This article highlights some of the most important variables in the process and offers tips for achieving consistency.
A common misunderstanding is that thickness of ink deposition depends on the thickness of the stencil. That just isn’t true—except at the image-non-image edge, the 0.4 mm mentioned above. As line or halftone resolution becomes finer, however, image edges become closer. Thus, with fine lines or halftone dots, the thickness of the stencil affects the ink deposit. If the stencil is too thick, high resolution detail may not print at all; and if it does print, the resultant high column of ink may collapse. If the print is process color, an excessive initial ink deposit can interfere with the lay-down of successive colors that overlap. With his characteristic good humor, MacDermid Autotype’s Steven Abbott, PhD, calls this phenomenon PITS (piling ink tone skewing). As a general rule of thumb, EOM should not exceed 10% of total mesh thickness for high-resolution line or halftone work, and 25% for general printing. For consistent results, use a thickness gauge to measure EOM.
Excessive stencil thickness is a problem for the inexperienced who multicoat direct-emulsion screens using intermediate dryings and face coats to achieve a low Rz value. Without care and skill, direct-emulsion EOM can reach 20 µm, even as the Rz values lessen to acceptable levels. Capillary films (Figure 4), which have a backing sheet in place during drying to prevent contraction of the film emulsion into the mesh structure on the substrate side of the mesh and which hold the emulsion flat and independent of the mesh weave during drying, carry their own risks—especially if the film thickness isn’t properly correlated to the mesh count. Even if it is, while the Rz value will be low, the EOM is typically 5-7 µm, though with at least one dual-cure capillary film on the market, it’s possible to achieve an EOM of 2-3 µm.
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