This article highlights some of the most important variables in the process and offers tips for achieving consistency.
If the non-image areas of the artwork lack clarity—if they are translucent, but not transparent—and if the density is low, then the artwork is hopelessly compromised and the stencil will be, too. If the image edges lack acuity, even prints made from low-Rz, high-quality stencil materials will look sawtoothed, with poor mesh bridging. Having said that, in the real world, one can get by with inkjet artwork with a density of 2.8-3.5, though that is, again, a compromise. The point is that photographic stencils can never be better than the artwork used to generate them, nor can the final print be better than the stencil.
Vacuum contact, fiat lux
The contact frame should draw a vacuum of 20 in. Hg at a minimum—and beyond that, beware of Newton rings. The vac-uum blanket should be black, and the contact glass should be cleaned annually, whether it needs it or not. The exposure unit (Figure 2) should be of high quality in terms of spectral output that matches the sensitivity curve of the stencil material (call it efficiency), light intensity, and light geometry, by which I mean the extent to which the light emanates from a point, so that the light incidence to the stencil is as nearly perpendicular as possible.
If the exposure distance is moveable, it should be set to expose at a distance that is at least 1.5 times the diagonal of the image area of the screen. This ensures even illumination of the stencil, avoiding hotspots and underexposed areas on the same stencil. Spectral output should be measured with a spectrophotometer; intensity with a radiometer. The above doesn’t apply to shops that use digital direct-exposure equipment, which does not use any intermediates at all.
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