These days UV LED systems are used as a matter of course particularly for adhesive curing and inkjet printing processes. And, because of the many advantages it offers, LED technology is now also breaking new ground in the field of UV curing applications. By 2018, sales of UV LEDs in the curing sector are expected to overtake UV lamp technology.
by Achim Herzog
Thanks to further development and an increase in demand for UV LED chips, the light output of UV LEDs has increased considerably in recent years. As a result, LEDs are now not only more robust and capable of working with higher currents (greater output), they are also more efficient. Whereas in 2011 the efficiency level of 395 nm UV LEDs was around 25% (25% of the energy input was released in the form of UV light energy), it is now above 50%. The efficiency of UV LEDs can be expected to increase to approximately 60%. By comparison, the efficiency of standard white LED household lighting is currently 65% and is likely to rise to 80% in the future.
Short-wave LED chips efficient?
The physical properties of UV LEDs mean that such high levels of efficiency cannot be achieved with every wavelength. Currently for example, roughly 25–30% of the 365 nm wavelength can be reached. In contrast, for a UV-C LED with an emission wavelength of 275 nm the figure is just 2.5–5%. In addition to their poor efficiency, short-wave UV LED chips in the UV-C range are also extremely expensive because they are difficult to manufacture, the small quantities involved and the high production scrap rate.
If the costs of a 275 nm LED are assumed to be 100%, a 365 nm LED chip would only cost 5% of this figure and a 395 nm chip less than 1%. Taking into account the output emitted, a short-wave UV-C LED cannot be considered to be cost efficient at present. Therefore it will be some time before LEDs can supply the broadband spectrum (UV-C, UV-B and UV-A) of a UV arc lamp to suit industrial requirements.
Systems with mixed-wavelength
IST Group and its subsidiary Integration Technology Ltd (ITL) have been using mixed-wavelength UV LED systems since 2011. This produces an improvement in surface curing in certain areas, but the poorer efficiency of the shorter wavelength tends to outweigh the somewhat reduced reactivity of the ink with longer-wave, but more powerful, 385 nm or 395 nm chips.
Until greater efficiency and a higher output can be attained with shorter wavelengths, it will continue to be necessary to make use of suitable UV LED-curable inks formulated for long-wave UV-A (365 to 405 nm).
The interaction of all the individual components involved is an important factor in attaining maximum UV LED chip efficiency. In the case of a UV LED system, cooling and optics in particular have a crucial influence on the efficiency of the system as a whole. For this reason, system cooling should take place as closely as possible to the actual LED chip.
As a rule of thumb, it can be assumed that an increase in temperature of one degree Kelvin will cause the output of the chip to drop by 0.5%. A temperature difference of 20K thus results in output losses of around 10%. To stop this from happening, ITL products employ Varicool technology for air-cooled systems or Steadycool technology for water-cooled systems to keep the UV LED chip at a constant temperature level.
Determing cooling temperature
Thermal simulation for all components at the start of each product development process also makes it possible to precisely determine the cooling temperature required for water-cooled systems. Bearing in mind the requirements of the individual areas of application, the systems can be developed for largely non-condensing operation with a water supply temperature of approx. 20–24 °C without having to make any concessions in terms of cooling of the overall system. The constantly low “junction temperature” is guaranteed.
Good cooling of the back of a UV LED chip is essential to optimise the efficiency of the front of the chip. The optics and the materials used are of particular significance in this respect. As UV is an extremely aggressive wavelength, it is important to optimise all the materials in the area of the radiation source with regard to reflection or transmission. No matter how powerful a UV LED chip may be, the system will not be efficient if the output fails to achieve the chemical reaction level.
New UV LED booster technology
The standard versions of all new UV LED systems from the IST Group feature innovative XT8 UV LED booster technology. The newly developed XT8 booster can increase the UV output by up to 30% for attaining maximum UV LED system efficiency on a press. LEDs offer specific properties: Immediate readiness for use, compact individual chip dimensions, clearly defined emission spectrum. As UV LEDs do not require any run-up times, the LEDs can be turned on and off very quickly. The switching process does not cause any ageing of the LEDs, which means that they can be turned on and off as often as necessary without any losses.
