What are the opportunities for LED UV technology in the graphic arts industry?

The basics of LED UV technology (Part 2)

When professionals from the graphics art industry are asked about the future of the UV curing of printing inks, they also mention LED UV technology as well. The first part of this series of articles talks about the basic technology behind the Light-Emitting Diode (LED). This second part discusses where the opportunities lie for LED UV technology in general and how it could be developed for its specific application in the graphic arts segment.

Thanks to the attractive advantages offered by light-emitting diodes, particularly from the viewpoint of energy consumption, service life and compact size, several branches of industry are extremely interested in this technology. Of these, the branches with the greatest overall economic importance are lighting manufacturers and the automotive industry. For both industries, the generation of white light is at the top of their wish list for the current technical development of LED technology. They are partly motivated by circumstances such as the announcement by the Australian government of its desire to replace all conventional bulbs in private households with energy-saving alternatives such as LED lights. Similar proposals are emerging in other countries. The automotive industry also uses LEDs in tail lamps, for example. LED headlamps are currently also making their way onto the market.

One man’s meat is another man’s poison

These examples show the huge market potential of LED technology, particular in the area of white light LEDs. In contrast, the use of LED UV for curing printing inks and varnishes is advancing at a rather modest pace. Nevertheless, these two areas are more closely tied than one would suspect. Since light-emitting diodes are only capable of radiating the colours of one spectral range, LEDs can only produce white light in a roundabout way. As already described in Part 1 of these articles, use is made of UV LEDs embedded in an illuminant layer for converting the radiated short-wave light into white light. As a result, the market for LED UV in the graphic arts industry - which may be small in terms of volume - can directly benefit from this limiting feature. The market power of the lighting and automotive industries could therefore give LED UV technology in the printing industry a considerable boost.

Despite this, Dr. Bernd Brandl, Product Manager of LED UV systems at the IST Metz corporation, anticipates a delay of several years before a widespread market launch is on the cards.

Entry via inkjet and high ink layer thickness

We are currently witnessing the first applications of LED UV technology in the graphic arts industry, as demonstrated in presentations by inkjet suppliers at the Drupa trade fair. Above all, LED UV was presented on large format printing systems using the multipass method. Also on display were two single-pass inkjet printing units for narrow-web label printing: a system from Atlantic Zeiser, designed for a printing width of 36 mm and a printing speed of 60 m/min, and an LED system from Summit UV, which prints labels at a speed of 25 m/min using the inkjet rotary process. The LED UV systems can therefore be used both for freezing or pinning the ink droplets and for curing the inkjet inks.

In a study of an LED UV application, the company IST METZ demonstrated the printing of a PP film in 10 µm of white screening ink at a speed of 50 m/min on an LED laboratory system. At the present time, LED systems for surface curing with a UV output from 1 - 4 W/cm² at a wavelength of 395 nm, and 0.5 - 2 W/cm² at 365 nm are commercially available from a varied range of suppliers. We can expect to have to upwardly adjust these figures soon, as in the past the performance of UV light-emitting diodes has mostly almost doubled year on year.

In focused spotlights, it is already possible to achieve considerably higher UV intensity in the mm² range.

At present, commercially available LEDs with wavelengths right up to the UVB range do not achieve the necessary UV output or performance to be considered for industrial curing applications. These developments must, of course, be pursued with interest.

Major hurdles for LED UV in sheet-fed offset printing

Whether the performance described above is sufficient for LED UV curing to establish itself in the field of sheet-fed offset printing is a matter of speculation. It also remains to be seen whether the large amount of general interest was really beneficial for this new LED technology, or whether all the fuss at the Drupa even had a negative effect by raising unrealistic expectations, even though several vital preconditions have yet to be satisfied before the technology can enjoy widespread use in industrial sheet-fed offset printing.

One obstacle to the use of LED UV in sheet-fed offset printing is the printing inks, for example. The UV inks and varnishes commonly in use today are not suitable for this new technology, because their absorption spectrum is designed for conventional medium-pressure UV lamps. On the other hand, the wavelength range emitted by LED systems is limited to a narrow spectrum, with the main focus on the UVA range with peaks at 365 nm and 395 nm. The absorption spectrum of the inks and varnishes must conform to a narrow wavelength range around these peaks.

