Drying, Curing & UV LED UV curing & monitoring
of low migratory inks Tom Kerchiss, chairman at RK Print Coat Instruments, says today’s drying and curing technology provides the converter and industrial processors with more options than at any previous time – and that’s a good thing
N
ot so many decades past a typical oven had a small fan and relied almost solely on heat. Control of air velocity was absent, so in effect a dryer largely resembled a baker’s oven. The move away from solvents to water-based inks, coatings and adhesives together with and increasingly varied base stock, faster machine speeds and new application areas spurred on the development of more sophisticated hot air drying systems and infrared drying and UV curing.
Coating (inks, etc.) success depends in large measure to matching drying/ curing technology to the application. For instance, some coatings don’t take kindly to being hit with a lot of heat. If the substrate being coated is of lightweight and air hits it too aggressively, the coating might dimple or pucker or the substrate itself may be marred. Hot air systems have been subject to many developments. For instance, impingement systems generate controlled turbulence through the dryer. In this way air hits the substrate and breaks through the boundary -layer of air associated with fluid dynamics (the Bernoulli effect) to promote heat/drying efficiency. In contrast to convection ovens, which are used to drive off solvents or water and apply heat to cure coatings, UV curing cures by polymerisation rather than evaporation, thereby VOCs and HAP emissions are virtually eliminated.
The benefits of, for example UV flexo, are well documented and include a superior finish that is resistant to scratches, scuffs and chemicals. But no process or technology is without its drawbacks, some of which centre around the management of heat and of limitations regarding the efficacy of curing specific compounds of colour. A better understanding of light/energy manipulation technology and heat management
that can be integrated in with RK Print Coat Instruments FlexiProof LED UV are from the Phoseon Firejet UV range of products. Typically the lamps offer a tailored output either at the important 385 or 395nm wavelength.
LED lamps emitting UV-A energy at 395nm can effectively cure white and highly pigmented inks. White inks can be problematic and difficult to cure at wavelengths below 385nm due to additives such as titanium dioxide particles absorbing and preventing a thorough cure. Lamps functioning at 395nm offer peak irradiance, the longer wavelength is able to effect optimum penetration into heavily pigmented
together with pre-press support has made implementing the next generation of LED UV products a realistic proposition.
One of the drawbacks in the past was that there was a lack of suitable inks formulated to react to the spectral output of LED UV light sources. This is no longer the case.
Ink manufacturers have developed low odour, low migration products that meet health and safety requirement, particularly for filmic flexible packaging, pharmaceuticals, medical devices and disposables, electronics and where relevant and appropriate other products.
A significant benefit of LED UV technology is that there is minimal heat transfer to the substrate; essential given the fact that filmic and combo structures used in conjunction with low migration inks are heat sensitive.
A drawback with the traditional mercury arc lamp is that it peaks at around 365 nm but there are extraneous spikes in the visible and infrared bands, which in turn is responsible for the heat that a mercury arc lamp generates. This heat can distort sensitive substrates.
LED UV lamps operate within a relatively narrow part of the UVA spectrum. The lamps
formulations and products.
There are a number of additional benefits associated with LED UV when compared with mercury arc lamps. LED UV lamps do not require warm up time: they are fully operational at the flick of a switch. LED lamps need only a small power supply and the lamps are ozone free. Quality control devices play a significant role in ensuring substrates, inks and process variables perform to expectations and that inks are properly cured. It would be wrong to assume that low odour low migratory inks are ready to go. Every aspect of processing and production needs to be considered. Pre-press, proofing, colour matching, colour adjustment and printability are areas where watchfulness pays off.
Pre-press colour communication or proofing devices such as the FlexiProof UV and the FlexiProof LED UV can be used for colour matching and for determining how the ink and substrate and other elements interact and perform over time. Issues surrounding printability such as gloss, durability, flexibility, rub resistance can be evaluated and unfamiliar materials and formulations trialled prior to production machine commitment.
18
June 2023
www.convertermag.com
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