This article highlights some of the most important variables in the process and offers tips for achieving consistency.
Indirect films offer the best combination of readily achievable low Rz value and thin EOM. Further, their EOM can be altered subtly by adjusting the exposure time. Indirect films typically have very low Rz values at an EOM of 3-4 µm. Properly processed indirect films can hold up for runs of 2000-4000 impressions—well within the requirements of many jobs—although they are best used with mesh counts of 156 threads/in. or higher.
Consistent coating of direct emulsion is best achieved with an automated coating machine. This allows controlled coating speed, angle, and pressure—variables that affect coating thickness. At present, few coating machines maintain an emulsion-coating-trough fill level, which is another significant variable, especially when working with large screens. The coating trough (Figure 5) should be selected carefully, too, as the thickness of its leading edge will also affect coating thickness. Be consistent in the coating regimen. Measure and document variations.
Follow the screen-preparation procedures recommended by the manufacturer of the stencil material you use to the letter. Indeed, follow the directions at all stages of the stencilmaking process. It is in the manufacturers’ best interest that you succeed—and in yours, too.
The wetter the better
Capillary and indirect films can, and should, be applied to the mesh immediately following the degreasing rinse, obviating the drying step required before coating direct emulsion and, perhaps more importantly, virtually eliminating any opportunity for pinhole-causing airborne dust or lint to settle on the mesh. This is a good place to mention the all-too-widespread practice of placing dry screens over capillary film and wetting through the mesh with a spray bottle. This is illogical and compromises the factory-coated emulsion thickness that is a principal advantage of using capillary film in the first place. Consider this: Just as mesh meters the ink during printing, it should be used to meter water when adhering capillary film. The spray-bottle technique invites uneven wetting and water-dwell times on the film, with consequently different stencil thicknesses across the same stencil.
The drier the better
A commercial stencil-drying cabinet with filtered air input is a good investment. Dry stencils at temperatures no higher than 100ºF. Measure the dryness of emulsion or capillary film with a moisture meter to be sure that the stencil is dry enough to expose. Moisture meters can detect residual or interior moisture.
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