EB alternative

With the pressures of energy costs and environmental issues, an interesting and exciting alternative to solvent or water based flexo printing has come onto the scene over the past few years, allowing wet-on-wet printing on the CI flexo press and using an electron beam system for final cure. No inter-colour drying or final drying on the bridge is necessary.

Electron beam (EB) curable inks have been available for wet-on-wet, web offset printing for over 25 years, originally for printing liquid packaging laminates and folding carton substrates, but more recently for flexible packaging films, as demonstrated at drupa 2008. In this configuration the inks are printed wet-on-wet and finally cured through an EZCure electron beam unit developed by Energy Sciences Inc (ESI), of the US. Several EB offset lines for flexible packaging films are now installed both in the US and Europe.

Several such machines are now installed and running in Europe providing graphics equivalent to top gravure quality.

The WetFlex process developed by Sun Chemical has great potential. However, the relationship between process and printing machine, plates, anilox and handling of the inks, requires close co-operation of the supplier partners to optimise the process.

The use of EB technology to cure flexo inks or varnishes is an environmentally friendly process that delivers no VOC or CO2 emissions, whilst no photo-initiators are used in the inks. The inks are applied wet-on-wet to the substrate, with no inter-colour drying required. After the final colour, the complete print is then cured 100 per cent by the one EB unit.

Electrons penetrate through the inks to cure down to, and (in the case of paper) into the substrate and are not affected by pigment density.

Compared to conventional systems that use solvent or water based inks, WetFlex allows for a much higher resolution of the graphics. This is due to a finer plate screen, finer aniloxes and a more homogeneous dot structure. In addition to the high resolution capability, it is the excellent ‘trapping’ behaviour, the brilliance and high gloss, the density stability and the durability (scuff and abrasion resistance) of the cured ink that ensures high quality flexo printing graphics.

The process is also ideal for using an expanded gamut printing process, which generally uses five, six, seven and more standard colours to create a broad range of colours. This offers a significant reduction of change-over times and elimination of left-over or waste ink. Compared with conventional flexo presses, there are no dryers and no thermal oxidation systems or similar VOC abatement. This also has the benefit that there are no CO2 emissions and, as the inks are cured under an inert atmosphere, no ozone is produced.

After EB curing, the product treated is completely cured and available for further converting operations. As the reaction is initiated by the creation of free radicals by high energy electrons, rather than by thermal effects, electron treatment is at ambient temperatures, which is useful for treating heat sensitive substrates.

EZCure EB systems are available for electron energies in the range of 70-125 kV. Curing of coatings and inks must take place under an inert gas, which is produced from liquid nitrogen stored on site. According to ESI, the consumption of nitrogen is minimised by the design of the process zone, which limits loss of N2 through the web entry and exit slots of the EB unit.

To summarise, the advantages of EB processing are: no VOCs or CO2 emissions; it is a ‘cold’ curing process; low energy consumption; no dry-in and re-solubility issues; no viscosity control necessary; higher colour consistency, gloss, better scratch, abrasion and chemical resistance; higher definition graphics; and fewer wash-up cycles. The drying energy of an EB system can also be precisely slaved to line speed, even during acceleration and deceleration.

EB inks can cure/dry only in the EB unit and not in the printing units.

Although the inks and printing machinery are being constantly developed and optimised, some excellent results have been achieved over the past year. Clearly, in the coming years the process will gain popularity for those converters who strive for higher print quality, lower energy consumption and VOC-free production.

Lawrence Wild is director of Edlon Machinery, UK representative for Energy Sciences Inc