor Blade
Impression roller
Ink pan
Inking system
The printing cylinder consists of seamless tube of either steel, aluminum, plastic or composite materials. It is then electroplated with copper and then the image is etched (engraved) so that the print areas are below the surface of the cylinder. The next step is to chrome plate the engraved copper cylinder to increase the life of the engraving.

The doctor blade is the device that removes the excess ink from the non engraved portions of the printing cylinder. The doctor blade contact angle and distance from the printing nip is key to balance the print quality, ink drying and press speed. The doctor blade is mounted on a precise and rigid mechanism to insure that it is straight and ripple-free. It is pneumatically loaded onto the printing cylinder and oscillates back and forth across the printing cylinder, reducing cylinder wear and helping to remove debris build-up which can be trapped under the blade.

The impression roller is a rubber covered sleeve, that is mounted on a steel mandrel. It is friction driven, with the primary purpose of squeezing the substrate against the printing cylinder. For substrates that have a surface roughness, electrostatic assist (ESA) is a normal practice to improve the printing quality. ESA applies an electric field between the printing cylinder and the impression roller.

The inking system consists of an ink pan, ink holding tank, ink pump with delivery and return ink hoses. Automatic ink viscosity control systems and fine ink filters are normally connected within the inking system and contribute in maintaining high and consistent print quality levels.

The dryer main purpose is to dry the wet ink on the substrate, prior to next printing unit. Dryer sizes are determined based on required printing speeds, ink types and ink laydown volume. A chill roller is mounted at the exit point of each dryer to insure the web enters the next print station in ambient temperature.

Precise color to color registration is made possible, via an automatic side and length register control system.

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