The authors discuss the most commonly overlooked variables in the screen-printing process and how to control them.
The squeegee is the power plant of the process. Essentially, it creates a wave in the ink as it passes across the screen. A wave is a pressure system, so as the ink wave moves over the stencil in front of the squeegee, the pressure causes it to fill mesh openings and push through the stencil until it makes contact with the substrate.
Once the ink is in contact with the substrate, another mechanism takes effect. This is the relationship between the surface energy of the substrate and the surface tension of the ink. If the surface energy of the substrate is higher than the surface tension of the ink, the ink will be attracted to the substrate and will be pulled out of the mesh openings as the screen snaps off the material. This, in simple terms, is the wetting of the substrate by the ink. Note that if the substrate is greasy or naturally has a low surface tension, wetting may not occur and ink transfer will be either incomplete or non-existent.
This wetting effect will only occur efficiently if the tension of the mesh is sufficient to overcome the tackiness of the ink. Should the mesh lie on the ink film for an extended period after the squeegee has passed, ink transfer will only be partial, and much of the ink will be retained in the mesh, resulting in a mottled-looking print. But with proper tension, snap off will occur immediately after the screen has moved on, resulting in a complete transfer of ink from the top of the mesh to the substrate.
The parameters to consider when selecting and using a squeegee include durometer (hardness), flexibility, profile, length, unsupported height, angle, speed, and pressure. Looking solely at durometer, we recommend using harder rather than softer squeegees because they are more stable and last longer. Softer squeegees tend to absorb more solvents from the ink, which causes them to distort. Use of soft squeegees is only advised when the surface to be printed is uneven, and a harder squeegee will not maintain stencil contact with the substrate.
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