Keeping cool

14 October 2008



David Pelling looks at one UV curing development that helps solve some of the problems of curing on heat sensitive substrates


We’ve all seen the results of overheating on sensitive substrates: distortion, shrinkage, drying out, surface cracking and embrittlement. For converters it can mean poor print register due to substrate movement or web breakage due to heat weakening, and for everyone the worst case scenario could be ‘fire’.

With surface temperatures of a UV lamp reaching around 800 deg C during curing, UV can be problematic for common substrates such as flexible PVC, polyester and PP, all of which distort at between 60 and 85 deg C. But with all the advantages that UV curing brings, it cannot be ignored for heat sensitive substrates.

In a UV lamp system, the mercury plasma contained in the quartz envelope (the lamp) has a core temperature up to 6,000 deg C, so the surface temperature of the quartz has to run in excess of 800 deg C in order to maintain the plasma core temperature and consequent mercury vapour output spectrum.

Output from the lamp is generally only 30 per cent UV energy, the rest being made up of 55 per cent heat energy and 15 per cent visible light, so to maximise the potential of the UV energy a reflector system is used to focus the energy (intensity) onto the substrate surface. But this also gathers heat energy and directs it to the substrate, making its temperature rise.

Over the years, several technologies have been introduced to reduce the heat reaching the substrate: multi-layer lay-down of dichroic coatings to absorb long wave length heat energy; reduction of bulb diameter to reduce IR radiation; heat absorbers such as cooling tubes and hot mirrors between the lamp and the substrate to interfere with the direct heat energy; and lastly by indirect radiation redirecting the UV from the lamp to the substrate via an angled reflector.

All of these methods have been effective to a degree, but all have their downsides. Dichroic reflector facings do reduce reflected heat energy, but direct energy still impinges on the substrate; reducing bulb diameter does lower the amount of heat energy, but again direct energy still impinges on the substrate and with the water cooling tubes or hot mirrors it is difficult to achieve high intensity focus, and the resulting loss of UV energy through the cooling tubes is too high. Indirect radiation provides the lowest temperatures, with little loss of UV energy if close to the substrate, but makes it difficult to focus energy and achieve high intensity.

But now there is a totally new concept for UV curing - the TwinRay, an indirect radiating UV module containing two 120 W/cm lamps with a specially designed reflector to overcome the problems on heat sensitive substrates.

Using an indirect reflective method provides the lowest temperatures, as already outlined, and dichroic coatings on all the reflector surfaces greatly reduce the direct heat energy radiating onto the substrate; whilst the special reflector design ensures that the UV energy is focused onto the substrate

as closely as possible; small, 18mm diameter lamps reduce the direct heat energy even further. Double peak energy is generated, similar to early single lamp modules, which has been proven to deliver good through cure; even though the dose is slightly reduced, overall curing performance is maintained.

With all the heat minimising technologies in TwinRay, delta T is reduced by more than 50 per cent compared with a conventional UV lamp system fitted with dichroic reflectors, minimising heat related issues and keeping the surroundings cooler.

As an added bonus, the dual-emitter design enables standard and speciality lamps to be used side-by-side in the same unit, ensuring optimum cure and extending the process potential. For example, a WhiteCure lamp can be used for depth cure of opaque white inks alongside a standard lamp for surface cure.

As one of the most compact modules on the market, at 120mm² cross-section, incorporating plug and play connections for supply components, replaceable ‘easy change’ dichroic reflector inserts and tool-free technology for plug-in lamp changes in as little as 30 sec, handling, service and maintenance of TwinRay modules is easier than with most other UV systems. In extensive testing by a major press manufacturers and end users, many plastics substrates suffered little distortion, print register was maintained and stack temperatures were reduced dramatically, making TwinRay ideal for coating and converting applications where low temperature UV curing is a must.

David Pelling is managing director of AdPhos UK


Compact, easy to handle design TwinRay double peak irradiation for better cure

External weblinks
Converting Today is not responsible for the content of external internet sites.

AdPhos UK

TwinRay double peak irradiation for better cure TwinRay double peak irradiation for better cure
Compact, easy to handle design Compact, easy to handle design


Privacy Policy
We have updated our privacy policy. In the latest update it explains what cookies are and how we use them on our site. To learn more about cookies and their benefits, please view our privacy policy. Please be aware that parts of this site will not function correctly if you disable cookies. By continuing to use this site, you consent to our use of cookies in accordance with our privacy policy unless you have disabled them.