Due to the limited choice of raw materials in this wavelength range and the cost of development, the price of an LED-optimised ink can almost certainly be expected to be higher than commercially available UV inks.

If, one day, such sophisticated applications as sheet-fed offset are to be covered by LED UV, efforts must be made to produce UV LEDs in larger volumes, in order to lower production costs. At the same time, it will obviously be necessary to further raise the UV output. But even then, LED UV technology will not initially have much impact on the market success of the tried and tested medium-pressure mercury lamp. With this lamp, conventional UV technology has been able to assert itself in virtually all common printing processes over the last four decades. Rather, the company regards LED technology as a complementary technology, which opens up new opportunities and applications, and lends feasibility to the idea of hybrid systems consisting of medium-pressure UV lamps and LED units. Whether or not use of the new LED technology in industry is practicable in terms of technical production issues and cost efficiency must be examined on the basis of detailed technical studies and a cost-of-ownership calculation.

Current developments in terms of energy consumption

The claim that LED UV technology can achieve energy savings of 70 to 80 % compared with conventional UV dryers sounds especially attractive to many printing firms.

However, in the light of current development trends in both technologies, this claim needs to be challenged when high UV outputs are required. In order to achieve faster curing speeds and drying qualities comparable to those of UV lamp systems, suppliers of LED units are currently increasing UV output at the expense of efficiency. For LED systems, too, a higher output of UV energy means an increase in electrical input. We also have to bear in mind the fact that current LED units, with their greatest output at 395 nm, have an efficiency of approx. 8 - 12 %, whereas the efficiency of a medium-pressure mercury lamp is 28 %. The efficiency of a 365 nm LED unit is currently less than 10 %.

Current advances in established medium-pressure lamp systems run counter to this trend, and enable energy savings of 40 % in rotary printing, for example, by increasing efficiency - as demonstrated by the BLK-5 UV system for energy-minimised printing that was launched at the Drupa and certified by the German Professional Association for Printing and Paper Processing.

If the current trend continues, the energy consumption of the two technologies will gradually cease to differ that much. This trend is set to continue in the next few years, and will further reduce the advantage of LED systems in terms of energy consumption, unless the efficiency of UV LEDs is increased - as is already the case for longer wave LEDs right up to the red visible range.

At the same time, it must also be borne in mind that both the production and operating costs of LED technology increase in proportion to the working width. The number of LEDs - and therefore the price - of an LED unit one metre long is twice that of a system for a 500 mm-wide web. In mercury discharge lamps, the price between the different lamp lengths does not vary as greatly.

The shorter the wavelength, the higher the price

An LED UV system in a sheet-fed printing press, premiered at the Drupa, works in the 365 nm range.

Other UV suppliers, on the other hand, prefer systems with an emission range of 395 nm, as these offer a considerably more favourable price/performance ratio. They are cheaper to purchase, and also offer a much higher UV output.

Adapting press designs to LED UV

Since LED UV systems radiate less heat, they can usually be positioned very close to the surface of the substrate, i.e. at a distance of 10 to 20 mm. This advantage really comes into its own in UV inkjet systems, but is this kind of distance also possible in modern sheet-fed offset presses? Here, the distance between the UV source and the substrate surface is currently 100 mm or more. In order to position LED lamps closer to the surface of the substrate, the design of existing presses would have to undergo significant modifications.

Thus, before LED UV technology can assert itself to a wide extent in the printing industry, an array of technical hurdles still needs to be overcome. In individual fields such as inkjet printing, the technology is already sufficiently solid to allow the first applications to be introduced. In a few years’ time, LED UV systems will achieve the necessary market maturity to allow them to be positioned as a viable alternative to traditional UV technology, in all fields in which their particular advantages can be brought into play.

 

The basics of LED UV technology (Part 2) as pdf-file