Drying, Curing & UV
Selectable drying and curing options
ByTom Kerchiss, Chairman of RK Print Coat Instruments Ltd
applied selectively and in precise masked areas. In addition, the compact confi guration size of systems makes it easy to fi t in with existing process lines to achieve increased output in conjunction with or as a replacement for other heating/drying methods. Since IR is a surface drying process it is harder to dry very heavy coatings and it can be diffi cult to use with fragile temperature sensitive materials. It is worth pointing out that temperatures which are too high can cause some coating materials to form a skin and blister. So, what about UV? In contrast to convection type ovens that drive off solvents or water by applying heat to dry off the ink or another coating, UV energy cures by polymerisation rather than evaporation, therefore very little in way of VOCs and HAP are emitted. UV can provide for a superior fi nish and is resistant to scratches, scuff s and chemicals. Another benefi t is that as a process UV is easily controllable since the coating or ink remains fl uid until exposed to the energising of the UV lamp or array. Units are generally compact and operating costs are reasonable.
P
rior to the advent of modern retail methods of marketing, an era some might defi ne as the time of brown box packaging, a typical dryer was relatively unsophisticated and bore more than a passing similarity to an under powered baker’s oven. Typically, the oven had a small fan and there was little in the way of air velocity and no fi nesse when it came to control. The impetus to redefi ne and improve drying techniques was due in large measure to the increasing complexity of the production process and the need to print, coat and convert a broader range of substrates, including of course multi-layered plastic fi lmic materials and laminated structures. With the development of new materials, opportunities arose in fl exible packaging and in new segments of industry such as medical and in electronics, automotive and much more besides not just for more capable hot air systems, but also for infrared (IR) and UV curing and electron beam (EB).
When drying a coating, it is critical that the drying technology is optimised for the application, taking into account the substrate and the various processing elements. This has always been the case but perhaps more so with the move towards water-based, water-wash inks and other substrates and materials that while reducing carbon footprint can pose processing challenges.
24 July/August 2025
A drawback with mercury arc lamps used for many UV applications is that they peak at around 365 nm but there are extraneous spikes in the visible and infrared bands, which in turn is responsible for the heat that these lamps generate and which can distort sensitive substrates. Though not suitable for every application LED or light emitting diode technology and other methods of heat management minimise many of the diffi culties associated with curing heat sensitive materials, broadening the range of products that can be subject to UV curing.
LED UV lamps do not require warm up and only require the fl ick of a switch. They also only need a small power unit and the energy/heat output is signifi cantly lower than with mercury arc lamps; they are also ozone-free.
Water-based inks dry by evaporation and depending upon the type of substrate may require a signifi cant amount of heat to dry off the water and leave the surface with a suffi ciently cured fi lmic layer that is rub resistant and is durable and meets other printability requirements. The drying of many multi-web fi lms for packaging may require powerful dryers with good airfl ow. Infrared provides certain advantages over conductive and convective heat transfer. To begin with, energy is transferred, not heat. The material converts the radiated energy to heat by absorption so in eff ect infrared is a surface heating technology. It has some similarities with conductive and convection heating, but it can provide very much more in the way of ‘through’ heating- that is a defi ned depth of drying, a requirement for some materials such as translucent plastics, paper and textiles. Infrared is often selected for spot applications, where coatings and adhesives are
Typically, the LED UV lamps supplied with RK Print Coat Instruments FlexiProof LED UV fl exo colour communication or proofi ng device off er a tailored output either at 385 or 395 nm wavelength. The FlexiProof LED UV can be used to determine how an ink/coating and substrate perform over time.
As alluded too earlier, every drying/curing technology off ers advantages and disadvantages; the same applies to coating applicator and print technologies, which is why pilot print and coating systems such as the VCML or the VCM pilot/production machine off ers a wide range of custom confi gurable and selectable drying/curing technology including hot air, IR and UV curing. Sometimes though, even mainstream methods of drying/curing are not appropriate and it is necessary to apply a degree of lateral thinking. For the drying of medical patches a VCML coater has been supplied that uses steam.
www.convertermag.com
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