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In Pursuit of the Perfect Stencil

(October 2007) posted on Tue Oct 09, 2007

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

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With direct emulsion, the factors that are important in controlling the stencil parameters are the solids content/viscosity of the emulsion and the coating procedure that is employed. High solids content is desirable, as it minimizes shrinkage on drying. Shrinkage of the wet emulsion layer on drying leads to high Rz values and poor print quality, even if you’re using a high-solids-content emulsion, unless you pay particular attention to the method of coating.

In order to optimize stencil profile, and minimize Rz, you must optimize the coating procedure for each application. In general, with a high-solids-content emulsion of around 40% solids, it is possible to achieve good results with simple wet-on-wet coating procedures. For very coarse screen mesh, such as 61 threads/in., the open weave and high percentage open area of the fabric requires but two coats on the print side followed by one coat on the squeegee side. For 110-thread/in. mesh, two coats on each side should suffice. Once we get to 230-thread/in. mesh, two coats on the on the print side and three on the squeegee side will duplicate the results that we’d achieve with capillary film. The additional coats on the squeegee side of the screen in effect cause a build up of emulsion on the print side, which is where we need our stencil. The only time when an additional coats is necessary, after the initial coats have dried, is when printing four color process with UV curable inks.

The very high mesh counts, such as 380 and 460 threads/in., which are best at minimizing ink deposit, are also good at preventing emulsion buildup during coating. The easiest way to minimize both stencil profile and Rz value for this highly demanding application is to face coat the screen after drying. This ensures that the thin stencils required to minimize ink deposit will also provide a tight gasket fit onto the substrate and prevent ink from bleeding beyond the image area under pressure from the squeegee, a condition that causes sawtooth lines and the star-shaped halftones that lead to excessive dot gain.

Emulsions formulated with lower solids content are unable to bridge the coarsest mesh counts effectively with simple wet-on-wet coating methods. This property effectively limits the mesh count range on which they can productively be used (Figure 3).

 

Exposure


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