A polymer layer for electromechanical engraving

After ten years of intensive research and development work, the time did finally come: In 2020, Rossini officially launched Ecograv, the world’s first automatic production line for the polymer coating of gravure cylinders. The intention is to replace the galvanic process steps “copper plating” and “chrome plating”.

“However, the best thing about this solution is that the special polymer layer is suitable for electromechanical engraving. This is most important due to the fact, that still about 90% of all gravure cylinders produced are engraved using this well-established process,” says Flavio Losa, Development Engineer at Rossini.

The Italian company Rossini Spa is a well-known solution provider of sleeves and rollers with elastomer and polymer covers for applications in the flexo, gravure and coating industries. “Our decades of experience in this area served as an ideal starting point for a very ambitious, long-term research project that we initiated more than ten years ago under the personal leadership of Felice Rossini. “The objective of this project was to offer an alternative to the traditional electroplating process in gravure cylinder production,” explains Flavio Losa. “Even though the electroplating of gravure cylinders with copper and chrome is well established in the industry, it is still a highly complex and energy-intensive process. Due to the related health and environmental risks, it has to meet the highest safety standards. ”

The current state

Chromium trioxide, also known as chromium (VI) oxide, is used in the electrolyte solution for hard chrome plating of gravure cylinders. However, chromium trioxide in its galvanic (liquid) state is environmentally hazardous and highly toxic. In contrast, the metallic chrome deposited on the cylinder is completely harmless.

Chromium trioxide is listed in REACH Annex 14 by the European Chemicals Agency (ECHA). Use is therefore generally prohibited unless authorization has been granted. In this regard, the gravure printing industry has achieved some success. On December 21, 2020, the responsible EU Commission approved the application for authorization for further use of chromium trioxide until September 21, 2024.

“This provides the gravure printing industry with a transitional period to adjust to this new situation. Efforts are being made to continue using chromium (VI) beyond 2024, but with the Ecograv system, Rossini now offers a long-term and future-proof solution compared to the previous galvanic process,” Flavio Losa continues.

Designed for electromechanical engraving

The Ecograv system developed by Rossini is a four-step process for gravure cylinder production that replaces the copper and chrome layers with a wear-resistant monolayer. The layer, which consists of a special polyurethane-based polymer, is explicitly designed for imaging using electromechanical engraving.The production of this new type of cylinder provided with a polymer monolayer takes place on an automated production line.

A pilot system is scheduled to go into operation in the last quarter of 2021 at the technology center at Rossini’s Italian headquarters. “There, customers and interested parties can experience the new process concept on site and convince themselves of its numerous advantages. As Rossini’s development partners, the two Italian companies Gerosa and ICR Spa significantly contributed to the success of this project,” says Flavio Losa.

Development partners

The Italian package printing company Gerosa, which also has its own cylinder production facility, contributed intensively to the implementation of the process concept and is engaged with Rossini for the delivery of the first Ecograv system.

ICR, Italy’s leading gravure cylinder manufacturer, also supported the project by taking over the engraving part and electromechanically engraved numerous test cylinders provided with the special polymer layer.

A four-step process

Instead of the previous eight to twelve process steps, the single-layer Ecograv process only requires four steps for the preparation of gravure cylinders:

1. Coating
   Application of the Ecograv polymer layer on any gravure cylinder instead of a copper/chrome layer
2. Curing of the polymer layer
   The polymer layer is thermally treated in a static oven
3. Surface finishing
   Grinding and polishing of the hardened polymer surface. The polymer surface of the gravure cylinder is grounded exactly to the circumference and is also given a defined surface roughness below 0,1 Ra
4. Electromechanical engraving
   The image is electromechanically engraved into the polymer surface (stylus engraving)

Diametrical properties

For Rossini, the challenge in developing this new type of coating was to meet two essential properties of a gravure cylinders, which are usually diametrically opposed to one another. On the one hand, the polymer layer must meet the typical properties of a hard chrome layer (hexavalent chrome), and on the other hand, it has to provide the appropriate cutting properties for electromechanical imaging with a diamond stylus.

“Therefore, a major challenge and thus a focus of our research and development activities was at the doctoring abilities of the polymer layer. In gravure printing, sufficient surface roughness is most important for the build-up of a non-printing lubrication film between the surface of the gravure cylinder and the doctor blade, preventing the cylinder from running dry and thus bearing the risk of the cylinder surface being destroyed. Our new polyurethane-based polymer layer not only approximates the surface roughness of chrome but can also be electromechanically engraved. This should be mentioned as previous experience with synthetic composite materials exclusively relates to direct laser engraving. In addition, it must also be emphasized, that on a global scale stylus engraving is still the mayor process used for gravure cylinder preparation. However, it is quite possible, that in the near future Rossini may also introduce Ecograv gravure cylinders with a laser-engravable surface layer,” Flavio Losa continues.

As the Ecograv gravure cylinders are highly conductive, no build-up of electrostatic charges occurs on the polymer layer. In addition, their mechanical and chemical resistance properties are excellent and they are suitable for all commercially available water- and solvent-based inks and varnishes. To refurbish worn-out Ecograv cylinders for re-coating, the engraved layer is turned off and the surface then grinded and polished. With the application of the new polymer layer the cylinder is then ready for re-engraving. “The resulting polymer waste can be disposed like usual domestic waste,” Flavio Losa explains.

Intensive print tests

At the Rossini research center, Ecograv cylinders have already been used for several intensive monochrome print tests at speeds between 200 and 250 m/min. A test chart was printed on a 12-micron polyester film with a run length of 370,000 meters. Standard doctor blades and standard solvent-based ethyl acetate inks were used.

“Rossini was extremely satisfied with the test results,” says Flavio Losa. “No tonal changes occurred during the production run and we were amazed that compared to chrome, the electromechanically engraved polymer layer achieved better ink release properties. Therefore, no ESA system commonly used in gravure printing to improve release properties of the cells was necessary. High colour densities, smooth tonal gradations, fine line elements as well as homogeneous solid areas were consistently printed out over the entire run. In the near future we will carry out further print tests with other images, parameters and substrates with systematic approach.”

In addition, Rossini will soon start a series of tests under production conditions in collaboration with his development partner company Gerosa. In this context, a very sophisticated 7-colour job consisting of monochrome background, barcodes, Arabic characters, fine and coarse line elements as well as solid areas and special Pantone colours will be printed using Ecograv cylinders.

Environmental and economic aspects

The four-step Ecograv process for gravure printing cylinder production not only includes the generation of a polymer layer for electromechanical engraving, but also an automatic inline process consisting of the production steps layer application, layer hardening as well as surface grinding and polishing. The subsequent imaging or electromechanical engraving is performed offline.

Compared to the conventional electroplating process for copper and chrome, the materials used for the Ecograv process are non-toxic. They do not cause any pollution by waste water or exhaust air and therefore are not subject to any official approval procedure.

For the application of the 2 mm thick polymer layer all market available gravure printing cylinders can be used. This means that all printing presses in operation do not require any adjustments when using Ecograv cylinders. Flavio Losa says: “Considering the fact that gravure print shops usually keep thousands of cylinders in stock, this is real cost saving. As far as space requirements of the Ecograv line are concerned, this is comparable to conventional electroplating lines. However, the polymer coating of Ecograv gravure cylinders requires less investment and fewer production steps. In addition, it is much more difficult to keep the conventional electroplating process stable.”

Conclusion

With the four-stage Ecograv process developed by Rossini, a special polymer layer suitable for electromechanical engraving is applied to the surface of the gravure cylinder. Therefore, this is a real alternative to the established electroplating process in gravure cylinder production.

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