Saw-toothing in the stencil and print can also be caused by low-resolution digital images... but that problem should be addressed with the digital image or film positive provider. Film problems aside, let’s address the areas under the control of the screen technician.

EMULSION SOLIDS CONTENT AND MESH BRIDGING
The solids content of an emulsion is an indicator of what proportion of the emulsion remains after all the water has evaporated. Low solids content in some older emulsions contributed to poor mesh bridging and sawtoothing. Modern emulsions have higher solids content to allow faster emulsion build-up with fewer coating passes, which bridges the mesh more effectively, resulting in sharper, cleaner stencils with improved print definition. Ensure that you are using the proper emulsion for your application.

Deni Breard Technical Services Associate,SGIA

Ms Breard recently joined the SGIA Technical Services Team following her work as Technical Services Manger at Sericol (Hong Kong)Lit. Earlier ,she had also worked as Training Manager for sericol at there US headquarters in Kansas city, Kansas by developed and presented technical training curricula and served as a resource for Sericol's Technical Department. She is a speaker and workshop leader for industry shows and exhibutions and contributes to industry publication with over 20 articles in print, inaddition to her many special purpose articles for specific training programmes. Ms Denise is a former member of the SGIA board of directors and has participated on various SGIA committees, including the education committee for which she served as chair. She has serve on the Board of Advisors for printing programmes, including that of SPTF.

COATING METHOD & STENCL EOM AND RZ
A smooth stencil of optimum thickness will form a gasket between the substrate and the knuckles of the mesh, allowing a sharp, crisp, and smooth transfer of ink through the stencil. Your coating methods will affect both stencil thickness and smoothness. Both characteristics can be measured, so consult your emulsion supplier for assistance.

As water evaporates during emulsion drying, the coated emulsion layer can shrink down into the mesh knuckles, which affects both stencil thickness and smoothness. In some cases, the shrinkage can be enough to leave a rough or jagged stencil surface, which can cause saw-toothing in the printed image and can contribute to excessive dot gain in four-color process printing.

Select a clean, dry, degreased screen at proper screen tension. Evenly apply one or two coats to the substrate side of the screen and immediately inspect the mesh from the squeegee side to ensure there is a smooth and glossy coating of emulsion, free of mesh marks. If the squeegee side is not glossy, apply more coats from the substrate side. Repeat until the squeegee side is glossy from emulsion. This ensures that the mesh openings are filled with emulsion instead of air and eliminates pinholes caused by air bubble entrapment.

Follow the substrate-side coats with one or two coats of emulsion on the squeegee side. Additional coats to the squeegee side (wet-onwet) will increase the stencil build, or thickness of emulsion over mesh. Dry the freshly coated screens with the substrate side down and the squeegee side up. Test and measure to determine how many coats from the squeegee side are needed for your application.

Stencil smoothness (Rz) can be enhanced by applying a face coat (or fill coat) to the substrate side of the screen after the initial emulsion coats dry. Apply one or two face coats with the sharp side of the coater. Face coats do not substantially increase the stencil thickness; they simply fill in the stencil surface “valleys” left between the mesh knuckles when the water evaporates during screen drying. This improves the gasket effect of the stencil and enhances print definition. Rz can be measured with an instrument called a Surface Profilometer.

Optimum stencil thickness will vary with different applications. Coarser mesh counts have thicker mesh knuckles and tend to require more emulsion over mesh to achieve the desired gasket effect. Direct projection screens are in another group altogether, as they may show only 1-2 microns EOM.

Optimum stencil thickness will vary with different applications. Coarser mesh counts have thicker mesh knuckles and tend to require more emulsion over mesh to achieve the desired gasket effect. Direct projection screens are in another group altogether, as they may show only 1-2 microns EOM.
An ElectronicThickness Gauge is an instrument for ensuring consistent, precise stencil coating by measuring the thickness of the stencil. EOM (or Emulsion Over Mesh) is especially important in four-color process printing and for printing with UV curable inks. Emulsion over mesh (EOM) is the thickness in microns of emulsion that is greater than the thickness of the mesh. EOM is sometimes expressed as a ratio or per-centage of the total thickness of mesh plus stencil, but the actual measurement in microns is a more com-mon expression.

HOW CAN I TELL IF MY SCREENS ARE UNDEREXPOSED
Symptoms obvious before going to press include visible film positive edges and tape marks, slimy stencil during developing, and foaming in sink or on screen during washout. Symptoms on press include pinholes, scumming, breakdown during print the run, and even bits of stencil flaking off the screen. Symptoms after press include difficult reclaiming, and yellowish-brown mesh staining where the emulsion used to be.
Any of these symptoms - or any changes in emulsion, mesh, exposure equipment, or procedures - indicate that a conscientious screen technician should conduct a series of exposure calculator tests. Following these simple steps will enable your screen technician to achieve consistent and predictable screens, and stencil sawtoothing will be a thing of the past.

This article was published in Specialty Graphic Imaging Association’s SGIA News, May 2005 issue. Reprinted with Permission. Copyright 2005. All rights reserved. Visit www.sgia.org for more information about the SGIA and other resources enjoyed by members of SGIA International

CONCLUSION: AS SIMPLE AS 1-2-3

1. Select a good quality high-solids emulsion.
2. Follow good coating procedures.
3. Expose the stencil properly.