Guide to UV Measurement
Using Performance to Determine Exposure
There are quite sophisticated laboratory methods for determining “percent conversion,” “degree of crosslinking,” molecular weight, and so forth. None of these methods are very handy on the shop floor.
What really counts is the performance of the cured ink, coating or adhesive. Some of the simplest tests are the best and quickest indicators of how well the optical part of the cure system is balanced. To differentiate surface cure from deep cure, we need only to find test sensitive to these layers. For example, tackiness, cotton fiber attachment, talc sticking, and fingernail scratch are generally surface tests. Thumb twist or tape pull adhesion will tell about deeper cure. Solvent rubs will give some information about degree of cure.
Here are some steps to take to evaluate the lamp selection for a given ink series or coating. These same steps can be useful in evaluating inks and their behavior.
- Identify a meaningful, simple pair of tests for surface cure and deep cure; Make these tests over a range of production speed and see what fails first;
- If surface finish deteriorates first as test speed increases, there is probably not enough short wavelength energy;
- If adhesion deteriorates first, increased peak irradiance will improve it, in addition to the use of longer wavelengths (lamp and/or photoinitiator).
If surface properties are achieved at very different speeds from deep or bulk properties, then UV energy is being wasted. These steps will also quickly identify how much “safety margin” a cure process has. These are the best clues for what needs to be monitored on line or periodically.