Most technical data sheets will provide either the total energy required or time and intensity needed for UV cure. However, understanding the many factors that affect the UV cure helps ensure the appropriate process for each unique application is achieved.
Testing is the best way to verify full cure. There are several ways to test, but if possible, adhesion tests on your actual components produce the most accurate results. If adhesion strength cannot be increased with additional UV cure time you’ll know your process is sufficient.
Keeping in mind that bulb intensity will drop off with the number of hours used, a safety margin will allow you to use the same bulb for a longer period of time. So if at minimal bulb output, full cure is still achieved – at maximum output, there is excess cure.
At minimum bulb output watch for signs of oxygen inhibition which can occur when the intensity of the bulb falls below the threshold intensity needed for that product to achieve a dry surface. At maximum bulb output, watch for signs of heat distortion, scorching or coloration.
Overexposed UV cure adhesives may become harder than expected with reduced elongation. These properties can affect bond integrity especially in applications that involve bonding substrate with different thermal expansion rates. It may not be apparent whether the symptoms (heat distortion, scorching, coloration) are a result of the light or the heat. Applying a fan to remove the heat is a quick way to be sure.
To facilitate the longest bulb life, thoroughly test the process to determine the upper and lower UV cure limits.
For more information on each, see Factors Affecting UV-Cure
