Despite continuous advances in the past decade, flexo printing using solvent-based or water-based inks still has some limitations, including problems encountered in obtaining high quality and stable printing results, because At line 157, there is a limit to the amount of halftone dot transfer, as well as a significant effect on the results due to solvent evaporation and an increase in cost. Water-based inks may also contain 5% to 15% organic solvents. The development of UV-curable flexo inks for wide-format (800~1300mm) flexographic flexo presses has significantly promoted the advancement of printing technology in this field and greatly improved the quality of printing results.

It has been nearly 30 years since UV-curable inks were used on machine-type flexographic presses. This technology is so common that 80% of all narrow web printers sold worldwide are printers that can use UV curable inks, while the remaining 20% ​​are designed for water-based or solvent-based inks. It is not an easy task to develop a printing technology that uses UV inks on satellite printers with a width of more than 800mm. One of the reasons is that a large amount of heat generated during the operation of a UV light source (60% of the radiation amount of the UV light source is infrared) is difficult to eliminate. There are only a few companies in the world manufacturing satellite-based UV printers, but the results and benefits of this model show that flexo will no doubt be dominated by this technology. For example, Czech's SOMA Engineeing, a manufacturer of satellite printing presses, confirmed that in the past three years, the company has provided six 800-mm wide eight-color printing presses to Central and Eastern Europe, and the demand continues to grow. At the same time, the rapid development of new-generation UV-curable inks and the continuous reduction in their prices have further promoted this demand.

Because many people have always believed that the price of UV inks is relatively high, this has limited the popularity of this technology to some extent. And as detailed analysis shows, the cost of printing with UV inks is almost the same as using solvent inks.

User expectations and development trends
Flexo printing is constantly evolving to meet packaging user expectations such as:

â—† Higher and more stable printing quality must be comparable to rotary gravure.
â—† Short version of live production capacity.
â—†Real-time printing.
â—† consistent print repeatability.
â—† Expand the product range and production capacity of printing plants.
â—† Make printed packaging materials cheaper.
â—† Can accurately determine and continuously improve the health and safety standards for packaging production.

Only those modern, well-organized printing companies can meet these expectations. They have the most advanced presses and are optimized for user needs in terms of print width, print speed, number of print colors, and pressroom. The best technology that may be required to meet these needs is flexographic printing with UV-curable inks.

At present, the development of the flexo process and technology mainly focuses on the following aspects:

â–  Continuously improve the printing quality, mainly through the development of new photosensitive resin plates, more precise ink transfer system, accurate anilox roller engraving, design sleeve clips, maintain and control the tension of the web and a higher degree of color Registration and other ways to achieve.
â–  Faster job conversion speed, which is mainly achieved through the introduction of pneumatic connection sleeve technology, to achieve a convenient and rapid replacement operation on the operator side.
â–  Eliminates gears and mechanical shafts and replaces them with direct or electronic drives and automatic cleaning systems.
â–  Reducing the loss rate and achieving reproducibility at the time of printing, using a digital system that uses print process control and color adjustment.
â–  Reduce the impact of volatilization on the environment, use solvent-free UV-curing inks, and reduce the cost of solvent catalytic combustion.
â–  Increase production capacity, adopt higher printing speed, electronically control print parameters, unwind/unwind the unit, and adjust the printer's width according to the width of the print job.
â–  Improve printing accuracy by using UV-curable inks to achieve higher halftone dot density.
â–  Expansion of press selection and production capacity, which includes increasing the number of printing units and additional processing units, such as adding cold seals, hot stamping, automatic clearing, inspection systems using full-frame cameras, compounding, etc.

In addition, a new video control system for the continuous automatic monitoring of printing jobs is also very important. It can indicate, locate and archive printing errors, and can also monitor colors, bar codes and related parameters.

UV technology on satellite presses
One of the things involved in applying UV technology to satellite printers is the use of special inks that dry immediately when exposed to ultraviolet light. The ink contains no solvent or water, and no other ingredients. Therefore, after the UV light source provided between the satellite printing units and a UV light source disposed downstream are irradiated, all printing ink components remain on the substrate. The length of the UV light source is generally 5 cm wider than the widest possible print width. It can produce the maximum amount of UV-C light. The wavelength of this light is less than 300 nm, which can make the ink's curing efficiency the best .

The UV curing process uses only about 30% of the amount of light radiation, while the remaining 70% is mainly infrared, visible light and a small amount of ozone. Of the heat generated by it, 20% is absorbed by a water-cooled satellite drum, 30% is absorbed by the water-cooled circulation system of the UV light source, and the rest is discharged via the air cooling system. Ozone is not a major problem because it usually decomposes into oxygen when it is discharged outside or through a catalytic filter.

In order to achieve full operation capability, a UV light source rated at 200 W/cm should be used. Therefore, an 800-mm 8-color printer runs at 200 m/min and consumes 128 kW (800 mm × 8 colors × 0.2 kW = 128 kW). The 5-color printing with a running speed of 100 m/min consumes 40 kW.

For this application, GEW in the United Kingdom has introduced a UV light source with a metal case and an external water tank. It uses a reflective material and the reflector also rotates with the water cooling tank to ensure that when the press has stopped Printed substrates will not be destroyed. This light source also has an additional water cooling tank for absorbing any other extra heat. Each light source has an independent power supply, air discharge system and water cooling system connection. It is also very convenient to replace the UV lamp in the light source. According to data provided by GEW, this light source has a lifetime of at least 2,000 hours.

Conventional photopolymer plates can also be printed with UV inks, and because of the lack of solvents and other properties that can damage their structure, their print resistance and service life have been improved. This is also the case with the pumps, hoses, rubber rollers and anilox roller sleeves used. However, some ink suppliers recommend the use of a photosensitive resin plate specifically designed for UV ink printing. They also recommend the use of ceramic coated anilox roller sleeves with special ink pocket shapes and very narrow edges. Satellite printers equipped with UV-curable ink systems can be kept clean without frequent cleaning. Because the ink on the printing plate does not dry, no blockage of the dots occurs, and there is no need to manually clean the plate.

Because the UV ink does not dry on the press, the printing process can be suspended for several days. The crew part used to print CMYK only needs occasional cleaning. There is also no need to measure and control the viscosity of the ink, as its viscosity is constant as long as the temperature of the ink remains constant. There is also no risk of fire and explosion, so there is no need to use expensive anti-static systems and fire suppression systems, which also means that printing companies can reduce the cost of insurance. The printing quality obtained with the UV ink technology is higher than that obtained with the water-based or solvent-based inks, mainly because the former has a higher ink transfer capacity and printing capacity above the 178 line, resulting in halftone dots. And the density of text is also higher.

The printing quality of UV flexographic printing can be comparable to that of rotogravure printing, but the cost of its pre-installation facilities and printing preparation is much lower. UV prints are clearer, brighter and more glossy, and they are more resistant to mechanical, chemical, and temperature changes. It is generally not necessary to add additional protective agents or polishes. Because UV-curable inks have good adhesion to most substrates, there is no need to add some primer to enhance adhesion. In addition, UV inks can be printed on thinner substrates, such as films with grammage less than 1 g/m2, due to the higher color ratio and 100% dryable content. Almost all known substrates (including Aluminum Foil, PVC, other film materials, and metal foils) can be printed with UV inks. Because there is no solvent, there is of course no need for expensive solvent residue testing.

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