Today, the concept of "only those who adapt to change, survive" is well understood.

The graphic arts and printing industries have also undergone rapid evolution with many of the traditional analogue processes being replaced by digital methods. This extends from the front end, whereby all prepress operations are now almost exclusively digital, through to direct to plate or press. 

There is a perceived growth in digital printing in those markets requiring print-on-demand, variable print or coding. Inkjet has been proven to offer advantages as a non-contact printing process, UV ink providing superior material performance for more demanding applications encountered within the industrial sector.
 
UV curing

The UV process is well established and applies to traditional printing methods. The change in state from liquid (wet) to solid (dry) is almost instantaneous as the ink is passed at high speed under a UV light source. The ink usually consists of 100 per cent solids and is photo-reactive such that it will harden to give the required performance on exposure to UV.

The composition of a free radical acrylated ink system includes the oligomer, which gives the overall mechanical properties; reactive diluent to control viscosity and curing characteristics; photo-initiator to kickstart the polymerisation reaction; pigments to impart colouration; and additives to provide the final properties.

The lower viscosity can increase the rate of oxygen diffusion and inhibit cure. Some inks are therefore cured under nitrogen. 

The photo-initiators are responsible for absorbing UV light at specific wavelengths: the UV-A region (320-390nm) is usually required for most effective through curing and the shorter wavelength UV-C (<280nm) for surface cure. The cure speed of inks can vary widely across suppliers, particularly for difficult to cure colours such as black. Application thickness must also be controlled, as the greater the thickness, further the UV light has to penetrate. Inks are applied typically within the range of 6-25 microns.
 
UV light generation
 
Alternative methods of generating UV light suitable for inkjet applications include classic arc, microwave powered UV and more recently LED technologies. Arc lamps have wide bore and are supplied in a range of lengths with different types of end-fittings for electrical connection. In comparison microwave bulbs are narrow bore, limited to 250mm or 150mm curing width and contain no electrodes (see Figure 1).

The light from the UV source is collected and directed onto the ink by means of reflectors. Dichroic coated reflectors are now more commonly used than polished aluminium as they offer more effective heat management and also can exhibit much higher reflectivity in the UV-C region.

A cooling mechanism is required to maintain the lamp and reflector at the optimum operating temperature, in addition to directing infrared or heat away from the substrate. Heat-management is therefore often a critical part of providing a UV solution and systems include water cooling, air cooling or a combination of both.

With classic arc technology, a high voltage is applied between the electrodes to ionise the inert gas within the lamp and form the arc. Due to the rapid increase in temperature, the small amount of mercury (and other metal halide additives) also contained within the lamp then vapourises and enters the arc to form a plasma. Inter-atomic collision with the high-speed moving electrons stimulates photon emission.

The Medium Pressure Mercury Arc (MPMA) lamp gives broad spectral output over the entire UV spectrum. Use of metal halide additives such as iron or gallium in doped lamps shifts the spectral output to longer wavelengths.

The alternative method of generating UV light is to use a microwave powered UV system. A uniform microwave field is created by means of a magnetron to energise the lamp and stimulate photon emission. An RF screen consisting of fine wire gauze positioned at the front of the reflector maintains the microwave energy within the cavity.

All microwave powered UV systems use high-velocity positive air cooling of the magnetrons, bulb, reflectors and substrate. Bulb life extends beyond arc lamps as there are no electrodes to deteriorate. However, magnetrons also have a finite life and need to be replaced.

Unlike classic arc or microwave powered UV, LEDs are relatively low- powered and are usually arranged in arrays or clusters to increase UV output. Increased power is limited by heat generation, which ultimately reduces life. The alternative is to pulse the LED or provide water cooling.

The UV output is very narrow-band and the selection of photo-initiators is limited to longer wavelengths. Combining LED sources of different types within the same cluster is one method of extending the spectral distribution. 

LED technology can be used effectively for 'pinning' or partially curing the droplets immediately following contact with the substrate so as to reduce any absorption or spread and change in dot size. Final curing can then be provided downstream by more conventional UV sources.
 
Specific requirements of a UV curing system for inkjet application

Irrespective of which technology is used to generate UV light, most of the UV curing equipment for use with inkjet printing has been adapted rather than designed specifically for the market. Traditional UV lamp systems are often too large, heavy and use simple high velocity air for cooling that can disrupt the ink droplets. Those systems operating at standard focal lengths can also cause pre-curing on the ink nozzles.

Often, the main requirement is to provide a small and compact high-power UV curing system that can be readily integrated into the process. 

Future single pass digital printing machines will likely utilise smaller printheads and operate at higher line speeds. Hence there is a need for higher-power lamp systems with high-dose efficiencies, yet these must emit little heat, be lightweight, small-size and low-cost. In addition, there is a requirement for lower power UV systems for 'pinning' on single or multi-colour applications.

Potential UV solutions

LED solutions are now available suitable for 'pinning'. The next generation of higher-power LED systems require water cooling to ensure long life. The LED array can be connected to an umbilical cord supplying power and heat management. In this way, the array can be fitted into extremely small spaces.

In cases where extreme high power is not required and heat sensitivity of the substrate is not critical, simple air-cooled arc lamp technology can be used for 'pinning' or final UV curing.

As these types of system normally support the lower-cost end of the market, the lamphead is usually fitted with a polished aluminium rather than dichroic reflector. Standard mercury or doped lamps often operate at a fixed power level from a ballast power drive.

Although microwave-powered UV has been successfully used for industrial inkjet applications, the physical size constraint makes this a less attractive solution for some applications requiring direct integration within a printing machine. However, these systems can be used with remote blowers to limit size. Some systems now offer an exhaust box accessory that allows UV exposure through a sealed quartz plate whilst extracting the exhaust by means of a separate fan.

One example of a high-power UV curing system that has been designed specifically to meet the needs of the ink jet market is shown in Figure 2.

The system relies on high purity de-ionised water being re-circulated through a single quartz IR filter located beneath the lamp, dichroic reflectors and lamphead (Figure 3).

The direct IR is absorbed by the water and removed via a heat exchanger. The system is ideally suited to a wide range of substrates including very heat-sensitive thin gauge filmic materials such as LDPE, PVC and foil-coated paper. Being totally water-cooled, there is no ozone generation and therefore no need for air extract.

Due to the effective heat management, the lamphead can be positioned very close to the substrate to provide extreme high intensities. The reduced operating distance also eliminates any pre-curing on the nozzles. Being water-cooled, there is no disruption of the ink droplets from high velocity air. The irradiance profile is also very uniform across the total curing width, the slight fall-off towards the electrodes at the ends of the lamp being compensated by providing a lamp of slightly longer length.

The UV system designers continue to make advances towards the ultimate solution that will meet all requirements of ultra-small size, lightweight and high power. One example of such a system is shown schematically in Figure 4. The golf ball gives an indication of overall scale. Again water cooling is used to effectively control heat management and totally eliminate the need for air cooling and exhaust extraction.

For further information, please visit: www.nordson-uv.com