Emulsion Exposure Tips

Emulsion Exposure

Light from exposure units can vary in both strength and in wavelength output.  Exposure lamps  come in a variety of types from fluorescent tubes to high output metal halide. All can vary widely  in the wavelengths they emit as well as the amount of UV light they emit.  Just because a lamp  emits light does not mean it can expose the emulsion to it’s maximum potential. Just as cars have varying degrees of horsepower and performance, so do exposing lamps.  

During the exposure process light activates the photo sensitizer to cross link the emulsion’s  chemistry. Emulsions need a multi spectral output bulb with high wattage for optimum performance. The wavelengths of the light should spike between 380 and 420 angstroms, the wavelengths that interact best with the sensitizer to cause the screen to expose. Often the box  the lamp comes in has a histogram of the wavelengths generated, if not ask for one from your supplier, or request the spectral wavelength output of the lamp you are using.

It should show multiple spikes if it is multi spectral, or it may only show one wavelength where it emits UV light. Multi-spectral bulbs interact more with the sensitizer producing better cross linking of the emulsion product. Single spectral bulbs may only activate a portion of the sensitizer to that wavelength while other components of the sensitizer receive no activation. Sensitizer is the diazo or SBQ component(s) of the emulsion.  

The strength (wattage) of the exposure lamp needs to be strong enough to penetrate the entire  emulsion film thickness for optimum cure as well.  Weak fluorescent light systems and lower wattage single spectrum metal halide lamps simply do not have enough energy to crosslink the  entire emulsion film completely.  If you print plastisol you can still obtain a good screen with these  systems but you will have difficulty making strong plastisol, waterbase, discharge, and thick film  screens destined for automatic press production. An exposure system that can work for a  manual printer with small print runs may produce inadequate screens for automatic press production and longer runs, the usual issue being dramatic increases in pin holes and stencil  breakdown. Exposure systems with multi-spectral wavelength bulbs and a wattage of 3K or more will produce durable screens for plastisol, water base, discharge and high density printing.