Bron Wolff discusses how you can assess and improve your UV-dryer performance by learning from his company's experience.
By Bron Wolff
I'd like to draw your attention to Figure 1A, which charts our results in curing a yellow ink. Using a double lamp system with each lamp set at 125 watts/in. and a belt speed of 110 ft/min, we were able to cure the ink successfully. However, when different colors from the same ink line were cured under those conditions, these other colors failed. (An ink failed when it couldn't pass cross-hatch and tape testing or exhibited blocking after prints were stored overnight under a 50-lb weight.)
Our tests also showed how the curing window changed when the same ink was printed on different substrates. The nature of the pigment, photoiniators, and other ink components, along with the energy absorption/reflectance characteristics of the substrate all influence the curing window. Ink pigment, however, is the biggest obstacle in curing.
Through such testing and lots of mistakes, we learned to use the curing window for each ink to our advantage. Basically, the ink with the largest curing window goes first, while the color with the narrowest window goes last. We use this approach to determine color sequence on each job we print and set our dryers accordingly for each color. Our goal is to run each color at the lowest possible power setting and highest belt speed that will ensure a good cure.
We tend to run process colors faster than spot colors because UV light more easily penetrates transparent process-color inks, allowing us to cure them more quickly. On jobs that incorporate both spot and process colors, we run the process colors first. The spot colors, with their greater pigment load, require the most curing energy and have the greatest potential for exposure to heat in the curing system, so we save them for last when possible.
Whether jobs incorporate spot or process colors, the first colors typically get the lowest lamp settings and highest speeds. This reduces the potential that initial colors will become overcured as subsequent colors are also printed and cured. Like T-shirts underbases, our initial colors get a "flash" cure and rely on subsequent print/cure cycles to complete the cure.
Worth the effort
Our modified curing systems work. But recreating the same results in your operation will take time, effort, and help from lamp and curing-system manufacturers. Modified lamps will do you no good without the correct power. And increasing power without modifying the lamps will only give you more heat, not more energy to penetrate the inks.
What we developed is a system, one that's matched to our specific inks, substrates, and operating environment. It took us to years and countless hours to achieve, and involved a lot of sweat, time, and money. Before you commit to a project like this, ask your customers what they need. Our customers see the improvements and think our efforts were worth it. If your customers get better product quality and faster turnaround on their orders, they may, too.
Author's note: I read a lot and don't necessarily believe a lot of what I read, but every once in a while I get inspiration from someone else. Richard W. Stowe of Fusion UV Systems, Gaithersburg, MD, has written several articles on UV curing over the last five years that have appeared in various publications (including Screen Printing). His work helped us get through several issues as we were re-engineering our curing systems.
If you're thinking about a similar overhaul of your curing equipment, I recommend two things: patience and good maintenance/electrical personnel. I usually lack patience, but Joliet Pattern is lucky to have the talented Zmolek brothers, Rich and Steve. Without them, our UV project may never have seen the light.
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