FIGURE 5. Excimer the principle is the same. As a point of interest, LED arrays can impart heat onto the web, if the coating doesn’t absorb all of the emitted energy. A Rule of Thumb in LED curing for converting is that systems up to 16 W/cm² output are air-cooled and over 16 W/cm² tend to be water-cooled. The cooling is for the electronics and drivers needed for the LEDs themselves, and this produces internal heat, which needs to be managed. Similar to UV, the higher the output, the more heat needs to be managed, but here it is in the housing and not web-facing. Although LED technology offers a lot of benefits, there are some drawbacks that need to be considered before making a transition. Many converting formulations are not available as LED-curable, and those that are don’t always offer the same end result as a conventional system. As they have evolved, LED-array outputs have increased, but they still suffer from having to have the substrate relatively close to the emitting window, which can be a challenge in some converting lines. LEDs have good longevity, but this can be influenced easily by poor maintenance, unsuitable working conditions and poorly calibrated electronics. Therefore, housekeeping and maintenance need to be clearly defined and regularly undertaken. When compared to UV, an output-matched LED system has a higher price tag. It will have lower operating costs as it doesn’t have consumables, and it can be switched on as needed. However, the production cycles need to be carefully evaluated to ensure an accurate comparison. As an example, a short-run line that has many stops and makereadies will have a different calculation than a line running 24/7. Enter the Excimer Excimer is a specialized emitting source (see Figure 5). It is
monochromatic at 172 nm and, therefore, produces a hammer of energy to the tune of 7.4 eV, which is high enough to break the bonds in hydrocarbons and create microfolds in the surface of coatings (a few microns thick). This offers an alternative to expensive formulations that create matte effects, as the light is diffused when it hits the microfolds. In breaking the bonds of hydrocarbons, Excimer can be used for surface cleaning (prevalent in display manufacturing). That same energy also can be used for surface modification to change the energy characteristics. Excimer can be seen as more of a modification tool as opposed to a curing method. It is an augmentation to radiation curing. One disadvantage of Excimer is that nitrogen is needed to create an inertization chamber (devoid of oxygen), and this adds to the cost of operating the technology. Depending on the viscosity and opacity of the coating, this may require a pre-gelling unit to achieve the matte required, meaning additional capital expenditure. Another point to consider, Excimer bulbs usually run up to 1,500 hrs and are relatively expensive to replace, so the operating costs of Excimer must be carefully considered. Excimer (width-matched) has the highest capital expenditure out of all three systems, but there are distinct cost savings in the right production environment, as it is an alternative to multiple costly coatings and surface modifcation. Hybrid radiation curing As illustrated, each system has clear advantages and can be combined (more and more frequently) to provide a hybrid platform that gives an ideal outcome, as opposed to a compromise or change in formulation that can be time-consuming and costly to implement. This offers converters a much wider process window and, therefore, their customers a superior continued on page 87 2020 Quarter 1 • www.convertingquarterly.com
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