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Twenty Questions and Answers about UV Curing and Related Concerns

(February 2009) posted on Wed Feb 18, 2009

Trying to adjust to the realities of working with UV screen-printing inks? Use this Q&A discussion to clear up any misunderstanding about the inks, the curing process, and other aspects of UV technology.

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By Bea Purcell

Regardless of whether the ink is fast or slow curing, the important point to consider is the end application. An-other important factor to keep in mind is the curing equipment. The ink may be fast curing, but if the curing equipment lacks the efficiency to cure, the ink will be slow curing or worse, will not cure at all.

Why does polycarbonate film become yellowish when I use UV inks?

Polycarbonate is sensitive to UV wavelengths of less than 320 nanometers. The yellowing on the surface is caused by the breakage of the molecular bonds caused by photo-oxidation. UV energy is absorbed by the molecular bonds of the plastic and generates radicals. The radicals react with oxygen in the atmosphere to generate appearance and physical changes within the plastic.

How can the yellowing on polycarbonate’s surface be avoided or even eliminated?

If UV inks are used to print polycarbonate, the yellowing on the material’s surface may be minimized but may not be completely avoided. The use of additive curing unit bulbs, such as iron and gallium, has been proven to minimize this yellowing. These bulbs emit less UV energy in the shorter wavelengths detrimental to polycarbonate. In addition, curing each ink color properly to minimize exposure of the substrate to UV energy and keeping the curing unit well maintained will go a long way in reducing surface degradation of polycarbonate materials.

Are UV inks considered green, and if so, why?

UV inks are green compared to solvent-based inks. UV-curable inks are consider-ed to be 100% solids, meaning that just about everything in the ink is reacted into the final ink film. Solvent-based inks evaporate solvents into the atmosphere as the ink dries. Solvents are considered volatile organic compounds and not environmentally friendly.

What is the unit of measure for the density numbers a densitometer provides?

Optical density numbers are unit-less. Densitometers measure the amount of light reflected or transmitted by a printed substrate (Figure 2). An electronic photocell eye is connected to a meter that mathematically converts percentage of reflected or transmitted light into logarithmic density values. The use of logarithmic scale compresses large measurement numbers into small and manageable values. The basic mathematical formulas are as follows.

What factors affect density?


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