And what’s more?
When switched off, the LEDs do not consume any power or use up any of their service life. This means: Given intermittent LED system operation with equal curing times and intervals (50% curing – 50% interval), users can achieve 50% energy savings whilst doubling the lifetime of the system.
If actuated correctly, the compact dimensions of each individual chip can also be used to good advantage. As LED chips are always operated together in a “string”, it is not possible to actuate each individual chip (approx. 1 mm2) separately, but the individual strings or segments can be operated independently of one another. Such segmented actuation means that LEDs only have to be switched on at the points where they are required for curing. Ideally, only the actual printed image will then be exposed to UV light.
In 2004, in the early days of UV LED development, it was recognized that that e a UV LED emits monochromatic light (a single wavelength only) which inevitably means that no infrared light (IR) is released. Although heat is generated by the absorption of UV light into the substrate and the ink, and the chemical curing reaction is also exothermic (energy is released), far less heat is produced than with a UV arc lamp, where IR radiation accounts for a large part of total emissions. This enables machines to be built of a simpler and more compact design, as cooling rollers, cooled under shielding or the like are then not necessary. In addition, the UV LED systems can be positioned closer to the substrate, thus minimising scattered radiation and ensuring maximum UV yield.
Which is the right system?
When considering the variety of UV LED systems available, the question arises: Which is the right system for the application concerned? If inks are only to be dried to the extent required to stop them penetrating into the substrate or “bleeding” on the substrate, a low-output system is sufficient. With its low UV output, it produces a skin on the printed dot or increases the viscosity of the ink.
This so-called “pinning” is primarily used in digital printing between the individual inks or ink sets. As little space is available because of the mechanical design of the machine, the pinning systems have to be highly compact. To prevent UV light undercutting at the print head, use is also made of optics to focus the Lambertian distribution of the LED light onto a narrow strip. The UV system output required depends, to a great extent, on the reactivity of the ink and on whether the inks are LED-curable or cured using medium pressure vapour lamps. In contrast to final curing, pinning of conventional (non-LED-curable) UV inks is possible in many cases.
Water- or air-cooled UV LED systems
There are basically two categories of systems for final curing: Water-cooled and air-cooled UV LED systems. The more efficient, more direct water cooling method permits higher power densities than is the case with air-cooled systems. As cooling takes place directly via the back of the system, water-cooled systems such as the Solidcure2 series from ITL or the LUV from IST are very compact and can be neatly integrated into the machine. An added advantage of water cooling is that the exchange of heat occurs away from the machine in the heat exchanger, which can be located at a strategically favourable position in the production shop.
If there is a need to avoid the tubes and pipes required for cooling water and to be absolutely certain that no condensate forms on the unit or tubes – regardless of the ambient temperature and humidity – use can also be made of air-cooled UV LED systems with a somewhat lower output.
Simple integration of air-cooled systems
These are slightly larger on account of the cooling fins fitted directly in the head and have the negative side effect of causing air to circulate or the machine/machine components to heat up. Nevertheless, their simple integration can make air-cooled systems a good alternative to water-cooled systems.
Once the question of cooling has been clarified, attention can be turned to the required output. The most important consideration is the fitting position. Whereas ITL systems such as the new SZ or VZ or the Solidcure2 series are designed to be positioned very close to the print carrier, the LUV 80 and the LUV 20 from IST are intended to be used a certain distance away, with integrated optics to optimise the beam path accordingly.
Over a distance, a good deal of the peak output of a non-focused LED system will be lost, which can have a negative influence on the curing process. However, an optimum curing result does not just depend on the peak value (W/cm2). It is also essentially governed by the dose (mJ/cm2), which can be calculated from the dwell time of the substrate under the UV system. To sum up, maximum efficiency can only be attained if the overall system is optimally balanced in line with the machine requirements.