Requirements finesse of printing for markets such as luxury, agri-food and security involves a choice of different rasterized cylinders. Indeed, the size and depth of the alveoli, burning technique,… are parameters must be defined before realizing a gravure cylinder.
Attention to detail, finesse and precision are so assets implemented when burning in cylinders of impressions, to hope to eventually get an impression of quality.
THE REMOVAL OF THE COPPER
The first step, which is common to all the techniques of engraving, is removal of a thin layer of copper on the soul of the cylinder base most often steel. But today more and more of its cylinders are made in lighter materials to reduce their costs and to facilitate their handling.
Copper is deposited by electroplating, that is, through a continuous electric current to be put down a layer of metal (copper here) to the surface of the object. It is initially in the form of cations in solution in a solvent (typically water).
The cylinder is then consistently polite, in order to obtain a smooth touch as possible. It can also be ground or cut according to customer demand.
THE DIFFERENT TYPES OF BURNING
o electromechanical engraving
The electromechanical engraving is one of the most used techniques in the market. A stylus driven by electromechanical impulses diamond serious cells in the copper layer. It is the set of engraved cells that makes up the image to smear.
This technique ensures a high print quality. Indeed, depending on the size and depth of the engraved cells we can adjust the amount of ink deposited and the print quality.
o direct laser engraving
This technique requires you to first put a layer of zinc (by electroplating) over the copper layer. It is then a laser beam that will come to create the image on this outer layer.
The alveoli can be of shapes and sizes, so that different levels of fineness. Indeed, most cells are small, more finesse and the print quality will be high. Direct laser engraving is used for printing on aluminium.
o burning laser and chemical attack
This technique requires you to first put a protective layer of a polymer (3-5 µm of thickness) black, above that of copper. The laser beam just reproduce the design by selectively removing the protective resin, without affecting deep copper (it marks only on the surface). A solution composed of ferric chloride finally comes to attack the copper layer, where it is no longer protected by the black resin. Finally, this protective layer is removed through a final wash.
This technique is known as being the first technique of illustration used in roto-gravure. This method is characterized by its very high engraving resolution, and is for example used for printing in the area of security threads.
Here, it’s a milling head coming to burn on the surface of copper because the customer wants embossing. In general, it is necessary to have two embossed cylinders (a ‘male’ and ‘female’) when printing with this technique.
It’s a print finishing technique very appreciated for its wide range of applications, and that is very often used for packaging cardboard printing (ex: packs of cigarettes).
PROTECTION BY CHROME
The final stage of preparation of the cylinders in rotogravure is chrome. As for copper and zinc, chromium is deposited by electroplating. This helps protect the copper layer and the engraved image against corrosion, discolouration, wear…
Mickaël Reynier, R & D engineer