Guide to UV Measurement
Heat and Infrared
Heat can benefit UV curing when it is used to enhance flow-out (for gloss) or mobility (for cure rate and adhesion). It becomes a problem primarily when, for example, the substrate cannot tolerate it or volatilization of the coating occurs. In precision screen print, keeping a low surface temperature to maintain low dot gain may be necessary.
Infrared (IR) energy is produced by all UV lamps. It is, in fact, emitted by the quartz bulb envelope. Larger bulbs emit more IR than smaller diameter bulbs, because of their larger surface area. This suggests that for heat-sensitive applications, a smaller diameter bulb may be a preferred choice.
There are several extremely simple methods to reduce surface temperature. Air cooling is a very effective method of removing heat – it is easily designed into the curing chamber and its air flow system. The second method is almost absurdly obvious: move faster. Parts pick up less thermal energy when exposed for a shorter time. If it is possible to speed up the process, do it.
In some instances, moving the lamp out of focus (farther from the work surface) will reduce the rate at which IR arrives at the work surface and, depending on the conductivity of the substrate, can reduce the surface temperature a little. However, if the cure requires the high peak of focused irradiance to achieve depth of cure, this can be counter-productive.
A method of last resort is to use dichroic reflectors in the lamp. These work by reducing some of the infrared radiation that is reflected toward the work surface.