Excellence in stencilmaking yields screens that stand up to production wear and produce detailed, high-quality graphics. Find out how to regulate the instabilities in the process that can lead to less than desirable results.
By Ross Balfour
The type of artwork used can also have a big effect on the properties of the finished stencil. Most film positives have a dense black image area (a high Dmax) and a clear background (a low Dmin). Vellum, on the other hand, rarely achieves a Dmax much greater than 1.5, and at the same time, the Dmin is usually around 0.30. What this means is that the vellum only allows 50% of the light to reach the stencil. Before optimum exposure is reached, the insufficient Dmax has let light penetrate to the image area, compromising washout properties and detail. The expression about stuck between a rock and a hard place definitely applies to vellum.
Mesh preparation should also not be ignored as an area that can affect stencil performance. Although screen mesh is thoroughly washed after manufacture, dust and oils from handling—along with adhesive overspray—cause contamination that you should remove prior to coating.
Degreased mesh, although it may be squeaky-clean, is, with the exception of stainless-steel wirecloth, not very conducive to good stencil adhesion. Polyester mesh is woven from slick, smooth PET fibers. Photoemulsion, or water-based paint, does not stick well to untreated PET. For this reason you’ll to prepare the mesh properly in order to maximize stencil adhesion. You can improve physical adhesion by lightly roughening the surface of the mesh with a specially designed abrasive degreaser. Improving chemical adhesion can be as simple as treating the mesh with a formula that contains a wetting agent. After rinsing, an adhesion-promoting surface primer remains on the mesh that enables the stencil to stick more effectively. Mesh-prep systems are even available that combine degreaser, abrasive, and wetting agent all in one product.
The improvements seen in adhesion are most noticeable at underexposure. Photopolymer stencil materials benefit the most of all from good mesh preparation, because they do not contain a diazo that bonds to the mesh during exposure.
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