Facts and figures illustrate optimal techniques.
By Neil Bolding
Continued from How Suitable Is UV LED Exposure for Your Stencil System?
Once you understand the output characteristics of your exposure unit and the peak absorption of the stencil system you wish to use, you can better gauge its compatibility. Figure 9 shows the typical absorption curves for the three types of stencil systems (Red: Acrylate sensitizer – Combined with diazo in dual cure emulsions; Blue: Diazo – Alone in single cure emulsions; Green: SBQ – Photopolymer emulsions.) Provided that an exposure system generates sufficient energy at the wavelengths where the stencil systems are reactive, the crosslinking necessary to harden the stencil will take place.
To return to LED systems, most of the ones on the market now appear to have been tuned to provide peak outputs at either 385 or 395 nm. Some units have a secondary source at 405 nm. LEDs have a very narrow emission profile (typically +/- 10 nm), but all of the typical emulsions and capillary films in use today are sensitive in those wavelengths. The questions are how efficient the reactions will be and whether they will provide suitable stencils for printing.
First, you must have the correct procedures in place to produce optimal stencils. Making sure that coated screens are thoroughly dried prior to exposure is essential because water will detract from the crosslinking in the diazo reaction. Earlier, we observed that diazo reacts with the hydroxyl group of the polyvinyl alcohol. As you all know, the water molecule is H2O – or, written another way, H-OH. This shows that water also has a hydroxyl group that can be an unwanted part of the reaction. If the diazo reacts with water instead of the polyvinyl alcohol, crosslinking with another polyvinyl alcohol chain will not happen and a weakened stencil will result.
Also be certain that the exposure time is optimized. The common tool for determining this is an exposure calculator, which incorporates neutral density filters to produce multiple levels of exposure (five, in the case of the exposure calculator shown in Figures 10 and 11) in a single operation. This tool is ideal for diazo and diazo/photopolymer stencil systems.
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