This article highlights some of the most important variables in the process and offers tips for achieving consistency.
Once beyond the screen fabric, artwork, and light source, the first step toward stencilmaking standardization is the selection of the stencil system as to available stencilmaking skills, length-of-run capability, and imaging properties. Then look at the particular stencilmaking product that can best meet the requirements of the job as to ink and washup-solvent compatibility, exposure speed—fast for a weak light source, or for rapid stencil throughput in high-volume production—and imaging properties.
Imaging properties refer to the stencil’s ability to impart to the print the two principal measures of image quality: acutance, often referred to as edge sharpness; and resolution, which is fineness of detail. With the highest resolution work, the two are interrelated but, most of the time, they can be evaluated independently. It is possible to have superb acutance and low resolution, as in the case of a print made with a knife-cut stencil.
Prints can also have high resolution but poor acutance, as with fine mesh using too few coats (or no face coats) of emulsion and image-non-image edges that are sawtoothed. Poor acutance can also result from lower quality emulsions that have low solids content or that are manufactured with coarse filler particles that are large and granular. You can measure resolution and acutance (see SGIA’s Technical Guidebook, B184).
EOM and Rz value
The thickness of the stencil on the substrate side of the fabric—the EOM (Figure 3), or thickness of emulsion over mesh—also affects both the Rz value (the stencil’s surface roughness) and the thickness of the ink deposit, though only at the image-non-image edge, and for a distance of about 0.4 mm. The importance of a low Rz value in gasketing ink and avoiding sawtoothing is well understood, and the Rz can be measured with a profilometer as part of production documentation. The effect of an excessive EOM, and how it can result from the best of intentions to reduce the Rz value, is not as widely understood.
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