Lawrence Wild, sales director of Edlon Machinery, profiles electron beam technology
The WetFlex process developed by Sun Chemical has great potential, however, the relationship between process and printing machine requires close co-operation of the partners. The inks are applied wet-on-wet to the substrate with no inter colour drying required. After the final colour, the complete print is cured safely by one EB unit.
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 scuff and abrasion resistance of the cured ink that ensures high quality flexo printing graphics.
The process is ideally suited for using an expanded gamut printing process: generally with five, six, seven and more standardized 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. When compared with conventional flexo presses there are no dryers and no thermal oxidation systems or similar VOC abatement. Additionally, there are no CO2 emissions and since the inks are cured under an inert atmosphere, no ozone is produced.
Electron processes are used to modify the molecular structure of materials. EB curing involves electron initiated free radical polymerization. In this process the materials are modified in a few milliseconds, after which the product treated is completely cured and available for further converting operations. Since 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.
Our principal Energy Sciences Inc offers the EZCure range of electron beam processors with multiple cathodes. These systems are available for electron energies in the range of 70-125kV.
The processors have several filament wires arranged inside a vacuum chamber in parallel to the web direction. The filaments emit electrons that are accelerated to high energies by a high voltage. The electrons emerge through a thin metallic window into atmospheric conditions and penetrate the material to be treated.
The penetration depth of electrons depends on their energy (in kilo-electron-volt, keV) and the density of the material to be treated. Therefore, the penetration for a given electron beam energy may be expressed in terms of the weight of material irradiated per unit area, commonly expressed in grams per square metre. The surface weight is equal to the penetration in microns for materials of density 1 (eg, 20g/m2 = 20 micron). To determine the required electron energy, penetration curves are used for ESI equipment.
The dose is the amount of energy required to cure the ink. The absorbed energy in the material treated is expressed in Megarad (Mrad) or in Europe, kiloGray (kGy):
1Mrad = 10kGy = 2.4cal/g = 108erg/g
The dose can be influenced by the EB current output and the production speed. It is directly proportional to the beam current output and inversely proportional to the product throughput rate. If the current output is doubled at constant production speed, the dose is also doubled. This means a required dose can be obtained by adjusting the current output and/or the production speed.
The dose rate defines the throughput that can be obtained. It is expressed in Mrad metre per minute (Mrad m/min), and in Europe, in kiloGray metre per minute (kGy m/min). An EB processor with a dose rate of 1,200Mrad m/min can apply a dose of 3Mrad (typical of these inks) to a product at a speed of 400m/min (or 4Mrad at 300m/min).
Curing of coatings and inks must take place under an inert gas. Oxygen is a quencher of the free radicals reacting faster with the free radicals of the chemistry to be cured than with the radicals amongst themselves. A perfect curing of the surface is inhibited if the process takes place in air. The inert gas used is nitrogen.
The advantages of EB processing are: no VOCs or CO2 emissions; it is a ‘cold’ curing process requiring low energy consumption; no dry-in and re-solubility issues; no viscosity control required; higher colour consistency and gloss; better scratch, abrasion and chemical resistance; higher definition graphics; and fewer wash-up cycles.
The drying energy of an EB system can be precisely slaved to line speed. EB inks dry only with electrons and not in the printing unit.
With EB processing you meet all environmental protection requirements with the lowest possible energy consumption along with high production speeds, low operating costs and faster equipment amortization.