The use of LEDs in UV curing systems is becoming more and more widespread. With the apparent advantages of LEDs such as reduced heat radiation, instant on-and-off switching and the absence of moving parts LED UV curing systems offer certain benefits that have to be balanced against the higher initial cost and the particularities of this technology.
The British UV systems manufacturer GEW (EC) Limited has been actively engaged in researching the potential of UV LEDs for many years but is also committed to further develop conventional arc lamp technology for label printing, metal decorating and industrial applications. So as to correctly understand the possibilities and potential pitfalls of LEDs a closer examination of LED technology is essential.
Malcolm Rae, Managing Director at GEW explains: “At 25 to 30%, the percentage of electrical energy converted to UV radiation by an LED is actually very similar to that of an arc lamp. The total power requirement for an LED UV curing system is also very similar to that of an arc lamp solution at around 100W/cm.” This may come as a surprise considering the energy-saving label that LEDs have assumed.
“It is true, LED’s are substantially more efficient than older UV systems but the differences compared to modern arc lamp systems in operation are minimal,” says Rae. “The real energy advantage for LEDs is that they switch on and off instantly, not requiring the warm up and standby cycles of arc lamps. This reduces the non-production energy consumption significantly and can make LEDs an attractive energy saving alternative in environments with short production runs and high standby cycles.”
The second important consideration is lifetime cost efficiency. LED arrays potentially last over 20,000 hours, about seven to ten times longer than an arc lamp with 2 to 3,000 hours.
“But it is important to remember that a replacement LED array can cost around 15 times more than all the equivalent lamps over the same period.” continues Rae. “We must also acknowledge the fact that accidents happen. Repairing an arc lamp housing after an incident is an inconvenient cost, but replacing a damaged LED array is a substantial expense”.
With similar power usage and efficiency, the unwanted heat produced by LEDs is also similar to an arc lamp. However, with LEDs most of this heat is absorbed by water-cooled heat sinks behind the light source, and in general only UV radiates down towards the substrate. With an arc lamp, both UV and heat are radiated in all directions and special reflectors are required to return the UV to the substrate and absorb as much of the heat as possible. Thus LEDs are able to cure on heat sensitive materials without water-cooled rollers, although this must be verified on a case-by-case basis.
So in what particular circumstances would one use UV LEDs? LEDs produce a narrow band of high intensity long wave UV, compared to the broad spectral output of arc lamps. This can penetrate thicker, heavily pigmented inks much better than UV from arc lamps, and helps support higher running speeds in what have traditionally been difficult applications. Some special low migration ink formulations also demonstrated faster running speeds when cured with UV LEDs. In addition, the lack of short wave radiation means there is no ozone production, removing the need for extraction and saving significant air extraction costs. Extraction may, however, still be desirable to prevent ink and curing odours from accumulating in the production area. LEDs are solid state components and thus a highly accurate and infinitely controllable UV source. Converters can more reliably tune the power of the array to their requirements and control the UV output and product quality of their process more accurately than ever before. The arrays can be width-switched, e.g. turning off the outer edges when a narrower web is being run, and there are even choices available on the spectral output.
LEDs are therefore more flexible and customisable. ‘In an industry where flexibility and speed are valued increasingly by the end customer, LEDs are a solution today for the cutting edge and specialist applications and are a future solution for all applications when efficiency, power and cost become competitive against existing arc technology,’ says Rae. ‘We at GEW believe that uncomplicated solutions are the best. LEDs are the simplest and most effective solution for some printing applications, particularly for inkjet printing. Our LED UV curing solutions for this industry can very easily be fitted to a Flexo press, but this does not mean it is automatically the best option for every customer.”
The costs of LED inks and curing systems are likely to come down and the performance of both will increase further. Today LEDs are a more expensive alternative but if current trends continue, they will become an attractive proposition in the future. Considering that label converters will not want to duplicate their ink stocks with special LED-compatible formulations and have to master a new production process, demand for energy-efficient arc lamp systems will continue to grow.
“At GEW we are committed to working with ink manufacturers to explore and develop the opportunities for LED curing in all sectors of the industry, but we remain equally committed to building on the strengths of proven, viable technology. GEW offers its customers experienced and objective advice on whether LEDs or arc lamps are best suited to their production scenario.” says Rae. “In parallel with the development of LED UV curing solutions, GEW has been working on the new RHINO electronic power supply that further increases the performance and efficiency of our E2C low-energy arc lamp systems.”