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Mesh Matters

(June 2009) posted on Wed Jun 03, 2009

The level of detail you can print, the quantity of ink you lay down during the print stroke, and the ability to reproduce an image accurately and consistently are but a few of the print characteristics that are influenced by your screen. Here you

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Artwork In line work, the width of the narrowest lines and the registration tolerances among colors establish important parameters for fabric selection. In process-color printing, the smallest element in the artwork is the shadow dot, which can be lost through a mesh opening or behind a thread if it is not considered during fabric selection. As screen rulings (lines/in.) go higher, the printable tonal range gets smaller because that 5 or 10% shadow dot simply cannot be resolved or printed with the available screenmaking materials.

The two mesh criteria that are the most critical in process-color printing are mesh count and thread diameter. The mesh-to-dot ratio refers to the mesh count versus the halftone line count and is typically used to minimize the occurrence of moiré patterns (Figure 5). Though moiré can never be completely eliminated, higher mesh-to-dot ratios can reduce the incidence of global moiré. A minimum ratio of 3.5:1 is recommended, but results improve significantly at a ratio of 5:1 or higher. So at 3.5:1, a 65-line/in. halftone would require the use of a 230-thread/in. fabric (3.5 threads/line x 65 lines/in. = 227.5 threads/in.), and, at a 5:1 ratio, a 75-line/in. halftone would require a 380-thread/in. screen (5 threads/line x 75 lines/in. = 375 threads/in.).

In addition to mesh count, thread diameter is an important factor because the threads can block small dots or whole rows of dots, creating a visible moiré pattern. This is commonly seen in highlight dots and with high halftone line counts. For this type of fine-detail printing, first determine the mesh-to-dot ratio and then select the finest thread diameter available in that mesh count. Some experimentation june be needed with differing line counts, tonal ranges, and screen angles.

Many charts and mathematical formulas are available to help determine the smallest imageable element on any screen mesh, but there is a simple visual test you can perform to see whether your artwork can be reproduced on a given type of mesh. Place the film positive over the mesh and view both the mesh and positive under a 30x or 60x microscope. Look for signs that the image will either be blocked by the mesh threads or too small to be supported by the mesh.

Ink characteristics Frequently, ink selection is predetermined by the substrate or post-printing performance requirements, as in the case of electronic parts and membrane switches. The nature of the ink required should be taken into account when selecting a mesh.

For instance, when printing with metallic inks, the mesh opening must be at least three to four times the width of the metallic particle size or the ink will not completely transfer to the substrate. Very narrow lines or fine halftone dots would not be advisable in this situation because the mesh required to print them might not provide an acceptable mesh opening.

Stencil selection Rather than influencing which mesh you select for a job, the stencil system you use should be matched to the chosen screen fabric. Keep in mind that the resolution and edge quality characteristics of emulsions and stencil films vary from product to product. You should look for a stencil material that will provide the needed resolution and edge definition with a percent solids content that will be compatible with your mesh count. Very high solid content emulsions will often leave too much emulsion on fine mesh counts, leading to difficulties in printing a complete image. The best choice of stencil material is one the provides the optimum stencil thickness (emulsion-over-mesh ratio) with a smooth stencil surface (low Rz).


Focus on the fabric

Pairing the right mesh with the job at hand means smoother production and better print quality. By combining the guidelines presented here with your own workplace practices and experimentation, you’ll be able to identify the best mesh for every application and use it to its full potential. 




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