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UV Curing

(November 2000) posted on Tue Nov 14, 2000

Bron Wolff discusses how you can assess and improve your UV-dryer performance by learning from his company's experience.

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By Bron Wolff

As I mentioned previously, one side effect of the curing process is heat. Only a small portion of the power used by a UV lamp is converted to UV-curing energy. Most of the power is converted directly to heat. The high levels of heat produced by conventional lamps causes a range of problems including substrate shrinkage, poor intercoat adhesion, brittle inks, plasticizer migration. The heat also makes the finished piece more difficult to diecut, overlaminate, and form.

We asked ourselves the following question: If heat is such a problem, and most of the power used by the lamps is converted to heat, why don't we change the curing system so that it converts more power into UV light and less into heat? This way, even if the voltage drops slightly, the more efficient lamps would still be able to provide enough energy to get a successful cure.

We started our exploration with a radiometer and a heat gun. The curing system was a 300 watt/in double-lamp unit with focused reflectors that were cleaned twice a week and lamps that were rotated regularly. We ran 4-mil vinyl, printed with red ink through the system at 50 ft/min, measuring and recording both the heat and the UV energy output. We then sped up the belt to 60 ft/min and repeated the measurement. We continued this exercise using the same type of vinyl and ink and speeding up the dryer 10 ft/min each time. At 100 ft/min, the print was still curing fine, but the amount of heat to which the prints were exposed only dropped marginally.

The thing to understand about UV curing is that the difference between dosage and irradiance. Dosage is the amount of light energy accumulated by the print/substrate during the time it spends under the lamp. Irradiance is the intensity of the energy, which influences the depth and speed at which the ink cures. The irradiance is influenced by the wattage of the lamp, the efficiency of the mercury gas within the lamp, the condition and focus of the reflectors, and age of the lamps.

More time inside a curing unit does not necessarily equate to a better cure--it only means more exposure to heat. That's why multiple passes through the dryer do no good with UV inks.


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