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Best Practice in Screen Printing

(January 2001) posted on Thu May 31, 2001

The authors discuss the most commonly overlooked variables in the screen-printing process and how to control them.

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By Carol Swift, Peter Kiddell

If you sharpen the squeegee, do so with great care. The edge provided by the manufacturer is better than what you'll be able to obtain with any but the most effective squeegee-sharpening equipment. The practice of sharpening a squeegee immediately after it's removed from the press also should be avoided because it produces an unstable, and often uneven, printing edge that is more open to attack by solvents in your inks. We recommend allowing squeegees to rest for 24 hours before sharpening them.

Screen and stencil

he stencil defines the image by providing a boundary for the ink and metering how much ink is deposited by the mesh. The accuracy of image replication depends on a stencil's ability to form a gasket-like seal between image and non-image areas during the squeegee stroke.

Stencil thickness and flatness (Rz), as well as the bridging properties of the emulsion across the mesh openings, affect the accuracy of edge definition in your printed images. If the stencil is too smooth (very low Rz), the stencil will tend to stick to the substrate, resulting in reduced print quality. Mesh geometry (mesh count, thread diameter, etc.) determines how much ink will be metered through the mesh. Stencil thickness only effects the ink film thickness on the edge of the image. If the image consists of fine lines or fine dots, stencil thickness can have a considerable effect on the ink deposit. But with large printing areas, stencil thickness only affects edge definition.

Tension in the mesh provides the hidden energy of the process. A screen should be tensioned within the range recommended by the mesh manufacturer. As the tension is increased, snap off can be reduced. Reducing the snap off also reduces stretching of the image and improves image quality. Tension gives stability to the image. However, too much tension can make the screen more fragile and can take the mesh close to or past its elastic limit, resulting in irrecoverable tension loss. For multicolor work, each screen used in the job should have a tension level that falls within 1 N/cm of the other screens for that job.


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