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The Mysteries and Myths of UV Curing

(March 2002) posted on Wed Mar 13, 2002

Discover how UV inks differ from other formulations and what it takes to print and cure them successfully.


By Bea Purcell

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• multifaceted, which reflects energy at the widest focal width

One final consideration about curing equipment concerns heat management. High temperature created by the IR energy UV lamps emit is an unfortunate byproduct of the curing process. Preventing this heat from building up and damaging sensitive substrates is a critical concern in UV curing. Systems for heat management in modern curing units run the gamut from water cooling systems and fans to dichroic mirrors (which reflect UV but absorb IR energy) and automatic shutter systems.
 

What processing issues must be considered when using UV? In screen printing UV inks, controlling ink-film thickness is of paramount importance. Mesh type, screen-tension level, emulsion thickness, and related screenmaking factors all contribute to the amount of ink deposited on the substrate. These variables have to be controlled in order to consistently achieve the ink-film thickness recommended by the ink manufacturer. Similarly, press-setup parameters must also be controlled, including squee-gee hardness, screen off-contact, and squeegee/floodbar speed and pressure.

The type of substrates you are printing on also influences the results you can expect from UV curing. On polycarbonate substrates, for example, UV energy at shorter wavelengths (below 300 nm) tends to affect the substrate surface. This phenomenon, known as photo-oxidation, appears as a gradual yellowing of the polycarbonate surface that occurs with repeated exposure to the UV energy. The subtle change in the polycarbonate surface affects ink adhesion, especially if multiple colors are printed. However, the use of gallium lamps, which release UV energy in the shorter wavelength range, has been shown to minimize this photo-oxidation of polycarbonate.

Polyester is another material commonly printed with UV. The main factor to keep in mind when working with po-lyester is that this material needs to be pretreated (flame or corona treatment) or coated with some form of primer to achieve good ink adhesion.

The last issue in using UV technology is safety. While UV inks aren't characterized by dangerous solvents, the monomers they contain can cause allergic reactions in users who are sensitive to this chemistry. Monomers used in UV inks are tested for toxicity (skin irritation) and assigned a rating from one to ten, where one is least toxic and ten most toxic. But even with low-toxicity monomers, it is always a good idea to wear protective clothing, including eye and hand protection, when handling UV inks.


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