This guide examines stencil-material characteristics, issues related to light sources, and methods for testing you can use each time you create a stencil.
No color change occurs with photopolymer stencils. Although the type of exposure calculator just discussed might be useful for determining the degree of resolution available at different exposure levels, it does not indicate the extent of cure. An alternative method involves the use of a grayscale-sensitivity guide, which is a phototool that features a 21-step non-halftone grayscale, with density increases of 0.15 at each step. More grayscale steps harden as exposure time increases. The sensitivity guide, when used correctly, helps determine optimum exposure with only one test.
Yet another test method uses no film at all. Instead, it involves the use of a digital radiometer to determine the point at which all of the sensitizer in the coating is used up. Here, a 365-nm filter is fitted into the light integrator’s photocell, and the photocell in placed in the vacuum frame behind the coated mesh. Exposure is then started. No light is able to reach the photocell at the beginning, because the sensitizer is highly absorbent. During exposure, as sensitizer is used up, the emulsion coating allows more light to reach more areas of the emulsion coating. An increasing amount of light is measured by the radiometer, and then the light gradually levels off.
The light integrator
The light integrator steps in to compensate for lamp degradation and power fluctuations. Matching the photocell filter to the sensitivity curve of the emulsion is important, because a disproportionate amount of stencil hardening is caused by those wavelengths that are most penetrating and usefully absorbed. Depending on the type of stencil material, the required wavelengths might reach outside the UV spectrum and into the blue and violet range. The photocell filter must be able to tell the integrator when adequate amounts of those wavelengths have reached the stencil.
Post exposure can be useful for improving the resistance properties of a stencil, but the benefits depend on the type of emulsion used.
Diazo emulsion or film A yellow undercast signifies unused diazo sensitizer in an underexposed diazo stencil. The partially exposed sensitizer doesn’t wash out from the stencil during processing, because it has already reacted with and attached to the polymers and resins that make up the stencil. After drying, it is possible to re-expose the screen, bleaching out the remaining diazo and further crosslinking the stencil to improve its solvent or water resistance.
Dual-cure emulsion or film The situation is the same with dual-cure products when underexposed. But unlike purely diazo stencils, correctly exposed dual-cure stencils can benefit from post exposure. That’s because the secondary crosslinking system can be made to polymerize further, even after all of the diazo is used up. This typically improves solvent resistance and can simplify reclaiming.
Photopolymer emulsion and film These emulsions benefit most from post exposure. Unlike diazo, which can be used with 100% efficiency when exposure time is long enough, photopolymer molecules can be very stubborn. Only some molecules react very quickly to create the short exposure times for which photopolymers are known. The rest of the molecules aren’t aligned correctly and crosslink only with difficulty. Increasing exposure time causes a loss of resolution and detail with little payback in terms of improved durability.
Unused photopolymer During development, when the stencil is wet, some of the unreacted molecules will realign and be available for crosslinking the second time around, thereby resulting in improved solvent and water resistance.
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