UV curing

Curing of UV inks, hybrid inks and coatings is an issue that hasn't yet received a good solution. There is no objective way to determine whether inks or coatings have cured. The industry relies on several methods that are either empirical or indirect.

Empirical methods
MEK test examine the resistance of a coating to chemicals. The user has to rub the coating with MEK and note down the number of rubs that it takes for the coating to break down. Results may vary depending on the user however. Moreover, only resistance to chemicals is tested, and not actual curing. Furthermore, when a printed product has to go through the entire converting process, the chemical properties might change and the printer might end up with unexpected results.
The reticulating pen techique involves making a stroke with a standardized pen and observing how the ink behaves on top of the coating.
Potassium Permanganate Testing involves dropping a stain and seeing the color change on the coating.
For inks, one commonly testing method used is IPA, quiet similar with MEK.

It is advised to use all three tests, and if the results of two of them are the same, then this might provide an answer as to whether there is curing, overcuring, or undercuring.

We recently did a UV adhesion test at PSC testing all possible light configurations, two speeds, and two sets of inks and coatings. What impressed me was the the same coating behaved differently when used with different inks. I am still trying to figure out why; what are the variables involved. Yet, I haven't come up with a good answer.

The best a printshop could do is to establish its onw standards for the inks and coatings. Then, whenever a job is run, conduct a simple test to determine whether curing takes place.

FOGRA has come up with ACET. A device that tests varnishes and inks, and possibly even coatings. The tests are the same with MEK and IPA, only that it takes out the subjectivity of the operator, since it's a device that makes the test.

On the category of indirect tests, we talk mainly of testing the lamps. This is a big issue, and yet not quite objective. UV lamps' performance deteriorate over time, shifting the wavelengths of their light emissions. Since different wavelenghts have different curing capabilities, there is a need of a system that monitors the life of the lamps. Moreover, the lamps are not performing equally in the center and on their edges (imagine you are having a 64 inch press). Still, there are systems (LM9000) that allow constant monitoring of the performance of the lamps and they allow you to determine a warning level based on whether the light emissions are sufficient for curing. In this case as well, each printshop has to determine it's own standards.

Second thing to do is to put some strips that change color when they receive UV radiation. These can be measured spectrally, with a radiometer, or even visually. This doesn't tell us if there is cure or not, only if the UV lamps reach a point that emit not sufficient UV radiation. I measured spectrally some strips from the test we run. I think that the prefered wavelength is at 600nm, since there is where I observed the highest variation from the strips (there is no point in the documentation of the strips -UV FastChecks- that says in which wavelength to measure).

There are also several rub tests, that test adhesion. This two involves a printshop to determine it's own standards.

These are the methods I have tested so far. If I come up with a better one I will let you know. If you know of a better method, please let me know.

There is information on the web about this issue. www.uvprocess.com has a great number of products you might wish to check out.

-D