In gravure printing, the image is etched or engraved into the gravure cylinder. Unlike other printing processes, gravure prints from ink-filled and depressed cells produced on the copper cylinder surface. Cell ink then gets transferred to the desired substrate. A number of methods exist when it comes to transferring the ink to the desired substrate.
Diffusion Etch and Direct Transfer
The Diffusion-Etch or conventional gravure engraving process is the oldest gravure cylinder engraving technique. The method utilizes two film positives. One is the image's film positive and the other a kind of special gravure screen. These contain anywhere between 100 to 200 lines per inch. This screen is utilized to convert any solid image into multiple cells. Each cell is a small square oriented at 45-degree angle towards web travel direction through the press. The screen and positive image are positioned on top of carbon tissue and water soluble paper encased with gelatin resist. The latter is also sensitive to light. These are then exposed to the ultra-violet light. Cells have the same size in conventional gravure engraving. Cell wall-the membrane thickness between cells- remains constant. The quantity of received light determines cell depth. Light areas and highlights result in deeper cells. The deeper cells hold more ink.
Another gravure cell engraving method is Direct-Transfer Process. It is similar to conventional gravure system. The difference between the two techniques is the resist composition. The resist is a substitute for carbon tissue with the high resolution and high contrast photopolymer emulsions. This emulsion is directly applied to the gravure cylinder's copper plated surface. Similar to diffusion etch process, the non-image or exposed areas become hard and the image or unexposed areas continue to be soft. The unexposed resist is washed away by the solvent. Cells thus produced by photopolymeric resist prints with considerably more smoother edges compared to cells that got etched by any electromechanical engraving.
Mechanicals
The third gravure cell engraving method is Electromechanical engraving. The process consists of a diamond stylus controlled by electronic means to cut cells into gravure cylinder surface. A computer is used to scan original copy to digitize it. The scanned copy is then converted into dots similar to halftones. Each is an electronic signal, which may range in intensity beginning from zero to 100 percent. This depends on whether the image is a dark one or a light one. The latest gravure engraving method is Laser Cutting Process. It utilizes computer directed lasers.
