In a couple of week’s time many of us will head over to Berlin for this year’s Fespa show and it’s a good bet that many of the new printers on show will be fitted with LED lamps for UV curing.
A Light Emitting Diode is basically a semiconductor that produces light when connected to an electric current. Since it’s a solid material it’s relatively robust and can last a long time – typically 15,000 to 20,000 hours, as opposed to conventional mercury or metal halide lamps, which have an average life span of just 1000 hours.
LED arrays are immediately ready to work and can be just turned on and off as needed to cure the inks; there’s no need to wait for the lamps to warm up or to keep the printer running to maintain the working temperature of the lamps so the lifespan really means just the actual printing time.
Equally, an LED either works or it doesn’t. It gives exactly the same output up from new until the point the lamp fails so there’s no need for the operator to compensate for a fading lamp, which in turn gives a more consistent result.
LEDs are more power efficient, requiring less energy, which translates directly into lower electricity bills. Typically, LEDs do not produce much heat so they work well with heat-sensitive materials. However, it’s worth noting that the heat produced by mercury lamps can also help with the ink adhesion.
However, there are some limitations in the use of LEDs. UV-curable inks use ultraviolet light to start a chemical reaction that cures the ink. These inks are polymer-based, using monomer and oligomer polymers as a carrier, together with photo initiators that react to ultraviolet light. This reaction starts a polymerisation process that cross links the polymers together, curing the ink.
So one of the main limitations of LEDs is the wavelength of the light that they produce, which in turn limits the photo initiators that can be used as they must be tuned to work at that wavelength. Jon Harper Smith, marketing manager at Fujifilm Europe’s Graphic division, says: “Initially we were stuck with 395 nanometres which is only just in the UV range which has been the industry standard in the West but in Japan it’s 385nm but there is not much to choose between the two.”
Another issue is that oxygen in the atmosphere inhibits the curing process. Harper Smith explains: “With any UV system based on free radical UV where we have an interface with air or oxygen, the surface reacts with the air and it kills the reaction.” The result is that the surface of the ink, which is exposed to oxygen, cures at a different rate from the body of the ink. This problem is amplified with smaller drop sizes, where the surface area makes up a greater proportion of the droplet.
Harper Smith adds: “385 and 395nm – the common initiators are very good at through-cure but not brilliant at surface-cure. So we use various tricks to get around that particular problem. You can have systems that have mixed wavelengths such as 395nm and 365nm in the same unit.” This in turn means that the ink has to contain two different photo initiators, which potentially increases the cost of the ink.
Another solution is to use Nitrogen to hold the oxygen back from the ink. Fujifilm, for example, demonstrated its Eucon system for flexible films at the last Drupa. This is a UV inkjet printer that uses LED combined with nitrogen gas to fully cure the inks to create pouches and other packaging suitable for food though European health and safety regulations limit this to the Japanese market for now. Meanwhile, Screen introduced a label press last year, the L350UV+LM, which is aimed at producing packaging for food products. This uses low migration inks combined with a nitrogen purge mechanism to ensure that the inks cure fully.
That said, most vendors are working to develop water-based ink solutions for flexible packaging, which is a more sensible direction than nitrogen-enhanced LED UV curing. However, we are seeing LED arrays appear with a greater variety of wavelengths and with more powerful output, which is leading directly to faster printers. It’s still the case that the fastest wide format printers use mercury lamps for curing but most small and mid-volume wide format devices as well as many narrow web inkjet label printers do use LED curing. We’re also seeing a trend towards UV inks and LED curing amongst flexo and offset presses. There are several UV systems that can switch between conventional and LED curing and some flexo inks contain dual initiators so that they can be used with both mercury and LED curing.
Finally, it’s also worth noting that other industrial processes are also making use of LED curing. This includes adhesive curing for bonding and sealing applications, UV coatings for wood and other coated surfaces as well as medical applications such as decontamination and sterilisation of medical instruments. This can only be good news for the printing industry because the more industrial processes that use LEDs, so the more R&D money will be spent on further developing the technology.